CO2 directional fracturing target layer crack propagation simulation control method based on flow regulation and control

Abstract

The invention discloses a CO2 directional fracturing target layer crack propagation simulation control method based on flow regulation and control. The method comprises the steps that a plurality of experimental coal samples are manufactured, and drill holes are formed; the fracturing pump inputs CO2 with set flow into the coal sample through the CO2 injection pipeline; the high-frequency dynamic pressure sensor collects fluid pressure data in the coal sample crack; the acoustic emission sensor monitors and obtains the crack initiation moment and crack initiation pressure of the coal sample; the method comprises the following steps: carrying out cracking and crack width monitoring on a coal sample, calculating a theoretical value of crack initiation pressure and a theoretical value of CO2 injection flow of the coal sample, injecting supercritical CO2 fluid into the coal sample, obtaining a first-stage correction coefficient and a second-stage correction coefficient of the CO2 injection flow, and calculating a corrected CO2 injection flow value. According to the method, under the multi-physical-field coupling environment, the crack initiation pressure of the coal body can be accurately measured and calculated, the correlation between the CO2 injection flow and the crack width of the coal body is established, and the CO2 injection flow better meets the actual requirements of field operation through a CO2 injection flow grading correction mechanism.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coalbed methane development and CO2 mineralization sequestration, and particularly relates to a CO2 directional fracturing target layer fracture propagation simulation control method based on flow regulation. BACKGROUND

[0002] CO2 has strong solubility, easy expansion and high permeability, can enter smaller pores of coal seams, and can erode the internal structure of coal rock body through the joint action with water and coal rock body, effectively improve the fracture network development degree of coal rock body, reduce the fracturing pressure, and improve the fracturing volume, so the CO2 mineralization fracturing sequestration technology has been widely applied in coalbed methane development. At present, the CO2 directional fracturing target layer operation mainly has the following defects or deficiencies in actual application:

[0003] (1) In the fracture initiation and expansion judgment, the existing technology mainly relies on empirical speculation and simplified model, and it is difficult to accurately depict the actual process, which is specifically shown as follows: the traditional fracture initiation pressure calculation only takes into account the basic mechanical parameters, and does not consider the thermal-flow-solid coupling effect unique to CO2 fracturing, such as the thermal expansion of coal body caused by the change of CO2 injection temperature, the pore structure evolution caused by CO2 dissolution, etc., which leads to a significant deviation between the theoretical value of coal body fracture initiation pressure and the actual value; the monitoring of coal body fracture expansion is limited to surface observation or pressure curve inversion, which cannot quantify the width, extension direction and network distribution of internal micro-cracks of coal body, and no "fracture morphology-permeability" correlation model is established, which makes it difficult to evaluate whether the effective extraction threshold is reached; the acceleration effect of CO2 and coal body erosion reaction on coal body fracture expansion is ignored, which further aggravates the lagging nature of fracturing prediction and the risk of misjudgment.

[0004] (2) In the CO2 injection parameter regulation, the existing technology mainly adopts fixed parameters or empirical adjustment strategy, which cannot match the demand of fracture dynamic expansion, which is specifically shown as follows: the CO2 injection flow rate setting lacks quantitative correlation with the initiation pressure and fracture width, which is easy to cause problems such as difficult initiation due to low flow rate or disordered expansion of fractures, early leakage of CO2, etc.; the regulation feedback is lagging and lacks accuracy, and does not dynamically optimize in combination with real-time monitoring data, which may cause fracture through the layer or CO2 ineffective consumption; the matching relationship between coal body elastic modulus evolution and injection rate and other multi-parameter synergistic effects are not considered, which is easy to cause uncontrolled expansion of fractures, and ultimately restricts the coalbed methane extraction efficiency. SUMMARY

[0005] In order to solve the problems in the prior art to some extent, the present application provides a CO2 directional fracturing target layer fracture propagation simulation control method based on flow regulation, which specifically comprises the following steps:

[0006] A CO2 directional fracturing target layer fracture propagation simulation control method based on flow regulation, comprising the following steps:

[0007] S1. Prepare multiple experimental coal samples and drill holes perpendicular to the end face of the coal sample along the central axis of the coal sample. A fracturing pump is connected to the drill hole through a CO2 injection pipeline to input CO2 at a set flow rate into the coal sample. A high-frequency dynamic pressure sensor is installed at the bottom of the drill hole to collect fluid pressure data in the cracks of the coal sample. An acoustic emission sensor is set on the outer surface of the coal sample to obtain the crack initiation time and crack initiation pressure of the coal sample.

[0008] S2, a triaxial pressure was applied to the coal sample using a rock force triaxial testing system, and cracking and crack width were monitored using an industrial CT detection system and a non-contact full-field strain measurement DIC system.

[0009] S3, Calculate the theoretical value of the crack initiation pressure of the coal sample. Theoretical value of CO2 injection flow rate ;

[0010] S4. Based on the theoretical value of CO2 injection flow rate, supercritical CO2 fluid is injected into the coal sample using a fracturing pump. A set of actual fracture initiation pressures of the coal sample fractures are collected, and the first-order correction factor for the CO2 injection flow rate is calculated. ;

[0011] S5, calculate the crack initiation width of the coal sample and set a crack width threshold. When the crack initiation width of the coal sample > the crack width threshold, simultaneously record the actual crack initiation pressure of a set of coal samples. Calculate the second-order correction factor for CO2 injection flow rate. ;

[0012] S6, based on the first-level correction factor and second-order correction factor Through formula Calculate the corrected CO2 injection flow rate.

[0013] Furthermore, the theoretical value of the fracture initiation pressure of the coal sample is calculated using the following formula. :

[0014] ,

[0015] In the formula: This is the theoretical value of the crack initiation pressure; The minimum horizontal principal stress; This represents the maximum horizontal principal stress. The stress coefficient of the porous elastic medium; This represents the original formation pressure; The elastic modulus of the reservoir coal; Poisson's ratio; is the coefficient of thermal expansion of coal. The temperature at which the fluid reaches the bottom of the well; The original formation temperature; This represents the tensile strength of the coal in the reservoir.

[0016] Furthermore, the theoretical value of CO2 injection flow rate is calculated using the following formula. :

[0017] ,

[0018] In the formula: Theoretical value for CO2 injection flow rate; This is the correction factor for the permeability of the coal sample; This represents the cross-sectional area of ​​the coal sample perpendicular to the direction of crack propagation. This is the theoretical value of the crack initiation pressure; This represents the original formation pressure; The dynamic viscosity of CO2 under formation conditions; This represents the effective seepage path length of CO2 within the coal sample. This represents the density of CO2 under formation conditions. The temperature at which the fluid reaches the bottom of the well; CO2 compressibility factor; is the CO2 gas constant.

[0019] Furthermore, step S4 includes the following specific steps:

[0020] S401, arbitrarily select a coal sample, the coal sample injection flow rate is... The supercritical CO2 fluid, high-frequency dynamic pressure sensor and acoustic emission sensor synchronously monitor the actual crack initiation pressure of coal sample in real time;

[0021] S402, observe and record in real time whether the coal sample is cracking. If no cracking signal is detected, stop injecting supercritical CO2 fluid, replace with a new coal sample and adjust the flow rate of supercritical CO2 fluid, and restart the CO2 fracturing operation.

[0022] S403, Collect a set of actual crack initiation pressure data for coal sample fractures. , ,..., (n≥5), calculate its average value. Through formula First-order correction factor for calculating CO2 injection flow rate .

[0023] Furthermore, a criterion for determining the initiation of cracking in coal samples is established. A coal sample is considered to have initiated cracking if any of the following conditions are met:

[0024] The pressure curve shows a sudden drop with a decrease of >10% and a duration of >1 second;

[0025] The acoustic emission sensor captured a signal energy rate increase of >500%, and three consecutive monitoring points detected a rupture signal with the same frequency characteristics.

[0026] An industrial CT inspection system observed new cracks with a length >2 mm around the borehole of the coal sample;

[0027] Furthermore, the flow rate adjustment rules for supercritical CO2 fluid are as follows:

[0028] If the fluctuation range of the actual crack initiation pressure of the coal sample in the previous experiment is less than 5%, the flow rate increment of the supercritical CO2 fluid is set to 8%-10%;

[0029] If the fluctuation range of the actual crack initiation pressure of the coal sample in the previous experiment was 5%-10%, the flow rate increment of the supercritical CO2 fluid should be set to 5%-8%.

[0030] If the actual crack initiation pressure of the coal sample cracked in the previous experiment However, there was no sudden drop, and the flow rate increment of the supercritical CO2 fluid was set to 3%-5%.

[0031] Furthermore, step S5 includes the following specific steps:

[0032] S501, Calculate the crack initiation width of the coal sample after crack initiation. ;

[0033] S502, Set seam width threshold If a single-size proppant is used, the slit width threshold is... , This represents the median proppant particle size; if a mixed particle size proppant is used, the slit width threshold is... , The maximum particle size in the mixed proppant;

[0034] S503, Calculate the second-order correction factor for CO2 injection flow rate. :like Immediately stop injecting supercritical CO2 fluid into the coal sample, replace it with a new coal sample, adjust the increase in the flow rate of the injected supercritical CO2 fluid, and repeat the fracturing experiment until... ;

[0035] when At that time, record the actual crack initiation pressure. Collect a set of actual crack initiation pressure data for coal samples. , ,..., (n≥5), calculate its average value. Through formula Calculate the second-order correction factor for obtaining the CO2 injection flow rate. .

[0036] Furthermore, the crack initiation width of the coal sample after cracking is calculated using the following formula:

[0037] ,

[0038] In the formula: The crack initiation width; This represents the total distance over which the CO2-induced fracture surface is acted upon by the fluid. The effective stress coefficient of the crack; The fluid pressure within the fracture during CO2 fracturing fracture propagation; Vertical stress; The angle of inclination of the crack; This represents the maximum horizontal principal stress. The azimuth angle of the maximum horizontal principal stress; The azimuth angle of the crack; The minimum horizontal principal stress; The stress coefficient of the porous elastic medium; This refers to the distance at which the pre-applied acid solution acts on the seam surface. The Young's modulus of the rock after acid etching; The distance at which the fracturing fluid acts on the fracture surface; The Young's modulus of un-acid-etched coal.

[0039] Furthermore, in step S1, an electric heating jacket is installed on the CO2 injection pipeline, and the CO2 temperature is set to be stably controlled within the range of 35℃-45℃; the CO2 injection pressure is pre-pressurized to the range of 12MPa-20MPa by a fracturing pump.

[0040] Furthermore, triaxial pressure was applied to the coal sample through staged loading and temperature control. During the staged loading process, the pressure increment of each stage was set within the range of 5MPa-10MPa, and the pressure stabilization time was ≥30mins. The temperature control accuracy was set to ±0.5℃, and the heating rate of the coal sample from room temperature to the target formation temperature was ≤2℃ / min.

[0041] Based on the above technical solution, the method described in this invention has the following beneficial effects:

[0042] In a multi-physics coupling environment, the coal fracturing pressure can be accurately calculated, the correlation between CO2 injection flow rate and coal fracture width can be established, and the CO2 injection flow rate graded correction mechanism can help the CO2 injection flow rate better meet the actual needs of on-site operations.

[0043] It helps reduce CO2 injection consumption and mineralization storage-related costs, significantly improves coalbed methane extraction efficiency, and supports the improvement of resource utilization efficiency and economic benefits.

[0044] It can effectively reduce the risk of CO2 leakage, which is of positive significance for improving the safety of mineralization storage and reduces potential environmental hazards to a certain extent. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the overall process of the method described in this invention. Detailed Implementation

[0046] The technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this disclosure or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0047] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure pertains.

[0048] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0049] In all examples shown and discussed herein, any specific value should be interpreted as merely exemplary and not as a limitation; therefore, other examples of exemplary embodiments may have different values.

[0050] Unless otherwise specified, CO2 mentioned in this embodiment refers to supercritical CO2 fluid.

[0051] This embodiment describes a simulation control method for crack propagation in a CO2-directed fracturing target layer based on flow rate regulation. Figure 1 The overall flowchart of this method is given, which includes the following steps.

[0052] S1. Collect coal or rock blocks on-site at the mine, cold process them to make multiple experimental coal samples, and drill holes along the central axis of the coal samples.

[0053] The fracturing pump is connected to the borehole via a high-pressure pipeline and is used to input a set flow rate of CO2 into the coal sample;

[0054] A high-frequency dynamic pressure sensor is installed at the bottom of the borehole to collect changes in fluid pressure within the cracks after the coal sample has cracked.

[0055] Acoustic emission sensors are uniformly arranged on the outer surface of the coal sample to monitor the acoustic emission signals generated by micro-fractures inside the coal sample. Combined with the abrupt change characteristics of the pressure curve, the crack initiation time and crack initiation pressure of the coal sample are obtained.

[0056] In this step:

[0057] (1) Collecting coal blocks or rock blocks on-site at the mine and preparing coal samples through cold processing methods such as cutting can ensure that the geological parameters of the coal samples are the same as or similar to those of the mine to the greatest extent. Under the premise of ensuring that the geological parameters are the same as or similar to those of the mine, other methods commonly used in this field can also be used to prepare coal samples, which will not be elaborated here.

[0058] (2) The size of the coal sample should meet the size requirements of the experimental equipment. For example, in this embodiment, a rock force triaxial test system is preferred to be used to test the crack propagation of the coal sample. Therefore, the size of the coal sample should be made according to the size requirements of the rock force triaxial test system. This will not be elaborated further.

[0059] (3) Use diamond thin-walled drill bit to drill a hole with a diameter of 5mm-10mm and a depth of 2 / 3 of the coal sample height perpendicular to the end face of the coal sample. The verticality deviation of the drill hole shall not exceed 0.5°.

[0060] (4) The connection between the fracturing pump and the borehole is sealed with hydrogenated nitrile rubber to ensure no leakage under 30MPa pressure.

[0061] (5) An electric heating jacket is installed on the CO2 injection pipeline (high-pressure pipeline) to stably control the CO2 temperature within the range of 35℃-45℃. This temperature range is higher than the critical temperature of CO2 of 31.1℃, which can better simulate the supercritical CO2 fracturing environment during actual operation.

[0062] (6) The CO2 injection pressure is pre-pressurized to the range of 12MPa-20MPa by a fracturing pump. This pressure range is higher than the critical pressure of CO2 (7.38MPa), which ensures that CO2 enters the coal sample in a supercritical state for the experiment. Preferably, a pressure sensor with an accuracy of 0.1%FS and a flow meter with an accuracy of 0.5% can be installed on the CO2 injection pipeline to monitor the CO2 status parameters in real time and ensure the consistency between the experimental environment and the on-site fracturing conditions.

[0063] (7) The acoustic emission sensors are evenly distributed on the outer surface of the coal sample using a coupling agent, and the sensor spacing is ≤ 1 / 2 of the side length of the coal sample.

[0064] S2 uses a rock force triaxial testing system to apply triaxial pressure to coal samples through graded loading and temperature control. This ensures that the parameters such as pressure, temperature, maximum horizontal principal stress, and minimum horizontal principal stress borne by the coal samples during the experiment are the same as or similar to the geological parameters of the mine, and is used to simulate the loading of the formation environment.

[0065] Industrial CT inspection systems and non-contact full-field strain measurement (DIC) systems are preferred for monitoring cracking and crack width in coal samples.

[0066] To achieve excellent experimental results and fully reflect the actual working environment, the preferred option is:

[0067] (1) During the triaxial pressure stage loading process, the pressure increment of each stage is controlled within the range of 5MPa-10MPa, and the pressure stabilization time is ≥30mins to ensure that the stress and strain of the coal sample reach a stable state.

[0068] (2) Temperature control adopts a combination of a wrap-around heating jacket and a platinum resistance temperature sensor. The temperature accuracy is controlled within ±0.5℃. The heating rate from room temperature to the target formation temperature is ≤2℃ / min to avoid thermal damage to the coal sample due to sudden temperature changes.

[0069] (3) Intermittent scanning of industrial CT detection system: Before CO2 injection, the initial internal structure image of the coal sample is obtained. During CO2 injection, a scan is performed every 5 mins-10 mins. The scanning range should cover the entire coal sample. The initiation location and extension trajectory of internal cracks in the coal sample are identified through image comparison and analysis.

[0070] (4) The non-contact full-field strain measurement DIC system sprays random speckle patterns on the outer surface of the coal sample and uses two high-speed cameras to simultaneously acquire surface deformation images of the coal sample. The surface displacement field is calculated by digital image matching algorithm, and the crack width is derived. For internal cracks that are not visible on the surface, the crack contour is extracted by combining the gray value changes of industrial CT images and using image segmentation and edge detection algorithms to calculate the crack width, ensuring that the width measurement accuracy reaches 10 μm.

[0071] S3. Currently, the commonly used methods for calculating the initiation pressure of coal fractures are based solely on fundamental mechanical parameters such as the horizontal principal stress and vertical stress of the coal body, as detailed below:

[0072]

[0073] In the formula:

[0074] This is the theoretical value of the crack initiation pressure, in MPa.

[0075] The minimum horizontal principal stress is expressed in MPa.

[0076] The maximum horizontal principal stress is expressed in MPa.

[0077] This represents the tensile strength of the reservoir coal, expressed in MPa.

[0078] This traditional formula fails to comprehensively consider the porous media characteristics of coal, the thermal expansion effect induced by CO2 injection, and the influence of the tensile strength of the coal itself on the coal seam fracturing process. In the supercritical CO2 fracturing scenario, because the thermal-fluid-structure interaction is not reflected, the theoretical calculation value of the coal fracturing pressure deviates from the actual fracturing pressure, and the deviation generally exceeds about 20%, which makes it impossible to meet the precise control requirements of CO2 directional fracturing.

[0079] To obtain a more accurate theoretical value of the coal seam fracture initiation pressure and to scientifically and effectively guide CO2-directed fracturing operations, this embodiment provides a formula for calculating the theoretical value of the coal sample fracture initiation pressure, as follows:

[0080]

[0081] In the formula:

[0082] This is the theoretical value of the crack initiation pressure, in MPa.

[0083] The minimum horizontal principal stress is expressed in MPa. The maximum horizontal principal stress is expressed in MPa.

[0084] The stress coefficient of porous elastic media is dimensionless, ranging from 0.3 to 0.8, and can be determined by laboratory core pore pressure dissipation experiments.

[0085] This represents the original formation pressure, expressed in MPa.

[0086] The elastic modulus of the reservoir coal is expressed in GPa.

[0087] Poisson's ratio is dimensionless.

[0088] The coefficient of thermal expansion of coal is expressed in °C. -1 ;

[0089] The temperature of the fluid when it reaches the bottom of the well, in °C;

[0090] The original formation temperature is expressed in °C.

[0091] This represents the tensile strength of the reservoir coal, expressed in MPa.

[0092] The formula described in this embodiment can effectively solve the defects in the current calculation method of the theoretical value of coal fracture initiation pressure, specifically as follows:

[0093] (1) The porous elasticity effect term was introduced. "Based on Biot's porous elasticity theory, the regulation effect of the original formation pressure on the effective stress of the coal body is quantified, which breaks through the limitation of traditional calculation methods that ignore the influence of the pore structure of the coal body and is more in line with the nature of the porous medium of the coal body."

[0094] (2) A thermal expansion effect term was set. This quantifies the contribution of coal thermal deformation caused by the difference between CO2 injection temperature and formation temperature to the fracturing pressure, fully reflects the unique thermal-fluid-solid coupling characteristics of CO2-induced fracturing, and solves the problem of the singularity of existing calculation methods that only consider mechanical effects.

[0095] (3) A tensile strength compensation item was set up. This corrects the simplification of the existing calculation method that ignores the tensile strength of the coal body, making the calculation process more consistent with the actual mechanical behavior of the coal body from stress accumulation to fracture, and avoiding the underestimation of the crack initiation pressure due to ignoring tensile characteristics.

[0096] S4. Currently, the industry generally uses the single-phase fluid steady-state seepage formula based on Darcy's law to calculate the CO2 injection flow rate. This formula is the core basis for flow rate design in traditional fracturing engineering, as detailed below:

[0097]

[0098] In the formula:

[0099] The theoretical CO2 injection flow rate calculated using traditional methods is expressed in m³ / s.

[0100] This is the basic permeability of the coal sample, in m² (usually the initial permeability measured in the laboratory is used, without dynamic correction).

[0101] The cross-sectional area of ​​the coal sample perpendicular to the direction of crack propagation is expressed in m².

[0102] The theoretical value of crack initiation pressure calculated using traditional methods is given in Pa.

[0103] This represents the original formation pressure, expressed in Pa.

[0104] CO2 dynamic viscosity, in units of

[0105] This represents the length of the CO2 seepage path within the coal sample, expressed in meters (m).

[0106] This traditional formula uses a fixed base permeability. It cannot reflect the dynamic impact of coal sample porosity differences and CO2 dissolution on permeability; and it assumes that CO2 is a single-phase conventional fluid, without considering the significant changes in physical properties such as density and compressibility under supercritical conditions.

[0107] Based on the shortcomings of traditional CO2 injection flow rate calculation formulas, and referring to the geological parameters of the mine and the theoretical value of the fracture initiation pressure of the coal sample, this embodiment provides the following formula for calculating the theoretical value of CO2 injection flow rate:

[0108]

[0109] In the formula:

[0110] The theoretical flow rate for CO2 injection, in m³ / s;

[0111] This is a dimensionless correction factor for coal sample permeability, ranging from 0.8 to 1.2, based on the porosity of the coal sample. The corrected fitting formula is as follows: ;

[0112] The cross-sectional area of ​​the coal sample perpendicular to the direction of crack propagation is expressed in m².

[0113] This is the theoretical value of the crack initiation pressure, in MPa.

[0114] This represents the original formation pressure, expressed in MPa.

[0115] The dynamic viscosity of CO2 under formation conditions is given by 100 kJ / m³. The values ​​were obtained through laboratory measurements using a high-temperature, high-pressure viscometer.

[0116] The effective seepage path length of CO2 inside the coal sample, in meters, is taken as 1 / 2 of the characteristic length of the coal sample (if the coal sample is a cube, take its side length; if it is a cylinder, take its diameter, which can be measured by vernier calipers).

[0117] The density of CO2 under formation conditions is expressed in kg / m³ and is calculated using the Peng-Robinson equation of state.

[0118] Temperature of the fluid when it reaches the bottom of the well, in Kelvin (K).

[0119] The CO2 compressibility factor is dimensionless and is determined by laboratory PVT experiments or calculated using the SRK equation of state.

[0120] The CO2 gas constant has a value of [value missing]. .

[0121] Compared to the traditional CO2 injection flow rate calculation formula based on Darcy's law, the formula provided in this embodiment achieves accurate adaptation to supercritical CO2-induced fracturing conditions through multi-dimensional optimization, specifically as follows:

[0122] (1) The permeability correction coefficient k based on the dynamic correction of coal sample porosity is introduced, which breaks through the limitation of the traditional formula using a fixed basic permeability k0, which cannot reflect the influence of coal heterogeneity and CO2 dissolution on permeability. It can reduce the flow rate calculation deviation caused by the solidification of permeability parameters by about 8%-12%.

[0123] (2) Through The synergistic effect of physical properties such as density, bottom hole temperature, and compressibility factor of supercritical CO2 is quantified. All parameters are based on actual formation working conditions. This effectively solves the deviation caused by the traditional formula assuming CO2 is a single-phase conventional fluid and ignoring the abrupt change in physical properties under supercritical conditions. It is fully adapted to the fracturing environment of supercritical CO2.

[0124] (3) The accurate crack initiation pressure calculated based on the multiphysics coupling model of this invention Difference from the original formation pressure Coupled with CO2 physical properties through a product, it better conforms to the nonlinear seepage law of supercritical CO2 in the pores of coal, which can improve the accuracy of flow calculation to over 90%, and at the same time provide a reliable theoretical basis for subsequent first- and second-level flow correction, avoiding the flow runaway problem caused by traditional empirical adjustments.

[0125] S5, based on the theoretical value of CO2 injection flow rate, supercritical CO2 fluid is injected into the coal sample through a fracturing pump. A set of actual fracture initiation pressures of the coal sample fractures are collected, and the first-level correction factor of CO2 injection flow rate is calculated. .

[0126] The specific steps are as follows:

[0127] S501, arbitrarily select a coal sample, and use a fracturing pump to inject the sample through a borehole at a flow rate of [missing information]. The supercritical CO2 fluid is controlled within ±5% of its flow rate fluctuation. A high-frequency dynamic pressure sensor and an acoustic emission sensor are used to monitor the actual crack initiation pressure of the coal sample in real time.

[0128] S502 sets the criteria for determining the initiation of cracking in coal samples. A coal sample is considered to have initiated cracking if any of the following conditions are met:

[0129] (1) The pressure curve shows a significant sudden drop (the drop is greater than 10% and the duration is greater than 1 second);

[0130] (2) The acoustic emission sensor detected a sudden increase in signal energy rate (increase > 500%), and three consecutive monitoring points detected a break signal with the same frequency characteristics.

[0131] (3) The industrial CT detection system observed new cracks with a length >2 mm around the borehole of the coal sample.

[0132] S503: Observe and record in real time whether the coal sample has cracked. If no cracking signal is detected, immediately stop injecting supercritical CO2 fluid, replace with a new coal sample, and restart the CO2 fracturing operation.

[0133] Preferably, the increase in the flow rate of supercritical CO2 fluid injected in each experiment should be less than 10% of the flow rate of the previous experiment, until the coal sample initiates fracturing and the actual fracturing pressure is recorded. .

[0134] In this embodiment, the specific increment of the supercritical CO2 fluid flow rate should be determined based on the pressure change trend of the previous experiment. Those skilled in the art should understand that a 10% increase in the supercritical CO2 fluid flow rate is merely a preferred example, and other values ​​within the range of ≤10% can also be used. If the previous experimental pressure rose slowly without significant fluctuations, the flow rate increment is 8%-10%; if the previous experimental pressure fluctuated slightly (5%-10%), the flow rate increment is 5%-8%; if the previous experimental pressure was close to... However, there was no sudden drop, and the traffic increase was 3%-5%.

[0135] S504, Collect actual crack initiation pressure data of ≥5 groups of coal samples. , ,..., (n≥5), calculate its average value. Through formula First-order correction factor for calculating CO2 injection flow rate .

[0136] S6. Currently, the calculation of fracture initiation width in coal seams generally suffers from the following problems, resulting in low accuracy of the obtained fracture initiation width. This makes it unsuitable for effectively guiding CO2 fracturing operations in practical applications. The specific problems are briefly described below:

[0137] (1) The distance of the pre-acid solution action on the fracture surface and the Young's modulus of the rock after acid etching were not included, which could not be adapted to the common acid pretreatment conditions on site, resulting in a deviation of more than 30% in the calculation of the fracture width in the acid-etched area;

[0138] (2) The crack dip angle and azimuth angle are not coupled, and the vertical stress and horizontal principal stress are not decomposed in direction. Therefore, it cannot reflect the influence of the actual stress field distribution on the crack width and can only calculate the width of ideal vertical or horizontal cracks.

[0139] (3) The difference in Young's modulus between the fracturing fluid and acid action zones was not distinguished. A single Young's modulus was used for calculation, and the model was oversimplified, resulting in insufficient calculation accuracy and failing to meet the requirements for proppant particle size matching.

[0140] To effectively address the aforementioned issues, this embodiment provides the following formula for calculating the crack initiation width, used to accurately calculate the crack initiation width of the coal sample after cracking. And set a threshold for coal sample slot width. ,when Simultaneously, the actual crack initiation pressure of a set of coal samples was recorded. Calculate the second-order correction factor for CO2 injection flow rate. The specific steps are as follows:

[0141] S601, the crack initiation width of the coal sample after cracking is calculated using the following formula:

[0142]

[0143] In the formula:

[0144] The crack initiation width is expressed in meters (m).

[0145] This represents the total distance over which the CO2-induced hydraulic fluid acts on the fracture surface, in meters.

[0146] The effective stress coefficient of the fracture is dimensionless, ranging from 0.6 to 0.9, and is determined through laboratory effective stress experiments (using a triaxial rock mechanics testing system, measuring the compressive strength of coal samples under different pore pressures, fitting a linear relationship between compressive strength and pore pressure, and the absolute value of the slope is the effective stress coefficient). The testing standards refer to GB / T 50266-2013 "Standard for Test Methods of Engineering Rock Mass");

[0147] The fluid pressure within the CO2-driven fracturing fracture during its extension is expressed in Pa.

[0148] This is the vertical stress, measured in Pa.

[0149] The crack dip angle is expressed in rad.

[0150] The maximum horizontal principal stress is expressed in Pa.

[0151] The azimuth angle of the maximum horizontal principal stress is expressed in rad.

[0152] The crack azimuth angle is in rad.

[0153] The minimum horizontal principal stress is expressed in Pa.

[0154] The stress coefficient of porous elastic media is dimensionless, ranging from 0.3 to 0.8, and can be determined by laboratory core pore pressure dissipation experiments.

[0155] The distance at which the pre-applied acid solution acts on the seam surface, in meters;

[0156] This represents the Young's modulus of the rock after acid etching, in Pa.

[0157] The distance at which the fracturing fluid acts on the fracture surface, in meters;

[0158] This represents the Young's modulus of un-acid-etched coal, expressed in Pa.

[0159] In the above formula:

[0160] (1) Total distance of fluid action on CO2 fracture surface Using industrial CT scan images, image measurement software was used to measure the straight-line distance from the borehole wall to the crack tip along the crack extension direction, and the average of three measurements was taken.

[0161] (2) Fluid pressure within the fracture during CO2 fracturing fracture propagation The average pressure value of the high-frequency dynamic pressure sensor within 10 seconds after the coal sample cracks is read and used as the fluid pressure inside the crack.

[0162] (3) Crack dip angle β and crack azimuth angle The surface displacement field obtained by the non-contact full-field strain measurement DIC system was fitted with the crack trajectory line, and the angle between the crack and the horizontal plane (crack inclination angle β) and the direction of the maximum horizontal principal stress (crack azimuth angle) were calculated. ), with an accuracy of ±1°;

[0163] (4) Distance of action of pre-acid solution on seam surface If pre-treatment with acid is used in the experiment, the calculation is based on the acid injection volume and the cross-sectional area of ​​the coal sample, using the following formula: ( (This refers to the volume of acid injected). If not used, take... ;

[0164] (5) Young's modulus of the rock after acid etching Multiple points (≥5 points) were tested in the acid-etched area using a nanoindenter, and the average value was taken. The test load was 500-1000 μN.

[0165] (6) Distance of action of fracturing fluid on the fracture surface :equal (when hour, );

[0166] (7) Young's modulus of un-acid-etched coal The uniaxial compression test was conducted on the un-acid-etched area of ​​the coal sample using a rock force triaxial testing system, and the slope of the elastic stage was taken as the Young's modulus.

[0167] The formula for calculating the crack initiation width of coal samples after cracking, as described in this embodiment, has the following advantages compared to the commonly used calculation methods at present:

[0168] (1) Introducing Lacid, Eacid and Young's modulus of unacidified coal ( A segmented modulus calculation model can be constructed, and parameters can be dynamically adjusted according to whether acid pretreatment is used to accurately adapt to different on-site processes.

[0169] (2) The direction decomposition of vertical stress is achieved by crack inclination angle (β), and the direction correction of horizontal principal stress is achieved by crack azimuth angle (θ) and maximum horizontal principal stress azimuth angle (γ). The effect of actual stress field on crack width is accurately quantified, and the width of cracks with arbitrary inclination angle and azimuth angle can be calculated.

[0170] (3) The total distance of fluid action on the CO2 fracture surface (Leff) and the distance of fluid action on the fracture surface (Leff) Based on this, we ensure that the calculated parameters are highly matched with the fluid action range in the actual fracturing process, and avoid the parameters from being out of touch with the actual working conditions.

[0171] S602, Set the slit width threshold according to the particle size corresponding to the target proppant. The specific rules are as follows:

[0172] If a single-size proppant is used, the slit width threshold ( (This refers to the median particle size of the proppant, in meters), ensuring that the proppant can smoothly enter the crack and form effective support.

[0173] If a mixed particle size proppant is used, the slot width threshold ( (This refers to the maximum particle size in the mixed proppant, in meters), to prevent large-diameter proppant from clogging cracks.

[0174] S603, Calculate the second-order correction factor for CO2 injection flow rate. .

[0175] like Immediately stop injecting supercritical CO2 fluid into the coal sample and replace it with a new coal sample to repeat the fracturing experiment. The increase in supercritical CO2 fluid flow rate in each experiment should be less than 10% of the previous experiment's flow rate. After each adjustment, recalculate the fracture initiation width in the coal sample until... .

[0176] when At that time, record the actual crack initiation pressure. Collect actual crack initiation pressure data from ≥5 groups of coal samples. , ,..., (n≥5), calculate its average value. Through formula Calculate the second-order correction factor for obtaining the CO2 injection flow rate. .

[0177] S7, the first-level correction coefficient obtained in step S5. and the secondary correction coefficient obtained in step S6 Through formula Calculate the corrected CO2 injection flow rate.

[0178] The above steps were used to determine the theoretical initiation pressure of coal sample fractures. and crack width Based on accurate calculations and repeated experiments, as well as the judgment of the actual crack initiation pressure and crack width of coal samples, this embodiment can make reasonable adjustments to the theoretical value of CO2 injection flow rate, thus solving the technical problem in the prior art of "being unable to timely and accurately control the CO2 injection rate and injection volume during crack initiation and propagation".

[0179] In actual coalbed methane extraction operations, the corrected CO2 injection flow rate obtained in this embodiment can be used as the initial control parameter for on-site CO2 injection, and combined with real-time on-site monitoring data of pressure, flow rate, and fractures (such as ground microseismic monitoring), to... Dynamic fine-tuning (fine-tuning range ≤5%) is performed to achieve precise matching between "simulation and field", ultimately achieving the technical effects of improving the accuracy of fracture directional propagation (directional deviation ≤15°), reducing ineffective CO2 consumption (consumption reduced by 15%-20%), and increasing coalbed methane extraction rate (extraction rate increased by 10%-15%).

[0180] The following section further illustrates the method described in this embodiment by calculating actual parameters from coal samples collected from a coal mine.

[0181] The first step was to collect coal blocks from the No. 3 coal seam (buried at a depth of 800m) in a certain anthracite coal mine and process them into cubic coal samples with dimensions of 100 mm × 100 mm × 100 mm. The coal sample porosity was 8.5% and the permeability was 1.2 mD.

[0182] The geological parameters of the No. 3 coal seam in this anthracite mine are as follows:

[0183] The original formation pressure was 8 MPa, and the original formation temperature was 35℃.

[0184] Maximum horizontal principal stress σ H The minimum horizontal principal stress is σ = 25 MPa. h The vertical stress is 18 MPa, σ. v It is 20 MPa;

[0185] The elastic modulus E is 3.5 GPa, Poisson's ratio μ is 0.25, and the tensile strength σ t The pressure is 1.8 MPa, and the coefficient of volumetric thermal expansion is α. m for ;

[0186] The stress coefficient η of the porous elastic medium is 0.5, and the CO2 injection temperature T w It is 25℃;

[0187] The target proppant is quartz sand with a median particle size d. 50 Set the seam width threshold to 0.8 mm. .

[0188] The second step is to calculate the theoretical value of the crack initiation pressure:

[0189]

[0190] The theoretical value of crack initiation pressure was calculated.

[0191] The third step is to calculate the theoretical CO2 injection flow rate:

[0192] The dynamic viscosity of CO2, μ, is CO2 density ρ CO2The sample density is 780 kg / m³; the compressibility factor Z is 0.85; the cross-sectional area A of the coal sample is 0.01 m²; the effective seepage path length L is 0.05 m; and the permeability correction coefficient k is 0.957 (calculated from a porosity of 8.5%).

[0193]

[0194] The theoretical value of CO2 injection flow rate was calculated. ,Right now .

[0195] Step 4: Calculate the first-level correction factor Secondary correction coefficient and the corrected actual CO2 injection flow rate.

[0196] Level 1 correction factor :

[0197] according to Injection, average actual crack initiation pressure from 5 experiments ,but .

[0198] Secondary correction coefficient :

[0199] Adjust the flow rate to (Increment 9.1% < 10%), the crack width was measured. The average actual crack initiation pressure in 5 experiments ,but .

[0200] Corrected actual CO2 injection flow rate:

[0201] .

[0202] Field application effect

[0203] Will The method was applied to the No. 3 coal seam of the mine for on-site fracturing. Ground microseismic monitoring showed that the directional extension deviation of the fracture was 12°. The CO2 consumption was reduced by 18% compared with the traditional empirical method, and the coalbed methane extraction rate was increased from 15% to 28%, achieving the expected technical effect.

[0204] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation, characterized in that, Includes the following steps: S1. Prepare multiple experimental coal samples and drill holes perpendicular to the end face of the coal sample along the central axis of the coal sample. A fracturing pump is connected to the drill hole through a CO2 injection pipeline to input CO2 at a set flow rate into the coal sample. A high-frequency dynamic pressure sensor is installed at the bottom of the drill hole to collect fluid pressure data in the cracks of the coal sample. An acoustic emission sensor is set on the outer surface of the coal sample to obtain the crack initiation time and crack initiation pressure of the coal sample. S2, a triaxial pressure was applied to the coal sample using a rock force triaxial testing system, and cracking and crack width were monitored using an industrial CT detection system and a non-contact full-field strain measurement DIC system. S3, Calculate the theoretical value of the crack initiation pressure of the coal sample. Theoretical value of CO2 injection flow rate ; S4. Based on the theoretical value of CO2 injection flow rate, supercritical CO2 fluid is injected into the coal sample using a fracturing pump. A set of actual fracture initiation pressures of the coal sample fractures are collected, and the first-order correction factor for the CO2 injection flow rate is calculated. ; S5, calculate the crack initiation width of the coal sample and set a crack width threshold. When the crack initiation width of the coal sample > the crack width threshold, simultaneously record the actual crack initiation pressure of a set of coal samples. Calculate the second-order correction factor for CO2 injection flow rate. ; S6, based on the first-level correction factor and second-order correction factor Through formula Calculate the corrected CO2 injection flow rate.

2. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 1, characterized in that, The theoretical value of the fracture initiation pressure of the coal sample is calculated using the following formula. : ， In the formula: This is the theoretical value of the crack initiation pressure; The minimum horizontal principal stress; This represents the maximum horizontal principal stress. The stress coefficient of the porous elastic medium; This represents the original formation pressure; The elastic modulus of the reservoir coal; Poisson's ratio; is the coefficient of thermal expansion of coal. The temperature at which the fluid reaches the bottom of the well; The original formation temperature; This represents the tensile strength of the coal in the reservoir.

3. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 1, characterized in that, The theoretical value of CO2 injection flow rate is calculated using the following formula. : ， In the formula: Theoretical value for CO2 injection flow rate; This is the correction factor for the permeability of the coal sample; This represents the cross-sectional area of ​​the coal sample perpendicular to the direction of crack propagation. This is the theoretical value of the crack initiation pressure; This represents the original formation pressure; The dynamic viscosity of CO2 under formation conditions; This represents the effective seepage path length of CO2 within the coal sample. This represents the density of CO2 under formation conditions. The temperature at which the fluid reaches the bottom of the well; CO2 compressibility factor; is the CO2 gas constant.

4. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 1, characterized in that, Step S4 includes the following specific steps: S401, arbitrarily select a coal sample, the coal sample injection flow rate is... The supercritical CO2 fluid, high-frequency dynamic pressure sensor and acoustic emission sensor synchronously monitor the actual crack initiation pressure of coal sample in real time; S402, observe and record in real time whether the coal sample is cracking. If no cracking signal is detected, stop injecting supercritical CO2 fluid, replace with a new coal sample and adjust the flow rate of supercritical CO2 fluid, and restart the CO2 fracturing operation. S403, Collect a set of actual crack initiation pressure data for coal sample fractures. , ,..., (n≥5), calculate its average value. Through formula First-order correction factor for calculating CO2 injection flow rate .

5. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 4, characterized in that, Establish a criterion for determining the initiation of cracking in a coal sample. The coal sample is deemed to have initiated cracking if any of the following conditions are met: The pressure curve shows a sudden drop with a decrease of >10% and a duration of >1 second; The acoustic emission sensor captured a signal energy rate increase of >500%, and three consecutive monitoring points detected a rupture signal with the same frequency characteristics. An industrial CT inspection system observed new cracks with a length greater than 2 mm around the borehole of the coal sample.

6. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 4 or 5, characterized in that, The flow rate adjustment rules for supercritical CO2 fluid are as follows: If the fluctuation range of the actual crack initiation pressure of the coal sample in the previous experiment is less than 5%, the flow rate increment of the supercritical CO2 fluid is set to 8%-10%; If the fluctuation range of the actual crack initiation pressure of the coal sample in the previous experiment was 5%-10%, the flow rate increment of the supercritical CO2 fluid should be set to 5%-8%. If the actual crack initiation pressure of the coal sample cracked in the previous experiment However, there was no sudden drop, and the flow rate increment of the supercritical CO2 fluid was set to 3%-5%.

7. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 1, characterized in that, Step S5 includes the following specific steps: S501, Calculate the crack initiation width of the coal sample after crack initiation. ; S502, Set seam width threshold If a single-size proppant is used, the slit width threshold is... , This represents the median proppant particle size; if a mixed particle size proppant is used, the slit width threshold is... , The maximum particle size in the mixed proppant; S503, Calculate the second-order correction factor for CO2 injection flow rate. :like Immediately stop injecting supercritical CO2 fluid into the coal sample, replace it with a new coal sample, adjust the increase in the flow rate of the injected supercritical CO2 fluid, and repeat the fracturing experiment until... ; when At that time, record the actual crack initiation pressure. ; Collect a set of actual crack initiation pressure data for coal samples. , ,..., (n≥5), calculate its average value. Through formula Calculate the second-order correction factor for obtaining the CO2 injection flow rate. .

8. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to claim 7, characterized in that, The crack initiation width of the coal sample after cracking is calculated using the following formula: ， In the formula: The crack initiation width; This represents the total distance over which the CO2-induced fracture surface is acted upon by the fluid. The effective stress coefficient of the crack; The fluid pressure within the fracture during CO2 fracturing fracture propagation; Vertical stress; The angle of inclination of the crack; This represents the maximum horizontal principal stress. The azimuth angle of the maximum horizontal principal stress; The azimuth angle of the crack; The minimum horizontal principal stress; The stress coefficient of the porous elastic medium; This refers to the distance at which the pre-applied acid solution acts on the seam surface. The Young's modulus of the rock after acid etching; The distance at which the fracturing fluid acts on the fracture surface; The Young's modulus of un-acid-etched coal.

9. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to any one of claims 1-8, characterized in that, In step S1, an electric heating jacket is installed on the CO2 injection pipeline, and the CO2 temperature is set to be stably controlled within the range of 35℃-45℃; the CO2 injection pressure is pre-pressurized to the range of 12MPa-20MPa by a fracturing pump.

10. The method for simulating and controlling the propagation of cracks in a CO2-directed fracturing target layer based on flow rate regulation according to any one of claims 1-8, characterized in that, Triaxial pressure was applied to the coal sample through staged loading and temperature control. During the staged loading process, the pressure increment of each stage was set within the range of 5MPa-10MPa, and the pressure stabilization time was ≥30mins. The temperature control accuracy was set to ±0.5℃, and the heating rate of the coal sample from room temperature to the target formation temperature was ≤2℃ / min.

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