US2880802A - Recovery of hydrocarbons from oil-bearing strata - Google Patents

Description

United States Patent RECOVERY OF HYDROCARBONS FROM OIL-BEARING STRATA Paul G. Carpenter, BartlesviHe, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application March 28, 1955 Serial No. 497,421

Claims. (Cl. 166-11) This invention relates to a process for the recovery of hydrocarbons from an oil-bearing earth formation or stratum. A specific aspect of the invention is concerned with secondary oil recovery.

The recovery of oil from a subsurface formation may be accomplished by various means. The oil may be initially produced by natural means such as gas or rock pressure, natural water drive, or solution gas pressure. When these natural forces are insuflicient to cause the oil to flow from producing wells at an economical rate, the oil may be pumped or raised by any desired known method. When pumping methods are no longer effective in producing oil from a particular formation at an economical rate, additional oil may be recovered by the employment of secondary recovery methods, e.g., by water flooding or repressuring the partially depleted sands. These methods involve the injection of water, gas, air, or a combination of these into the formation through one or more input wells and the withdrawal of oil through one or more output or producing wells. It is also the practice in some instances to produce artificial fractures in the producing formation so as to open up the same to flow of additional oil therefrom. It has also been known to burn a fuel gas in the formation surrounding a bore hole so as to cause heavy and semi-solid to solid hydrocarbons (kerogen) in the formation to become sufliciently fluid to flow out. This procedure is disclosed in the Us. patent of Frederick E. Frey, 2,382,471. The technique of hydrafracing is disclosed in copending application of I. W. Marx and H. Parker, Serial No. 473,238. filed December 6, 1954, now Patent 2,813,583. In said application, superheated steam and/or hot water are injectedinto the artificial fractures to heat up the formation and drive additional oil therefrom.

In the secondary recovery of hydrocarbons from subsurface strata by the methods known in the art the ultimate recovery of oil has not been as great as desired and, of course, any oil remaining in the oil-bearing formation after secondary recovery methods have been terminated results in economic waste and loss which is measured by the amount of valuable oil remaining in the formation. 1 have devised a method which is eflfective in increasing the ultimate recovery of oil from an oilbearing subsurface formation.

The principal object'of the invention is to provide a process for the recovery of oil from a subsurface oilbearing formation which requires an artificial driving force to cause the oil to flow out of the formation. Another object is to provide a process for rendering the hydrocarbons in an oil-bearing formation more fluid so that they flow out of the formation more readily. Another object is to provide a process for increasing the ultimate recovery of hydrocarbons from a hydrocarbonbearing stratum. A further object of the invention is to taining the same. It is also an object of the invention to ice provide a method of distilling and cracking hydrocarbons in strata containing the same so as to effectively drive the distilled and cracked hydrocarbons therefrom. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

The invention comprises forming artificial fractures in an oil-bearing stratum followed by burning a combustible material in the artificial crevices so formed so as to heat up the formation and render the hydrocarbons therein more fluid, followed by driving (or otherwise flowing) the same out of the formation into output wells where they are recovered by conventional means.

Hydraulic fracturing to produce artificial cracks or crevices in a formation may be effected by any suitable means, such as the conventional methods of the art, wherein a heavy fluid such as gelled liquid hydrocarbon (napalm) is injected into the formation around the bore hole with sufiicient pressure to fracture the formation and thereafter the gelled liquid hydrocarbon (usually gelled gasoline) is rendered more fluid by the heat of the formation or by the injection into the bore hole and the gelled gasoline in the artificial fractures of a gel-breaking agent or peptizer which converts the gelled material into a readily flowable liquid. Hydraulic fracturing is also effected with other suitable heavy liquids such as heavy crude and/or refined oil. In most instances, it is desirable to introduce a propping agent, such as suspended coarse sand, in the fracturing liquid to hold the cracks open after release of pressure. Any hard solid particulate material may be used as a propping agent. A recent technique entails very rapidly injecting a large volume of heavy oil in a short period of time and this technique has met outstanding success. Sand is used in the last batch of oil injected to hold open the cracks after release of pressure. The same technique can be used after effecting fracturing with gelled gasoline. In the hydraulic fracturing method extremely high pressure is built up in the bore hole utilizing any desired number of pumps working simultaneously to produce the same. Actual fracturing of the formation is accompanied by a sudden drop in pressure which is readily readable on the pressure gauges connected with the injection apparatus.

The process of the invention involves the formation of horizontal fractures in the oil-bearing stratum being produced which extend from an input well to one or more output wells usually positioned in a generally circular pattern around the input well. It has been found that the hydraulic fluid utilized in fracturing a stratum has been recovered in the surrounding output wells at a distance of ,41 mile from the injection well Within 24 hours from the fracturing operation. The spacing of the output wells from the input well or wells is in the range of about 3 8 to /2 mile depending upon the character of the formation through which the bore holes extend. It may be advantageous to space the wells even closer or more remote in formations which warrant such spacing.

After fracturing has been effected so that artificial cracks extend generally horizontally through the formation from one well to another, a combustible material is burned in the crevices so as to heat up the formation and thereby fluidize and drive the oil out of the formation into the crevices from which it flows to the output well or wells. The burning may be initiated and conducted as dislosed in the above-identified Frey patent or by any other suitable means whereby the heat of combustion is imparted to the rock or sand formation above and below the artificial cracks in which the fluid is burned. A mixture of natural gas or other fuel gas and air or oxygen may be utilized as the combustible material. It is also feasible to introduce air at the required temperature for combustion and burn fluid hydrocarbon present in the cracks in the formation after fracturing, thereby utilizing the natural material in the formation for the heating process. One technique used is to conduct the burning step for a given period and follow this with fluid pressure drive through the input well to recover the resulting more fluid hydrocarbon in the surrounding output wells and follow this procedure with another burning step to impart additional heat to the formation and thereby heat up and/or crack additional hydrocarbon from more remote parts of the formation.

A special advantage is obtained when using a gelled gasoline or other gelled hydrocarbon of this character in that this fracturing fluid or semi-fluid provides an excellent combustible material for the burning step and requires only the addition of hot oxygen or an oxygencontaining gas, such as air, to effect the burning. Any viscous combustible fluid which is rendered less viscous when subjected to formation heat and/or the effects of thinning agents are adapted to use in the invention. In operations where it is desired to utilize alternate burning and producing or recovery steps, gelled gasoline may be utilized in the fracturing and first burning step and also in the succeeding burning steps but it is also feasible to utilize in succeeding burning steps another type of combustible material, such as natural gas or the natural hydrocarbon present in the cracks, as the combustible material.

The fracturing and burning procedure of the invention may be advantageously combined with a burning step prior to the fracturing whereby oil and solid hydrocarbon in the formation surrounding the bore hole is rendered more fluid so that it can be readily recovered from the bore hole in conventional manner. Following this preliminary recovery step, the formation is fractured and the procedure of the invention is followed with burning of combustible material in the artificial crevices formed by the fracturing step, followed by fluid drive of the more fluid hydrocarbons (resulting from the burning step) to the surrounding output wells. The invention, as can be readily seen, is applicable to the production or recovery of hydrocarbon material from wells in which a conventional burning procedure has been utilized so as to recover additional hydrocarbon therefrom and increase the ultimate yield of hydrocarbon from the formation.

Another embodiment of the invention entails forcing coarse grit into the artificial fractures or cracks at any stage in the process hereinbefore described, after the initial burning step. The function of the coarse grit forced into the cracks in the formation is to render these cracks less permeable and to render the permeability of the formation to horizontal flow more nearly uniform so that fluid drive through the formation, rendered more permeable by the burning step, is more effective in driving the more fluid oil out of the entire cross section of the formation. The coarse grit injected into the artificial cracks in the formation should be sufficiently large to avoid plugging of the pores of the formation and, of course, not too large to enter the artificial cracks therein. The coarseness of the grit to be used in any given application of the process depends upon the natural pore size of the formation and the thickness of the cracks. Any hard solid granular material, such as sand or rock particles, may be utilized for this purpose.

The following example is presented as illustrative of the invention and is not to be construed as unnecessarily limiting the invention.

Example A fracturing fluid is made up by adding about 3-8% by weight of a hydroxy calcium soap to about 20-30' barrels of crude oil. The hydroxy soap comprises 40-50% of a calcium laurate or a saturated fatty acid soap and 60-50% of a calcium naphthenate. A flow type mixer is used at the well head and the soap and oil are mixed to produce a gel. When substantially the maximum gelation of the hydrocarbon has been obtained, it is pumped into a confined zone of the well where the fracture is to be produced, i.e., a zone which has been isolated by one or more well packers.

The actual type and amounts of fracturing fluid to be used will vary with the size of fracture desired, type of formation, etc. In a well about 4000 feet deep in a sandy formation the above fracturing fluid functions adequately. When the viscosity of the gel reaches 200-300 centipoises, the fluid is injected into the wall at a rate of about 4 barrels a minute. Sand, in the amount of 0.1 pound per gallon of liquid, is injected into the fracture fluid prior to its entry into the well. The sand serves as a prop to hold the fracture open after the pressure is released. After injection of the fracture fluid, there is pumped into the well several barrels of oil to separate the fracturing fluid and the gel breaker which is to follow. This gel breaker consists of 20 barrels of gasoline containing 30 gallons of 60% oil-soluble sulfonates and 40% aromatic petroleum solvents. Following the gel breaker there is pumped into the formation an additional 20-30 barrels of crude oil to displace the gel breaker from the tubing into the well formation.

When the viscous fluid reaches the formation a substantial increase in the pressure will be noted on the surface. The pressure rises until the breakdown pressure is reached, indicating fracturing has occurred. In the sandy formation described this pressure is 2000-4000 p.s.i.g. The depth of the well, nature of the formation, folding of the formation and the like will cause the breakthrough pressure to vary.

After a passage has been established from the input to the output well, a thermite bomb is dropped down the inlet well followed by air at 50-200 p.s.i.g. and combustion is initiated. The small amount of oil and fracturing fluid in the fractured area burn and the hot combustion gases are removed at the outlet well. The burning is continued with about 400 M c.f.d. (1,000 cu. ft. per day.) of a mixture of 10% fuel gas and air at 50 p.s.i.g. The formation is thereby heated so that the oil in areas adjacent the fracture becomes more fluid and flows to the outlet well. In this manner the temperature through the fracture reaches about 11'00 F. The temperature of the adjacent unburned area farthest from the fracture in a 25 foot sand thickness reaches a temperature in the range of -300 F. By increasing the temperature of a typical 14 API crude oil from 100 to F. its viscosity drops from about 630 to 67 centipoises. This great change in viscosity permits higher oil recoveries.

After one year of heating, the plug in the inlet Well is drilled out and natural gas injected into the formation at a pressure of about 200 p.s.i.g. This gives an additional drive to the oil in the heated area and after 10 years the anticipated oil recovery would be 3,'-000-10,'000 barrels per acre. In certain cases it may be advantageous to use water as the driving fluid rather than *gas.

The specific technique to be applied to the recovery of oil from any given formation depends upon 'the'character of the formation as may be readily determined by conventional procedures, such as core drilling and the like. Some of the factors involved in'determining the particular procedure to be used include permeability, porosity and pore size, kerogen content and specific gravity of the oil, thickness of the formation, formation temperature and pressure, concentration of connate water, etc.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessa'ry limitations on the invention.

I claim:

1. A process for fluidizing and driving hydrocarbons from an oil bearing formation penetrated by an input well and an output well comprising forming fractures connecting said wells; propping said fractures open; introducing and depositing a liquid fuel substantially thruout the fractures in the fractured formation; and burning said fuel thruout said fractures by contacting same with air at ignition temperatures injected thru said input well so as to heat the adjacent formation above and below said fractures, thereby rendering the hydrocarbons therein more fluid and driving same into said output well; and recovering produced hydrocarbons from said output well.

2. The process of claim 1 wherein said fuel comprises a gelled liquid hydrocarbon.

3. The process of claim 1 wherein a mixture of normally gaseous hydrocarbon and air is burned in said fractures after substantially complete consumption of said fuel so as to continue the heating of said formation and producing of hydrocarbons therefrom.

4. The process of claim 1 wherein, after the burning step, a gritty particulate material is injected into said fractures to decrease the permeability thereof, the particle size of said gritty material being too large to clog the pores of the surrounding formation; and including subjecting the resulting formation to fluid drive to recover additional hydrocarbons.

5. The process of claim 1 wherein said formation is fractured by forcing gelled hydrocarbon into same under fracturing pressure, whereby said hydrocarbon is distributed thruout the fractures to provide said fuel.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,733 Farris Nov. 10, 1953 2,272,673 Kennedy Feb. 10, 1942 2,596,844 Clark May 13, 1952 2,642,943 Smith June 23, 1953 2,780,449 Fisher et al Feb. 5, 1957 OTHER REFERENCES Development of Subsurface Combustion Drive, by Hester et al., The Petroleum Engineer, November 1954, pp. B-89 and 3-90.

Claims (1)

1. A PROCESS FOR FLUIDIZING AND DRIVING HYDROCARBONS FROM AN OIL BEARING FORMATION PENETRATED BY AN INPUT WELL AND AN OUTPUT WELL COMPRISING FORMING FRACTURES CONNECTING SAID WELLS; PROPPING SAID FRACTURES OPEN; INTRODUCING AND DEPOSITING A LIQUID FUEL SUBSTANTIALLY THRUOUT THE FRACTURES IN THE FRACTURED FORMATION; AND BURNING SAID FUEL THRUOUT SAID FRACTURES BY CONTACTING SAME WITH AIR AT IGNITION TEMPERATURES INJECTED THRU SAID INPUT WELL SO AS TO HEAT THE ADJACENT FORMATION ABOVE AND BELOW SAID FRACTURES, THEREBY RENDERING THE HYDROCARBONS THEREIN MORE FLUID AND DRIVING SAME INTO SAID OUTPUT WELL; AND RECOVERING PRODUCED HYDROCARBONS FROM SAID OUTPUT WELL.