Test sample and design method of compact sandstone far-field hydraulic fracturing simulation experiment

Abstract

The application provides a test sample and a design method for a compact sandstone far-field hydraulic fracturing simulation experiment, and relates to the fields of oil and gas drilling and completion. The test sample comprises a rock sample body and a sleeve. The top surface of the rock sample body is provided with a hole along the axis of the rock sample body. The sleeve is arranged in the hole. The sleeve is composed of two or more fracturing sections. The fracturing sections are separated by pistons. The test sample and the design method for the compact sandstone far-field hydraulic fracturing simulation experiment can more accurately reflect the crack change characteristics in the process of hydraulic fracturing of a compact sandstone reservoir under far-field conditions.

Description

Technical Field

[0001] This invention relates to the field of oil and gas drilling and completion, and in particular to a test specimen and design method for a far-field hydraulic fracturing simulation experiment of tight sandstone. Background Technology

[0002] Hydraulic fracturing is a necessary means to achieve effective extraction of tight oil and gas, and the post-fracturing effect is closely related to the complexity of the fracture network. Currently, due to the lack of accurate and effective field monitoring methods, the effect of hydraulic fracturing on underground reservoirs cannot be directly observed, leading to a lack of understanding of the extension laws of hydraulic fractures. This, in turn, results in a certain degree of blindness and randomness in the design and layout of current hydraulic fracturing well networks. Hydraulic fracturing physical simulation experiments are a reliable and effective means of understanding fracture extension laws, and are of great significance for the design of hydraulic fracturing extraction schemes, technical modifications, and improvement of oil and gas resource recovery in tight reservoirs.

[0003] Currently, most hydraulic fracturing physical simulation experiments focus on shale and cement specimens, with few experimental studies specifically targeting tight sandstone. In well completion design, experiments often employ single-stage fracturing, rarely considering the impact of multi-stage, multi-cluster fracturing completion methods on the geometry of hydraulic fractures, thus neglecting the number of perforation clusters and the influence between different clusters during fracturing. Regarding parameter design, some physical model experiments ignore scaling between in-situ and laboratory scales, only qualitatively describing fracture propagation patterns in tight sandstone, making it difficult to objectively and quantitatively characterize the in-situ fracture network propagation patterns. Furthermore, while some experiments scale experimental parameters based on similarity principles, they overlook the different controlling factors influencing fracture morphology at different experimental scales. Therefore, it is necessary to establish different parameter calculation models based on the experimental observation scale. Chinese patent application No. 202010952485.X discloses a method for evaluating the propagation characteristics of hydraulically fractured fracture networks in shale and mudstone. The method for evaluating fracture network propagation characteristics includes: preparing shale samples, conducting triaxial hydraulic fracturing physical simulation experiments, and determining the influence of full bedding and natural fractures at different strata on fracture network propagation. However, this patent uses full-diameter shale samples encased in cement mortar, which cannot accurately reflect the mechanical characteristics of the reservoir rock. Furthermore, the patented sample only has one fracture initiation segment, making it impossible to simulate the interaction between hydraulic fractures.

[0004] In view of this, based on years of experience in production and design in this and related fields, the inventor has designed a test specimen and design method for simulating far-field hydraulic fracturing of tight sandstone through repeated experiments, in order to solve the problems existing in the prior art. Summary of the Invention

[0005] The purpose of this invention is to provide a test specimen and design method for a far-field hydraulic fracturing simulation experiment of tight sandstone, which can more accurately reflect the fracture change characteristics during the hydraulic fracturing process of tight sandstone reservoirs under far-field conditions.

[0006] To achieve the above objectives, the present invention proposes a test specimen for a far-field hydraulic fracturing simulation experiment of tight sandstone, wherein the test specimen includes a rock sample body and a casing, the top surface of the rock sample body has an opening along the axis of the rock sample body, the casing is inserted into the opening, and the casing is composed of two or more fracturing sections, each of the fracturing sections being separated by a piston.

[0007] This invention also proposes a design method for a far-field hydraulic fracturing simulation experiment of tight sandstone, wherein the test specimens as described above are prepared; a triaxial stress test system is used and the loading stress is calculated; a pumping parameter calculation model is established; and a multi-stage hydraulic fracturing simulation experiment is conducted based on the loading stress and the pumping parameters to determine the crack initiation and propagation laws of the tight sandstone.

[0008] Compared with the prior art, the present invention has the following features and advantages:

[0009] The test specimen for the far-field hydraulic fracturing simulation experiment of tight sandstone proposed in this invention has a casing composed of two or more fracturing sections, which allows for multi-stage fracturing experiments. This is more similar to the hydraulic fracturing operation of underground reservoirs and can more realistically reflect the effect of hydraulic fracturing.

[0010] The design method for far-field hydraulic fracturing simulation experiments of tight sandstone proposed in this invention can conduct multi-stage fracturing experiments, and through calculation, make the loading stress and pumping parameters more consistent with the scaling law of in-situ and laboratory scales, thereby enabling a more objective and quantitative characterization of the propagation law of the in-situ fracture network. Attached Figure Description

[0011] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely illustrative to aid in understanding the invention and do not specifically limit the shapes and proportions of the components. Those skilled in the art, guided by the teachings of this invention, can select various possible shapes and proportions to implement the invention according to specific circumstances.

[0012] Figure 1 This is a schematic diagram of the test specimen for the far-field hydraulic fracturing simulation experiment of tight sandstone proposed in this invention;

[0013] Figure 2 This is a flowchart of an embodiment of the design method for a far-field hydraulic fracturing simulation experiment of tight sandstone proposed in this invention;

[0014] Figure 3 This is a schematic diagram of the pressure change and stress-strain in the upper section of the wellbore in one embodiment of the present invention;

[0015] Figure 4A This is a diagram of acoustic emission events in the upper half of the cluster in one embodiment of the present invention;

[0016] Figure 4B This is a schematic diagram (a) of the crack initiation in the upper half of the crack cluster in one embodiment of the present invention;

[0017] Figure 4C This is a schematic diagram (II) of the crack initiation in the upper 1 / 2 cluster of cracks in one embodiment of the present invention;

[0018] Figure 4D This is a schematic diagram (III) of the crack initiation in the upper 1 / 2 cluster of cracks in one embodiment of the present invention;

[0019] Figure 4E This is a diagram of acoustic emission events in the third cluster of the upper section in one embodiment of the present invention;

[0020] Figure 4F This is a schematic diagram (a) of the initiation of the third cluster of cracks in the upper section in one embodiment of the present invention;

[0021] Figure 4G This is a schematic diagram (II) of the crack initiation of the third cluster of cracks in the upper section in one embodiment of the present invention;

[0022] Figure 4H This is a schematic diagram (III) of the crack initiation of the third cluster of cracks in the upper section in one embodiment of the present invention;

[0023] Figure 4I This is a diagram of acoustic emission events at the tip of a test specimen in one embodiment of the present invention;

[0024] Figure 4J This is a schematic diagram (a) of the crack initiation of the entire test specimen in one embodiment of the present invention;

[0025] Figure 4K This is a schematic diagram (II) of the crack initiation of the entire test specimen in one embodiment of the present invention;

[0026] Figure 4L This is a schematic diagram (III) of the crack initiation of the entire test specimen in one embodiment of the present invention; Figure 5 This is a schematic diagram of the pressure change and stress-strain in the lower section of the wellbore in one embodiment of the present invention;

[0027] Figure 6A This is a diagram of acoustic emission events in the first cluster of the lower segment in one embodiment of the present invention;

[0028] Figure 6B This is a schematic diagram (a) of the initiation of the first cluster of cracks in the lower section in one embodiment of the present invention;

[0029] Figure 6C This is a schematic diagram (II) of the crack initiation of the first cluster of cracks in the lower section in one embodiment of the present invention;

[0030] Figure 6D This is a schematic diagram (III) of the initiation of the first cluster of cracks in the lower section in one embodiment of the present invention;

[0031] Figure 6E This is a diagram of acoustic emission events during the reactivation of the first cluster in the upper section according to an embodiment of the present invention;

[0032] Figure 6F This is a schematic diagram (a) of the reactivation of the first cluster of cracks in the upper section in one embodiment of the present invention;

[0033] Figure 6G This is a schematic diagram (II) of the reactivation of the first cluster of cracks in the upper section in one embodiment of the present invention;

[0034] Figure 6H This is a schematic diagram (III) of the reactivation of the first cluster of cracks in the upper section in one embodiment of the present invention;

[0035] Figure 6I This is a diagram of acoustic emission events at the bottom of the test sample in one embodiment of the present invention;

[0036] Figure 6J This is a schematic diagram (a) of the crack initiation of the entire test specimen in one embodiment of the present invention;

[0037] Figure 6K This is a schematic diagram (II) of the crack initiation of the entire test specimen in one embodiment of the present invention;

[0038] Figure 6L This is a schematic diagram (III) of the crack initiation of the entire test specimen in one embodiment of the present invention;

[0039] Explanation of reference numerals in the attached figures:

[0040] 100. Test specimen; 10. Rock sample body;

[0041] 20. Casing; 21. Fracturing section;

[0042] 22. Cracks appear. Detailed Implementation

[0043] The details of the present invention can be more clearly understood by referring to the accompanying drawings and the description of specific embodiments. However, the specific embodiments of the present invention described herein are for illustrative purposes only and should not be construed as limiting the invention in any way. Under the teachings of this invention, those skilled in the art can conceive of any possible modifications based on the invention, and these should all be considered to fall within the scope of the invention.

[0044] The present invention proposes a test specimen 100 for a far-field hydraulic fracturing simulation experiment of tight sandstone. The test specimen includes a rock sample body 10 and a casing 20. The top surface of the rock sample body 10 has a hole opened along the axis of the rock sample body 10. The casing 20 is inserted into the hole and is composed of two or more fracturing sections 21, which are separated by a piston.

[0045] The test specimen 100 for the far-field hydraulic fracturing simulation experiment of tight sandstone proposed in this invention has a casing 20 composed of two or more fracturing sections 21, which can then be used for multi-stage fracturing experiments. This is more similar to the hydraulic fracturing operation of underground reservoirs and can more realistically reflect the effect of hydraulic fracturing.

[0046] In an optional embodiment of the invention, the piston is a movable rubber plug within the orifice, equivalent to a packer used in fracturing operations. After fracturing of one fracturing section is completed, the piston is moved to open the perforation of another fracturing section and seal the perforation of the already fracturing section, allowing for fracturing again. This allows for testing of two different fluids and injection parameters on two fracturing sections of a rock sample.

[0047] In an optional embodiment of the present invention, fracturing grooves 22 are respectively provided on the outer and inner walls of each fracturing section 21; the fracturing grooves 22 can simulate the perforations formed in the casing and wellbore during well completion, so as to make the fracturing experiment more similar to the actual pressure operation.

[0048] In one optional example of this implementation, multiple fracturing grooves 22 are provided on each fracturing section 21. The multiple fracturing grooves 22 are equidistantly arranged to act as perforation clusters, thereby simulating a multi-stage, multi-cluster fracturing completion method.

[0049] In an optional embodiment, crack initiation grooves 22 are symmetrically provided on the two opposite sidewalls of the sleeve 20, and each crack initiation groove 22 is provided along the circumferential direction of the sleeve 20.

[0050] In an optional embodiment of the invention, the rock sample 10 is a tight rock selected from the reservoir.

[0051] In one optional example of this implementation, the axis of the rock sample 10 is parallel to the bedding plane of the rock sample.

[0052] In an optional embodiment of the present invention, the sleeve 20 is a low-carbon steel pipe.

[0053] In an optional example of this embodiment, the sleeve 20 is fixed inside the hole with epoxy resin.

[0054] In an optional example of this implementation, an acoustic sensor 30 is embedded on the surface of the rock sample 10 to detect acoustic emission events during the experiment.

[0055] In an optional embodiment of the present invention, the rock sample 10 is rectangular and prepared according to the experimental design dimensions. The long side (axis) of the rock sample 10 is parallel to the bedding plane of the rock sample.

[0056] This invention also proposes a design method for a far-field hydraulic fracturing simulation experiment of tight sandstone, including: preparing a test sample 100 and simulating the on-site well completion method; using a triaxial stress test system and calculating the loading stress; establishing a pumping parameter calculation model; and conducting a multi-stage hydraulic fracturing simulation experiment on the test sample based on the loading stress and pumping parameters using the triaxial stress test system to determine the fracture initiation and propagation laws.

[0057] The design method for far-field hydraulic fracturing simulation experiments of tight sandstone proposed in this invention can conduct multi-stage fracturing experiments, and through calculation, make the loading stress and pumping parameters more consistent with the scaling law of in-situ and laboratory scales, thereby enabling a more objective and quantitative characterization of the propagation law of the in-situ fracture network.

[0058] In an optional embodiment of the present invention, the triaxial stress testing system is a true triaxial stress testing system.

[0059] In an optional embodiment of the present invention, the loading stress is calculated based on the in-situ stress environment of the reservoir obtained from the rock body of the test sample.

[0060] In this embodiment, the in-situ stress value of the reservoir (σ) is obtained. H0 , σ h0 , σ v0 ) and pore pressure P p Calculate the effective reservoir stress (σ) H ',σ h ',σ v ') f .

[0061] Specifically, the effective stress is calculated by subtracting the pore pressure σ0 - Pp from the corresponding in-situ stress. In indoor experiments, the rock sample contains unsaturated fluid, so the total stress is consistent with the effective stress. That is, the stress applied to the rock sample during the experiment should be the effective stress. To scale the stress state to the conditions achievable in indoor experiments, the ratio of the stress difference R must remain constant.

[0062] Based on the effective stress value of the reservoir (σ) h ',σ h ',σ v ') Calculate the shape factor R:

[0063] R=(σ H '-σ h ') / (σ v '-σ h ') (1)

[0064] In formula (1), R is the shape factor, and σ H ' is the effective horizontal maximum principal stress, σ h ' is the effective horizontal minimum principal stress, σ v 'This represents the effective vertical stress.'

[0065] Keeping the shape factor and effective stress ratio constant, the pressure conditions are scaled to a laboratory scale; then, the applied stress is calculated based on the equipment load limits.

[0066] If the calculated effective stress does not exceed the mentioned equipment load, then triaxial stress loading is applied to the rock sample 10 according to the effective stress. If the equipment load of the triaxial stress test system is less than the effective stress, the loading stress needs to be scaled while ensuring that the shape factor remains unchanged. For example, the maximum stress that a press equipped with a jack system in a triaxial stress test system can provide is 5000-6000 psi. Considering the elastic effect of the rock sample under closed conditions, for conservatism, the maximum stress is set to the lower limit of the maximum load of 5000 psi. The maximum value of the triaxial stress of the rock sample should be as close as possible to 5000 psi, while ensuring that R remains constant, and then the loading stress in the laboratory is designed.

[0067] In an optional embodiment of the present invention, the pumping parameters include the experimental fluid viscosity and pumping rate. Establishing the pumping parameter model includes: designing the experimental fluid viscosity and pumping rate based on on-site construction parameters and rock mechanics parameters. Specifically:

[0068] Under far-field conditions, the fracture propagation mechanism is mainly controlled by the relationship between fracturing fluid viscosity and rock fracture toughness, which is applicable to the disk-shaped fracture theory. A time-varying dimensionless parameter M is introduced:

[0069] in

[0070] In formula (2), t is the crack propagation time. m It is the characteristic time for the crack to transform from a viscosity-controlled crack to a toughness-controlled crack, where μ is the fluid viscosity, Q0 is the pumping rate, E′ is Young's modulus, and K is the fracture toughness.

[0071] Based on the similarity criterion derived from the theory of disk-shaped cracks, the dimensionless parameter M in the laboratory... l It needs to be consistent with the dimensionless parameter M on site. f To maintain consistency, the laboratory fluid viscosity μ is obtained according to formula (1). l Viscosity μ of in-situ fracturing fluid f Relationship:

[0072]

[0073] In formula (3), μ l The viscosity of the experimental fluid is μ. f Q represents the viscosity of the fracturing fluid used in on-site operations. l Q is the pump displacement for the experiment. f For fracturing operation displacement, E f E represents the reservoir Young's modulus. l K represents the Young's modulus of the rock sample. l For rock sample fracture toughness, K f For reservoir fracture toughness, t l max t represents the maximum propagation time of the crack in the laboratory. f max This represents the maximum propagation time of the crack at the site.

[0074] According to the theory of disk-shaped cracks, the maximum crack propagation time t max With the maximum crack propagation radius R max satisfy:

[0075]

[0076] In formula (4), Q is the pumping displacement, E is Young's modulus, and K is fracture toughness.

[0077] Substituting equation (4) into equation (3), we obtain the calculation model for laboratory fluid viscosity and pump displacement as follows:

[0078]

[0079] In formula (5), μ l The viscosity of the experimental fluid is μ. f Q represents the viscosity of the fracturing fluid used in on-site operations. l Q is the pump displacement for the experiment. f For fracturing operation displacement, E f E represents the reservoir Young's modulus. l K represents the Young's modulus of the rock sample. l For rock sample fracture toughness, K f For reservoir fracture toughness, R l max R is the maximum half-length of the crack in the laboratory. f max This represents the maximum half-length of the crack at the site.

[0080] The fracturing displacement Q is determined based on the fracturing design. f fracturing fluid viscosity μ f Based on the maximum half-length of the fracture, the Young's modulus E of the reservoir was calculated using conventional logging data. f and reservoir fracture toughness K f The Young's modulus E of the rock sample was calculated based on rock mechanics experimental data. l And rock-like fracture toughness K l According to formula (5), the experimental fluid viscosity μ is designed.l and pump displacement Q l .

[0081] In an optional embodiment of the present invention, the test specimen (rock block) is loaded to the final stress state, and fracturing fluid is injected into the wellbore at a designed pumping rate to conduct a hydraulic fracturing physical simulation experiment. It should be noted that the final stress refers to the triaxial stress applied to the rock sample reaching the designed stress state, and is unrelated to the fracturing pressure.

[0082] Based on the pressure changes during the experiment, the rupture pressure was determined. After the test specimen ruptured, core samples with cylindrical cross-sections were taken around the casing to determine the distribution of internal fractures. Simultaneously, the number, propagation time, and extent of fractures were determined by real-time monitoring of the dynamic acoustic wave signals emitted during specimen rupture using an acoustic emission instrument. The evolution of the farthest acoustic emission event at the fracture endpoint was tracked along different coordinates to establish a planar trajectory model of the fracture endpoint's evolution over time.

[0083] In an optional embodiment of the invention, the design method further includes changing the loading stress and pumping parameters during hydraulic fracturing to compare the effects of different factors on fracture propagation.

[0084] In an optional example of this implementation, different loading stresses and pumping parameters are used for different sections of the fracturing section 21.

[0085] In one alternative example, different experimental parameters such as perforation method, triaxial stress, fracturing fluid viscosity, and pumping rate are changed in different fracturing sections of the same test specimen to compare fracture complexity and analyze the influence of different geological and engineering factors on fracture propagation in tight sandstone.

[0086] In an optional example of this invention, the multi-stage hydraulic fracturing process is as follows: first, the first fracturing stage is fracturing using one triaxial stress, fracturing fluid viscosity, and pumping rate; then, the second fracturing stage is fracturing using a second triaxial stress, fracturing fluid viscosity, and pumping rate, and so on. Simultaneously, controlled variables, such as using different pumping rates in the upper and lower fracturing stages, can be used to compare the impact of pumping rate on fracture morphology and to change other parameters.

[0087] Please refer to Figures 1 to 6L The design method for the far-field hydraulic fracturing simulation experiment of tight sandstone proposed in this invention will now be described in detail with reference to an embodiment.

[0088] This invention uses a tight sandstone reservoir as an example to illustrate the specific implementation process. The gas field in this embodiment is structurally located in the northwest of the Yishan Slope, with six strata developed from bottom to top. One of these strata belongs to a tight sandstone gas reservoir characterized by "low porosity, low permeability, and low gas saturation." Due to the influence of the reservoir's lithology and physical properties, this tight sandstone gas reservoir requires large-scale volumetric fracturing to form an effective artificial fracture network, thereby improving the reservoir's permeability. Based on regional geological and engineering data, the design method for the far-field hydraulic fracturing physical simulation experiment of this gas-bearing section of the tight sandstone is as follows:

[0089] The first step involves cutting the natural outcrop of tight sandstone in this stratum into large rectangular rock samples (sample 10) measuring 28”×28”×36”, with its long side parallel to the bedding plane. A borehole with a diameter of 1-1 / 6” is drilled from the center of the 28”×28” top surface along the long side, and a prefabricated low-carbon steel casing (casing 20) with an outer diameter of 1” and an inner diameter of 0.68” is bonded into the borehole to serve as the wellbore. A binary completion method is used, employing a piston system to separate the upper and lower fracturing sections 21. Fracturing grooves 22 with a depth of 3 / 4” are cut every 3” on the fracturing section 21 to simulate multiple fracturing clusters (e.g., Figure 1 Thirty-eight acoustic sensors were embedded in the surface of the rock sample to record acoustic emission events during the hydraulic fracturing process.

[0090] The second step involves determining the vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress (σ) at a reservoir depth of 2900m. v0 , σ H0 , σ h0 The pressures were 68 MPa, 51 MPa, and 43 MPa, respectively, and the pore pressures (P) were... p The effective vertical stress, effective maximum horizontal principal stress, and effective minimum horizontal principal stress (σ) of the reservoir were calculated to be 28 MPa. v ',σ H ',σ h The pressure is divided into 40MPa, 23MPa, and 15MPa. According to formula (1), the shape factor R is calculated to be 0.32. The maximum effective load of the experimental system is 34.5MPa. Keeping the shape factor and effective stress ratio unchanged, the pressure conditions are scaled to the laboratory scale, and the experimental loading stresses are designed to be σ. v (σ NS ) = 34 MPa, σ H (σ EW ) = 19.55 MPa, σ h (σ TB =12.75MPa.

[0091] The third step is to determine the viscosity μ of the slickwater in the field, based on the on-site well fracturing design. f =5cp, injection rate Q f =3m3 / min, maximum half-length of crack R f max =200m; Calculate the Young's modulus E of the reservoir rock based on well logging data. f =30GPa, reservoir rock fracture toughness K f = 2.21 MPa.m 1 / 2 Rock waste from the rock sample cutting process was processed into cylindrical plunger samples, and indoor rock mechanics experiments were conducted to determine the Young's modulus E of the rock samples. l =33.39 GPa, rock sample fracture toughness K l = 2.68 MPa.m 1 / 2 Based on the size of the tested rock samples, the maximum half-length of the laboratory fracture is determined by the distance from the wellbore to the edge of the rock block, R. l max = 34.21cm. Calculate the pump displacement Q for different pumping rates according to formula (4). l The maximum crack propagation time was determined at flow rates of 100, 80, 60, 40, and 20 mL / min to ensure sufficient time for crack propagation monitoring. Therefore, Q was selected. l =20mL / min is the experimental pump flow rate. According to formula (5), the laboratory fluid viscosity μ is calculated. l =6000cp.

[0092] The fourth step involves placing the test rock sample in a multiaxial press, increasing all stresses from 5 MPa to 12.75 MPa, then increasing the north-south and east-west stresses to 19.55 MPa, and finally increasing the north-south stress to 34 MPa. A fracturing fluid with a viscosity of 6000 cp is injected into the upper section of the wellbore at a pumping rate of 20 mL / min to conduct a hydraulic fracturing physical simulation experiment. Based on the wellbore pressure changes, the fracture pressure in the upper section is 24.8 MPa (e.g., ...). Figure 3 Combined with acoustic emission monitoring results, the third fracturing layer developed into a disc-shaped fracture, while the first and second clusters of fractures showed almost no activity. The evolution of the farthest acoustic emission events at the fracture endpoints was traced along different coordinates (e.g., Figures 4A-4L Four circles are plotted along the farthest events of the X and Y branches at the XY cross-section of the acoustic emission location. The cumulative envelope is traced, and a model of the evolution of the crack endpoint over time is fitted, where the crack endpoint radius is in mm and time T is in minutes.

[0093] R X1 =355.6+1000×1.45×sqrt((T–86.63) / 60);

[0094] R X2 =355.6—1000×1.35×sqrt((T–86.63 / 60);

[0095] R Y1=355.6+1000×1.3×sqrt((T–86.63) / 60);

[0096] R Y2 =355.6-1000×1.5×sqrt((T-86.63) / 60);

[0097] The fifth step involves conducting a hydraulic fracturing physical simulation experiment in the lower fracturing section of the rock sample, with the pump injection rate intermittently changed every 15 seconds to transmit pressure pulses to the wellbore. The fracturing pressure in the lower section is 35.87 MPa (e.g., Figure 5 The first cluster of cracks developed into a disc-shaped ring and extended to the edge of the rock sample, while the first cluster of cracks in the upper section was activated (e.g., Figures 6A-6L This indicates that fluctuating the pump injection rate can increase fracture complexity, promote uniform fracture initiation, and improve reservoir stimulation. It is recommended that this process be applied to in-situ hydraulic fracturing operations.

[0098] Among them, regarding Figures 4A-6L My detailed explanation is as follows:

[0099] Figure 4A The horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample, showing the acoustic emission events in the upper half of the cluster;

[0100] Figures 4B to 4D The location of acoustic emission events in the upper half of the shaft is perpendicular to the shaft. The horizontal axis is the X (east-west) direction, and the vertical axis is the Y (north-south) direction, showing the fracture initiation and propagation at different times.

[0101] Figure 4E The horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample. It shows the acoustic emission events of the third cluster in the upper section, which is also the most important fracture.

[0102] Figures 4F to 4H The location of acoustic emission events is perpendicular to the wellbore. The horizontal axis is the X (east-west) direction, and the vertical axis is the Y (north-south) direction, showing the initiation of the third cluster of fractures and their propagation at different times.

[0103] Figure 4I The horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample, showing the acoustic emission events of the entire rock sample (upper and lower sections).

[0104] Figures 4J to 4L The location of acoustic emission events is perpendicular to the wellbore. The horizontal axis is the X (east-west) direction, and the vertical axis is the Y (north-south) direction, showing the initiation of all fractures in the entire rock sample and their expansion at different times. Figure 6AThe horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample, showing the acoustic emission events of the first cluster in the lower section;

[0105] Figures 6B to 6D The location of the first cluster of acoustic emission events in the lower section perpendicular to the wellbore, with the horizontal axis representing the X (east-west) direction and the vertical axis representing the Y (north-south) direction, shows the crack initiation under pressure pulses and the expansion after different times.

[0106] Figure 6E The horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample. It shows the acoustic emission events of the first cluster in the upper section, which are reactivated fractures.

[0107] Figures 6F to 6H The location of acoustic emission events in the direction perpendicular to the wellbore, with the horizontal axis representing the X (east-west) direction and the vertical axis representing the Y (north-south) direction, shows the expansion of the fracture at different times after the reactivation of the first cluster of fractures in the upper section below the pressure pulse;

[0108] Figure 6I The horizontal axis represents time, and the vertical axis represents the location of acoustic emission events in the longitudinal direction (top and bottom) of the rock sample, showing the acoustic emission events of the entire rock sample (upper and lower sections).

[0109] Figures 6J to 6L The position of acoustic emission events is perpendicular to the wellbore. The horizontal axis is the X (east-west) direction, and the vertical axis is the Y (north-south) direction, showing the expansion of all fractures in the entire rock sample at different times under pressure pulse.

[0110] The detailed explanations of the above embodiments are intended only to explain the present invention so as to facilitate a better understanding of the present invention. However, these descriptions should not be construed as limiting the present invention for any reason. In particular, the various features described in different embodiments can be arbitrarily combined with each other to form other embodiments. Unless there is an explicit description to the contrary, these features should be understood to be applicable to any embodiment, and not limited to the described embodiments.

Claims

1. A design method for a far-field hydraulic fracturing simulation experiment of tight sandstone, characterized in that, Prepare test specimens; the test specimens include a rock sample body and a casing, the top surface of the rock sample body has an opening along the axis of the rock sample body, the casing is inserted into the opening, the casing is composed of two or more fracturing sections, each of the fracturing sections is separated by a piston, a triaxial stress test system is used and the loading stress is calculated; establish a pumping parameter calculation model; conduct multi-stage hydraulic fracturing simulation experiments based on the loading stress and the pumping parameters to determine the initiation and propagation laws of tight experimental fractures; Specifically, the loaded stress is calculated based on the in-situ stress environment of the reservoir obtained from the test sample. The in-situ stress environment includes the in-situ stress value and pore pressure of the reservoir. The effective stress of the reservoir is then calculated based on the in-situ stress value and pore pressure. The shape factor is calculated based on the effective stress value of the reservoir. : （1） In formula (1), The shape factor, For the effective horizontal maximum principal stress, For the effective horizontal minimum principal stress, For effective vertical stress; While keeping the shape factor and effective stress ratio constant, the pressure conditions are scaled to a laboratory scale, and the loading stress is calculated based on the equipment load limit. The pumping parameters include the experimental fluid viscosity and pumping rate. The pumping parameter model is established by designing the experimental fluid viscosity and pumping rate based on the on-site construction parameters and rock mechanics parameters. Introducing a dimensionless parameter that varies with time : （2） In the formula, It is the crack propagation time. It is the characteristic time of crack transformation from a viscosity-controlled crack to a toughness-controlled crack. For fluid viscosity, For pump discharge, For Young's modulus, For fracture toughness; Based on the similarity criterion derived from the theory of disk-shaped cracks, the dimensionless parameter in the laboratory... Requires matching with dimensionless parameters on site Maintaining consistent laboratory fluid viscosity Viscosity of fracturing fluid in the field Relationship: (3) In formula (3), For the experimental fluid viscosity, This refers to the viscosity of the fracturing fluid used in on-site operations. To test the pump injection displacement, For fracturing operation displacement. The reservoir's Young's modulus, The Young's modulus of the rock sample. The fracture toughness of the rock sample, For reservoir fracture toughness, The maximum propagation time of the crack in the laboratory. This represents the maximum propagation time of the crack at the site. Maximum crack propagation time With the maximum radius of crack propagation satisfy: （4） In formula (4), For pump discharge, For Young's modulus, For fracture toughness; The laboratory fluid viscosity and pump displacement calculation model is as follows: （5） In the formula, For the experimental fluid viscosity, This refers to the viscosity of the fracturing fluid used in on-site operations. To test the pump injection displacement, For fracturing operation displacement. The reservoir's Young's modulus, The Young's modulus of the rock sample. The fracture toughness of the rock sample, For reservoir fracture toughness, This represents the maximum half-length of the crack within the laboratory. This represents the maximum half-length of the crack at the site.

2. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 1, characterized in that, The design method also includes changing the loading stress and the pumping parameters to compare the effects of different factors on the propagation of cracks in tight sandstone.

3. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 2, characterized in that, Different loading stresses and pumping parameters are used for different sections of the fracturing section.

4. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 1, characterized in that, Each of the fracturing sections is provided with a fracturing initiation groove.

5. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 4, characterized in that, Each of the fracturing sections is provided with multiple fracturing grooves, which are equidistant from each other.

6. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 1, characterized in that, The axis of the rock sample is parallel to the layers of the rock sample.

7. The design method for a far-field hydraulic fracturing simulation experiment of tight sandstone as described in claim 1, characterized in that, An acoustic sensor is embedded in the surface of the rock sample.

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