Experimental method and system for simulating hydraulic fracturing condition under in-situ condition

Abstract

The invention provides an experimental method and system for simulating the hydraulic fracturing fracture condition under the in-situ condition, and the method comprises the steps: S1, placing a fracturing pipe in a mold, and adding a prefabricated first number of cement-based composite materials into the mold; s2, adding a rock core into the mold, inverting the rock core, and then adding a second amount of cement-based composite material until the mold is filled with the cement-based composite material, so as to obtain a sample; s3, applying triaxial pressure to the sample to simulate formation pressure of an extension group; s4, after the confining pressure is stable, pressure liquid is injected into the rock core at a preset speed, and a fracturing experiment is started; and S5, observing the fracturing curve data graph to obtain the time point and fracturing pressure of rock core fracture, stripping the rock core from the sample, and analyzing the fracture condition of the rock core.

Description

Technical Field

[0001] The present invention relates to the enhanced production technology and fracturing technology fields of low-permeability and tight reservoirs, and particularly relates to an experimental method and system related to the fracture propagation behavior during the fracturing process. Background Art

[0002] A series of measures have been taken for the development of low-permeability reservoirs at home and abroad. However, the core problem affecting the success of fracturing, namely the fractures generated by the fracturing operation and their development and distribution patterns, still remains unresolved. Under the action of in-situ stresses in all directions, the rock fracture modes and pressure indicators caused by hydraulic fracturing are different. A series of problems often occur during the hydraulic fracturing of low-permeability reservoirs: (1) Excessively high fracturing pressure parameters can cause irregular fracture initiation, leading to a change in the actual seepage path; (2) Excessively high injection pressure can cause the rupture of weak partition layers, resulting in fluid cross-layer problems, and preventing the corresponding fractures from appearing in the target oil layer; (3) The actual water absorption, creep, etc. of the oilfield mudstone will determine the final fracturing effect. Under high-pressure water injection, the actual water absorption capacity of the mudstone increases significantly, resulting in a certain creep deformation of the mudstone layer, endangering the safety of the water injection casing; (4) Most low-permeability reservoirs are composed of heterogeneous sandstones, which are highly compact and have a relatively large actual brittleness value. The fractures that occur during the hydraulic fracturing process will increase their heterogeneity, which will also greatly affect the disturbance problems between and within layers and the actual fracturing effect. Summary of the Invention

[0003] The purpose of the present application is to provide an experimental method and system for simulating the situation of hydraulic fracturing under in-situ conditions, so as to effectively conduct indoor fracturing simulation to explore the fracture initiation, development, and distribution patterns under actual fracturing conditions.

[0004] To achieve the above purpose, on the one hand, the present application provides an experimental method for simulating the situation of hydraulic fracturing rupture under in-situ conditions, including: S1: Place a fracturing pipe in a mold, and add a pre-prepared first quantity of cement-based composite material to the mold; S2: Add a core to the mold, invert the core, and then add a second quantity of cement-based composite material until the mold is filled to obtain a sample; S3: Apply triaxial pressure to the sample to simulate the formation pressure of the Yanchang Formation; S4: When the confining pressure is stable, inject the pressure liquid into the core at a preset speed and start the fracturing experiment; S5: Observe the fracturing curve data graph to obtain the time point of core fracture and the fracturing pressure, and, peel the core from the sample and analyze the fracture situation of the core.

[0005] Preferably, S1 further includes: applying silicone oil to the inner side of the mold; the height of the first quantity of cement-based composite material is one-third of the height of the mold.

[0006] Preferably, S3: The triaxial pressure includes applying a first pressure in the X-axis direction of the sample, a second pressure in the Y-axis direction of the sample, and a third pressure in the Z-axis direction of the sample.

[0007] Preferably, different magnitudes of pressure are applied in the three axes to simulate under the condition that there is a pressure difference in the three axes and it acts together with the injection pressure of the pressure liquid.

[0008] Preferably, the oil pressure method is used for pressure application, and the maximum confining pressure is set to 50 MPa.

[0009] Preferably, S5 further includes: acquiring the acoustic wave signal transmitted by the core fracture; converting the acoustic wave signal into an electrical signal and increasing the electrical signal to a preset voltage; using an independent channel controller for measurement, analyzing the acoustic emission signal, and obtaining the fracture initiation, development, and distribution areas through the relative time reaching the preset position.

[0010] Preferably, S5 further includes: after the fracturing experiment, using a linear cutting machine to strip the cement-based composite material outside the core, and when the cutting part reaches the internal core, using a hand saw blade for cutting to obtain the internal core.

[0011] Preferably, after S5, it further includes: drying the core, and then saturating the core with distilled water; using the X-ray computed tomography method to detect the internal structure of the core, and analyzing the fracture position and extension direction according to the scanning results.

[0012] On the other hand, the present application provides an experimental system for simulating the in-situ conditions in the case of hydraulic fracturing fracture, using the above experimental method, including: a layered in-situ stress simulation device for restoring the formation conditions during the simulation of fracturing; a fracturing fluid injection device for controlling the liquid flow rate injected into the fracturing pipe; and a fracture monitoring device for determining the fracture propagation mechanism and its morphology.

[0013] Preferably, the layered in-situ stress simulation device includes: a pressure control module for controlling the magnitude of the pressure input to apply pressure under the condition of maintaining a pressure difference in the triaxial pressure; a surrounding rock temperature control module for performing preheating treatment to make the temperature of the sample area the temperature at the actual core position underground.

[0014] The technical solution provided by the present application can achieve the following beneficial effects:

[0015] 1. A physical simulation of a full-diameter core is carried out under the coexistence of confining pressure and pore pressure; the reproduction accuracy of the fracture propagation process is optimized;

[0016] 2. By adopting a triaxial confining pressure loading system, the underground confining pressure environment can be simulated more realistically, ensuring the natural propagation form of fractures;

[0017] 3. Combining seepage-stress coupling control can accurately simulate the influence of pore pressure on crack propagation and improve the reliability of experiments;

[0018] 4. Adopting a three-dimensional crack reconstruction method can accurately obtain the spatial shape of cracks and improve the crack identification accuracy;

[0019] 5. Combining numerical simulation to optimize the prediction ability of crack propagation, making the experimental data more consistent with theoretical analysis. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0021] Figure 1 It is a flowchart of the preparation method of cement-based composite materials provided by the embodiment of the present application;

[0022] Figure 2 It is a flowchart of the experimental sample preparation method provided by the embodiment of the present application;

[0023] Figure 3 It is a schematic diagram of the experimental sample preparation process provided by the embodiment of the present application;

[0024] Figure 4 It is a flowchart of the hydraulic fracturing experiment method provided by the embodiment of the present application;

[0025] Figure 5 It is a schematic diagram of the comparison before and after the sample is fractured provided by the embodiment of the present application;

[0026] Figure 6 It is a schematic diagram of the fracturing curve provided by the embodiment of the present application;

[0027] Figure 7 It is a schematic diagram of the crack initiation and distribution direction of the core after fracturing provided by the embodiment of the present application;

[0028] Figure 8 It is a schematic diagram of the hydraulic fracturing simulation CT scan provided by the embodiment of the present application;

[0029] Figure 9 It is a flowchart of the fracturing calculation provided by the embodiment of the present application;

[0030] Figure 10 It is a schematic diagram of the numerical simulation of three groups of experiments after applying confining pressure provided by the embodiment of the present application;

[0031] Figure 11 It is a schematic diagram of numerical simulation when the external cladding material provided by the embodiment of the present application is applied with confining pressure and pore pressure;

[0032] Figure 12 It is a schematic diagram of numerical simulation after full-diameter core fracturing provided by the embodiment of the present application;

[0033] Figure 13 It is a schematic diagram of fracturing simulation by combining the overall external material and the internal core provided by the embodiment of the present application;

[0034] Figure 14 It is a diagram of the three-dimensional fracturing physical simulation experimental device at the full-diameter core scale provided by the embodiment of the present application. Detailed implementation manners

[0035] The embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals indicate the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention, and should not be construed as a limitation of the present invention.

[0036] In the description of this specification, the description referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0037] Embodiment

[0038] New high-strength anti-pore-pressure material

[0039] Cement-based material is selected as the main base material for the experiment. It can be understood that the cement-based material has good plasticity, which enables better wrapping of the full-diameter core, and the high-strength cement-based material can resist a compressive capacity of more than 100 MPa. The core used in this example experiment has a compressive strength of about 30 - 50 MPa, which meets the requirements of this example in terms of compressive strength.

[0040] When considering the pore pressure situation, it is necessary to consider both a relatively high flexural strength. At the same time, factors such as the large loss of cement materials and poor corrosion resistance are all problems that need to be urgently solved for cement-based materials. Nanomaterials have the characteristics of reducing porosity and increasing the degree of cement hydration. Nanomaterials with particle sizes in the range of 1 - 100 nm are widely used in cement-based composites.

[0041] The base material is selected as silicate. A method of preparing an additive by promoting the hydrolysis, condensation of trifluoropropyltrimethylsiloxane and forming a nano-silica-carbon nanotube composite structure with carbon nanotubes through high-temperature activation of nano-silica is adopted. The additive and the base material are compounded to form a new composite silicate-based high-strength pore pressure-resistant material. After being modified by nanomaterials, this material has high pore pressure resistance and high density performance, and has achieved the effect of simulating the formation.

[0042] In one example, first, nano-silica is activated at 80 °C. The activated nano-silica accelerates the hydrolysis of trifluoropropyltrimethylsiloxane to form more silanols. The -OH on the surface of nano-silica undergoes a dehydration condensation reaction with the silanols, and further forms a nano-silica-carbon nanotube composite structure with carbon nanotubes. The silanols can continue to undergo a dehydration condensation reaction with the calcium hydroxide in the cement hydration products, so that the nano-silica-carbon nanotube composite structure combines more tightly with calcium hydroxide to achieve the effect of filling pores.

[0043] Specifically, the experimental chemical reagents used are nano-silica (99.9% AR), ultra-fine silicate cement (3000 mesh), carbon nanotubes (AR), trifluoropropyltrimethylsiloxane (96% AR), defoamer (99% AR), water reducer (99% AR), and deionized water.

[0044] Among them, the physical property parameters of nano-silica and carbon nanotubes are shown in Table 1 below:

[0045]

[0046] Table 1

[0047] Among them, the component contents of the ultra-fine silicate cement are shown in Table 2 below;

[0048]

[0049] Table 2

[0050] In one example, several experimental groups are set, and the component contents (mass fraction) are shown in Table 3 below:

[0051]

[0052] Table 3

[0053] A specific preparation method is as follows: Add nano-silica into trifluoropropyltrimethylsiloxane, heat to 80 °C, stir for 2 h, then add carbon nanotubes, and continuously stir at 80 °C for 2 h. Prepare experimental groups according to the specific mixing ratios in Table 3. Among them, in experimental group 1, corresponding graphitized carbon nanotubes (G-CNT) are added; in experimental group 2, hydroxyl graphitized carbon nanotubes (OH-G-CNT) are added; in experimental group 3, short-arm carbon nanotubes (S-MWNT) are added; in experimental group 4, hydroxyl short-arm carbon nanotubes (OH-S-MWNT) are added; in experimental group 5, pure cement without any additives is used; in experimental group 6, nano-silica is added into trifluoropropyltrimethylsiloxane and stirred at room temperature for 2 h. Then, add carbon nanotubes and continuously stir at room temperature until the mixture is uniform, and configure the control group according to the mixing ratios in Table 3.

[0054] The preparation steps are as Figure 1 shown.

[0055] Add the mixed cement slurry into a stirrer according to a water-cement mass ratio of 0.38, with a rotation speed of 800 r / min, and continuously stir for 20 min. After stirring evenly, take out the paste and put it into a mold. Among them, for mechanical property testing, a cement triple mold with dimensions of 40 mm × 40 mm × 160 mm is used, and the experimental samples are kept at a temperature of 25 °C and a humidity of 98% in a constant temperature and humidity chamber for 28 days of curing.

[0056] Dissolve the prepared composite material in xylene, then pour it into acetone, filter and dry it. Take a certain amount of the product after washing and purifying with toluene solvent and dry it. After fully dissolving the product in the solvent with deuterated acetone as the solvent, load it into a nuclear magnetic tube, and use a nuclear magnetic resonance spectrometer to perform nuclear magnetic resonance hydrogen spectrum testing on the product; in addition, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetry-isothermal calorimetry, hydration heat, scanning electron microscopy observation, N2 adsorption and desorption, freeze-thaw damage resistance, electrochemical corrosion, contact angle testing, dielectric constant-resistivity, and mechanical property testing, etc. can also be carried out on the composite material to ensure that the composite material can be applied to fracturing simulation experiments.

[0057] Core

[0058] In this example, a real full-diameter core from the Yanchang Formation in the Ordos Basin is used, and the above high-strength pore pressure-resistant material is used to conduct three-dimensional wrapping on the core to carry out a three-dimensional fracturing physical simulation experiment at the core scale, aiming to ensure that the materials around the core are not fractured under stress, while cracks are generated in the internal core due to the combined action of confining pressure and net port pressure.

[0059] In one example, the core is taken from the Yuan 284 block in the Changqing Oilfield. The lithology of the oil-bearing formation series is mainly feldspar sandstone intercalated with purple mudstone. The detrital components are mainly feldspar (with an average of 34.82%) and quartz (with an average of 29.33%). The types of cements are mainly clay, carbonate, and siliceous. The sensitive minerals are mainly clay minerals, with illite as the main one; the carbonate minerals are mainly ferrocalcite (with an average of 2.7%) and ankerite (1.5%); the siliceous minerals are mainly quartz (1.02%), with occasional pyrite (0.02%) and siderite (0.04%); the samples are prepared into cylinders with a diameter of 25 mm and a height of 50 mm.

[0060] Experimental method

[0061] Under high pore pressure conditions, a physical simulation experiment of fracture propagation in the reservoir is carried out to achieve the effect of simulated volume fracturing. Specifically, standard low-permeability reservoir rock samples are selected, three-dimensionally coated with high-strength pore pressure-resistant materials, and then a physical simulation experiment of the fracture propagation mechanism is carried out.

[0062] Specifically, first prepare cubic samples, as Figure 2 shown, including the following steps:

[0063] S11: First, prepare a 300 mm × 300 mm × 300 mm mold, with a 10-mm-diameter small hole drilled in the exact middle of the bottom for placing the fracturing pipe. Four feet are welded to the bottom of the mold for fixing the mold.

[0064] S12: Apply silicone oil to the four sides of the mold to prevent the sample from sticking to the mold during sampling.

[0065] S13: Place the fracturing pipe in the exact middle of the bottom of the mold, and drill screws from the bottom to fix the fracturing pipe.

[0066] S14: Prepare 25 kg of cement-based composite material according to the mass fraction ratios of experimental groups 1 - 3 in Table 3. Add the pre-prepared cement-based composite material to the mold. When the material is filled to 10 cm, add the pre-drilled core, and invert the core, then continue to add the cement-based composite material until the upper part is completely filled.

[0067] S15: Fix the mold, add external fixing bolts, and clean the excess cement-based material around.

[0068] The preparation process of the cubic sample is as Figure 3 shown.

[0069] After preparing the samples, a three-dimensional fracturing physical simulation experimental device with a full-diameter core scale is used to conduct fracturing tests at room temperature (22 °C). The maximum confining pressure is set to 50 MPa, and the pore pressure (i.e., the net pressure at the port during fracturing) can be transmitted back to the test software by the sensor. According to the coring position of the full-diameter core, the temperature around the core is set to 70 °C.

[0070] As Figure 4 shown, the fracturing experiment includes the following steps:

[0071] S21: Place the sample into the holder;

[0072] S22: Combine the true stress difference of the formation where the full-diameter core is located and apply triaxial pressure to the sample to simulate the formation pressure of the Yanchang Formation. Specifically, apply triaxial pressures of 33 MPa in the X-axis, 36 MPa in the Y-axis, and 39 MPa in the Z-axis to simulate the formation pressure of the Yanchang Formation;

[0073] S23: After the confining pressure is stable, continuously inject fracturing fluid (deionized water) from the top of the holder into the core at a constant flow rate using a constant-current pump and start the fracturing experiment. The injection rate is 8 mL / min, and observe the fracturing curve data graph;

[0074] S24: When the fracturing pressure suddenly drops at a certain time point, it is judged that the core has fractured;

[0075] S25: After the fracturing experiment is completed, take out the large core and use a linear cutting machine to strip the cement outside the natural core, and analyze the initiation, development, and distribution of cracks in the core.

[0076] In another example, variable confining pressure ratio + dynamic pore pressure control is adopted to simulate different reservoir conditions. The controllable confining pressure range is 10 - 50 MPa, and it has the function of real-time monitoring of pressure changes; the PID control algorithm is adopted to dynamically adjust the pore pressure.

[0077] First, set different confining pressure ratios, corresponding to three schemes of equal confining pressure, uniaxial high confining pressure, and biaxial high confining pressure. The initiation direction, length, and branching of cracks can be recorded, and the influence of different stress conditions on cracks can be analyzed. In the equal confining pressure scheme, σX = σY = σZ, corresponding to a uniform formation environment; in the uniaxial high confining pressure scheme, σX > σY ≈ σZ, corresponding to the environment near the fault. In the biaxial high confining pressure scheme, σX ≈ σY > σZ, corresponding to the heterogeneous reservoir environment.

[0078] Furthermore, through the PID control system, three working conditions of low pore pressure, medium pore pressure, and high pore pressure are simulated. Among them, the low pore pressure is set to 5 MPa to simulate the low-pressure reservoir; the medium pore pressure is set to 15 MPa for benchmark testing; the high pore pressure is set to 25 MPa to simulate the high-pressure reservoir.

[0079] Preferably, an incremental PID is combined with a fuzzy PID. Among them, the incremental PID is used to stabilize the pressure and reduce overshoot, and is applicable to the control of high confining pressure (>15 MPa); the fuzzy PID adaptively adjusts the PID parameters and is applicable to low pressure (5 MPa) or situations where the pressure changes violently. It can be understood that when the pressure error is small, the incremental PID is used; when the error is large or the confining pressure / hole pressure changes rapidly, it switches to the fuzzy PID.

[0080] When the pressure error e(k) is small (for example, ∣e(k)∣<2 MPa), the incremental PID is used:

[0081]

[0082] When the pressure error e(k) is large (for example, ∣e(k)∣≥2 MPa), or the error change rate Δe(k) is fast, then switch to the fuzzy PID:

[0083]

[0084] Furthermore, in order to ensure that the control algorithm can switch at the appropriate time, a decision rule is defined:

[0085] If ∣e(k)∣<2 MPa, use the incremental PID; if ∣e(k)∣≥2 MPa or ∣Δe(k)∣≥1 MPa / s, use the fuzzy PID.

[0086] In a calculation example, the target hole pressure P’ = 15 MPa, the initial hole pressure P = 10 MPa, the sampling interval dt = 0.1 s, the calculation time T = 10 s, and the calculation results of the first 5 steps are shown in Table 4:

[0087]

[0088] Table 4

[0089] Analyzing Table 4, it can be obtained that at the beginning, the error is large, and the fuzzy PID is used for rapid adjustment (reducing the error by 2 - 3 MPa). When the error decreases, it automatically switches to the incremental PID to provide stable convergence.

[0090] Experimental result analysis

[0091] The method for verifying the performance of the high-strength anti-hole pressure material in this example is, first, to observe whether the outer cladding material is damaged before and after fracturing. Since it has a high anti-hole pressure capacity, the sample will still remain in its original state unchanged after the fracturing simulation experiment. The comparison of the sample before and after fracturing is as Figure 5 shown.

[0092] Due to the excellent mechanical properties of the high-strength pore pressure-resistant material, it can be clearly observed before and after fracturing that although small-area damage appears at the corners of each sample, this is caused by a small stress concentration at the corners, and the overall sample shows good integrity. After cutting the sample with a linear cutting machine, it can be further seen that no cracks appear in the external three-dimensional cladding material. After analyzing the fracturing curve, it can be seen that the internal core has completed the physical fracturing simulation experiment and cracks have occurred.

[0093] The fracturing curve is an important reference data for the generation and propagation of cracks. The fracturing curve is as Figure 6 shown. In order to restore the original underground situation, according to the rock mass fracture criterion, when the triaxial pressure difference is greater than 3 MPa, cracks will be generated, so the triaxial pressure difference is set, where the confining pressure X = 33 MPa, Y = 36 MPa, Z = 39 MPa, and the temperature is 70 °C. The temperature can be received and monitored in real time by temperature sensors in 6 directions.

[0094] It can be seen from the fracturing curve that as time increases, the fracturing pressure gradually increases. The fracturing fluid is selected from the well water used for fracturing in Well Yuan 284 of Changqing Oilfield, China, with a pressure of about 41 MPa, and then the pressure drops rapidly, indicating that the cracks begin to break rapidly.

[0095] Figure 7 This is an observation diagram after cutting the sample with a linear cutting machine and taking out the core. The dotted area is the crack generated after the sample breaks, and the crack shows a tendency to expand in a single direction. After measurement, only one crack about 31.4 cm long appears in the core, and the effective corrosion area is 0.

[0096] In order to further prepare to observe the position and development direction of the internal cracks in the core, non-destructive testing of the internal structure is carried out by using the X-ray computed tomography (CT) analysis method. The basic working principle is to use X-rays to irradiate the sample, and parts with different densities and mineral contents will show different degrees of attenuation of X-rays, and then be presented in different gray levels in the image. Use the three-dimensional visualization software Avizo-Comsol to dock with the finite element software Abaqus to perform RVE three-dimensional model fracturing simulation. Before CT scanning, the sample needs to be dried. The sample is placed in a drying oven at 120 °C and dried for 24 h, and then the sample is saturated with distilled water.

[0097] Specifically, a German Diondo D2 type X-ray computed tomography scanner is used for CT scanning, with a set scanning height of 30 cm and a scanning diameter of 10 cm; a cross-section is scanned at intervals of 0.5 mm, and the resolution of this equipment can reach 38 μm.

[0098] The scanned cross-sectional data is subjected to threshold segmentation using VGStudioMAX to extract the crack area, and the crack area is fitted with the three-dimensional sandstone structure to verify whether the crack area is correctly extracted.

[0099] The specific data processing results are as Figure 8 shown, Figure 8 -a is the CT scan result before the fracturing simulation experiment, Figure 8 -b is the scan result after the fracturing simulation experiment, and it can be clearly seen the crack position and the extension direction; Figure 8 -ch and 8-d are the data extracted after threshold segmentation using the software respectively. The middle area in the figure is the crack initiation position, and a main crack can be clearly observed; Figure 8 -e is to observe the crack at the selected interfaces in the X / Y / Z three directions; Figure 8 -f is the top view of the core scan; Figure 8 -g and 8-h are the left view and the right view of the scan respectively. The CT scan results are obvious, proving that the hydraulic fracturing simulation only causes a main crack and no other microcracks are generated.

[0100] The physical fracturing simulation experiment combined with the X-ray computed tomography (CT) results shows that only one main crack is caused in the full-diameter core inside the hydraulic fracturing experiment, and the fracture surface is relatively flat.

[0101] Stress balance equation

[0102] This example combines the triaxial confining pressure and pore pressure conditions set in the physical simulation to simulate the crack initiation, development and distribution mode under ideal conditions. The actual stress balance parameters of the porous medium can be expressed by the principle of virtual work: the virtual work of the rock within a specific time is equal to the virtual work of the body force and the surface force acting on the rock. By not considering the fluid viscosity in the rock and finally simplifying the treatment, the following formula (2-1) can be obtained:

[0103]

[0104] In the formula, D ep is the corresponding elastoplastic matrix; t represents the time data; m = [1, 1, 1, 0, 0, 0]T; K s represents the specific compression modulus parameter of the solid particles; S0 represents the actual saturation parameter; p0 represents the actual liquid pressure; t represents the corresponding rock surface force; f represents the actual rock body force parameter; δε represents the actual virtual displacement; δu represents the actual virtual strain; dV represents the actual volume element; dS represents the actual area element.

[0105] Establishment of the continuity equation

[0106] For a specific volume of rock, it can be obtained through the analysis of the mass conservation equation that the mass of the fluid flowing through the rock within a certain period of time is equal to the sum of the fluid increase and the outflow. Assuming the actual seepage law is Darcy's seepage law, the continuity equation can be obtained through calculation and derivation by this law as shown in the following formula (2-2):

[0107]

[0108] In the formula, k0 represents the product obtained between the initial permeability coefficient tensor and the fluid density; ρ0 represents the liquid density in the calculation; k r represents the specific permeability coefficient in the actual calculation; g is the specific vector of the calculated gravitational acceleration; n represents the actual rock porosity data; K0 represents the liquid volume data in the rock.

[0109] Boundary condition

[0110] The flow boundary condition is as shown in the following formula (2-3):

[0111]

[0112] In the formula, n is the unit normal trend corresponding to the flow boundary; k is the coefficient tensor parameter of the permeability: q o represents the total liquid volume passing through the boundary; The pore pressure boundary condition The pore pressure boundary condition can be expressed as p o = p o b, that is, it is considered that the pore pressure on the boundary is a certain value p o b.

[0113] ABAQUS Finite Element Discretization Method and Stress-Seepage Coupling Equation

[0114] Define the function as shown in the following formula (2-4):

[0115]

[0116] In the formula, N u and B represent the actual vector matrices; ū represents the unit node displacement data in the calculation process; represents the unit node pore pressure data therein; N p represents the shape function used in the calculation.

[0117]

[0118] In the formula, is the control equation; is the continuous boundary equation.

[0119] Substitute formula (2-4) into formula (2-5), and after simplification, the solid-phase finite element equation (2-6) can be obtained:

[0120]

[0121] Transpose equations (2-2) and (2-3) so that the right side of the equation is 0. Using the Galerkin method, let the corresponding polynomials on the left side of equations (2-2) and (2-3) replace the in equation (2-5). Further, for ε and p in equation (2-4) o Substitute the established shape functions into equation (2-6) and simplify it to equation (2-7):

[0122]

[0123] Combine equations (2-6) and (2-7) to obtain the corresponding stress-seepage coupling equation (2-8). Rely on Abaqus to solve and calculate this part of the equations, and detailed basic data such as stress, strain, displacement, and porosity can be obtained in this way:

[0124]

[0125] In the formula:

[0126]

[0127] In the formula, q ab is the fluid flow rate on the boundary.

[0128] Fracture initiation criterion

[0129] Due to the influence of the actual structure and self-weight stress of the reservoir rock, the fracture surface will change in different situations under the influence of the corresponding compressive-shear stress. The development direction of the crack tip will also change under the comprehensive influence of the tensile-shear stress. The generation of cracks is mainly affected by the tensile stress in the principal stresses. In the model analyzed in this application, by analyzing and calculating the influence of the stress intensity factors of type I and type II on the actual initiation and propagation mechanisms, it is concluded that in the case of initiation, the opening-mode failure is the main failure mode of the rock. Therefore, the initiation in this analysis is expressed by the fracture toughness of type I, and the judgment criterion is as shown in equation (2-10):

[0130] K1 = K 1C (2-10);

[0131] In the formula, K1 is the stress intensity factor of type I; K 1C is the critical fracture stress intensity factor, that is, the fracture toughness.

[0132]

[0133] Among them, Γ is the integration contour; n is the normal of the contour; ds represents the specific differential arc length on the relevant integration contour.

[0134] In formula (2-11), the J integral can be further simplified to the form shown in formula (2-12):

[0135] J = J (1) + J (2) + M (1，2) (2-12);

[0136] Among them:

[0137]

[0138] Finally, the expression of the M integral is (2-14):

[0139]

[0140] During the fracturing process, the initiation and propagation of fractures are closely related to the basic properties of the sample and the injected fluid. Therefore, corresponding assumptions need to be made to simplify the analysis of the hydraulic fracturing failure mechanism. The liquid flow mode in the rock mass during fracturing belongs to laminar flow, which is driven by the fluid pressure difference between the wellbore and the rock mass. It is assumed that the initial pore pressure of the reservoir rock is atmospheric pressure. Considering that the pressure increase rate during the fracturing experiment depends to a large extent on the injection rate, and the fluid compressibility and filtration rate change with the increase of the injection pressure. For simplicity, a constant pressure increase rate C is used. Summarizing the on-site fracturing construction experience, setting the injection speed at 20 mL / min can achieve a pressure increase speed of about 0.5 MPa / s. Most traditional theories about the fracture breakdown pressure are based on linear elastic fracture mechanics. For isotropic and homogeneous rocks, when the maximum tensile stress reaches the tensile strength of the rock on the wellbore wall, fracture will start, and the maximum tensile stress on the wellbore wall is expressed in cylindrical coordinates as formula (2-15):

[0141]

[0142] Among them, a is the wellbore radius; σθ max is the maximum tensile stress; are the circumferential stresses caused by the confining pressure and the wellbore fluid pressure respectively, and they are respectively:

[0143]

[0144] For an axisymmetric cylindrical specimen, σ H and σ h are two horizontal stresses, equal to the confining pressures σ1 and σ2. When the injection pressure reaches a certain value, a pair of symmetric fractures are generated on the wellbore wall, and the breakdown pressure P can be obtained by substituting into equation (2-16).

[0145] Expressed as Equation (2-18):

[0146] P b = σ t - 2σ3 (2-18);

[0147] Equation (2-18) represents the extreme case where the rock is completely impermeable. However, the high pore pressure of the fracturing fluid in the permeable zone not only causes additional circumferential stress changes but also changes according to the effective stress law, so the following modifications should be made:

[0148]

[0149] where p(r) is the pore pressure at a distance from the wellbore center; the elastic stress caused by the radial stress can be used to calculate the changing pore pressure by integrating it.

[0150]

[0151] where r is the Poisson's ratio, and the Biot coefficient a = (2 - C / Ch), where C and Ch are the rock compressibility and the overall compressibility, respectively.

[0152] Fractures are initiated on the wellbore wall and are independent of the pore pressure distribution inside the rock. The breakdown pressure of the permeable rock can be obtained by substituting Equation (2-20) to get Equation (2-21):

[0153]

[0154] The net pressure at the rock's port is lower than the breakdown pressure of a rock with the same tensile strength. However, the above formula still does not consider the influence on the net pressure at the port due to the flow behavior of the fracturing fluid, nor can it explain the influence of the pressurization rate on the net pressure at the port. When the maximum tensile effective stress is equal to the rock's tensile strength, fractures will occur accordingly; in the case of tensile failure at a certain point in the rock, the propagation of the fracture becomes unstable, and the distance between this point and the wellbore surface is called the length of the rock's tensile failure (d). The pressure breakdown pressure of the stress model at this point is given by Equation (2-22):

[0155]

[0156] If the pressurization rate C → 0, t → 0, then the pore pressure around the wellbore is greater than the injection pressure, and the stress state at the wellbore changes greatly compared to before, resulting in the rock starting to fracture. The change in pore pressure caused by the failure length (d) can be expressed as Equation (2-23):

[0157]

[0158] If the pressure boost rate C → 0 and t → 0, the pore pressure around the wellbore is equal to the injection pressure, i.e., p = P, then the formula will be changed to:

[0159]

[0160] For the point stress model, the method of calculating the net pressure of the rock port depends to a large extent on the value of the characteristic length d. When the circumferential stress at a certain distance d from the fracture extension point to the fracture tip reaches the tensile strength of the rock, tensile fracture propagation starts from the pre-existing fracture, and the characteristic length d can be calculated by formula (2-25).

[0161]

[0162] where K IC is the fracture toughness of the rock.

[0163] Fracture propagation criterion

[0164] There is tensile-shear failure at the tip of the fracture surface during actual fracturing. Therefore, for the specific fracture propagation mechanism of the fracture surface, the combined influence of type-I and type-II fracture failures is necessarily required. So, during the analysis process, the combined influence of type-I and type-II factors on the propagation mechanism needs to be calculated as a whole. The fracture propagation law is determined by the loading method of the maximum axial stress, that is, the fracture will start to propagate along the direction of the maximum axial stress and where the actual shear stress parameter approaches zero. During the actual analysis process, the fracture front transforms into several points. By relying on finite element software to analyze specific stress, strain and other parameters, and studying the maximum axial stress parameters of such discrete points, the corresponding fracture propagation direction can be obtained. Combining the factor sizes and propagation sizes at the positions of such data points, the overall fracture propagation distance on the front fracture surface can be further judged.

[0165] The theoretical formula for the maximum circumferential stress is as follows:

[0166]

[0167] where, σ θθ and are the polar coordinate directions used in the calculation process; is the polar coordinate component used in the calculation process; K I and K H are the type-I and type-II stress intensity factors in the calculation.

[0168] Fracture propagation and mesh generation

[0169] In the analysis of this example, the calculation method of the "sub-model" is used, and re-meshing is carried out through an adaptive calculation mode. Compared with the overall re-meshing method, it can reduce a large amount of computing resources and improve the final analysis efficiency.

[0170] The concept of the sub-model is that, in order to control the computational scale and improve the overall computational efficiency, the crack propagation position is divided to determine it as the corresponding sub-model. The sub-model and the rest of the model are output separately. Only the research on the synthesis of new high-strength pore pressure-resistant materials and the rock mass fracture behavior needs to be incorporated into the sub-model to meet the computational requirements. At the same time, with the expansion of the sub-interval, the development and distribution trend of the overall crack can be redefined.

[0171] In another example, the extended finite element method is used to simulate crack propagation. An additional interpolation function is used to describe the crack, and there is no need to re-mesh:

[0172]

[0173] where N i (x) is the shape function, u i is the standard displacement variable, H(x) is the displacement jump function on the crack surface, a j is the crack propagation influence parameter, F(x) is the crack tip characteristic function, b k is the crack tip field variable.

[0174] Fracturing calculation process

[0175] In this example, the finite element software Abaqus is used to confirm the stress conditions of the full-diameter core under the condition of fracturing simulation, and then to deduce the initiation, development and distribution mode of the crack. The key is to build a model to analyze and obtain the displacement and stress fields under the influence of the load; confirm the strength factors in the initiation and propagation scenarios through Abaqus, combine the analysis data to determine the propagation direction, and re-mesh the detailed crack surface. The calculation process of this part can be referred to Figure 9 as shown:

[0176] The calculation steps include:

[0177] S31: Establish a finite element model related to fracturing, provide the corresponding stress load, the pressure in the wellbore and the external constraint mechanism of the full-diameter core; through finite element processing, and then build a crack model, regard the extended area as the sub-model interval, so as to achieve the effect of simulating the generation of cracks under the stress condition:;

[0178] S32: Use the finite element software to assign values and analyze to obtain the required initial stress and strain distribution information, and load the net pressure at the port on this basis;

[0179] S33: Read the sub-model, run the graphical crack, import the corresponding boundary conditions, re-mesh the internal full-diameter core and the external coating material, and re-integrate the sub-model with cracks and the rest of the model;

[0180] S34: Obtain the stress intensity factor at the crack tip through finite element analysis, that is, obtain the corresponding stress parameters, and finally obtain the specific strength factor parameters of the leading edge nodes.

[0181] S35: Combine the fracture criterion to determine the specific propagation mode. If it meets the fracture critical value, it is regarded as the existence of the corresponding crack propagation phenomenon, perform data update processing for crack generation, re-divide the corresponding tip grid, and simultaneously restart the calculation work to obtain the required stress and strain distribution information. Carry out the calculation in a repetitive cycle to further calculate the entire process of crack propagation; if it does not reach the propagation critical value, it is necessary to increase the fracturing data at the actual perforation position and analyze the critical state of fracture occurrence. Based on the experimental results of physical fracturing simulation, the reference value of the net pressure at the port during numerical simulation is controlled within ±3 Mpa of the actual data.

[0182] Numerical simulation of fracture morphology

[0183] Use Abaqus software for numerical simulation. The boundary conditions are set to 33 MPa in the X direction, 36 MPa in the Y direction, and 39 MPa in the Z direction. Since the external material did not rupture after the physical simulation experiment, the core and the external cement-based material have linear elasticity. The contact pressure and the liquid injection pressure are 35 MPa, 37 MPa, and 41 MPa respectively, and their values are based on the fracture initiation pressure of the physical simulation experiment. Compressive damage and tensile damage are added to the simulation process respectively, and the specific stress cloud distribution of the force is as Figure 10 shown.

[0184] To better display the change of the stress cloud diagram when the sample is under the combined action of confining pressure and pore pressure, in this example, hydraulic fracturing is carried out on the premise that the settings in the X direction are 33 MPa, the settings in the Y direction are 36 MPa, and the settings in the Z direction are 39 MPa remain unchanged. From Figure 10 the stress cloud diagram, it can be seen that after applying the confining pressure, the stress increases. Although there is a little stress concentration, the stress change is not significant enough to affect the sample strength. When the confining pressure gradually increases until cracks appear in the internal sample finally, no obvious cracks appear in the stress cloud diagram, which proves that the sample can withstand the confining pressure and pore pressure designed in the experiment.

[0185] To further observe the stress change of the external coating material under the coexistence of confining pressure and pore pressure, the sample is cut along the Z direction, and the stress change inside the sample can be directly observed. Excluding the pressure on the internal full-diameter core, it can be intuitively seen that the sample is intact and no stress concentration failure occurs, which proves that the sample strength meets the experimental requirements as shown in Figure 11 .

[0186] Figure 12It shows the stress cloud of the internal core under the coexistence of pore pressure and confining pressure. It can be clearly seen that the blue area in the hydraulic fracturing experimental group is more obvious, that is, the stress is mainly concentrated around the wellbore. It can be determined from the figure that the fractures are relatively single and limited in length.

[0187] Figure 13 It shows the fracturing simulation carried out by combining the overall external material and the internal core. When the external constraint pressure and the fracturing pressure exist simultaneously, a single fracture will be shown, and this simulation result corresponds to the result of the physical fracturing simulation hydraulic fracturing experimental group. The numerical simulation also confirms the reliability of the physical fracturing simulation experimental data from the side.

[0188] The analysis of the fracture morphology in the above example relies on manual observation or the results of finite element calculation. In another example, deep learning is used to analyze the fracture morphology. First, feature extraction is carried out, and the features include fracture length L, fracture width W, and fracture curvature κ:

[0189]

[0190] Train a convolutional neural network (CNN) to extract the fracture image features:

[0191] Feature = CNN(I) (2-29);

[0192] Use a support vector machine (SVM) for classification:

[0193]

[0194] The fracture types can be automatically classified through deep learning, improving the data processing efficiency. Combining with CT scan data can improve the classification accuracy.

[0195] In addition, acid fracturing simulation is also carried out in this application and compared with hydraulic fracturing. The net pressure at the acid fracturing experimental port is reduced by 4 MPa. On the basis of forming a main fracture, 2-3 secondary fractures are added. The net pressure at the port of the experimental group using acid fluid and thickener compound as the fracturing fluid is reduced by 6 MPa compared with the hydraulic fracturing experimental group. On the basis of forming a main fracture, multiple microfractures in different planes and irregular shapes are generated, and the runoff direction and erosion area of the synthetic polymer (thickener) in the core can be observed. The overall full-diameter core presents a volume fracturing mode.

[0196] This example provides an experimental system for simulating the hydraulic fracturing rupture situation under in-situ conditions. The experimental system includes a layered in-situ stress simulation device, a fracturing fluid injection device, and a fracture monitoring device.

[0197] Specifically, regarding the layered in-situ stress simulation device, since it is necessary to effectively restore the formation conditions during the simulation of the fracturing process, the core factors are the actual stress magnitude and distribution. Usually, there is a certain pressure difference in the triaxial confining pressure, and the in-situ stresses at different horizons also have different magnitudes. For hydraulic fracturing, the parameters of the actual triaxial stresses will affect the fracture propagation trend, the specific magnitude and distribution pattern of the stresses, and also the specific geometric shape of the fractures. Therefore, using a true triaxial physical simulation in the analysis can more ideally control the stress distribution state. The main parts of the true triaxial testing machine adopted in this example are composed of a mainframe frame (including pressure application in five directions and an upper protection pressing plate), an electro-hydraulic servo loading mechanism, a lifting mechanism, a pressure boosting mechanism, a measurement and control mechanism, a main control computer, a fracturing fluid pumping mechanism (including a pump for adding dye), etc. The physical picture of the test equipment is as shown in Figure 14 shown.

[0198] Regarding the fracturing fluid pumping device, in order to control the injection liquid displacement at a fixed value, an MTS booster pump is used to control the flow rate. The pumping flow rate is set at 20 mL / min, the actual volume data of the isolator is 700 mL, and the actual pressure bearing is 50 MPa.

[0199] Regarding the fracture monitoring device, an acoustic emission instrument is used to measure specific fracture propagation mechanisms and morphologies, etc., and an MTS data receiving device is used to complete signal detection. When changes such as internal core rupture, fracture initiation, and propagation occur, relevant acoustic wave signals will be released. Due to the acoustic wave signals transmitted by the core rupture, they are first converted into the required electrical signals by corresponding sensors (probes), and the signal amplifier is used to increase it to a specific voltage. Subsequently, an independent channel controller (ICC) is used to carry out the measurement work. An image processing software is used to store and process the data. During the acoustic emission monitoring process, multiple (the maximum number is 14) channels can be used, and there is also the effect of sound source localization. By analyzing the acoustic emission signals, through the relative time to reach a specific position, the specific fracture initiation, development, and distribution areas are further judged. In this analysis, 6 or 4 probes are used as a group during processing, and according to the set experimental scheme, the probes are further pasted onto the surface of the corresponding pressurized partition board. After the physical fracturing simulation experiment is completed, if the sample is broken in a brittle failure manner and the internal core is taken out, it will cause the core to undergo secondary cracking, affecting the actual effect of the fractures. Therefore, in order to ensure the integrity of the internal core, after the physical fracturing simulation experiment in this experiment, the sample is gradually cut using a linear cutting machine. When the cutting part reaches near the internal core, a hand saw blade is used for cutting to ensure that the internal core is not damaged to the greatest extent.

[0200] Figure 14 The overall drawing of the specific experimental equipment is given. Figure 14-a is the total control center and the pressure control module, which realizes the application of triaxial stress under the condition of ensuring the pressure difference by controlling the magnitude of the input pressure and observing the magnitude of the applied stress in the control center computer in a timely manner; Figure 14 -b is the surrounding rock temperature control module and the hydraulic pump. The surrounding rock control module can perform preheating treatment to ensure that the experimental temperature around the sample is the temperature at the actual core position underground; Figure 14 -c is the liquid injection tank, which can provide a maximum injection pressure of 25 MPa (net port pressure); Figure 14 -d is the internal situation diagram of the sample placement part. The pressure application pump of this equipment uses the oil pressure method for pressure application, and the maximum liquid supply pressure of each part that can be controlled is 60 MPa. Through this design method, physical simulation is realized by applying different stresses in each direction, and then realizing the combined action of triaxial stress with a pressure difference and the liquid injection pressure (pore pressure).

[0201] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. An experimental method for simulating in-situ conditions in hydraulic fracturing rupture, characterized in that, Including: S1: Place the fracturing pipe into the mold, and add a prefabricated first quantity of cement-based composite material into the mold; S2: Add the core into the mold, invert the core, and then add a second quantity of cement-based composite material until the mold is filled to obtain a sample; S3: Apply triaxial pressure to the sample to simulate the formation pressure of the Yanchang Formation; S4: When the confining pressure is stable, inject the pressure liquid into the core at a preset speed and start the fracturing experiment; S5: Observe the fracturing curve data graph to obtain the time point of core fracture and the fracturing pressure, and, peel the core from the sample and analyze the fracture condition of the core.

2. The method according to claim 1, characterized in that, S1 further includes: Apply silicone oil to the inner side of the mold; The height of the first quantity of cement-based composite material is one-third of the height of the mold.

3. The method according to claim 1, characterized in that, S3: The triaxial pressure includes: Apply a first pressure in the X-axis direction of the sample, apply a second pressure in the Y-axis direction of the sample, and apply a third pressure in the Z-axis direction of the sample.

4. The method according to claim 3, characterized in that Apply different magnitudes of pressure in the three axes to simulate under the condition that there is a pressure difference in the three axes and it acts together with the injection pressure of the pressure liquid.

5. The method according to claim 3, wherein Use the oil pressure method for pressure application, and set the maximum protection confining pressure to 50 MPa.

6. The method according to claim 1, wherein S5 further includes: Obtain the acoustic wave signal transmitted by the core fracture; Convert the acoustic wave signal into an electrical signal and increase the electrical signal to a preset voltage; Use an independent channel controller for measurement, analyze the acoustic emission signal, and obtain the fracture initiation, development, and distribution regions through the relative time to reach a preset position.

7. The method according to claim 1, wherein S5 further includes: After the fracturing experiment, use a linear cutting machine to peel the cement-based composite material outside the core. When the cutting part reaches the inner core, then use a hand saw blade for cutting to obtain the inner core.

8. The method according to claim 1, wherein After S5, it further includes: Dry the core, and then saturate the core with distilled water; Use the X-ray computed tomography method to detect the internal structure of the core, and analyze the fracture position and extension direction according to the scanning results.

9. An experimental system for simulating in-situ conditions in hydraulic fracturing rupture cases, using the method according to any one of claims 2-8, characterized in that, Including: A layered in-situ stress simulation device for restoring the formation conditions during the simulation of fracturing; A fracturing fluid injection device for controlling the liquid flow rate injected into the fracturing pipe; And A fracture monitoring device for determining the fracture propagation mechanism and its morphology.

10. The system according to claim 9, wherein, The layered in-situ stress simulation device includes: A pressure control module for controlling the magnitude of pressure input to apply pressure to the three axes while maintaining a pressure difference; A surrounding rock temperature control module for performing preheating treatment to make the temperature of the sample area the temperature at the actual core position underground.

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