WO2016167860A1

WO2016167860A1 - Through casing hydraulic fracture mapping

Info

Publication number: WO2016167860A1
Application number: PCT/US2016/017015
Authority: WO
Inventors: Qing H. LIU; Zhiru YU; Jianyang ZHOU
Original assignee: Duke University
Priority date: 2015-04-15
Filing date: 2016-02-08
Publication date: 2016-10-20

Abstract

Systems and methods for through casing hydraulic fracture mapping are disclosed. According to an aspect, an induction logging tool for mapping the earth's fractures is disclosed that includes a set of transmitting coils, a set of bucking transmitting coils, and a set of receiver coils. The induction logging tool may be lowered into a borehole at multiple depths. Various recordings of the electrical and magnetic signals may be captured using the receiver coils set. This data may subsequently be processed using an inverse algorithm to provide a mapping of the earth's fractures.

Description

THROUGH CASING HYDRAULIC FRACTURE MAPPING

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U. S. Provisional Patent Application Number 62/147,745, filed April 15, 2015 and titled THROUGH CASING HYRAULIC FRACTURE MAPPING, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to earth fracture mapping. More particularly, the present disclosure relates to systems and methods for through casing hydraulic fracture mapping.

BACKGROUND

[0003] Hydraulic fracturing has being performed for more than 60 years in more than a million wells. Despite the long history in hydraulic fracturing, the growth of fractures over time is not well understood. During a hydraulic fracturing process, high pressure fracturing fluid or pumping fluid is injected to a target geological formation (e.g., a tight shale formation) through a borehole and creates fractures in the target geological formation. Proppants (e.g., sand) are typically injected with fracturing fluid to keep fractures open. Once fractures are produced in the formation, natural gas and oil can then flow out from the formation to a production well.

[0004] The creation of hydraulic fractures can be monitored in real time via micro- seismic techniques. However, these techniques are only effective during a fracturing process. After hydraulic fractures are created, the growth of fractures remains unknown.

[0005] For at least the aforementioned reasons, there is a need of improved techniques to effectively characterize fractures.

SUMMARY

[0006] Embodiments of the present disclosure provide induction logging techniques for the electrical investigation of earth formations. The techniques include lowering of an induction logging tool into a borehole at multiple logging depths of the borehole. The techniques further include exciting transmitting coils using a sinusoidal signal at multiple frequencies. Further, the techniques include recording data using multiple receivers on the induction logging tool at different logging depths, and the data so recorded is processed using an inverse algorithm so as to generate a mapping of the earth formations, such as fractures. [0007] Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the present disclosure are described in detail herein and are considered a part of the present disclosure. For a better understanding of the present disclosure with advantages and features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing summary, as well as the following detailed description of various embodiments, is better understood when read in conjunction with the drawings provided herein. For the purposes of illustration, there is shown in the drawings exemplary embodiments; however, the presently disclosed subject matter is not limited to the specific methods and instrumentalities disclosed.

[0009] FIG. la depicts an environment for injection of contrast agents in fractures.

[0010] FIG. lb is a line diagram to depict interactions of EM waves and received EM responses from fractures.

[0011] FIG. lc is a block diagram of a logging tool design according to embodiments of the invention.

[0012] FIG. 2 is a block diagram of the transmitter control of the induction logging tool.

[0013] FIG. 3 is a block diagram of the receivers control unit of the induction logging tool.

[0014] FIG. 4 is a block diagram of the bucking control unit of the induction logging tool.

[0015] FIG. 5 is a line diagram of an induction logging tool according to embodiments of the invention.

[0016] FIG. 6 is a flow chart describing an induction logging technique according to embodiments of the invention.

[0017] FIG. 7 is a line diagram depicting an apparatus for experimental validations.

[0018] FIG. 8 is a graph depicting validations in free space in a cased-hole environment.

[0019] FIG. 9 is a graph depicting conductivity reconstruction in tap water in a cased- hole environment.

[0020] FIG. 10 depicts measured data and simulation in tap water in a cased-hole environment.

[0021] FIG. 11 depicts conductivity reconstruction in salty water in a cased-hole environment. [0022] FIG. 12 depicts measured data and simulation in salty water in a cased-hole environment.

DETAILED DESCRIPTION

[0023] The presently disclosed subject matter is described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or elements similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, although the term "step" may be used herein to connote different aspects of systems and/or methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.

[0024] FIG. lc illustrates a block diagram of an induction logging system 100 according to embodiments of the present disclosure. Referring to FIG. lc, the induction logging system 100 includes a transmitters control unit 102, a bucking control unit 104, a receivers control unit 106, a reference clock 108, and a central control unit 110. The central control unit 110 may further include a temperature sensor 112 and a data storage 114. The central control unit 110 may be used for system initialization, logging control, and data storage. For example, the central control unit 110 may monitor changes in excitation currents on both transmitters and bucking transmitters. Further, the central control unit 110 may compensate changes if needed and collect measurement data from each receiver by controlling switch arrays.

[0025] FIG. 2 illustrates a block diagram of an example transmitters control unit 102 according to embodiments of the present disclosure. Referring to FIG. 2, the transmitters control unit 102 is configured to transmit electric or magnetic signals towards target formations or fractures. The transmitters control unit 102 can drive one or more transmitters 1028. Depending on the type of signals to be sent, transmitters can be, but are not limited to, small dipoles or magnetic coils oriented in three different axes that can emit electric or magnetic fields in arbitrary orientations. The transmitters control unit 102 can include a sub-processing unit 1024 that is configured to receive and implement commands from the central control unit 110 shown in FIG. lc. It can control the output amplitude and phase of programmable sine wave generator 1020, monitor the amplitude and phase of currents on transmitter 1028, and upload requested data to the central control unit 110. The transmitters control unit 102 can further include a waveform amplifier 1022 that is configured to enhance driving power for the transmitter 1028. This may be done so as to increase the incident signal level. In embodiments of the present disclosure, the waveform amplifier 1022 can be, but is not limited to, a linear power amplifier. Alinear power amplifier is an electronic circuit whose output is proportional to its input; however, it is capable of delivering more power to a load.

[0026] After system calibration, the amplitude of the phase currents on transmitter 1028 may be obtained through a transistor Rl 1026. The programmable sine wave generator 1020 is connected to the reference clock 108. The programmable sine wave generator 1020 is configured to compensate the fluctuation in currents due to changes in ambient temperature. Therefore, excitation currents on the transmitter 1028 remain the same during logging.

[0027] FIG. 3 illustrates a block diagram of an example receivers control unit 106 of the induction logging tool in accordance with embodiments of the present disclosure. Referring to FIG. 3, the receivers control unit 106 can include multiple receivers 1068a... 1068n. The multiple receivers 1068a... 1068n are configured to detect weak signals propagating back from the target formation. Further, the multiple receivers 1068a... 1068n are made with high sensitivity. The multiple receivers 1068a... 1068n are followed by corresponding linking pre-amplifiers 10602a... 10602η. The pre-amplifiers 10602a... 10602η are configured for low noise amplification, driving power capability enhancement, and common-mode interference rejection.

[0028] The receivers control unit 106 further includes a switch array 1066, which is configured for selecting signals from the multiple receivers 1068a... 1068n. The switch array 1066 is in further communication with an amplifier and band pass filter 1064. The amplifier and band pass filter 1064 may be configured to receive the selected signals from the switch array 1066 and further enhance and filter the signals thus received. The amplifier and band pass filter 1064 increase the signal-to-noise ratio of the selected pre-amplified signal. The useful weak signals selected from strong noise by using a lock-in amplifier 1060. The lock-in amplifier 1060 is also in communication with the reference clock 108. The receivers control unit 106 further includes a sub-processing unit 1062 which is in communication with the lock-in amplifier 1060 to receive the amplitude and phase of the signals processed by the lock-in amplifier 1060. This data can be then uploaded to the central control unit 110. Further, during measurement process, the sub-processing unit 1062 is configured to control the switch array 1066.

[0029] FIG. 4 illustrates a block diagram of an example bucking control unit 104 according to embodiments of the present disclosure. Referring to FIG. 4, even though contrasts of fractures to be detected are greatly enhanced by injected nanoparticles, secondary fields or EM responses from fractures are still small due to the small relative volume of the fractures. In order to make full use of the dynamic range and sensitivity of the receiver system, bucking may be needed.

[0030] There are multiple bucking techniques that may be implemented in accordance with embodiments of the present disclosure. In an embodiment of the present disclosure, a concentric active bucking technique may be utilized. This bucking technique can null the primary fields only at a receiver location without affecting the primary field outside of a borehole. The bucking control unit 104 may include multiple bucking transmitters 10402a... 10420η. The bucking control unit 104 may also include a switch array 1048, a waveform amplifier 1042, a programmable sine wave generator 1040, a sub-processing unit 1044, and a current detection resistor R2 1046. The sub- processing unit 1044, which is in communication with the central control unit 110, may take commands from the central control unit 110. It is further configured to control output amplitude and phase of the programmable sine wave generator 1040. It is also further configured to monitor the amplitude and phase of currents on bucking transmitters 10402a... 10402η and upload requested data to the central control unit 110. The bucking control unit 104 further includes a waveform amplifier 1042 that is configured to enhance driving power for the bucking transmitters 10402a... 10402η. This is done so as to increase the incident signal level. In an embodiment of the present disclosure, the waveform amplifier 1042 can be, but is not limited to, a linear power amplifier. A linear power amplifier is an electronic circuit whose output is proportional to its input; however, it is capable of delivering more power to a load.

[0031] After system calibration, the amplitude of the phase currents on bucking transmitters 10402a... 10402η may be obtained through the current detection resistor R2 1046. The programmable sine wave generator 1040 is connected to the reference clock 108. The programmable sine wave generator 1040 is configured to compensate the fluctuation in currents due to changes in ambient temperature. Therefore, excitation currents on the bucking transmitters 10402a... 10402η remain the same during logging. This bucking technique implementation may be implemented readily, because it applies the active bucking technique that allows the magnitude and phase of the bucking transmitter to be adjusted individually to optimize the nulling effect. This active bucking technique can also achieve minimum effects on secondary fields from fractures. Therefore, secondary fields received at receivers with bucking may be the same as secondary fields received at receivers without bucking transmitters.

[0032] FIG. 5 illustrates a line diagram of an example induction logging tool according to embodiments of the present disclosure. Referring to FIG. 5, the induction logging tool may include an elongated cylindrical body including a transmitting coil set 202. In embodiments of the present disclosure, the transmitting coil set may include multiple coils with different orientation. The orientation of the transmitting coils 202 may be in three different axes, i.e., each in axis u, axis v, and axis w. The induction logging tool may include a bucking coil set 204, which may also be configured with orientation of three coils in three different axes. Further, a receiving coil set 206 is also included in the induction logging tool 100. The receiving coil may be a three coil set with the three coils having orientation in different axes, namely u, v, and w. As mentioned, transmitters and receivers are placed in three orthogonal orientations. The orientations can be denoted as u, v, and w. Assume E_uv is the electric field transmitted by a transmitter oriented in u direction and received by a receiver oriented in v direction. There are a total of 9 components in electric field and 9 components in magnetic field to be collected, namely

[0033] Data in different components contains different information of the formation and fractures. When these data are processed, fractures can be properly characterized.

[0034] FIG. 6 is a flow chart depicting an example induction logging technique 600 according to embodiments of the present disclosure. Referring to FIG. 6, the technique 600 may begin at step 602 wherein the induction logging tool 100 is lowered into a borehole for mapping of fractures. In embodiments of the present disclosure, the borehole may be filled with borehole fluid or oil-based mud. These materials form a homogeneous background with weak or zero conductivity. In embodiments,, of the present disclosure, the borehole may be vertical to the ground (vertical borehole), deviated, or parallel to the ground (horizontal borehole). Outside of the borehole, there can be a metallic or fiberglass casing or casing made with other material that can support the near borehole structure. The casing has a certain thickness that ranges from a few millimeters to several centimeters. The metallic casing may have high conductivity and/or a certain level of magnetic permeability. A couple inches of cement layer may be placed outside of the casing. There may also be multiple casing-cement layers outside of the borehole.

[0035] At step 604 of the technique of FIG. 6, the depth of the induction logging tool 100 is determined. If the induction logging tool 100 has reached a pre-assigned logging depth, the technique 600 proceeds to step 612 wherein the data captured is processed using an inverse algorithm. However, if the induction logging tool 100 has not reached a pre-assigned logging depth at step 606, the transmitter coils 202 are excited. The transmitter coils 202 may be excited by a sinusoidal signal at one or multiple given frequencies between about 10 Hz and about 100 Hz. Transmitters with orientations of u, v, and w may be excited simultaneously or in a sequence. At step 608, the corresponding bucking coils 204 are excited and data are collected using the receiver coils 206. At step 610, the induction logging tool 100 is moved on to next position for data collection. This data is then processed at step 612 using the inverse algorithm. The data processed by the inverse algorithm at step 612 is outputted at step 614 as fracture mapping.

[0036] In an example implementation, EM fields received by the receivers 11068a... 1068n can be denoted as the total electric field, E^t, or the total magnetic field, H^t. Total fields can be considered a sum of incident fields (E H^l) and scattered fields (E^s, H^s). Incident fields are the electric and magnetic fields that propagate into the formations without the existence of fractures. Scattered fields are the results of interactions between incident fields and fractures. Therefore, the following equations result:

E^t = E^l + E^s (1) H^s (2)

According to the volume equivalent theorem, scattered fields can be viewed as radiated fields from equivalent volume sources. The equivalent sources are related to total fields and contrasts of fractures. As a result, total fields received by receivers on a logging tool can be described by

E^': ~ E^c ÷ ]ω ÷ - vV A + ~V x F (3)

(4) where ω is the angle frequency, e_b and μ_ύ are background permittivity and magnetic permeability and k_b is the background wave number. A and F are vector potentials that may be calculated by

A = ]ωμ_ί} j_y g(r, r') _e(r')D(r')dr' (5)

and

χ = ^JL (7)

*=b

Xu - (8) where, D and B are electric and magnetic flux densities, respectively, e_r and μ_τ are effective permittivity and magnetic permeability of fractures, respectively, and g is the scalar Green's functions in homogeneous background. χ_ε and χ_μ are dielectric and magnetic contrasts, respectively.

It can be seen that equations (3) and (4) are functions of contrasts and fractures characteristics. They can be rewritten as

M^l = Κχ_μ) (10)

Since the total fields are available at receiver locations, fracture contrasts and geometries can be solved. The above equations may be solved by using multiple methods. The methods can be either an iterative method, distorted born approximation method, numerical mode matching, or a BCGS- FFT method or its improved version the mixed-order BCGS-FFT method.

[0037] FIG. 7 depicts an apparatus 700 for experimental validation of the method 600. The scale factor for this system is 1 : 1000 in frequency and 40: 1 in conductivity and thus 1 :200 in dimensions.

[0038] The whole experiment is done under water inside a water tank 704. Water filled in the water tank 704 has a weak conductivity that simulates target formation environment. A waterproof metallic casing 706 is set up in the water tank. This metallic casing is 78 μπι in thickness and has an electric conductivity of 5.96xl0⁷ S/m. An induction logging tool 710 working at 100 kHz is made. It can move freely in air inside the metallic casing suspended in the water tank 702 through a linear motor 702. Athin layer of high conductivity material 708 is placed inside the water tank 704 to simulate the response of a contrast-enhanced fracture.

[0039] The forward modeling algorithm is validated by comparing secondary fields received in logging tool while logging in air with the existence of one fracture. The comparison is shown in FIG. 8. The logging curves from different receivers are shown. Excellent agreement is indicated in this comparison.

[0040] In the first experiment, the water tank 704 is filled with tap water that has a conductivity of 0.0293 S/m. The electric conductivity mapping result obtained from logging data is shown in FIG. 9. In FIG. 9, a fracture can be easily identified and its conductivity is clearly shown. This fracture is successfully mapped with the correct electric conductivity value. Receiving signals are estimated using the reconstructed fracture. The comparison of these estimated receiving signals and measured signals (logging data) is shown in Figure 10. Again, excellent agreement can be observed.

[0041] The second experiment is done in the water tank 704 filled with salty water. The salty water has higher conductivity at 1.02 S/m. As shown in FIG. 11, the fracture is correctly mapped in salty water background. And, the comparison of estimated receiving signal and measured data shown in FIG. 12 also indicates an excellent agreement.

[0042] As a conclusion, the experiments are very successful in mapping fractures with higher conductivity. Since these lab experiments are scaled by field measurements, this technique will also work in the field while the working frequency is lowered to around 10 - 100 Hz and dimensions are scaled up 200 times.

[0043] Aspects of the present subject matter are described herein with reference to flowchart illustrations and/or block diagrams of methods and apparatus (systems) according to embodiments of the subj ect matter. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

[0044] While there has been shown and described herein what are presently considered the preferred embodiments of the present disclosure, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the present disclosure as defined by the appended claims.

[0045] The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present subject matter. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

[0046] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the present disclosure. Indeed, the novel methods, devices, and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions, and changes in the form of the methods, devices, and systems described herein may be made without departing from the spirit of the present disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present disclosure.

Claims

CLAIMS What is claimed is:

1. A method comprising:

lowering an induction logging tool into multiple logging depths of a borehole to collect data associated with a geological formation;

exciting a plurality of transmitter coils by a sinusoidal signal at multiple frequencies at each logging depth;

using a plurality of receivers to record, at each logging depth, data that includes one or multiple components of electric or magnetic fields; and

processing the recorded data by an inverse algorithm to provide a mapping of the earth formation.

2. The method of claim 1, further comprising filling the borehole with one of borehole fluid and oil -based mud.

3. The method of claim 1, wherein the borehole is one of vertical, deviated, and horizontal.

4. The method of claim 1, wherein the borehole is surrounded by a support structure.

5. The method of claim 4, wherein the support structure is made of one of a metal and fiberglass.

6. The method of claim 5, wherein a cement layer is formed outside the support structure.

7. The method of claim 1, further comprising processing the data using an inverse algorithm.

8. The method of claim 7, further comprising using the processed data to estimate and map contrasts distribution within the geological formation.

9. The method of claim 8, wherein the mapping is three-dimensional (3D) mapping.

10. An induction logging system comprising: a transmitters control unit comprising one or more transmitters, wherein the transmitters control unit is configured to drive the one or more transmitters to emit electric and magnetic signals to a target geological formation;

a receivers control unit comprising one or more receivers, wherein the receivers control unit is configured to control sensitivity of the one or more receivers to detect weak signals emanating back from the target geological formation;

a bucking control unit comprising one or more bucking transmitters;

a reference clock configured to generate reference clock signals for the control unit transmitters, the one or more bucking transmitters, and the one or more receivers; and

a central control unit configured to be in connection with and control of the one or more transmitters control unit, the receivers control unit, and the bucking control unit.

11. The system of claim 10, wherein the transmitters control unit comprises a programmable sine wave generator, a waveform amplifier, a current detection resistor, and a sub-processing unit.

12. The system of claim 11, wherein the sub-processing unit is configured to receive and implement commands from the central control unit.

13. The system of claim 10, wherein the waveform amplifier is a linear power amplifier configured to enhance driving power for the one or more transmitters to increase incident signal level.

14. The system of claim 10, wherein the receivers control unit comprises a pre-amplifier, switch array, amplifier and band pass filter, lock-in amplifier, and a sub-processing unit.

15. The apparatus of claim 14, wherein the pre-amplifier is configured for low-noise amplification, driving power capability enhancement, and common-mode interference rejection.

16. The system of claim 14, wherein the switch array is configured to select signals from the one or more receivers.

17. The system of claim 14, wherein the sub-processing unit is configured to receive and implement commands from the central control unit and upload data.

18. The system of claim 14, wherein the one or more transmitters and the one or more receivers are placed in three orthogonal orientations.