US20110272153A1

US20110272153A1 - Method and System For Enhancing A Recovery Process Employing One or More Horizontal Wellbores

Info

Publication number: US20110272153A1
Application number: US13/132,868
Authority: US
Inventors: Thomas J. Boone; Ivan J. Kosik
Original assignee: ExxonMobil Upstream Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 2009-01-29
Filing date: 2009-12-07
Publication date: 2011-11-10
Also published as: CA2651527C; CA2651527A1; WO2010087898A1

Abstract

The present invention relates generally to a system and method for enhancing a recovery process employing one or more substantially horizontal wellbores. More particularly, the present invention relates to a method and system for enhancing a recovery process employing one or more horizontal wellbores by providing a zone of increased permeability in a hydrocarbon reservoir to facilitate vertical movement of flowable materials through the reservoir to thereby enhance the recovery process. In some embodiments, the reservoir comprises one or more vertical permeability impediments.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Canadian Patent Application 2,651,527 filed Jan. 29, 2009 entitled METHOD AND SYSTEM FOR ENHANCING A RECOVERY PROCESS EMPLOYING ONE OR MORE HORIZONTAL WELLBORES, the entirety of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to a system and method for enhancing a hydrocarbon recovery process. More particularly, the present invention relates to a system and method for enhancing a recovery process employing one or more horizontal wellbores by selectively altering vertical permeability in a reservoir.

BACKGROUND OF THE INVENTION

Oil and gas are nonrenewable natural resources relied upon heavily in the industrialized world. The rising demand for oil and gas, combined with declining conventional resources, has necessitated the development of alternative sources of oil, as well as methods of enhancing recovery from both conventional and alternative resources.
The Athabasca oil sands of Alberta, Canada, contain some of the largest deposits of hydrocarbons in the world. Oil sands are an important alternative source of crude oil, such as bitumen and heavy oil, that can be extracted and processed for fuel. Bitumen is classified as an extra heavy oil, with an API gravity of about 10° or less, referring to its gravity as measured in degrees on the American Petroleum Institute (API) Scale. Heavy oil has an API gravity in the range of about 22.3° to about 10°. The terms heavy oil and bitumen are used interchangeably herein since they may be extracted using similar processes.
Heavy oil can be recovered from oil sands by various methods, including in-situ oil recovery methods. In-situ heavy oil recovery methods are typically applied when a deposit cannot be mined economically due to the depth of overburden. The aim of most in-situ heavy oil recovery processes is to reduce the viscosity of heavy oil in the reservoir to enable it to flow into a well and be produced therefrom. Thermal in-situ heavy oil recovery processes utilize heat, typically provided by steam injection, to mobilize the bitumen. Hydrocarbon solvents and may also be utilized. Such processes frequently employ one or more horizontal wellbores and typically utilize gravity drainage as a fluid drive mechanism. Horizontal wellbores can also be employed in conventional oil or gas recovery processes, including sweeping and flooding processes that utilize displacement as a fluid drive mechanism, and have been shown to offer significant potential to maximize production or injection rates.
A key challenge with the practical application of any recovery process utilizing one or more horizontal wellbores, or otherwise relying on vertical movement of flowable material through a reservoir, is that permeability in most reservoirs is not homogeneous. Reservoir permeability generally refers to the capacity of formation material to permit the passage of fluid through it. The customary unit of measurement is the darcy (D) or millidarcy (mD). While the permeability of oil sand may generally be in the range of a few darcies to a few hundred darcies, the vertical permeability of an oil sands reservoir is frequently disrupted by layers of material having substantially reduced permeability. These layers or matrices can form vertical permeability impediments that inhibit or prevent the vertical flow of materials through the reservoir, relative to the more permeable pay zones, and thus limit production rate and resource recovery. The degree to which a vertical permeability impediment will hinder a recovery process will depend on such factors as the permeability and thickness of the impediment, the particular recovery process utilized and the viscosity of the hydrocarbon being recovered.
Methods of increasing vertical permeability in a reservoir, or enhancing hydrocarbon recovery from a reservoir having vertical permeability impediments, have been proposed. However, each suffer from disadvantages that will be apparent to persons skilled in the art.
A known method of increasing reservoir permeability in general involves hydraulic fracturing of a formation with viscous fracturing fluid containing a proppant material. Such fractures are typically induced from a production or injection well. However, attempts to propagate fractures through compartmentalized or stratified formations have historically yielded unpredictable and poor results, such as insufficient fracture extension and inadequate propping of fractures adjacent to less permeable areas of the formation.
U.S. Pat. No. 6,119,776 to Graham et al. describes a method of stimulating and producing a stratified reservoir by fracturing a horizontal injection well such that a vertical fracture extends through multiple layers of the formation and connects the injection well to a horizontal well below, thereby providing a passage for the flow of hydrocarbons. The lower well is sloped downward and intersects a vertical well that serves as a sump and a production well. Such a system is not designed for gravity drainage and is not suited for reservoirs where fracture extension occurs in a horizontal versus a vertical orientation. Moreover, a thick or substantially impermeable impediment, such as shale, may not be effectively propped open.
U.S. Pat. No. 2,280,851 (1942) to Ranney discloses early methods of drilling deviated wellbores for collection of oil. The wellbore may be undulating and may cross and re-cross a layer of impermeable material such that portions of the wellbore lie in productive stratum on either side of the impermeable layer in order to tap into both zones. The fluids that collect in the wellbore are vacuum pumped to the surface.
U.S. Pat. No. 6,708,764 to Zupanick discloses a drainage pattern of wellbores, which may include one or more undulating wellbores, extending from a main articulated wellbore to provide access to a large subterranean area from the surface. An undulating drainage wellbore may provide access to multiple layers of subterranean gas deposits separated by layers of impermeable material, such as a coal seam. The drainage pattern connects to a vertical wellbore having an enlarged cavity that can act as a sump.
U.S. 2007/0039729 to Watson et al. discloses a method of increasing vertical permeability in a reservoir by drilling a series of substantially vertical piercing wells upwardly and on various angles from a large service tunnel or workspace located deep within the formation. Each vertical well pierces the impermeable layer once from below to provide multiple openings in the impermeable layer. The openings cover a broad area in the horizontal plane of the impermeable layer. The entire process is carried out while maintaining isolation between the service tunnel and the formation fluids. Such a system for increasing vertical permeability in a reservoir is costly and inefficient. Tunneling is rarely practiced for this reason.
There exists a need for enhancing hydrocarbon recovery in general. In particular, there exists a need for enhancing hydrocarbon recovery processes employing one or more horizontal wellbores.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a method and system for enhancing a hydrocarbon recovery process employing one or more horizontal wellbores, which involves selectively altering permeability in the reservoir. The system and method of the invention are particularly useful in reservoirs having one or more vertical permeability impediments.
In a first aspect, there is provided a method of enhancing a hydrocarbon recovery process. The recovery process employs one or more substantially horizontal wellbores. The method comprises providing a zone of increased permeability in a hydrocarbon reservoir to facilitate movement of flowable materials through the reservoir to thereby enhance the recovery process. The zone of increased permeability is preferably provided in a region substantially above or below the one or more horizontal wellbores. In some embodiments, the zone of increased permeability extends generally in a direction substantially parallel the horizontal axis of the one or more horizontal wellbores along at least a portion of the length thereof. In some embodiments, the zone of increased permeability may be provided by an accessory conduit in the reservoir. In some embodiments, the reservoir has one or more vertical permeability impediments.
In another aspect, there is provided a method of enhancing a recovery process employing one or more substantially horizontal wellbores in a hydrocarbon reservoir having at least one vertical permeability impediment. The method comprises providing at least one accessory conduit in a region substantially above or below at least one said one or more substantially horizontal wellbores. The at least one conduit extends generally in a direction substantially parallel to the horizontal axis of the one or more horizontal wellbores along at least a portion of the length thereof.
In another aspect, there is provided a method of recovering hydrocarbons from a reservoir having at least one vertical permeability impediment. The method comprises providing a production well having a substantially horizontal portion for collection of production material comprising hydrocarbons and providing an injection well having a substantially horizontal portion for injection of injection material. The method further comprises providing at least one accessory conduit in the reservoir in a region substantially above or below the production well and/or the injection well and extending generally in a direction parallel to the production well and/or the injection well along at least a portion of its length. The conduit has one or more portions that extend through the vertical permeability impediment to facilitate movement of the production material and/or the injection material through the reservoir. The method further comprises recovering the hydrocarbons from the production material.
In another aspect, there is provided method of increasing permeability of at least one vertical permeability impediment in a reservoir to enhance a hydrocarbon recovery process employing one or more horizontal wellbores. The method comprises providing at least one accessory conduit in the reservoir in a region substantially above or below the one or more horizontal wellbores. The conduit has one or more portions that extend through the at least one vertical permeability impediment to facilitate movement of flowable material therethrough to enhance the recovery process.
In another aspect, there is provided a method of enhancing a hydrocarbon recovery process employing one or more horizontal wellbores in a reservoir having at least one vertical permeability impediment. The method comprises increasing the effective permeability of the at least one vertical permeability impediment in a region substantially above or below the one or more horizontal wellbores to facilitate the movement of flowable materials through the at least one vertical permeability impediment to enhance the recovery process.
In another aspect, there is provided a method of enhancing recovery process performance in a hydrocarbon reservoir having at least one vertical permeability impediment. The method comprises providing a first wellbore having a substantially horizontal portion for collection of hydrocarbons. The method further comprises providing at least one accessory conduit extending through the reservoir in a region substantially above or below the first wellbore. The accessory conduit has one or more portions that extend through the vertical permeability impediment to facilitate movement of flowable materials therethrough to enhance recovery.
In another aspect, there is provided an in situ heavy oil recovery process for recovering hydrocarbons from a reservoir having one or more vertical permeability impediments. The method comprised providing one or more horizontal wellbores in the reservoir, including a production wellbore, and increasing the effective permeability of the one or more vertical permeability impediments above the production wellbore in a region extending along at least a portion of the length of the production wellbore to facilitate the movement of flowable materials through the one or more vertical permeability impediments and toward the production wellbore.
In yet another aspect, there is provided a system for recovering hydrocarbons from a reservoir. The system comprises one or more substantially horizontal wellbores, including a production wellbore, and at least one accessory conduit located substantially above or below the one or more substantially horizontal wellbores for enhancing movement of flowable materials through the reservoir to thereby enhance recovery. In some embodiments, the reservoir has at least one vertical permeability impediment and the accessory conduit has one or more portions that extend through the at least one vertical permeability impediment to facilitate movement of flowable materials therethrough.
In some embodiments, the recovery process is a conventional oil recovery process or a gas recovery process. In some embodiments, the recovery process is a heavy oil recovery process. In some embodiments, the heavy oil recovery process is steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), steam flooding or a derivative thereof
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of embodiments of the invention in conjunction with the accompanying figures. The scope of the invention is not limited to the exemplary embodiments described herein. Furthermore, the figures are for illustrative purposes and may not be exact representations.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the attached figures, wherein:

FIG. 1 is an end view of a typical SAGD system in a reservoir;

FIG. 2 is an end view of a typical SAGD system in a partitioned reservoir having two vertical permeability impediments;

FIG. 3 is an end view showing multiple SAGD well pairs in a partitioned reservoir having two vertical permeability impediments;

FIG. 4 is an end view of a SAGD system in a partitioned reservoir having two vertical permeability impediments, wherein a zone of increased permeability is created in a region substantially above the SAGD well pair that extends through the vertical permeability impediments to facilitate movement of flowable material through the reservoir to enhance the recovery process, in accordance with an embodiment of the invention;

FIG. 5 is a side (a), top (b) and end (c) view of a partitioned reservoir showing an undulating accessory conduit that intersects two vertical permeability impediments to thereby create a zone of increased permeability to facilitate movement of flowable material through the reservoir to enhance the recovery process, in accordance with an embodiment of the invention;

FIG. 6 is a side (a), top (b) and end (c) view of a partitioned reservoir showing a pair of laterally offset undulating accessory conduits that intersect two vertical permeability impediments, in accordance with an embodiment of the invention;

FIG. 7 is a side (a), top (b) and end (c) view of a partitioned reservoir showing a pair of stacked undulating accessory conduits each intersecting a vertical permeability impediment, in accordance with an embodiment of the invention;

FIG. 8 is a side (a), top (b) and end (c) view of a partitioned reservoir showing a main lateral accessory conduit having lateral offshoots that intersect two vertical permeability impediments, in accordance with an embodiment of the invention;

FIG. 9 is a side (a), top (b) and end (c) view a partitioned reservoir showing two lateral accessory conduits each having perforations extending therefrom that puncture a vertical permeability impediment, in accordance with an embodiment of the invention;

FIG. 10 is a side (a), top (b) and end (c) view of a partitioned reservoir showing a main lateral accessory conduit with multiple propped fractures extending therefrom that penetrate two vertical permeability impediments, in accordance with an embodiment of the invention;

FIG. 11 illustrates a reservoir having two vertical permeability impediments, wherein a zone of increased permeability is created that extends through the vertical permeability impediments in a region substantially below the production well to improve primary natural gas, primary conventional oil, or water-drive conventional oil production from the reservoir, in accordance with an embodiment of the invention;

FIG. 12 illustrates a reservoir having two vertical permeability impediments, wherein a zone of increased permeability is created that extends through the vertical permeability impediments in a region substantially above the production well to improve primary oil or top-gas driven production from the reservoir, in accordance with an embodiment of the invention;

FIG. 13 illustrates a gas flooding process in a reservoir having two vertical permeability impediments, wherein zones of increased permeability are created that extend through the vertical permeability impediments in regions substantially below the injection well and substantially above the production well to facilitate the movement of flowable material through the reservoir to improve recovery, in accordance with an embodiment of the invention; and

FIG. 14 illustrates a liquid flooding process in a reservoir having two vertical permeability impediments, wherein zones of increased permeability are created that extend through the vertical permeability impediments in regions substantially above the injection well and substantially below the production well to facilitate the movement of flowable material through the reservoir to improve recovery, in accordance with an embodiment of the invention;

FIG. 15 shows the 2D results of a reservoir simulation modeling a SAGD operation in a continuous reservoir with Athabasca type properties and no vertical permeability impediments, wherein the temperature and gas saturation in the reservoir after 1000 and 2000 days of steam injection are shown;

FIG. 16 shows the 2D results of a reservoir simulation modeling a SAGD operation in a reservoir with Athabasca type properties and two vertical permeability barriers, wherein a zone of increased permeability is provided above the SAGD well pair and extending through the vertical permeability barriers, in accordance with an embodiment of the invention;

FIG. 17 shows graphical results of the reservoir simulations shown in FIGS. 15 and 16, comparing oil and water volumes between a SAGD operation in a reservoir with no vertical permeability impediments and a SAGD operation carried out in accordance with an embodiment of the invention in a reservoir having two vertical permeability barriers;

FIG. 18 shows the 3D results of reservoir simulations modeling a CSS operation in a partitioned reservoir with Athabasca Cold Lake type properties and one vertical permeability barrier, comparing gas saturation in a typical split pay reservoir with a split pay reservoir having a zone of increased permeability extending through the vertical permeability barrier above each of two adjacent horizontal CSS wells, in accordance with an embodiment of the invention;

FIG. 19 shows graphical results of the reservoir simulations shown in FIG. 18, comparing oil and water volumes between a CSS well in a split pay reservoir having a zone of increased permeability that extends through a vertical permeability barrier in accordance with an embodiment of the invention, with a CSS well in a split pay reservoir with no zone of increased permeability.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system for enhancing a hydrocarbon recovery process employing one or more substantially horizontal wellbores. In accordance with embodiments of the invention, the recovery process is enhanced by selectively altering the vertical permeability of the reservoir. It will become apparent to those skilled in the art that embodiments of the present invention may be applied to many different hydrocarbon reservoirs and recovery processes. A hydrocarbon reservoir includes one or more hydrocarbon-containing layers or zones and may also contain one or more permeability impediments. Although it is contemplated that the system and method of the invention can be utilized to enhance recovery process performance in substantially continuous reservoirs, embodiments of the invention are particularly advantageous in reservoirs having one or more vertical permeability impediments.
Vertical permeability impediments are well known the oil and gas industry and present significant challenges to the complete recovery of hydrocarbons, particularly in recovery processes employing one or more substantially horizontal wellbores. A substantially horizontal wellbore is a wellbore that is substantially horizontal along at least a portion of its length. It is understood in the industry that a substantially horizontal wellbore may include deviations from horizontal along its length but that the horizontal portions of such wellbores have an axis in a generally horizontal plane as opposed to a vertical wellbore. Examples of recovery processes that may employ one or more substantially horizontal wellbores include, but are not limited to, steam-assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), steam flooding and various derivatives thereof, as well as conventional oil and gas recovery processes, including various flooding and sweeping processes.
A vertical permeability impediment, as used herein, refers to a portion of a reservoir that has substantially reduced permeability compared to the primary pay zone(s) of the reservoir, such that vertical movement of flowable material through the reservoir is hindered or prevented. Flowable materials comprise fluids and/or gasses, including mobile hydrocarbons. Skilled persons will appreciate that the degree to which a particular vertical permeability impediment will impede the flow of mobile hydrocarbons in a recovery process will depend on such factors as the permeability and thickness of the impediment, the reservoir characteristics, the viscosity of the particular hydrocarbon being recovered, and the particular recovery process utilized. These and other factors should be considered in carrying out the methods of the invention.
A vertical permeability impediment may form a continuous, semi-continuous or discontinuous layer or layers in the reservoir and may be composed of one or more materials. There are many types of vertical permeability impediments that may be encountered. Common examples include baffles and barriers. A baffle in a heavy oil reservoir may, for example, have a permeability that is at least one order of magnitude less than the reservoir matrix permeability, thereby hindering vertical flow in a recovery process. A flow barrier, often referred to as a tight streak in the industry, in a heavy oil reservoir may, for example, have a permeability that is several orders of magnitude less than the reservoir matrix permeability, thereby substantially preventing vertical flow. Tight streaks are often composed of continuous or near continuous layers of shale or mudstone and often follow the bedding planes. They may be a few centimeters to several meters thick with extensive areal extent. Some thick tight streaks may be about 1 m to 3 m thick, and some particularly thick tight streaks may be about 4 m to 7 m, or even higher. The can pose significant impediments to vertical flow in a reservoir.
As an illustrative example, consider a heavy oil reservoir having a matrix permeability of about 1000 mD. A baffle may have a permeability in the range of about 100 mD to about 0.1 mD. A baffle having a relatively low permeability of about 10 mD or less would represent a significant vertical flow impediment in such a reservoir. A flow barrier may have a permeability of less than about 0.1 mD, or no effective permeability. Vertical permeability impediments having low to no effective permeability can essentially partition a reservoir into split pay zones. In such cases, hydrocarbon recovery will primarily occur from the pay zone in which a production well is actually located. It is understood that lighter oils and gases can move more easily through a given space than heavier oils, thus whether a particular impediment will act as a barrier or impediment will depend, in part, on the hydrocarbon encountering the impediment.
In accordance the invention, a recovery process employing one or more substantially horizontal wellbores is enhanced by selectively altering permeability in the reservoir. Vertical permeability is selectively altered by creating a zone of increased permeability at a targeted location or locations in the reservoir. The zone of increased permeability may be created in a region substantially above or below a horizontal wellbore employed in a recovery process. The zone of increased permeability may facilitate the migration of mobile hydrocarbons through a reservoir toward a production well to thereby enhance recovery. The zone of increased permeability may also facilitate the movement of injected materials, such as steam, gas, water, solvents, or polymers, through the reservoir to affect the mobilization or displacement of hydrocarbons in the reservoir. Flowable materials may be drawn into the zone of increased permeability by gravity drainage, pressure differential, or other factors.
In certain embodiments, the zone of increased permeability is created by providing one or more accessory conduits in the reservoir to improve or facilitate the movement of flowable material through the reservoir to thereby enhance recovery. In a reservoir having a vertical permeability impediment, the accessory conduit has one or more portions that extend through the vertical permeability impediment to improve the movement of flowable material therethrough. The conduit may be targeted to select areas of relatively low permeability in the reservoir, thereby creating a higher permeability region(s) to facilitate the movement of flowable materials. The accessory conduit may facilitate the movement of flowable materials from one location to another in a reservoir, or may permit flowable materials to move through a permeability impediment in the reservoir to enhance a recovery process. By selectively facilitating the vertical movement of materials in the reservoir, lateral movement of flowable materials through the reservoir may also be enhanced.
In certain embodiments, a method of enhancing a recovery process employing one or more substantially horizontal wellbores in a reservoir having at least one vertical permeability impediment is provided. The method comprises providing at least one accessory conduit in the reservoir thereby creating a zone of increased permeability. The at least one accessory conduit has a horizontal component, although it may also have vertical and lateral components as well. By horizontal component, it is meant that it extends in a generally horizontal direction in the reservoir along a portion of its length. By generally horizontal direction, it is meant that the conduit travels a horizontal distance in the reservoir, although the conduit may deviate from horizontal along its path and may also have segments or extensions thereof that deviate from horizontal. Preferably, the conduit extends in a direction substantially parallel to a horizontal wellbore used in the recovery process. The conduit may have one or more portions that extend through a vertical permeability impediment to facilitate movement of flowable materials therethrough during the recovery process.
The accessory conduit may be vertically spaced from the one or more horizontal wellbores. In some embodiments, the conduit lies substantially above or below the horizontal segment of one or more horizontal wellbores in the reservoir. Depending on the recovery process, the one or more horizontal wellbores may include a production well, an injection well, or both. In some embodiments, a single substantially horizontal well may serve as both an injection well and a production well. A production well refers to a well in which production materials collect. The production materials may be pumped to the surface from the production well or they may flow into another well for production. Production materials comprise hydrocarbons and may also comprise other flowable materials such as condensed steam, connate water and gas, depending on the application and the reservoir. Hydrocarbons recovered may include conventional oil, natural gas, mobilized bitumen or heavy oil, or combinations thereof.
In certain embodiments, a flowable injection material is injected into the reservoir to aid in mobilizing hydrocarbons through the reservoir. Means of mobilizing hydrocarbons may include, for example, reducing the viscosity of heavy oil or bitumen permitting it to flow by gravity drainage through the reservoir, or providing a drive mechanism to direct the flow of mobile hydrocarbons through a reservoir, or other means. In certain embodiments, injection materials, such as water, gas, steam, solvent, polymers or combinations thereof, may be administered via a substantially horizontal injection well.
In certain embodiments, the accessory conduit is located in a region substantially above an injection well, particularly when applied in a process that relies on gravity drainage.
The conduit need not be directly above the well and may optionally be laterally offset therefrom. However, in most cases, it is advantageous to position the conduit in substantially the same vertical plane as the injection well. The conduit creates a zone of increased permeability above the injection well that may facilitate the movement of flowable injection materials across a vertical permeability impediment. In an in situ heavy oil recovery process, the accessory conduit may facilitate the movement of flowable injection material into one or more upper regions of the reservoir, and may further facilitate the flow of production materials from the upper regions of the reservoir toward the production well below. Enhanced movement of flowable materials through a vertical permeability impediment permits fluid communication between previously separated productive formations and thereby enhances the recovery process. Fluid communication refers to the ability of flowable materials, such as fluid, gas, or oil, to move between different locations in a reservoir.
The location and profile of the accessory conduit can be optimized by the person skilled in the art having an understanding of the invention described herein. Factors to be considered include, for example, the hydrocarbon recovery process to be utilized, the formation characteristics, the location and characteristics of any vertical permeability impediments in the reservoir, and the location of other wellbores in the system, including any production or injection wells. The term accessory conduit indicates that it is not a primary injection or production wellbore in the recovery process.
In some embodiments, the recovery process employs a single horizontal production wellbore, such as in primary production of natural gas or conventional oil. In other embodiments, the recovery process utilizes a single horizontal wellbore as both an injection well and a production well, such as in cyclic steam stimulation (CSS) and various derivatives thereof In other embodiments, the recovery process utilizes separate injection and production wells, such as in steam-assisted gravity drainage (SAGD), and various derivatives thereof, or in flooding and sweeping processes that involve displacement of hydrocarbons in a reservoir.
The one or more accessory conduits may be drilled from the surface. Alternatively, in some embodiments, the one or more accessory conduits may be drilled laterally from another wellbore. For example, the accessory conduit may be drilled laterally from an upper hole section of a deviated injection or production well. In some cases, drilling a lateral wellbore has a significant economic advantage over drilling a new hole from the surface. The accessory conduit may be drilled while drilling a new production or injection wellbore. Alternatively, an accessory wellbore may be retrofit to a preexisting wellbore. In some embodiments, an accessory wellbore is retrofit to a wellbore in a preexisting recovery process to further stimulate recovery from the reservoir. In some retrofit scenarios, where the preexisting holes have already been completed, it may be preferable to drill a conduit from the surface.
Embodiments of the invention will be now described in more detail in reference to exemplary oil recovery processes. Reference to the drawings will also be provided. It should be understood that the scope of the invention is not limited to the particular embodiments described or those exemplified in the drawings.

Application to Heavy Oil Recovery Processes

Various non-limiting features and embodiments of the invention will now be described in reference to an exemplary in situ heavy oil recovery process.
Extraction of bitumen and heavy oil from oil sand reservoirs presents unique challenges due high viscosity, particularly if a heavy oil deposit is buried deep within a reservoir and cannot be surface mined due to the depth of overburden. In general, the aim of an in-situ heavy oil recovery process is to reduce the viscosity of heavy oil in a reservoir to enable it to flow into a well and be produced therefrom. Such recovery processes frequently utilize gravity drainage as an important fluid drive mechanism. Thermal in-situ oil recovery processes utilize heat, typically provided by injection of steam into a reservoir, to reduce the viscosity of the trapped oil and render it mobile. Hydrocarbon solvents or other solubilizing means may also be utilized.
Steam-assisted gravity drainage (SAGD) is a gravity-driven thermal in-situ oil recovery process invented by Roger Butler et al. (see, for example, Butler R. M., Thermal Recovery of Oil and Bitumen, GravDrain Inc., Calgary, Alberta, Canada, 1997). SAGD is used commercially to recover heavy oil or bitumen from reservoirs, particularly in the Athabasca region where the in-situ oil viscosity is very high, typically on the order of 1 million centipoises.
FIG. 1 illustrates an end view of a typical SAGD system and process of the prior art, in which two substantially horizontal wellbores are positioned in spaced-apart vertical relationship to one another in a hydrocarbon reservoir (10). A first substantially horizontal wellbore positioned near the bottom of the hydrocarbon reservoir serves as a production well (12). A second substantially horizontal well is positioned above the production well (12), in thermal communication therewith, and serves as an injection well (14). The wells are typically drilled as two separate wells from the surface using deviated drilling procedures well know to those skilled in the art. The upper hole sections of the wellbores may extend hundreds to thousands of feet into a reservoir. A steel casing is typically cemented into place to stabilize the upper hole sections of the wellbores. A screened or slotted liner typically lines the substantially horizontal portions of the injection well (14) and production well (12) to provide stability to the wellbores while maintaining fluid communication between the wellbore and the formation. Other completion tools and procedures may also be utilized. The drilling and completion of wellbores for an in situ oil recovery process is very time-consuming and expensive.
Once the system is in place, steam is injected into the reservoir via the injection well (14), typically from the surface. As the steam penetrates into the permeable matrix of the formation, a heated steam chamber (16) develops. The size and temperature of the steam chamber (16) increases with continued steam injection into the formation. As the heavy oil within the steam chamber (16) is heated, its viscosity is lowered and it becomes mobile. The mobile oil drains downward through the heated formation material and along the cooler edges (18) of the developing steam chamber (16) toward the production well (12) below. The mobile hydrocarbons collect in the production well (12), generally along with condensed steam and other production materials. The production materials collected in the production well (12) may be pumped to the surface for separation and processing.
The approximate drainage height of the reservoir is represented by h. Development of the steam chamber and flow of mobilized bitumen is limited, in part, by the permeability of the formation.
Butler et al. developed an Equation to estimate the flow rate of bitumen for a reservoir configuration as shown in FIG. 1:
$\begin{matrix} q = 2 L \sqrt{\frac{1.3 k g α ϕΔ S_{0} h}{m v_{s}}} & (1) \end{matrix}$
In Equation (1), q is the bitumen production rate, L is the horizontal well length, k is the reservoir effective permeability, g is the acceleration due to gravity, α is the thermal diffusivity of the reservoir, φ is the reservoir porosity, ΔS_ois the change in oil saturation, h is the drainage height as labeled in FIG. 1, m is parameter dependant upon viscosity-temperature properties of crude oil and ν_sis the kinematic viscosity of crude at steam temperature. This Equation has proven to be a reasonable approximation of bitumen flow rates in the field for reservoirs with sands that can be characterized as continuous clean sands. For these types of sands it is valid to assume constant permeability of the formation, as in Equation 1.
In a reservoir having a vertical permeability impediment, a developing steam chamber will expand vertically in the reservoir until the impediment is encountered, which impediment will inhibit or prevent further vertical expansion of the steam chamber. An exemplary case is illustrated in FIG. 2, wherein two continuous tight streaks (20 a and 20 b) partition a reservoir (22) vertically into thirds. As steam is injected into the injection well (24), the steam chamber (26) can only rise to the height of the first tight streak (20 a) and then it begins to spread laterally. Without wishing to be bound by any particular theory, it is anticipated that approximately one third of the oil in the reservoir will be produced via the production well (28), compared to a similar reservoir with no tight streaks, as shown in FIG. 1. Applying Equation 1, assuming all reservoir properties remain the same except the drainage height, h, which is now h/3, the bitumen production rate would now be q/√{square root over (3)}.
Referring to FIG. 3, one method to recover the heavy oil trapped in the pay zones above the tight streaks (30 a and 30 b) would be to drill an injection well (31) and a production well (32) in each productive zone. Although this is possible, two major disadvantages of drilling complete well pairs in each zone are cost and time. Applying Equation 1, and assuming all three wells pairs produce simultaneously, then the total oil production rate would theoretically be 3 times q/√{square root over (3)}, or √{square root over (3)} q. The total oil production rate increases but in proportion to the square root of the well count.
In accordance with an aspect of the invention, a significant increase in well pair performance and hydrocarbon recovery can be achieved by creating a zone of increased permeability in a region substantially above the SAGD well pair in a reservoir. The zone of increased permeability may be immediately above the SAGD well pair or may be laterally offset therefrom, so long as the location of the conduit facilitates the recovery process. In many cases, it is advantageous to locate the conduit immediately above or below a horizontal wellbore for both performance and cost reasons. If gravity drainage is utilized as a fluid drive mechanism, such as for heavy oil recovery, the zone of increased permeability is ideally provided in a region substantially above (i.e. in substantially the same vertical plane) the production well to facilitate drainage of production material toward the production well. Where a separate injection well is also employed, the conduit may be positioned above the injection well.
In the embodiment shown in FIG. 4, a zone of increased permeability (40) is created in the reservoir (42) substantially above a SAGD well pair (44, 45). The zone of increased permeability (40) is created by providing an accessory conduit (48) in the reservoir. In this embodiment, the conduit (48) has portions extending through two vertical permeability impediments (49 a and 49 b) thereby increasing fluid communication between the pay zones. In the embodiment shown, the vertical permeability impediments (49 a and 49 b) are above the SAGD well pair (44, 45). However, it is also possible that one or more vertical permeability impediments may be located between the production well and the injection well, in which case a conduit could be positioned between the production and injection wells to facilitate fluid communication therebetween. A suitable location and profile for the one or more accessory conduits can be determined by the skilled person. In subsequent sections, various exemplary conduit profiles will be described.
The removal of reservoir material to create an accessory conduit (48) above the SAGD well pair (44, 45) effectively creates a ‘chimney zone’ of increased permeability to facilitate the movement of flowable materials through the reservoir. For instance, the chimney zone allows the steam to rise upwards (white arrow) through the tight streaks toward the top of the reservoir and, also allows the hot bitumen to drain downward (black arrows) to the production well (44). Fluids drawn into the accessory conduit (48) will percolate through the permeable formation material in a gravity-assisted manner toward the production well (44). In the embodiment shown, the accessory conduit (48) permits fluid communication between three pay zones of a reservoir formerly separated by substantially impermeable tight streaks.
The accessory conduit preferably extends in a generally horizontal direction, along at least a portion of its length, substantially parallel a horizontal wellbore utilized in the recovery process. In other words, the conduit preferably has a portion that extends substantially above or below, including laterally offset from, a horizontal wellbore in the reservoir, such as an injection or production well, to facilitate movement of flowable materials through the reservoir. The accessory conduit is preferably substantially separated from the horizontal wellbore by permeable formation material.
It was surprisingly discovered that, by altering the permeability of a relatively narrow vertical region above a well pair in a partitioned reservoir, recovery performance comparable to, or even possibly exceeding, that of a reservoir without vertical permeability impediments can be achieved.
Following the reasoning provided above, assuming that the chimney zone is highly effective in allowing steam to rise and oil to drain, then the oil production rate, q_n, for a reservoir with n equal height layers can be written as:
q _n =√{square root over (n)}q ₁ (2)
where q₁is the flow rate for a case with no tight streaks and only one layer such that the rate is equivalent in this particular case to Equation (1). Stated simply, as the number of layers or tight streaks increases, the potential deliverability of the reservoir increases. Convergent flow in the reservoir chimney zone and other factors may impact the full potential for reservoir deliverability. Nonetheless, this is a remarkable and surprising finding since tight streaks are generally considered to have a negative impact on average reservoir permeability and therefore reservoir productivity in a recovery process.
If the chimney zone was not present and production occurred from only one layer of the reservoir, the production rate, q, would be
q=q ₁ /√{square root over (n)} (3)
which, as expected, is a reduction compared to the case with no tight streaks. Thus, embodiments of the present invention can significantly enhance recovery process performance in a reservoir having one or more vertical permeability impediments. The method and system of the invention provide an effective new means of overcoming a common problem in the industry.
Although SAGD is exemplified above, a skilled person will appreciate that the method and system of the present invention can be applied to any suitable recovery process utilizing one or more horizontal wellbores. The skilled person will further appreciate that there are many different methods that can be utilized to create the one or more accessory conduits to achieve an increase in vertical permeability in the reservoir to thereby enhance recovery process performance. Since the accessory conduit is neither an injection well nor a production well per se, the conduit does not have to meet the requirements (structural, completion, location, etc) of an injection or a production well, which offers significant freedom in designing the accessory conduit profile in order to optimize recovery process performance for a particular reservoir. There is also more freedom in selection of drilling equipment and tools. Using methods known to those skilled in the art, one or more accessory conduits may be drilled form the surface or may be drilled from an upper hole section of an injection or production well. If drilled as a lateral, the lateral conduit may be sealed from the injection or production well from which it was drilled. Although it may not be necessary to complete the conduits with any mechanical devices, the conduits may be completed if desired, utilizing known tools and methods. For instance, various types of slotted liners or sand screens could optionally be placed in the conduits to ensure sand does not collapse into and plug the holes. Alternatively, the conduits could be packed with high permeability gravel or ‘frac sand’. If desired, the diameter of the conduit in the reservoir may be increased by downhole tools, such as reamers, high pressure water jetting bits, section mills with expandable cutting arms, or the like. The latter two techniques would be particularly effective in soft shallow formations at increasing the size and extent of the chimney zone.
As will be appreciated by the skilled person, once having an understanding of the invention, there are a limitless number of different profiles that can be used in providing the one or more accessory conduits. The accessory conduit can have any desired profile that facilitates the movement of flowable materials through the reservoir to enhance recovery process performance. A suitable well profile for a particular application may be selected by a person skilled in the art.
Exemplary methods of providing an accessory conduit include, but are not limited to, drilling one or more conduits from the surface, drilling one or more lateral conduits from an existing wellbore, drilling multiple lateral conduits, drilling one or more conduits with lateral offshoots, drilling one or more conduits and perforating the main conduit, drilling one or more conduits and fracturing the main conduit, and variations, modifications, extensions and combinations thereof. Certain exemplary embodiments will be described further below.
In some embodiments, the accessory conduit may be drilled substantially in the horizontal plane of a vertical permeability impediment to carve a substantially horizontal channel through the impediment to permit vertical movement of flowable materials therethrough. It is understood that the vertical permeability impediment will follow dips and other natural deviations from the horizontal plane but it will nonetheless extend in a generally horizontal plane. The accessory conduit may follow a substantially straight path through the substantially horizontal plane of the vertical permeability impediment or it may follow a deviated path, such as a horizontally undulating path.
Rather than creating an accessory conduit entirely in the plane of the vertical permeability impediment, it may more practical to provide a conduit that extends in a generally horizontal direction while having vertical and/or lateral components such that portions of the conduit, including extensions thereof, extend through the impediment at multiple locations along its length.
The embodiment shown in FIG. 5 is a side (a), top (b) and end (c) view of an undulating accessory conduit (50) drilled laterally from an upper hole section of an injection well (51) to enhance vertical permeability above a well pair (51, 52). The accessory conduit (50) intersects two tight streaks (53, 54) in the reservoir (55). The conduit extends in a generally horizontal direction above the well pair (51, 52) and has both horizontal and vertical components. A key advantage of a lateral conduit design is that it can be drilled at relatively low incremental cost when drilled in conjunction with the steam injection well (51). The lateral hole may be drilled before or after the casing is set for the injection well (51). It is important to note that even a small diameter conduit has an effective permeability orders of magnitude greater than the original reservoir sand, since sand is removed from the reservoir to create it. Furthermore, even by providing a wellbore profile that penetrates the tight streaks (53, 54) at intervals rather than continuously, such as a sinusoidal type profile, the desired increase in effective reservoir permeability can still be achieved, and fluid communication between the different zones of the reservoir will be increased. The intervals may be regular or irregular, and may vary from a few meters to tens to hundreds of meters. A sinusoidal wellbore profile has been drilled in the past as a multi-penetration pilot (MPP) hole for the purposes of reservoir evaluation (see Lee, Extending rotary steerable capabilities to locate thin, complex sands. SPE/IADC Drilling Conference, Netherlands, 2005, SPE/IADC 92152).
FIG. 6 shows another embodiment, where multiple undulating conduits (60, 61) are drilled laterally from an injection well (62) in a reservoir (63) having two tight streaks (65, 66). Such a configuration may be advantageous if more frequent reservoir penetrations are required than can be delivered with a single conduit. Note that the conduits can be laterally offset from the well pair (62, 64) and still be effective.
FIG. 7 shows an alternative configuration where two stacked undulating lateral conduits (70, 71) are used to target two separate tight streaks (72, 73) in the reservoir (74) above a well pair (75, 76). In this configuration, it will generally be possible to penetrate the tight streaks (72, 73) many more times than with a single conduit that penetrates both tight streaks, as shown in FIG. 4. The geologic interpretation, separation of the vertical permeability impediments and drilling capabilities should all be considered by the skilled person in determining the preferred approach.
As an alternative to having more than one main lateral, the embodiment of FIG. 8 shows a multi-lateral conduit (80) drilled in a reservoir (81) having two tight streaks (82, 83). The multi-lateral conduit (80) comprises one main horizontal lateral drilled substantially parallel to the SAGD well pair (84, 85) and multiple short deviated laterals extending through the tight streaks (82, 83) from the main lateral. The short laterals could also be vertical or any other suitable orientation, length or shape. The short laterals are extensions of the conduit and provide the flow pathways through the tight streaks. These short laterals would preferably be placed in a region substantially above the SAGD wells (84, 85), i.e. above the SAGD well pair in substantially the same vertical plane, since this is the region where they will typically be most effective in a heavy oil recovery process utilizing steam or solvent injection and relying on gravity drainage.
Although a more costly approach, chimney zones above the injection well could be created by drilling a series of vertical wells from the surface that penetrate the tight streaks. These wells could also facilitate access to upper reservoir zones for reservoir surveillance purposes and improve the drainage of upper zone bitumen by supplemental injection of steam, gas, solvents and/or polymers. Supplemental production from vertical wells is also possible by producing mobilized bitumen outside the vicinity of the SAGD horizontal well pair, for instance, due to heat conduction from a zone below the tight streak.
Additional techniques, such as fracturing or perforating, may be used in conjunction with the concepts described above to increase the vertical permeability through the tight steaks. FIG. 9 shows an embodiment having two lateral accessory conduits (90, 91) drilled in close proximity to two tight streaks (92, 93) in a reservoir (94). The conduits are located above the well pair (95, 96). Short vertical holes are generated from the main laterals using perforating technologies. Perforations do not typically penetrate more than about a meter or, using special tools, a few meters, into a reservoir, so this configuration would ideally place the main lateral in close proximity to the tight streak. Perforations or similar high permeability tunnels emanating from the main accessory conduit could be generated using a variety of techniques such as explosive charges, lance perforating technologies, side wall coring or water jets. An advantage of this approach is that potentially many more closely spaced holes could be placed in a plane above the SAGD wells to increase the vertical permeability above the wells.
Techniques have been developed for generating multiple fractures along horizontal wells. Relatively small, closely placed fractures could be used in accordance with the invention to increase the effective permeability above a well pair. FIG. 10 again shows a reservoir (100) with two tight streaks (101, 102). In this embodiment, a lateral accessory conduit (103) is shown with spaced fractures (106) extending therefrom in a vertical plane oriented perpendicular to the SAGD wells (104, 105). The fractures may be packed with high permeability proppant to ensure effective permeability after fracturing. A similar approach could be utilized with fractures oriented in a vertical plane parallel to the SAGD wells. However, such horizontal fractures would typically need to be employed from a vertical wellbore.
In some situations, it may be desirable to increase the diameter of an accessory conduit beyond that achievable by normal drilling procedures. For instance, in situations where a tight streak is very thick or a large diameter accessory conduit is desired. As an example, high water pressure jetting bits with variable or adjustable jetting angle could be used to progressively expand the diameter of the precondition hole while circulating out the cuttings. Such jets can also be used to create perforation-like tunnels that emanate from a main accessory conduit, similar to those shown in FIG. 9. Alternatively, an accessory conduit could be a tunnel or cavern created through the plane of the tight streak, for example, by sequentially collapsing and cleaning out formation rock.
Acids, such as hydrofluoric acids, are commonly employed to dissolve some types of rock, and hydrochloric acids are very effective in carbonates. Depending on the mineral composition of the tight streak or other vertical permeability impediment, chemical or acid treatments can be used to create high permeability flow paths through the impediments. The acid could be administered via the accessory conduit and would penetrate the surrounding rock to create tunnels or paths through the vertical permeability impediment. This method would have particular application in carbonate reservoirs or in reservoirs with carbonate tight streaks.
Utilizing methods known in the art, data can be acquired and analyzed from new or existing wells in the reservoir to determine the mechanical properties of the reservoir and the location of any vertical permeability impediments. The data and information obtained may then be utilized in carrying out the methods of the invention. In one embodiment, the production well is the first well drilled in the system and is utilized to acquire reservoir data to determine the location of vertical permeability impediments in the reservoir before drilling an injection well or an accessory conduit.
The accessory conduit includes any lateral offshoots, perforations, fractures or the like extending therefrom to increase vertical permeability through a vertical permeability impediment. Although the accessory conduit may have vertical components such as undulations, lateral offshoots, perforations, fractures or the like, it is said to extend in a generally horizontal direction since the end of the conduit will be located a horizontal distance from the beginning of the conduit although possibly in a different horizontal plane.

Application to Conventional Oil and Gas Recovery Processes

Embodiments of the invention may be applied in conventional light oil or natural gas reservoirs as well to enhance horizontal well utilization. Horizontal wells offer significant potential to maximize production or injection rates. They can also reduce the surface environmental disturbance, or “footprint”, of onshore operations and minimize well accommodation space of offshore platforms. However, as is well known to those in the industry, horizontal well applications are limited by vertical permeability quality and compartmentalization. Operators sometimes try to improve recovery by drilling high angle production or injection wells, or ‘S’ wells, in order to penetrate multiple stratigraphic flow units. The method and system may be applied in continuous reservoirs, as well as complex stratified reservoirs and reservoirs segmented by one or more vertical permeability impediments.
In accordance with embodiments of the invention, the effective vertical permeability of a conventional oil or gas reservoir may be selectively altered to facilitate the movement of flowable material through the reservoir to enhance recovery. In some embodiments, the effective reservoir permeability is increased by providing at least one accessory conduit in the reservoir. In some embodiments, the reservoir may comprise one or more vertical permeability impediments. The accessory conduit may be positioned substantially above, below, lateral or distal to a production well or injection well in the oil or gas reservoir, depending on the particular application and reservoir characteristics.
FIGS. 11 to 14 show particular exemplary embodiments where an altered permeability zone is created substantially within the vertical plane of a horizontal production or injection well in the reservoir. Additional zones of increased permeability can be created at other locations in the reservoir, if desired. The altered permeability zone is beneficial to improve reservoir access and sweep of injection fluids. The reservoir engineer has the option to control the extent of the altered permeability zone and placement of the horizontal wells to best fit the specific reservoir geology and depletion plan. Furthermore, in designing the means of achieving the altered permeability zone, the design may include a mechanism to close off flow and restore the vertical permeability impediment. Early breakthrough of an injected fluid to the producer is an example of where it would be desirable to reverse the altered permeability zone. This could be achieved, for example, by cement squeeze of the preconditioning drill holes used to create the altered permeability zone in the described embodiments.
FIG. 11 shows an embodiment wherein a zone of increased permeability is created in a region substantially below a production well (140) in a reservoir vertical permeability impediments (141, 142). The zone of increased permeability is created by providing an accessory conduit (143) that extends through the two vertical permeability impediments (141, 142) to facilitate flow through the reservoir (black arrows) toward the production well (140). Such embodiment may be utilized, for example, for improving primary natural gas production, primary conventional oil production, or water-drive conventional oil production. Note that by providing a conduit (143) to facilitate vertical flow toward the production well (140), lateral flow in the reservoir is also enhanced. As with the embodiments described above for heavy oil applications, various conduit profiles could be used.
FIG. 12 shows an embodiment wherein a zone of increased permeability is created in a region substantially above a production well (150) in a reservoir. The zone of increased permeability is created by providing an accessory conduit (153) having portions that extend through two vertical permeability impediments (151, 152) in the reservoir to facilitate flow through the reservoir (black arrows) toward the production well (150). Such embodiment may be used, for example, for improving primary conventional oil production or top-gas driven production. Various conduit profiles could be used.
FIG. 13 shows a system for gas flooding, in accordance with an embodiment of the invention, wherein zones of increased permeability are created below a gas injection well (160) and above a production well (161) in the reservoir. The zones of increased permeability are created by providing a first accessory conduit (164) below the injection well (160) and a second accessory conduit (165) above the production well (161), each having portions that extend through two vertical permeability impediments (162, 163) in the reservoir to facilitate the flow of materials through the reservoir toward the production well (161). Various conduit profiles could be used.
FIG. 14 shows a system for liquid flooding, in accordance with an embodiment of the invention, wherein zones of increased permeability are created above a liquid injection well (170) and below a production well (171) in the reservoir. The zones of increased permeability are created by providing a first accessory conduit (174) above the injection well (170) and a second accessory conduit (175) below the production well (171), each having portions that extend through two vertical permeability impediments (172, 173) in the reservoir to facilitate the movement of flowable materials through the reservoir. Various conduit profiles could be used.
In many of the embodiments described herein, a reservoir having two vertical permeability impediments is exemplified. This is merely for consistency and ease in of comparison. It is understood that the invention may be applied in a substantially continuous reservoir or a reservoir having one or multiple vertical permeability impediments. A skilled person, having an understanding of the invention as described herein, will be able to extend and apply the concepts of the invention to various reservoirs and various recovery processes. The methods and systems of the invention are particularly useful for enhancing a recovery process utilizing one or more horizontal wellbores in a reservoir having one or more vertical permeability impediments.
Examples of in-situ heavy oil recovery processes to which embodiments of the present invention may be applied include, but are not limited to, steam-assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), steam flooding and various derivatives thereof, such as solvent-assisted SAGD (SA-SAGD or ES-SAGD), steam and gas push (SAGP), combined vapor and steam extraction (SAVEX), liquid addition to steam enhancing recovery (LASER), vapor extraction (VAPEX), constant steam drainage (CSD), and cyclic solvent process (CSP), as well as various flooding processes often utilizing polymers to enhance displacement. Examples of in-situ conventional hydrocarbon recovery processes to which embodiments of the present invention may be applied include, but are not limited to, primary production of light oil or natural gas, was well as flooding or sweeping processes, employing one or more horizontal wellbores, often utilizing polymers to enhance displacement. These recovery processes can employ one or more horizontal wellbores.

EXAMPLE 1

Heavy Oil Reservoir Simulations Using SAGD

The time and cost involved in conducting field testing is often prohibitive in the oil and gas industry. Therefore, reservoir simulations are commonly relied upon to test new systems and processes before or while field testing is performed. In the present examples, testing was conducted using proprietary reservoir simulation software, which utilizes numerical modeling to mimic fluid flow through porous reservoir media. Reservoirs were modeled using average properties for an Athabasca oil sands deposit.
FIG. 15 shows the 2D results of a reservoir simulation modeling a typical SAGD process in a continuous oil sands reservoir with Athabasca type properties and no vertical permeability impediments. Temperature and gas saturation in the reservoir are shown for 1000 days and 2000 days of steam injection. The injection and production wells are located near the bottom left corner of each block but are not shown. The results illustrate the development of the steam chamber (labeled “Hot”) over time during the SAGD process, with corresponding increase in gas saturation where mobilized bitumen has drained from the heated area into the production well.
FIG. 16 shows the 2D results of a reservoir simulation modeling a SAGD process in a partitioned reservoir with Athabasca type properties and two vertical permeability impediments. The SAGD well pair is located near the bottom left corner of each block but is not shown. The impediments were designed as substantially impermeable tight streaks, allowing conductive heat transfer only, that split the pay zone roughly into thirds.
In accordance with an embodiment of the invention, a zone of increased permeability was created above the SAGD well pair to facilitate the rise of steam and the drainage of heavy oil through the reservoir. The altered permeability zone, labeled as such in the figure, modeled a conduit having a vertical height sufficient to extend through the tight streaks and permit vertical communication between the pay zones within a narrow region above the well pair. The numerical permeability selected for the conduit was of sufficient magnitude to approximate the effective permeability of a 0.6 m×0.6 m simulation grid block penetrated by a wellbore. Sensitivity runs showed that the process was insensitive to the absolute value of the altered permeability zone in the simulator, provided it was sufficiently large relative to the matrix sand permeability. Any effective increase in permeability of the barriers in the region above the SAGD well pair would be expected to enhance the process.
The results show temperature and gas saturation in the reservoir after 1000 days, 1500 days and 2000 days of steam injection. It can be seen that, by creating the altered permeability zone above the SAGD well pair, the steam effectively penetrated the upper zones of the reservoir and all three zones became active SAGD zones after a period of time. The heated bitumen effectively drained through the reservoir from the upper zones and reached the production well in the lower zone, as evidenced by the increase in gas saturation. In the absence of the altered permeability zone, only the oil in the bottom pay zone would be recovered resulting in approximately ⅓ recovery. Thus, applying the method of the invention significantly enhanced a SAGD recovery process in a partitioned reservoir.
Many vertical permeability impediments will permit conductive heat transfer from one pay zone to an adjacent upper pay zone. Without fluid communication between the pay zones, this transferred heat is normally characterized as overburden heat loss, which is considered a detriment to SAGD thermal efficiency. In accordance with the invention however, this heat transfer actually benefits the operation since the zone of altered permeability permits fluid communication between the pay zones. Heat from the lower pay zone can advantageously be transferred to the upper pay zone to aid in mobilizing heavy oil in the upper pay zone, which mobilized oil can then drain downward through the reservoir toward the production well via the altered permeability zone. This voidage also causes a formation pressure drop and further promotes the injected steam to rise and contact cold bitumen in the upper pay zones. The coupled effect of an effective but relatively confined altered permeability zone with conductive preheating showed surprisingly good performance in the split pay reservoir simulation.
FIG. 17 shows the graphical results of a comparison of water and oil volumes for the two reservoir simulations described above. The dashed lines represent water and oil volumes for a typical SAGD operation in a reservoir with no tight streaks. The solid lines represent water and oil volumes for an enhanced SAGD operation carried out in a reservoir having two tight streaks wherein a zone of increased permeability is provided above the SAGD well pair. The oil rates were similar for the two cases for the first 3 years and the final cumulative oil volumes recovered were comparable. This is a significant finding since recovery would generally be much lower in the reservoir with tight streaks. Water rates between the two models were very similar, which is indicative of the steam rates as well.
Further reservoir simulations demonstrated practical application of the method of the invention in complex heavy oil reservoirs and reservoirs having top gas or bottom water.
Although the simulations performed focused on a SAGD process, using 2D and 3D grids, conclusions may reasonably be extrapolated to many applications and processes utilizing horizontal wellbores. The 2D and 3D simulations performed suggested that increasing the height, width and effective permeability of the altered permeability zone is directionally advantageous. However, for practical purposes, reservoir deliverability may be a limiting factor as given, for example, by Equation (1) or subsequent variations. It is also advantageous in most cases to position the altered permeability zone in a region substantially above the SAGD well pair (i.e. above the well pair along substantially the same vertical axis) and to continue the zone in a direction extending substantially parallel to the SAGD well pair along at least a portion of the horizontal length of the SAGD well pair. An altered permeability zone having sufficient vertical height (i.e. a chimney zone) would promote horizontal flow toward the conduit as well as vertical flow through the barriers toward the production well.
While various means of optimizing performance results are described above, it is important to note that performance results do not have to be optimal for the process to be highly effective. Even a narrow diameter conduit that intersects a vertical permeability impediment only at intervals along its length will enhance a SAGD recovery process significantly compared to a typical SAGD operation in such a reservoir. Reservoir characteristics, cost and practicality of creating the altered permeability zone should be considered in designing and implementing the altered permeability zone for a particular application. The characteristics of the vertical permeability impediment(s) should also be considered, including thickness and permeability.

EXAMPLE 2

Heavy Oil Reservoir Simulation of Split Pay Reservoir Using CSS

A numerical simulation study was conducted to evaluate applicability of the invention to cyclic steam stimulation (CSS) with horizontal wellbores. CSS is typically a single well recovery process where the same well receives a fixed volume of steam and is then produced following injection in a cyclic manner. Horizontal wells are increasingly being used to reduce the cost of field development. The horizontal wells are typically located at the base of the effective pay zone and therefore oil recovery is vulnerable to vertical permeability impediments.
A simulation model based on typical properties of the Cold Lake Clearwater formation was constructed. Referring to FIG. 18, the simulation was designed to mimic two adjacent horizontal wells (white) completed with spaced limited entry perforations (LEP), as discussed in SPE Paper 50429, “Targeted Steam Injection Using Horizontal Wells with Limited Entry Perforations”, T. Boone, D. Youck & S. Sun, 1998. The location of the LEP points for the two adjacent horizontal wells are at the vertical corners of the 3D block in the plane of the horizontal wells (white), as is evident by the gas saturation patterns. Two adjacent wells were used to reflect the staggered steam scheduling commonly implemented in a CSS operation.
Using typical cycle steam volumes and schedule the split pay reservoir was subjected to CSS with and without an altered permeability zone. Note the altered permeability zone spans the entire length of the short sides of the block model comprising a zone through the permeability impediment above and parallel with the horizontal wells. Simulation results show that the altered permeability zone allowed access to the upper pay zone and subsequently an increase in oil rate and recovery was achieved compared to the split pay reservoir without an altered permeability zone, as seen in FIG. 19.
FIG. 19 shows the graphical results of a comparison of water and oil volumes for the CSS reservoir simulations described above. The dashed lines represent water and oil volumes for a typical CSS operation in a split pay reservoir. The solid lines represent water and oil volumes for an enhanced CSS operation in a split pay reservoir, carried out in accordance with an embodiment of the invention, wherein a zone of increased permeability was provided above the CSS well, which penetrated a tight streak. Cumulative oil recovery was significantly increased in the presence of the altered permeability zone, without a significant increase in water volume.
The above-described embodiments of the invention are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.

Claims

1. A method of enhancing a hydrocarbon recovery process employing one or more horizontal wellbores, the method comprising:

providing a zone of increased permeability in a hydrocarbon reservoir to facilitate movement of flowable materials through the reservoir to thereby enhance the recovery process.

2. The method of claim 1, wherein the zone of increased permeability is provided in a region substantially above or below said one or more horizontal wellbores.

3. The method of claim 2, wherein the zone of increased permeability extends generally in a direction substantially parallel the horizontal axis of the one or more horizontal wellbores along at least a portion of the length thereof.

4. The method of claim 1, wherein the zone of increased permeability is provided by an accessory conduit in the reservoir.

5. The method of claim 4, wherein the hydrocarbon reservoir has at least one vertical permeability impediment and the accessory conduit has one or more portions that extend through the at least one vertical permeability impediment.

6. The method of claim 1, wherein the recovery process is a gas recovery process, a conventional oil recovery process or a heavy oil recovery process.

7. The method of claim 6, wherein the heavy oil recovery process is steam-assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), steam flooding or a derivative thereof.

8. The method of claim 7, wherein the heavy oil recovery process is SAGD.

9. The method of claim 8, wherein the one or more horizontal wellbores comprises a production wellbore and an injection wellbore, and wherein the at least one accessory conduit is provided in a region substantially above the injection wellbore.

10. The method of claim 9, wherein the accessory conduit is drilled laterally from an upper hole section of the injection wellbore.

11. The method of claim 4, wherein the at least one vertical permeability impediment is a baffle or a barrier.

12. A method of enhancing a recovery process employing one or more substantially horizontal wellbores in a hydrocarbon reservoir having at least one vertical permeability impediment, the method comprising:

providing at least one accessory conduit in a region substantially above or below at least one said one or more substantially horizontal wellbores, the at least one conduit extending generally in a direction substantially parallel to the horizontal plane of the one or more horizontal wellbores along at least a portion of the length thereof.

13. The method of claim 12, wherein the conduit has one or more portions that extend through the vertical permeability impediment to facilitate movement of flowable material therethrough during the recovery process.

14. The method of claim 12, wherein the one or more substantially horizontal wellbores comprises a production well for collection of flowable production material comprising hydrocarbons.

15. The method of claim 12, wherein the one or more substantially horizontal wellbores comprises an injection well for injection of flowable injection material into the reservoir.

16. The method of claim 15, wherein the injection material comprises water, steam, gas, solvent, polymer or a combination thereof to aid in mobilizing hydrocarbons in the reservoir.

17. The method of claim 12, wherein the at least one accessory conduit extends through the at least one vertical permeability impediment along the plane of the vertical permeability impediment.

18. The method of claims 12, wherein the at least one accessory conduit comprises an undulating conduit that crosses one or more said at least one vertical permeability impediment at intervals.

19. The method of claim 12, wherein the at least one accessory conduit comprises a substantially horizontal conduit having a plurality of lateral offshoots that extend through one or more said at least one vertical permeability impediment.

20. The method of claim 12, wherein the at least one accessory conduit comprises a substantially horizontal main conduit having a plurality of perforations extending therefrom that extend through one or more said at least one vertical permeability impediment.

21. A method of recovering hydrocarbons from a reservoir having at least one vertical permeability impediment, the method comprising:

providing a production well having a substantially horizontal portion for collection of production material comprising hydrocarbons;

providing an injection well having a substantially horizontal portion for injection of injection material;

providing at least one accessory conduit in the reservoir in a region substantially above or below the production well and/or the injection well and extending generally in a direction parallel to the production well and/or the injection well along at least a portion of its length, the conduit having one or more portions that extend through the vertical permeability impediment to facilitate movement of the production material and/or the injection material through the reservoir; and

recovering the hydrocarbons from the production material.

22. The method of claim 21, wherein the injection material comprises steam, and wherein the at least one accessory conduit creates a zone of increased permeability above the injection well that facilitates the vertical expansion of a steam chamber into one or more upper regions of the reservoir as well as gravity drainage of mobilized heavy oil from the upper regions toward the production well.

23. A method of increasing permeability of at least one vertical permeability impediment in a reservoir to enhance a hydrocarbon recovery process employing one or more horizontal wellbores, the method comprising:

providing at least one accessory conduit in the reservoir in a region substantially above or below the one or more horizontal wellbores, the conduit having one or more portions that extend through the at least one vertical permeability impediment to facilitate movement of flowable material therethrough to enhance the recovery process.

24. A method of enhancing a hydrocarbon recovery process employing one or more horizontal wellbores in a reservoir having at least one vertical permeability impediment, the method comprising:

increasing the effective permeability of the at least one vertical permeability impediment in a region substantially above or below the one or more horizontal wellbores to facilitate the movement of flowable materials through the at least one vertical permeability impediment to enhance the recovery process.

25. A method of enhancing recovery process performance in a hydrocarbon reservoir having at least one vertical permeability impediment, comprising:

providing a first wellbore having a substantially horizontal portion for collection of hydrocarbons; and

providing at least one accessory conduit extending through the reservoir in a region substantially above or below the first wellbore, the accessory conduit having one or more portions that extend through the vertical permeability impediment to facilitate movement of flowable materials therethrough to enhance recovery.

26. An in situ heavy oil recovery process for recovering hydrocarbons from a reservoir having one or more vertical permeability impediments, comprising:

providing one or more horizontal wellbores in the reservoir, the one or more horizontal wellbores including a production wellbore; and

increasing the effective permeability of the one or more vertical permeability impediments above the production wellbore in a region extending along at least a portion of the length of the production wellbore to facilitate the movement of flowable materials through the one or more vertical permeability impediments and toward the production wellbore.

27. A system for recovering hydrocarbons from a reservoir, the system comprising:

one or more substantially horizontal wellbores, including a production wellbore; and

an accessory conduit located substantially above or below the one or more substantially horizontal wellbores for enhancing vertical movement of flowable materials through the reservoir to thereby enhance recovery.