WO2018192481A1 - 一种co2压裂过程的应力场变化测量方法 - Google Patents

一种co2压裂过程的应力场变化测量方法 Download PDF

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WO2018192481A1
WO2018192481A1 PCT/CN2018/083339 CN2018083339W WO2018192481A1 WO 2018192481 A1 WO2018192481 A1 WO 2018192481A1 CN 2018083339 W CN2018083339 W CN 2018083339W WO 2018192481 A1 WO2018192481 A1 WO 2018192481A1
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dimensional
physical model
dimensional physical
model
fracturing
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PCT/CN2018/083339
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English (en)
French (fr)
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鞠杨
刘鹏
刘红彬
杨永明
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中国矿业大学(北京)
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Priority claimed from CN201710252945.6A external-priority patent/CN108732021B/zh
Application filed by 中国矿业大学(北京) filed Critical 中国矿业大学(北京)
Priority to US16/317,815 priority Critical patent/US10564080B2/en
Publication of WO2018192481A1 publication Critical patent/WO2018192481A1/zh

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
    • G01N3/10—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces generated by pneumatic or hydraulic pressure
    • G01N3/12—Pressure testing
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/16—Enhanced recovery methods for obtaining hydrocarbons
    • E21B43/164—Injecting CO2 or carbonated water
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
    • E21B49/006—Measuring wall stresses in the borehole
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T17/00—Three dimensional [3D] modelling, e.g. data description of 3D objects
    • G06T17/05—Geographic models
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y10/00—Processes of additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y80/00—Products made by additive manufacturing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/003—Generation of the force
    • G01N2203/0042—Pneumatic or hydraulic means
    • G01N2203/0044—Pneumatic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/006—Crack, flaws, fracture or rupture
    • G01N2203/0062—Crack or flaws
    • G01N2203/0066—Propagation of crack
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/022—Environment of the test
    • G01N2203/0244—Tests performed "in situ" or after "in situ" use
    • G01N2203/0246—Special simulation of "in situ" conditions, scale models or dummies
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/025—Geometry of the test
    • G01N2203/0256—Triaxial, i.e. the forces being applied along three normal axes of the specimen

Definitions

  • the invention claims the invention patent application filed in the Chinese Patent Office, the application number is 201710252945.6, and the application date is April 18, 2017, and the invention name is "a measurement method of stress field change of a CO2 fracturing process" and the application number is 201720408794.4
  • the application date is the priority of the utility model patent application entitled “A Stress Field Change Measurement System for a CO2 Fracturing Process” on April 18, 2017, the entire contents of which are incorporated herein by reference.
  • the invention relates to the technical field of internal stress field measurement, in particular to a method for measuring stress field variation in a CO 2 fracturing process.
  • the invention provides a method for measuring a stress field change of a CO 2 fracturing process, which solves the problem that the accuracy and reliability of the numerical simulation result which is easily affected by external influence in the prior art cannot be guaranteed.
  • a method for measuring a stress field change of a CO 2 fracturing process includes:
  • the CO 2 pumping system performs CO 2 fracturing experiments on multiple identical and transparent three-dimensional physical models of real reservoir cores
  • the computer performs digital reconstruction based on the CT scan results of the 3D physical model after the CO 2 fracturing experiment, obtains a three-dimensional digital model of the complete stitch net, and intercepts different crack states in the three-dimensional digital model of the complete stitch net to generate a three-dimensional digital model under different crack conditions;
  • the 3D printer prints according to the three-dimensional digital model in the different crack states, and obtains a three-dimensional physical model in a plurality of different crack states;
  • the CO 2 pumping system performs a CO 2 fracturing experiment under the corresponding target pressure on the three-dimensional physical model in the different crack states;
  • the temperature box performs cooling treatment on each three-dimensional physical model after performing the CO 2 fracturing experiment under the corresponding target pressure
  • the slicer slices each of the three-dimensional physical models after the temperature-lowering process, and obtains three orthogonal planar two-dimensional slices of each three-dimensional physical model;
  • the computer processes the stress fringe distribution to obtain the three-dimensional maximum shear stress of each point in each three-dimensional physical model.
  • the stress fringe distribution is obtained after two-dimensional photoelastic experiments of three orthogonal slices satisfying the experimental requirements.
  • the different crack states include: before the crack starts, the crack expands to a preset length, and the crack stops expanding;
  • the intercepting different crack states in the three-dimensional digital model of the complete seam net to generate a three-dimensional digital model in different crack states including:
  • the image of the CT scan result is subjected to region threshold segmentation, interception and reconstruction operations, and a three-dimensional digital model in which the crack is extended to a preset length is obtained.
  • the CO 2 pumping system performs a CO 2 fracturing experiment at a corresponding target pressure on the three-dimensional physical model in the different crack states, including:
  • the CO 2 pumping system injects supercritical CO 2 into the three-dimensional physical model before crack initiation until the injection pressure is lower than the fracture pressure by a preset pressure difference, stopping the injection of supercritical CO 2 and maintaining the current injection pressure;
  • the CO 2 pumping system injects supercritical CO 2 into a three-dimensional physical model with crack propagation to a preset length until the injection pressure is lower than the pressure at which the current crack is formed, stopping the injection of supercritical CO 2 and maintaining the current injection pressure. .
  • the method further includes:
  • the computer digitally reconstructs the scan result of the real reservoir core to obtain a three-dimensional digital model of the real reservoir core
  • the 3D printer prints according to the three-dimensional digital model of the real reservoir core, and obtains a plurality of identical and transparent three-dimensional physical models of the real reservoir core;
  • the warm box processes the three-dimensional physical model of the plurality of real reservoir cores, so that the properties of the three-dimensional physical models of the plurality of real reservoir cores are stable;
  • the triaxial loading device performs three-way servo loading on the three-dimensional physical model of the plurality of real reservoir cores until the target value is reached.
  • the method further includes:
  • the warm box processes the three-dimensional physical model in the different crack states to stabilize the properties of the three-dimensional physical model in the different crack states;
  • the triaxial loading device performs three-way servo loading on the three-dimensional physical model in the different crack states until the target value is reached.
  • the thermostat processes the three-dimensional physical model of the plurality of real reservoir cores, and stabilizes the properties of the three-dimensional physical models of the plurality of real reservoir cores, including:
  • the warm box slowly heats up the three-dimensional physical model of the plurality of real reservoir cores until the temperature is raised to 60 ° C, and the temperature is maintained for 1 hour, so that the properties of the three-dimensional physical models of the plurality of real reservoir cores are stable.
  • the thermostat processes the three-dimensional physical model in the different crack states to stabilize the properties of the three-dimensional physical model in the different crack states, including:
  • the three-dimensional physical model of the different crack states is slowly heated by the incubator until the temperature is raised to 60 ° C, and the temperature is kept constant for 1 hour, so that the properties of the three-dimensional physical model in the different crack states are stabilized.
  • the method further includes:
  • the temperature control system of the CO 2 pumping system regulates the injection pressure and temperature of the CO 2 .
  • the thermostat performs a cooling process on each three-dimensional physical model after performing a CO 2 fracturing experiment at a corresponding target pressure, including:
  • the incubator was cooled to room temperature at 2 ° C / h.
  • the computer processes the stress fringe distribution to obtain a three-dimensional maximum shear stress at each point in each three-dimensional physical model, including:
  • the computer extracts according to the stress fringe distribution by phase shift method to obtain a corresponding stress field
  • the computer calculates the three-dimensional maximum shear stress of each point in the three-dimensional physical model based on the maximum shear stress distribution ( ⁇ max ) xy , ( ⁇ max ) yz , ( ⁇ max ) zx in three orthogonal planes.
  • the present invention also provides a method for measuring a stress field change of a CO 2 fracturing process, comprising:
  • the distribution of the three-dimensional stress field is quantitatively displayed and analyzed by photoelastic experiments and phase shift analysis.
  • the step of acquiring a three-dimensional transparent physical model of the complex rock mass by using a 3D printing technology includes:
  • the 3D printer prints according to the three-dimensional digital model of the real core, and obtains multiple identical three-dimensional physical models; the three-dimensional digital model is obtained by CT scanning, image processing, and three-dimensional reconstruction of the real core.
  • the step of recording the three-dimensional stress field distribution characteristics of the different crack states by a "freezing" experiment of the three-dimensional model includes:
  • the three-dimensional physical model that reaches the "freezing" temperature is loaded by a triaxial loading device to simulate the original geostress state of the rock during the fracturing process;
  • the loaded three-dimensional physical model was subjected to a CO2 fracturing experiment using a CO2 pumping system to cause a specific crack state in the fracturing experiment;
  • the above steps are repeated to obtain stress field distribution features of a plurality of three-dimensional physical models having different crack states.
  • the specific crack state includes: before the crack starts, the crack expands to a preset length, and the crack stops expanding.
  • the steps of quantitatively displaying and analyzing the distribution of the three-dimensional stress field by photoelastic experiments and phase shift analysis include:
  • the three-dimensional physical model with different crack states after the temperature-lowering process is sliced by the microtome to obtain three orthogonal planar two-dimensional slices of each three-dimensional physical model;
  • the photoelastic analysis experiment is performed on the two-dimensional slice of each orthogonal plane by using the photoelastic stress test system, and the photoelastic fringe pattern of each slice is obtained;
  • I i is the light intensity of the incident light
  • I a is the modulation light intensity
  • f ⁇ is the stripe value of the model and h is the thickness of the model
  • the phase of the CO 2 gas is changed by adjusting the pressure and temperature of the gas to ensure that the fracturing fluid is in a specific gaseous or supercritical state.
  • CO 2 pumping system includes a temperature control system for regulating the injection pressure and temperature of CO 2.
  • the method for measuring the stress field change of the CO 2 fracturing process performs a CO 2 fracturing experiment on a plurality of identical and transparent three-dimensional physical models of a real reservoir core by a CO 2 pumping system; According to the CT scan results of the 3D physical model after the CO 2 fracturing experiment, digital reconstruction is performed to obtain a three-dimensional digital model of the complete stitch net, and different crack states are intercepted in the three-dimensional digital model of the complete stitch net to generate different a three-dimensional digital model in a crack state; and then printed by a 3D printer according to the three-dimensional digital model in the different crack states to obtain a three-dimensional physical model in a plurality of different crack states; then the CO 2 pumping system re-pairs the different cracks
  • the three-dimensional physical model in the state performs the CO 2 fracturing experiment under the corresponding target pressure; the three-dimensional physical model after the CO 2 fracturing experiment under the corresponding target pressure is cooled by the thermostat; the cooling is processed
  • the method for measuring the stress field change of the CO 2 fracturing process of the present invention is different from the numerical simulation method in the prior art, combined with digital reconstruction, 3D printing technology, CO 2 fracturing experiment, temperature control and
  • the dimensional photoelastic experiment can not only visually display the different crack states of the three-dimensional physical model, but also quantitatively record the internal stress phase diagram of the three-dimensional physical model, thereby realizing the visualization and quantitative characterization of the three-dimensional stress field distribution; and, truly reflecting the discontinuous structure
  • the above experiments are carried out simultaneously through a plurality of identical and transparent three-dimensional physical models, thereby ensuring the accuracy and reliability of the measurement results, and avoiding the values in the prior art that are susceptible to external influences due to the calculation accuracy. The accuracy and reliability of the simulation results cannot be guaranteed.
  • FIG. 1 is a flow chart of a method for measuring a stress field change of a CO 2 fracturing process according to an embodiment of the present invention
  • FIG. 2 is another flow chart of a method for measuring a stress field change of a CO 2 fracturing process according to another embodiment of the present invention
  • FIG. 3 is another flow chart of a method for measuring a stress field change of a CO 2 fracturing process according to another embodiment of the present invention.
  • thermo-optic curve diagram according to another embodiment of the present invention.
  • FIG. 5 is another flow chart of a method for measuring a stress field change of a CO 2 fracturing process according to another embodiment of the present invention.
  • the invention provides a method for measuring a stress field change of a CO 2 fracturing process, which solves the problem that the accuracy and reliability of the numerical simulation result which is easily affected by external influence in the prior art cannot be guaranteed.
  • the method for measuring the stress field change of the CO 2 fracturing process includes:
  • the S101 and CO 2 pumping systems perform CO 2 fracturing experiments on multiple identical and transparent three-dimensional physical models of real reservoir cores, that is, CO 2 pumping systems are similar to those of real reservoir cores.
  • the phase of the CO 2 gas can be changed by adjusting the pressure and temperature of the gas before the CO 2 injection to ensure that the fracturing fluid is in a specific gaseous or supercritical state.
  • the CO 2 pumping system includes a temperature control system for regulating the injection pressure and temperature of the CO 2 .
  • the three-axis loading device is required to fix and load the three-dimensional physical models of the real reservoir core respectively. After the three-dimensional physical models of the real reservoir core are fractured, the supercritical CO is stopped. Injecting 2 , unloading the external three-way stress of the triaxial loading device, and taking out the various three-dimensional physical models of the real reservoir core from the incubator.
  • the computer performs digital reconstruction according to the CT scan result of the three-dimensional physical model after the CO 2 fracturing experiment, obtains a three-dimensional digital model of the complete stitch net, and intercepts different crack states in the three-dimensional digital model of the complete stitch net. , generating a three-dimensional digital model in different crack states;
  • the complete fracturing process can be divided into three different crack propagation stages, that is, the different crack states include: before the crack starts, the crack expands to a preset length, and the crack stops expanding; The fracture pattern of different expansion stages is intercepted in the complete seam net model obtained by the structure.
  • the three-dimensional digital model in the three-dimensional digital model of the complete seam net is intercepted to generate a three-dimensional digital model in different crack states, including:
  • the image of the CT scan result is subjected to region threshold segmentation, interception and reconstruction operations, and a three-dimensional digital model in which the crack is extended to a preset length is obtained.
  • the preset length may be determined according to a specific application environment, and is not specifically limited herein, and is within the protection scope of the present application.
  • the S103 and the 3D printer print according to the three-dimensional digital model in the different crack states, and obtain a three-dimensional physical model in a plurality of different crack states;
  • the physical model is recreated by 3D technology, so that the physical model of three different stages in the crack propagation process is obtained, that is, the complete model without initial crack, the model with initial crack of a certain length, and the crack stop.
  • Extended physical model
  • the CO 2 pumping system performs a CO 2 fracturing experiment under the corresponding target pressure on the three-dimensional physical model in the different crack states;
  • the second CO 2 fracturing (ie, secondary fracturing) test in step S104 does not repeat the first CO 2 fracturing (ie, one-time fracturing) experiment in step S101.
  • the first CO 2 fracturing experiment in step S101 aims to obtain the final pressure crack network morphology of a plurality of identical and transparent three-dimensional physical models of real reservoir cores under specific temperature and stress conditions.
  • the CO 2 fracturing actually undergoes different cracking and expansion processes, and finally a complete pressure crack network is formed. . Therefore, in order to study the different expansion stages of cracks and the evolution of the stress field at the fracture tip, it is necessary to divide the complete fracture network obtained after the first CO 2 fracturing experiment into different expansion stages through step S102, that is, dynamic The crack propagation process is decomposed into different quasi-static crack expansion stages. Based on the image processing technology, the three-dimensional digital model of different stage expansion stages with a certain crack length is obtained by interception reconstruction.
  • step S103 the corresponding three-dimensional physical model is produced by using the 3D printing technology, and then the second CO 2 fracturing experiment can be performed on the three-dimensional physical model of each stage by step S104, so that the respective injection pressures are maintained and the stress is applied.
  • Freezing experiments show the transparent display and quantitative characterization of the three-dimensional stress field and its evolution during CO 2 fracturing.
  • different target pressures can be set.
  • the following pressure settings are selected:
  • the CO 2 pumping system injects supercritical CO 2 into a three-dimensional physical model in which the crack propagates to a preset length until the injection pressure is lower than the pressure at which the current crack is formed by a predetermined pressure difference, maintaining the current injection pressure constant.
  • the pressure that forms the current crack is also the pressure that drives the crack to continue to expand.
  • the preset pressure difference can be set according to a specific application environment, and is not specifically limited herein, and is within the protection scope of the present application.
  • the fracture pressure during the fracturing process refers to the pressure at which the crack begins to crack. That is, the peak point of the injected pressure-time curve obtained during a complete fracturing process. Once the injection pressure exceeds the fracture pressure, the crack begins to crack and the test piece (ie, the three-dimensional physical model of the experiment) is destroyed. In this step, the injection pressure of the three-dimensional physical model before the crack initiation is lower than the fracture pressure obtained by the first fracture, because it is guaranteed to be in a state of being about to be cracked, so as to facilitate the stress freeze experiment.
  • the subsequent CO 2 fracturing experiment was conducted to achieve stress freezing, so it is necessary to ensure that the injection pressure and the external confining pressure are maintained at a constant value without causing the crack to continue to expand. Therefore, the injection pressure of the subsequent CO 2 fracturing experiment is based on the stress value obtained in the first CO 2 fracturing test in the pressure-time curve, ensuring that the applied injection pressure is less than the driving force required to continue the crack propagation.
  • the temperature box performs cooling treatment on each three-dimensional physical model after performing the CO 2 fracturing experiment under the corresponding target pressure;
  • the incubator can be set to cool to room temperature at a rate of 2 ° C / h.
  • each three-dimensional physical model after the CO 2 fracturing experiment under the corresponding target pressure is taken out from the incubator.
  • S106 The slicer slices each three-dimensional physical model after performing the temperature-lowering process, and obtains three orthogonal planar two-dimensional slices of each three-dimensional physical model;
  • three three-dimensional physical models after cooling processing can be selected, and sliced along three orthogonal planes x-y, y-z, and z-x, respectively, to obtain three orthogonal planar two-dimensional slices of the three-dimensional physical model, and the slice thickness is 1 mm.
  • the computer processes the stress fringe distribution to obtain a three-dimensional maximum shear stress of each point in each three-dimensional physical model
  • the stress fringe distribution is obtained by performing two-dimensional photoelastic experiments on three orthogonal planar two-dimensional slices satisfying the experimental requirements.
  • the three orthogonal slices can be polished and polished to make the two-dimensional slice of the orthogonal plane satisfying the experimental requirements, and then subjected to two-dimensional photoelastic experiment to obtain the stress.
  • the stripe distribution is then obtained by step S107 to obtain the three-dimensional maximum shear stress of each point in the three-dimensional physical model.
  • the method for measuring the stress field change of the CO 2 fracturing process provided by the embodiment is different from the existing method for displaying the change of the fracturing stress field by using a numerical simulation, which comprehensively adopts CT scanning, digital reconstruction, 3D printing, CO 2 fracturing experiment, stress freezing and photoelastic measurement technology can not only transparently display the spatial distribution and expansion of internal cracks during fracturing of 3D physical model, but also obtain internal 3D stress phase during fracturing crack propagation.
  • the present embodiment can not only truly reflect the complex discontinuous structure of the solid interior, but also realize the transparent analysis and characterization of the internal stress field and its evolution by using multiple identical transparent three-dimensional physical photoelastic models to ensure the measurement results.
  • the accuracy and reliability of the existing numerical simulation methods and the traditional stress simulation techniques are generally difficult to ensure the accuracy and accuracy of the crack propagation calculation, the complex structure model is difficult to prepare and the fracture stress field cannot be continuously and transparently characterized. problem.
  • a method of measuring stress field fracturing a CO 2 process comprising:
  • the 3D printer prints according to the three-dimensional digital model of the real core, and obtains multiple identical three-dimensional physical models; the three-dimensional digital model is performed on the real core CT scan, image processing, and 3D reconstruction.
  • CO 2 pumping system Preparation of supercritical CO 2 :
  • the phase state of the CO 2 gas is changed by adjusting the pressure and temperature of the gas to ensure that the fracturing fluid is in a specific gaseous or supercritical state.
  • CO 2 pumping system includes a temperature control system for regulating the injection pressure and temperature of CO 2.
  • thermo-optic curve of the three-dimensional physical model shown in FIG. 4 The thermo-optic curve is generally obtained by recording the variation of the fringe order of the diaphragm under different temperatures at different temperatures.
  • the three-dimensional physical model is used to load the three-dimensional physical model to achieve the preset target pressure.
  • the target pressure here is to simulate the original geostress state of the rock during the fracturing process.
  • the geostress state of the reservoir rock with a depth of 1000 m to 3000 m can be selected, and of course, the stress state of other deep rocks can be used, which is not specifically limited herein.
  • CO 2 pumping system performs CO 2 fracturing experiments on three-dimensional physical models; including: ensuring physical model fracturing through precise displacement control and controlling injection pressure In the experiment, it can be in many different fracturing stages before cracking, crack propagation to specific length and so on.
  • the fracturing experiment before the model is in the process of cracking means that the CO 2 pumping system injects supercritical CO 2 into the three-dimensional physical model of the above-mentioned completed warming loading until the injection pressure is slightly lower than the model fracture pressure, maintaining the current injection. Constant pressure;
  • the fracturing experiment in which the crack is expanded to a specific stage means that the CO 2 pumping system injects supercritical CO 2 into the three-dimensional physical model of the above-mentioned completed warming loading until the model is broken, and when the crack expands to a preset length, The injection pressure is maintained constant and below the pressure at which the drive crack continues to expand.
  • the crack is extended to a specific stage and can be set according to a specific application environment, and is not specifically limited herein, and is within the protection scope of the present application.
  • the thermostat cools the model that completes the above fracturing experiment, ie, the temperature of the thermostat control model slowly decreases from the "freezing" temperature to room temperature.
  • the model can be set to cool to room temperature at a rate of 2 ° C / h.
  • other cooling rates can also be used, which are not specifically limited herein.
  • the CO 2 pumping system stops the injection, the injection pressure in the model drops to zero, the three-axis loading device unloads, and the model is removed from the oven.
  • the three-dimensional physical model obtained by “freezing” is obtained by slice, two-dimensional photoelastic experiment and quantitative analysis of photoelastic stripe to obtain the three-dimensional stress field of each fracturing stage. distributed.
  • the slicer slices three three-dimensional physical models having different fracturing stages after performing the temperature-lowering process, and obtains three orthogonal planar two-dimensional slices of each three-dimensional physical model; preferably, three cooling treatments may be selected.
  • the three-dimensional physical model is sliced along three orthogonal planes xy, yz, and zx, respectively, to obtain three orthogonal planar two-dimensional slices of the three-dimensional physical model, and the slice thickness is 1 mm.
  • the photoelastic stress test system performs the photoelastic analysis experiment under the specific optical path on the two-dimensional slice of each of the above orthogonal planes, and obtains the photoelastic fringe pattern of each slice: in a specific practical application, the three The two-dimensional slice of the orthogonal plane is polished and polished to make it into an orthogonal plane two-dimensional slice that meets the experimental requirements, and then subjected to two-dimensional photoelastic experiment to obtain the stress fringe distribution.
  • the computer obtains the distribution of the main stress direction angle ⁇ u of the full field range according to the color four-step phase shift method.
  • I i is the light intensity of the incident light.
  • I a is the modulation light intensity
  • f ⁇ is the stripe value of the model and h is the thickness of the model.
  • Another specific embodiment provides a method of processing three-dimensional physical model before fracturing CO.'S 2 according to the present invention, in the above embodiment of the above embodiment, prior to the step S101 of FIG. 1, see FIG. 2, further comprising:
  • S201 The computer performs digital reconstruction on the scan result of the real reservoir core to obtain a three-dimensional digital model of the real reservoir core;
  • the S202 and the 3D printer print according to the three-dimensional digital model of the real reservoir core, and obtain a plurality of identical and transparent three-dimensional physical models of the real reservoir core;
  • the substrate portion of the Vero Clear material having photoelastic properties can be printed, and the void or crack portion can be printed using the Fullcure 705 material.
  • the thermostat processes the three-dimensional physical model of the plurality of real reservoir cores, so that the properties of the three-dimensional physical models of the plurality of real reservoir cores are stable;
  • the thermostat slowly heats the three-dimensional physical model of the plurality of real reservoir cores until the temperature is raised to 60 ° C, and the temperature is maintained for 1 hour to make a three-dimensional physical model of the plurality of the real reservoir cores.
  • the nature is stable.
  • the three-axis loading device performs three-way servo loading on the three-dimensional physical model of the plurality of real reservoir cores until a target value is reached.
  • the temperature is kept constant for 1 hour to ensure that the three-dimensional physical model reaches a uniform freezing temperature inside and outside, and then the three-dimensional physical model is loaded into the target value by step S204.
  • the target value may be determined according to the specific application environment, and is not specifically limited herein, and is within the protection scope of the present application.
  • the method further includes:
  • the thermostat processes the three-dimensional physical model in the different crack states, so that the properties of the three-dimensional physical model in the different crack states are stable;
  • the warm box slowly heats up the three-dimensional physical model in the different crack states until the temperature is raised to 60 ° C, and the temperature is kept constant for 1 hour, so that the properties of the three-dimensional physical model in the different crack states are stabilized.
  • the three-axis loading device performs three-way servo loading on the three-dimensional physical model in the different crack states until a target value is reached.
  • the temperature is kept constant for 1 hour to ensure that the three-dimensional physical model reaches a uniform freezing temperature inside and outside, and then the three-dimensional physical model is loaded into the target value by step S204.
  • the target value may be determined according to the specific application environment, and is not specifically limited herein, and is within the protection scope of the present application.
  • the temperature box processes the above three-dimensional physical model, and uses 60 ° C as the freezing temperature of each three-dimensional physical model.
  • 60 ° C the freezing temperature of each three-dimensional physical model.
  • other temperatures can also be used, which are not specifically limited herein.
  • the thermo-optic curve is generally obtained by recording the change in the number of fringes of the disk under different pressures at different temperatures.
  • the photoelastic material Vero Clear used in the experiment can be tested in detail by designing a reflective photoelastic experiment at different temperature points.
  • the disk test piece of ⁇ 50 ⁇ 8 mm was printed by 3D printing technology. To eliminate the reflection in the experiment, one side of the disk test piece can be sprayed first. Then, the self-designed connecting rod device is used to load the disc in the high temperature box, and the high temperature box can be set with different temperature control curve programs and the shooting window is left.
  • the temperature was raised to the target temperature at 10 °C/h, and the temperature was kept for 15 minutes. The number of stripes at the center of the disc was recorded by the camera, and the thermo-curve curve of Vero Clear was drawn according to the stripe data of different temperature points. As shown in Figure 4.
  • the temperature of the transition state of the thermo-optic curve is selected as the target temperature, and the physical and mechanical properties of the printed model at the target temperature are tested.
  • the variation trend of the different temperature points in the integrated thermo-optic curve and the physical and mechanical properties of the material are selected.
  • the temperature of the state is 60 ° C as the freezing temperature. Of course, other temperatures may be selected, which are not specifically limited herein, and are all within the protection scope of the present application.
  • step S107 is as shown in FIG. 5, and includes:
  • S401 The computer extracts according to the stress fringe distribution by a phase shift method to obtain a corresponding stress field
  • the computer calculates a three-dimensional maximum shear stress of each point in the three-dimensional physical model according to a maximum shear stress distribution ( ⁇ max ) xy , ( ⁇ max ) yz , ( ⁇ max ) zx in three orthogonal planes.
  • the method for measuring the stress field change of the CO 2 fracturing process is different from the existing method for displaying the change of the fracturing stress field by using numerical simulation, which comprehensively adopts CT scanning, digital reconstruction, 3D printing, CO 2 fracturing experiment , stress freezing and photoelastic measurement technology can not only transparently display the spatial distribution and expansion of internal cracks during fracturing of 3D physical model, but also obtain internal 3D stress phase diagram during fracture crack propagation to achieve solid CO 2 fracturing

Abstract

一种CO 2压裂过程的应力场变化测量方法，其综合采用CT扫描、数字重构、3D打印、CO 2压裂实验、应力冻结及光弹性量测技术，不仅可以透明显示三维物理模型压裂时内部裂缝的空间分布与扩展形态，而且可以获得压裂裂缝扩展过程中模型内部三维应力相位图，实现固体CO 2压裂过程中三维应力场及其演化规律的透明显示与定量表征；同时利用多个相同透明的三维物理模型实现显示与表征，确保了测量结果的准确性和可靠性，解决了现有数值模拟方法中存在的计算精度低及准确性和可靠性难以保证的问题。

Description

一种CO 2压裂过程的应力场变化测量方法
本发明要求提交于中国专利局、申请号为201710252945.6、申请日为2017年4月18日、发明名称为“一种CO2压裂过程的应力场变化测量方法”的发明专利申请以及申请号为201720408794.4、申请日为2017年4月18日、发明名称为“一种CO2压裂过程的应力场变化测量系统”的实用新型专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明涉及内部应力场测量技术领域，特别涉及一种CO 2压裂过程的应力场变化测量方法。
背景技术
由于非常规油气水力压裂过程中不仅对水资源的消耗量巨大，而且压裂液中存在的大量有害添加剂容易造成生态环境的污染，因此，采用超临界CO 2压裂便是当前重点发展的一种无水压裂技术。
由于CO 2压裂过程的复杂性，水力压裂中的一些基础理论和方法并不完全适用于超临界CO 2的压裂研究，而对于超临界CO 2的现有研究大都处于对于CO 2压裂后形成缝网结构的定性分析上，无法实现其压裂过程的观测和表征。
现有技术中也存在采用数值模拟的方法，来定量分析裂缝扩展过程中应力场的分布演化规律的方案，但是数值模拟的方法计算精度容易受外界条件影响，使得数值模拟结果的准确性和可靠性无法保证。
发明内容
本发明提供一种CO 2压裂过程的应力场变化测量方法，以解决现有技术中由于计算精度易受外界影响而导致的数值模拟结果的准确性和可靠性无法保证的问题。
为实现上述目的，本申请提供的一种CO 2压裂过程的应力场变化 测量方法，包括：
CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验；
计算机根据CO 2压裂实验后的三维物理模型的CT扫描结果，进行数字重构，获得完整缝网的三维数字模型，并在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型；
3D打印机根据所述不同裂缝状态下的三维数字模型进行打印，得到多个不同裂缝状态下的三维物理模型；
CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验；
温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理；
切片机对进行降温处理之后的各个三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；
计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力；所述应力条纹分布为三个满足实验要求的正交平面二维切片进行二维光弹实验后得到的。
任选地，所述不同的裂缝状态包括：裂缝起裂前、裂缝扩展到预设长度及裂缝停止扩展；
所述在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型，包括：
基于图像处理技术，对所述CT扫描结果的图像进行区域阈值分割、截取及重构操作，得到裂缝扩展到预设长度的三维数字模型。
任选地，所述CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验，包括：
CO 2泵送系统向裂缝起裂前的三维物理模型中注入超临界CO 2，直至注入压力较破裂压力低预设压差时，停止注入超临界CO 2并维持当前注入压力；
CO 2泵送系统向裂缝扩展到预设长度的三维物理模型中注入超临界CO 2，直至注入压力较形成当前裂缝的压力低预设压差时，停止注入超临界CO 2并维持当前注入压力。
可选地，在所述CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验之前，还包括：
计算机对所述真实储层岩芯的扫描结果进行数字重构，获得所述真实储层岩芯的三维数字模型；
3D打印机根据所述真实储层岩芯的三维数字模型进行打印，得到多个相同且透明的所述真实储层岩芯的三维物理模型；
温箱对多个所述真实储层岩芯的三维物理模型进行处理，使多个所述真实储层岩芯的三维物理模型的性质稳定；
三轴加载装置对多个所述真实储层岩芯的三维物理模型进行三向伺服加载，直至达到目标值。
可选地，在所述CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验之前，还包括：
温箱对所述不同裂缝状态下的三维物理模型进行处理，使所述不同裂缝状态下的三维物理模型的性质稳定；
三轴加载装置对所述不同裂缝状态下的三维物理模型进行三向伺服加载，直至达到目标值。
可选地，所述温箱对多个所述真实储层岩芯的三维物理模型进行处理，使多个所述真实储层岩芯的三维物理模型的性质稳定，包括：
温箱对多个所述真实储层岩芯的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使多个所述真实储层岩芯的三维物理模型的性质稳定。
可选地，所述温箱对所述不同裂缝状态下的三维物理模型进行处理，使所述不同裂缝状态下的三维物理模型的性质稳定，包括：
温箱对所述不同裂缝状态下的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使所述不同裂缝状态下的三维物理模型的性质稳定。
可选地，所述CO 2泵送系统对真实储层岩芯的三维物理模型进行CO 2压裂实验之前，还包括：
CO 2泵送系统的温度控制系统调节CO 2的注入压力和温度。
可选地，所述温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理，包括：
温箱以2℃/h的速度降温至室温。
可选地，所述计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力，包括：
计算机根据所述应力条纹分布进行相移法提取，得到相应的应力场；
计算机根据公式
Figure PCTCN2018083339-appb-000001
计算得到载荷作用下三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx；
计算机根据三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx，计算得到三维物理模型内各个点的三维最大剪应力。
本申请还提供的一种CO 2压裂过程的应力场变化测量方法，包括：
通过3D打印技术获取复杂岩体的三维透明化物理模型；
通过三维物理模型的“冻结”实验来记录不同裂缝状态的三维应力场分布特征；
通过光弹性实验和相移法分析来定量地显示和分析三维应力场的分布。
可选地，所述通过3D打印技术获取复杂岩体的三维透明化物理模型的步骤包括：
3D打印机根据真实岩芯的三维数字模型进行打印，得到多个相同的三维物理模型；所述三维数字模型是通过对真实岩芯进行CT扫描、图像处理、三维重构所得到的。
可选地，通过三维模型的“冻结”实验来记录不同裂缝状态的三 维应力场分布特征的步骤包括；
利用升温装置对所述多个三维物理模型中的一选定三维物理模型升温以使所述三维物理模型达到“冻结”温度；
利用三轴加载装置对所述达到“冻结”温度的三维物理模型进行加载，以便模拟压裂过程中岩石所受的原始地应力状态；
利用CO2泵送系统对所述加载的三维物理模型进行CO2压裂实验以使其压裂实验中处于特定的裂缝状态；
利用温箱对完成处于特定的裂缝状态的三维物理模型进行降温处理，
重复上述步骤以便得到具有不同裂缝状态的多个三维物理模型的应力场分布特征。
可选地，所述特定的裂缝状态包括：裂缝起裂前、裂缝扩展到预设长度及裂缝停止扩展。
可选地，通过光弹性实验和相移法分析来定量地显示和分析三维应力场的分布的步骤包括：
利用切片机对进行降温处理之后的各个具有不同裂缝状态三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；
利用光弹应力测试系统对上述各个正交平面的二维切片进行光弹分析实验，得到各个切片的光弹条纹图；
利用计算机根据四步彩色相移法获得全场范围主应力方向角θ u的分布，
Figure PCTCN2018083339-appb-000002
且sin δ≠0，
其中，I i为入射光的光强；
根据改进的六步相移法获得等色线相位图δ u，
Figure PCTCN2018083339-appb-000003
其中，
Figure PCTCN2018083339-appb-000004
I a为调制光强
最终计算全场应力场的分布，
Figure PCTCN2018083339-appb-000005
其中，f σ为模型的条纹值，h为模型的厚度；
通过三个正交平面内的应力分量的合成，计算得到三维物理模型内各个点的三维应力场的分布。
可选地，在通过三维物理模型的“冻结”实验来记录不同裂缝状态的三维应力场分布特征的步骤前还包括；
在CO 2泵送系统中通过调节气体的压力和温度改变CO 2气体的相态，保证压裂流体处于特定的气态或者超临界态。
可选地，CO 2泵送系统包括温度控制系统，用于调节CO 2的注入压力和温度。
本发明提供的所述CO 2压裂过程的应力场变化测量方法，通过CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验；然后由计算机根据CO 2压裂实验后的三维物理模型的CT扫描结果，进行数字重构，获得完整缝网的三维数字模型，并在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型；再由3D打印机根据所述不同裂缝状态下的三维数字模型进行打印，得到多个不同裂缝状态下的三维物理模型；然后CO 2泵送系统再对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验；通过温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理；由切片机对进行降温处理之后的各个三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；最后通过计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力。也即，区别于现有技术中的数值模拟的方法，本发明所述CO 2压裂过程的应力场变化测量方法，结合数字重构、3D打印技术、CO 2压裂实验、温度控制和二维光弹性实验，不仅可以直观显示三维物理模型的不同裂缝状态，而且定 量化的记录三维物理模型内部应力相位图，从而实现三维应力场分布的可视化与定量表征；并且，真实反映了非连续结构体的复杂结构的同时，通过多个相同且透明的三维物理模型同时进行上述实验，保证了测量结果的准确性和可靠性，避免了现有技术中由于计算精度易受外界影响而导致的数值模拟结果的准确性和可靠性无法保证的问题。
附图说明
为了更清楚地说明本发明实施例或现有技术内的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述内的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。
图1是本发明实施例提供的CO 2压裂过程的应力场变化测量方法的流程图；
图2是本发明另一实施例提供的CO 2压裂过程的应力场变化测量方法的另一流程图；
图3是本发明另一实施例提供的CO 2压裂过程的应力场变化测量方法的另一流程图；
图4是本发明另一实施例提供的热光曲线图；
图5是本发明另一实施例提供的CO 2压裂过程的应力场变化测量方法的另一流程图。
具体实施方式
下面将结合本申请实施例中的附图，对本申请实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本申请一部分实施例，而不是全部的实施例。基于本申请中的实施例，本领域普通技术人员在没有作出创造性劳动前提下所获得的所有其他实施例，都属于本申请保护的范围。
本发明提供一种CO 2压裂过程的应力场变化测量方法，以解决现有技术中由于计算精度易受外界影响而导致的数值模拟结果的准确性和可靠性无法保证的问题。
具体的，该CO 2压裂过程的应力场变化测量方法，参见图1，包括：
S101、CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验，亦即，CO 2泵送系统对与真实储层岩芯性质相近的多个相同且透明的三维物理模型进行CO 2压裂实验；
具体的，可以在CO 2注入前通过调节气体的压力和温度改变CO 2气体的相态，保证压裂流体处于特定的气态或者超临界态。
也即，优选的，CO 2泵送系统包括温度控制系统，用于调节CO 2的注入压力和温度。
在CO 2压裂实验之前，需要三轴加载装置分别对真实储层岩芯的各个三维物理模型进行固定、加载，在真实储层岩芯的各个三维物理模型被压裂后，停止超临界CO 2的注入，卸载三轴加载装置的外部三向应力，将真实储层岩芯的各个三维物理模型从温箱中取出。
S102、计算机根据CO 2压裂实验后的三维物理模型的CT扫描结果，进行数字重构，获得完整缝网的三维数字模型，并在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型；
在具体的实际应用中，可以将完整的压裂过程分成三个不同的裂缝扩展阶段，即该不同的裂缝状态包括：裂缝起裂前、裂缝扩展到预设长度及裂缝停止扩展；然后在重构得到的完整缝网模型中截取不同扩展阶段的裂缝形态。
优选的，所述在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型，包括：
基于图像处理技术，对所述CT扫描结果的图像进行区域阈值分割、截取及重构操作，得到裂缝扩展到预设长度的三维数字模型。
该预设长度可以根据具体的应用环境而定，此处不做具体限定，均在本申请的保护范围内。
S103、3D打印机根据所述不同裂缝状态下的三维数字模型进行打印，得到多个不同裂缝状态下的三维物理模型；
根据上述不同裂缝状态的设置，利用3D技术重新制作物理模型，这样就得到裂缝扩展过程中三个不同阶段的物理模型，即没有初始裂缝的完整模型、带有一定长度初始裂缝的模型、裂缝停止扩展的物理模型。
S104、CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验；
值得说明的是，步骤S104中的第二次CO 2压裂(即二次压裂)试验，并不是重复步骤S101中的第一次CO 2压裂(即一次压裂)实验。步骤S101中的第一次CO 2压裂实验，其目的是得到真实储层岩芯的多个相同且透明的三维物理模型在特定的温度和应力条件下最终的压裂缝网形态。
根据上述内容可知，CO 2压裂实际上经历了不同的起裂和扩展过程，才最终形成完整的压裂缝网。。因此，为了研究裂缝的不同扩展阶段、压裂裂缝尖端应力场的演化，就需要通过步骤S102，将第一次CO 2压裂实验后得到的完整裂缝网络划分成不同的扩展阶段，即将动态的裂缝扩展过程分解为不同的准静态裂缝扩展阶段；并基于图像处理技术，截取重构得到带有一定裂缝长度的不同阶段扩展阶段的三维数字模型。再通过步骤S103利用3D打印技术，制作得到相应的三维物理模型，进而可以通过步骤S104对各个阶段的三维物理模型开展第二次CO 2压裂实验，使其各自维持相应的注入压力后进行应力冻结实验，即可得到CO 2压裂过程中三维应力场及其演化规律的透明显示与定量表征。
具体的，对于不同裂缝状态下的三维物理模型，可以设置不同的目标压力，优选的，选择如下压力设置：
CO 2泵送系统三维物理模型中注入超临界CO 2，直至注入压力较破裂压力低一个预设压差时，维持当前注入压力恒定；
CO 2泵送系统向裂缝扩展到预设长度的三维物理模型中注入超临界CO 2，直至注入压力较形成当前裂缝的压力低一个预设压差时，维持当前注入压力恒定。形成当前裂缝的压力也就是驱动裂缝继续扩张的压力。
该预设压差可以根据具体的应用环境进行设定，此处不做具体限定，均在本申请的保护范围内。
压裂过程中的破裂压力，是指裂缝开始起裂时的压力。也就是一次完整的压裂过程中，得到的注入压力-时间曲线的峰值点。一旦注入压力超过破裂压力，裂缝就开始起裂扩展，试件(即进行实验的三维物理模型)发生破坏。本步骤中对裂缝起裂前的三维物理模型的注入压力要低于第一次压裂得到的破裂压力，是因为要保证其处于即将起裂的状态，以便于进行应力冻结实验。
除第一次CO 2压裂实验外，后续开展的CO 2压裂实验是为了实现应力冻结，因此要保证注入压力和外部围压维持在一定定值，且不会使裂缝继续扩展。因此，后续CO 2压裂实验的注入压力以第一次CO 2 压裂实验得到压力-时间曲线中的应力值作为参考，保证施加的注入压力小于能够使裂缝继续扩展所需的驱动力。
S105、温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理；
优选的，可以设置温箱以2℃/h的速度降温至室温。
然后，即可卸载三轴加载装置的外部三向应力，将进行相应目标压力下的CO 2压裂实验后的各个三维物理模型从温箱中取出。S106、切片机对进行降温处理之后的各个三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；
具体的，可以选取三个进行降温处理之后的三维物理模型，分别沿三个正交平面x-y、y-z、z-x进行切片，得到三维物理模型的三个正交平面二维切片，切片厚度为1mm。
S107、计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力；
所述应力条纹分布为三个满足实验要求的正交平面二维切片进行二维光弹实验后得到的。
在具体的实际应用中，可以先通过对三个正交平面二维切片进行磨平抛光，使其成为满足实验要求的正交平面二维切片，再对其进行二维光弹实验，得到应力条纹分布，然后通过步骤S107得到三维物理模型内各个点的三维最大剪应力。
本实施例提供的所述CO 2压裂过程的应力场变化测量方法，通过上述过程，区别于现有的利用数值模拟显示压裂应力场变化的方法，它综合采用CT扫描、数字重构、3D打印、CO 2压裂实验、应力冻结及光弹性量测技术，不仅可以透明显示三维物理模型压裂时内部裂缝的空间分布与扩展形态，而且可以获得压裂裂缝扩展过程中内部三维应力相位图，实现固体CO 2压裂过程中三维应力场及其演化规律的透明显示与定量表征。并且，本实施例不仅可以真实地反映固体内部复杂的非连续结构，同时，利用多个相同透明的三维物理光弹模型实现了对内部应力场及其演化的透明分析与表征，确保了测量结果的准确性和可靠性，解决了现有数值模拟方法及传统应力实验技术中普遍存在的裂纹扩展计算精度和准确性难以保证、复杂结构模型难以制备和压裂扩展应力场无法连续及透明表征的难题。
在本发明的一个实施例中，提供了一个CO 2压裂过程的应力场变化测量方法，包括：
(一)获取复杂岩体的多个透明化三维物理模型：3D打印机根据真 实岩芯的三维数字模型进行打印，得到多个相同的三维物理模型；所述三维数字模型是通过对真实岩芯进行CT扫描、图像处理、三维重构所得到的。
(二)制备超临界态的CO 2：在CO 2泵送系统中通过调节气体的压力和温度改变CO 2气体的相态，保证压裂流体处于特定的气态或者超临界态。优选地，CO 2泵送系统包括温度控制系统，用于调节CO 2的注入压力和温度。
(三)在CO 2压裂应力可视化系统中，开展三维物理模型的应力“冻结”实验，以便记录不同压裂阶段的三维应力场特征，即记录能够反映真实应力场分布的模型应变或者变形：
1.升温达到三维物理模型的“冻结”温度：所述可视化系统的温箱对所述多个三维物理模型中将要进行实验的模型升温达到模型的“冻结”温度。优选地，采用60℃作为各个三维物理模型的冻结温度，当然也可以采用其他温度，此处不做具体限定。具体取值可以参见图4所示的三维物理模型的热光曲线，该热光曲线一般通过记录对径受压圆盘在不同温度下的条纹级数变化来得到。
2.模型外部三向应力加载：利用三轴加载装置分别对三维物理模型进行加载，达到预设的目标压力这里的目标压力是模拟压裂过程中岩石所受的原始地应力状态。优选地，可选用埋深为1000米到3000米深度的储层岩石所受的地应力状态，当然也可以采用其他深度岩石所受的应力状态，此处不做具体限定。
3.注入超临界CO 2进行压裂实验：CO 2泵送系统对三维物理模型进行CO 2压裂实验；包括：通过精准的排量控制、控制注入压力的大小，来保证物理模型在压裂实验中能够处于起裂前、裂缝稳定扩展到特定长度等多个不同的压裂阶段。
这里,模型处于起裂前的压裂实验是指：CO 2泵送系统向上述已完成升温加载的三维物理模型中注入超临界CO 2，直至注入压力略低于模型破裂压力时，维持当前注入压力恒定；
这里，裂缝扩张到特定阶段的压裂实验是指：CO 2泵送系统向上述已完成升温加载的三维物理模型中注入超临界CO 2，直至模型破裂，并在裂缝扩展到预设长度时，维持注入压力恒定且低于驱动裂缝继续扩展的压力。该裂缝扩展到特定阶段可以根据具体的应用环境进行设定，此处不做具体限定，均在本申请的保护范围内。
4.温箱对完成上述压裂实验的模型进行降温处理，即，温箱控制模型温度从“冻结”温度缓慢降低到室温。优选的，可以设置模型 以2℃/h的速度降温至室温。当然也可以采用其他降温速率，此处不做具体限定。CO 2泵送系统停止注入，模型内注入压力降为0，三轴加载装置进行卸载，然后将模型从温箱中取出。
(四)“冻结”得到的三维应力场的定量化分析：对“冻结”得到的三维物理模型通过切片、二维光弹性实验、光弹条纹的定量化分析获取各个压裂阶段的三维应力场分布。具体地，切片机对进行降温处理之后的各个具有不同压裂阶段三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；优选地，可以选取三个进行降温处理之后的三维物理模型，分别沿三个正交平面x-y、y-z、z-x进行切片，得到三维物理模型的三个正交平面二维切片，切片厚度为1mm。
(五)光弹应力测试系统对上述各个正交平面的二维切片进行特定光路下的光弹分析实验，得到各个切片的光弹条纹图：在具体的实际应用中，可以先通过对三个正交平面二维切片进行磨平抛光，使其成为满足实验要求的正交平面二维切片，再对其进行二维光弹实验，得到应力条纹分布。
(六)计算机根据彩色四步相移法获得全场范围主应力方向角θ u的分布，
Figure PCTCN2018083339-appb-000006
且sin δ≠0，
其中，I i为入射光的光强。
根据改进的六步相移法获得等色线相位图δ u，
Figure PCTCN2018083339-appb-000007
其中，
Figure PCTCN2018083339-appb-000008
I a为调制光强，
最终计算全场应力场的分布，
其中，f σ为模型的条纹值，h为模型的厚度。
Figure PCTCN2018083339-appb-000009
通过三个正交平面内的应力分量的合成，计算得到三维物理模型内各个点的三维应力场的分布。
本发明另一实施例提供了一个具体的CO 2压裂前三维物理模型的处理方法，在上述实施例的基础之上，在图1的步骤S101之前，参见图2，还包括：
S201、计算机对所述真实储层岩芯的扫描结果进行数字重构，获得所述真实储层岩芯的三维数字模型；
S202、3D打印机根据所述真实储层岩芯的三维数字模型进行打印，得到多个相同且透明的所述真实储层岩芯的三维物理模型；
具体可以采用具有光弹特性的Vero Clear材料打印基质部分，采用Fullcure 705材料打印孔隙或裂隙部分。
S203、温箱对多个所述真实储层岩芯的三维物理模型进行处理，使多个所述真实储层岩芯的三维物理模型的性质稳定；
优选的，温箱对多个所述真实储层岩芯的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使多个所述真实储层岩芯的三维物理模型的性质稳定。
S204、三轴加载装置对多个所述真实储层岩芯的三维物理模型进行三向伺服加载，直至达到目标值。
达到预设的冻结温度后，保持恒温1小时，保证三维物理模型内外均达到统一的冻结温度，然后即可通过步骤S204对三维物理模型进行三向伺服加载达到目标值。
该目标值可以视其具体应用环境而定，此处不做具体限定，均在本申请的保护范围内。
另外，在对所述不同裂缝状态下的三维物理模型进行应力冻结实验时，需要保证在试件达到冻结温度时，所受到的外部应力作用维持恒定，且试件处于线弹性阶段，不能发生破坏，因此，优选的，在图1的步骤S104之前，参见图3，还包括：
S301、温箱对所述不同裂缝状态下的三维物理模型进行处理，使所述不同裂缝状态下的三维物理模型的性质稳定；
优选的，温箱对所述不同裂缝状态下的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使所述不同裂缝状态下的三维物理模型的性质稳定。
S302、三轴加载装置对所述不同裂缝状态下的三维物理模型进行三向伺服加载，直至达到目标值。
达到预设的冻结温度后，保持恒温1小时，保证三维物理模型内 外均达到统一的冻结温度，然后即可通过步骤S204对三维物理模型进行三向伺服加载达到目标值。
该目标值可以视其具体应用环境而定，此处不做具体限定，均在本申请的保护范围内。
温箱对上述三维物理模型进行处理，采用60℃作为各个三维物理模型的冻结温度，当然也可以采用其他温度，此处不做具体限定，具体取值可以参见图4所示的三维物理模型的热光曲线，该热光曲线一般通过记录对径受压圆盘在不同温度下的条纹级数变化来得到。
具体可以针对实验中使用的光弹性材料Vero Clear，通过设计不同温度点的反射式光弹实验进行详细测试。利用3D打印技术打印得到φ50×8mm的圆盘试件，为消除实验中的反光，可以先对圆盘试件的一侧进行喷涂。然后在高温箱中利用自行设计的连杆装置对圆盘进行对径加载，高温箱可以设置不同的控温曲线程序并留有拍摄窗。实验中按照10℃/h的速度升到目标温度，恒温15分钟左右，利用摄像机拍摄记录下圆盘中心处的条纹级数，根据测得不同温度点的条纹数据绘制Vero Clear的热光曲线，如图4所示。
然后选取热光曲线过渡态阶段的各个温度作为目标温度，测试打印模型处于目标温度时的物理力学性质，本实施例综合热光曲线中不同温度点的变化趋势以及材料物理力学性质，选定过渡态温度60℃作为冻结温度，当然也可以选择其他温度，此处不做具体限定，均在本申请的保护范围内。
本发明另一实施例提供了一个具体的切片条纹图转换为三维应力场的分析方法，在上述实施例及图1至图3的基础之上，步骤S107如图5所示，包括：
S401、计算机根据所述应力条纹分布进行相移法提取，得到相应的应力场；
S402、计算机根据公式
Figure PCTCN2018083339-appb-000010
计算得到载荷作用下三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx；
S403、计算机根据三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx，计算得到三维物理模型内各个点的三维最大剪应力。
本发明提供的CO 2压裂过程的应力场变化测量方法，区别于现有利用数值模拟显示压裂应力场变化的方法，它综合采用CT扫描、数字重构、3D打印、CO 2压裂实验、应力冻结及光弹性量测技术，不仅可以透明显示三维物理模型压裂时内部裂缝的空间分布与扩展形态，而且可以获得压裂裂缝扩展过程中内部三维应力相位图，实现固体CO 2压裂过程中三维应力场及其演化规律的透明显示与定量表征；同时利用多个相同透明的三维物理模型实现上述显示与表征，确保了测量结果的准确性和可靠性，解决了现有数值模拟方法中存在的计算精度低及准确性和可靠性难以保证的问题。
本发明中各个实施例采用递进的方式描述，每个实施例重点说明的都是与其他实施例的不同之处，各个实施例之间相同相似部分互相参见即可。对于实施例公开的装置而言，由于其与实施例公开的方法相对应，所以描述的比较简单，相关之处参见方法部分说明即可。
对于前述的方法实施例，为了简单描述，故将其都表述为一系列的动作组合，但是本领域技术人员应该知悉，本发明并不受所描述的动作顺序的限制，也不受标号“一，二，三等”或者“1,2,3等”的顺序的限制，因为依据本发明，某些步骤可以采用其他顺序或者同时进行。本领域技术人员也应该知悉，说明书中所描述的实施例均属于优选实施例，所涉及的动作和模块并不一定是本发明所必须的。
还需要说明的是，在本文中，诸如第一和第二等之类的关系术语仅仅用来将一个实体或者操作与另一个实体或操作区分开来，而不一定要求或者暗示这些实体或操作之间存在任何这种实际的关系或者顺序。而且，术语“包括”、“包含”或者其任何其他变体意在涵盖非排他性的包含，从而使得包括一系列要素的过程、方法、物品或者设备不仅包括那些要素，而且还包括没有明确列出的其他要素，或者是还包括为这种过程、方法、物品或者设备所固有的要素。在没有更多限制的情况下，由语句“包括一个……”限定的要素，并不排除在包括所述要素的过程、方法、物。
以上所述，仅是本发明的较佳实施例而已，并非对本发明作任何形式上的限制。虽然本发明已以较佳实施例揭露如上，然而并非用以限定本发明。任何熟悉本领域的技术人员，在不脱离本发明技术方案范围情况下，都可利用上述揭示的方法和技术内容对本发明技术方案做出许多可能的变动和修饰，或修改为等同变化的等效实施例。因此，凡是未脱离本发明技术方案的内容，依据本发明的技术实质对以上实施例所做的任何简单修改、等同变化及修饰，均仍属于本发明技术方案保护的范围内。

Claims (17)

  1. 一种CO 2压裂过程的应力场变化测量方法，其特征在于，包括：
    CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验；
    计算机根据CO 2压裂实验后的三维物理模型的CT扫描结果，进行数字重构，获得完整缝网的三维数字模型，并在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型；
    3D打印机根据所述不同裂缝状态下的三维数字模型进行打印，得到多个不同裂缝状态下的三维物理模型；
    CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验；
    温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理；
    切片机对进行降温处理之后的各个三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；
    计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力；所述应力条纹分布为三个满足实验要求的正交平面二维切片进行二维光弹实验后得到的。
  2. 根据权利要求1所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述不同的裂缝状态包括：裂缝起裂前、裂缝扩展到预设长度及裂缝停止扩展；
    所述在所述完整缝网的三维数字模型中截取不同的裂缝状态，生成不同裂缝状态下的三维数字模型，包括：
    基于图像处理技术，对所述CT扫描结果的图像进行区域阈值分割、截取及重构操作，得到裂缝扩展到预设长度的三维数字模型。
  3. 根据权利要求2所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验，包括：
    CO 2泵送系统向裂缝起裂前的三维物理模型中注入超临界CO 2，直至注入压力较破裂压力低预设压差时，停止注入超临界CO 2并维持当前注入压力；
    CO 2泵送系统向裂缝扩展到预设长度的三维物理模型中注入超临界CO 2，直至注入压力较形成当前裂缝的压力低预设压差时，停止注入超临界CO 2并维持当前注入压力。
  4. 根据权利要求1至3任一所述的CO 2压裂过程的应力场变化测量方法，其特征在于，在所述CO 2泵送系统对真实储层岩芯的多个相同且透明的三维物理模型进行CO 2压裂实验之前，还包括：
    计算机对所述真实储层岩芯的扫描结果进行数字重构，获得所述真实储层岩芯的三维数字模型；
    3D打印机根据所述真实储层岩芯的三维数字模型进行打印，得到多个相同且透明的所述真实储层岩芯的三维物理模型；
    温箱对多个所述真实储层岩芯的三维物理模型进行处理，使多个所述真实储层岩芯的三维物理模型的性质稳定；
    三轴加载装置对多个所述真实储层岩芯的三维物理模型进行三向伺服加载，直至达到目标值。
  5. 根据权利要求1至3任一所述的CO 2压裂过程的应力场变化测量方法，其特征在于，在所述CO 2泵送系统对所述不同裂缝状态下的三维物理模型进行相应目标压力下的CO 2压裂实验之前，还包括：
    温箱对所述不同裂缝状态下的三维物理模型进行处理，使所述不同裂缝状态下的三维物理模型的性质稳定；
    三轴加载装置对所述不同裂缝状态下的三维物理模型进行三向伺服加载，直至达到目标值。
  6. 根据权利要求4所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述温箱对多个所述真实储层岩芯的三维物理模型进行处理，使多个所述真实储层岩芯的三维物理模型的性质稳定，包括：
    温箱对多个所述真实储层岩芯的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使多个所述真实储层岩芯的三 维物理模型的性质稳定。
  7. 根据权利要求5所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述温箱对所述不同裂缝状态下的三维物理模型进行处理，使所述不同裂缝状态下的三维物理模型的性质稳定，包括：
    温箱对所述不同裂缝状态下的三维物理模型进行缓慢升温，直至升温至60℃时，保持恒温1小时，使所述不同裂缝状态下的三维物理模型的性质稳定。
  8. 根据权利要求1至3任一所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述CO 2泵送系统对真实储层岩芯的三维物理模型进行CO 2压裂实验之前，还包括：
    CO 2泵送系统的温度控制系统调节CO 2的注入压力和温度。
  9. 根据权利要求1至3任一所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述温箱对进行相应目标压力下的CO 2压裂实验后的各个三维物理模型进行降温处理，包括：
    温箱以2℃/h的速度降温至室温。
  10. 根据权利要求1至3任一所述的CO 2压裂过程的应力场变化测量方法，其特征在于，所述计算机对应力条纹分布进行处理，得到各个三维物理模型内各个点的三维最大剪应力，包括：
    计算机根据所述应力条纹分布进行相移法提取，得到相应的应力场；
    计算机根据公式
    Figure PCTCN2018083339-appb-100001
    计算得到载荷作用下三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx；
    计算机根据三个正交平面内的最大剪应力分布(τ max) xy，(τ max) yz，(τ max) zx，计算得到三维物理模型内各个点的三维最大剪应力。
  11. 一种CO2压裂过程三维应力场变化的测量方法，包括：
    通过3D打印技术获取复杂岩体的三维透明化物理模型；
    通过三维物理模型的“冻结”实验来记录不同裂缝状态的三维应力场分布特征；
    通过光弹性实验和相移法分析来定量地显示和分析三维应力场的分布。
  12. 权利要求11的测量方法，其中：所述通过3D打印技术获取复杂岩体的三维透明化物理模型的步骤包括：
    3D打印机根据真实岩芯的三维数字模型进行打印，得到多个相同的三维物理模型；所述三维数字模型是通过对真实岩芯进行CT扫描、图像处理、三维重构所得到的。
  13. 权利要求11的测量方法，其中，通过三维模型的“冻结”实验来记录不同裂缝状态的三维应力场分布特征的步骤包括；
    利用升温装置对所述多个三维物理模型中的一选定三维物理模型升温以使所述三维物理模型达到“冻结”温度；
    利用三轴加载装置对所述达到“冻结”温度的三维物理模型进行加载，以便模拟压裂过程中岩石所受的原始地应力状态；
    利用CO2泵送系统对所述加载的三维物理模型进行CO2压裂实验以使其压裂实验中处于特定的裂缝状态；
    利用温箱对完成处于特定的裂缝状态的三维物理模型进行降温处理，
    重复上述步骤以便得到具有不同裂缝状态的多个三维物理模型的应力场分布特征。
  14. 权利要求13的测量方法，其中所述特定的裂缝状态包括：裂缝起裂前、裂缝扩展到预设长度及裂缝停止扩展。
  15. 权利要求11的测量方法，通过光弹性实验和相移法分析来定量地显示和分析三维应力场的分布的步骤包括：
    利用切片机对进行降温处理之后的各个具有不同裂缝状态三维物理模型进行切片，得到各个三维物理模型的三个正交平面二维切片；
    利用光弹应力测试系统对上述各个正交平面的二维切片进行光弹分析实验，得到各个切片的光弹条纹图；
    利用计算机根据四步彩色相移法获得全场范围主应力方向角 θu的分布，
    Figure PCTCN2018083339-appb-100002
    且sinδ≠0，
    其中，I i为入射光的光强；
    根据改进的六步相移法获得等色线相位图δ u，
    Figure PCTCN2018083339-appb-100003
    其中，
    Figure PCTCN2018083339-appb-100004
    I a为调制光强
    最终计算全场应力场的分布，
    Figure PCTCN2018083339-appb-100005
    其中，f σ为模型的条纹值，h为模型的厚度；
    通过三个正交平面内的应力分量的合成，计算得到三维物理模型内各个点的三维应力场的分布。
  16. 权利要求11的测量方法，其中，在通过三维物理模型的“冻结”实验来记录不同裂缝状态的三维应力场分布特征的步骤前还包括；
    在CO 2泵送系统中通过调节气体的压力和温度改变CO 2气体的相态，保证压裂流体处于特定的气态或者超临界态。
  17. 权利要求16的测量方法，其中，CO 2泵送系统包括温度控制系统，用于调节CO 2的注入压力和温度。
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