Pump stop pressure drop crack evaluation method, medium and equipment

Abstract

The invention provides a pump stop pressure drop crack evaluation method, medium and equipment, and relates to the technical field of unconventional oil and gas development, and the method comprises the steps: building a crack network model based on ground stress, natural crack distribution and fracturing construction parameters, calculating crack fracture and expansion, and obtaining a crack network form; establishing a shaft-fracture water hammer model according to the fracture network form, and performing discrete solution on the water hammer model by using a characteristic line method; acquiring a simulated water attack effect curve based on the solved water attack model; comparing the simulated water attack effect curve with the actual wellhead pumping stop pressure drop curve, and adjusting the fracture network model and the water attack model until the error is not greater than a set threshold value; introducing a seam length index and a branch index, and establishing a two-parameter evaluation chart based on the geometric morphology of the seam net; and introducing a boundary index and an attenuation coefficient, establishing a two-parameter evaluation chart based on the water attack effect, and calculating four evaluation parameters to obtain a crack evaluation result. According to the method, the pump stop water attack phenomenon under the complex seam network condition can be effectively diagnosed.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas development, and in particular to a method, medium and equipment for evaluating a pump-off pressure drop crack. Background Art

[0002] Hydraulic fracturing technology is currently an important means of reservoir reconstruction in unconventional oil and gas reservoirs. Artificial fractures form complex fracture networks by connecting natural fractures and faults in the reservoir, thereby improving the conductivity of the reconstruction area. Considering the complex geological conditions, geostress environment, the degree of development of natural fractures, and the influence of fracturing construction schemes on the distribution of hydraulic fractures, how to obtain the location and fracture network morphology of artificial fractures after fracturing is of great significance for evaluating the effect of fracturing reconstruction.

[0003] At present, the commonly used post-pressure fracture diagnosis methods include microseismic monitoring, near-well monitoring, inclinometer fracture monitoring, distributed acoustic sensor monitoring, etc. Due to the complexity of fracture monitoring, each monitoring technology has its own defects and limitations, such as the high cost of microseismic monitoring technology, the inclinometer cannot determine the size of single and complex fractures, and the acoustic sensor monitoring cannot determine the size of complex fracture networks. In addition, most of the existing pump-off water hammer analysis methods give the geometric dimensions of equivalent single fractures, and cannot perform water hammer diagnosis analysis on complex fracture networks. Therefore, the oil field site urgently needs a pump-off water hammer diagnosis method that is suitable for complex fracture network conditions, low cost, simple to implement, and highly accurate. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that the existing pump-off water hammer analysis method is difficult to effectively diagnose the pump-off water hammer phenomenon under complex fracture network conditions, and propose a pump-off pressure drop fracture evaluation method, comprising the following steps:

[0005] S1. Obtain the ground stress magnitude, natural fracture distribution and fracturing construction parameters in the study area, and establish a fracture network model;

[0006] S2. Calculate the fracture and expansion of fractures based on the fracture network model, obtain the fracture network morphology, establish a wellbore-fracture water hammer model according to the fracture network morphology, and use the characteristic line method to perform discrete solution on the wellbore-fracture water hammer model;

[0007] S3, obtaining a simulated water hammer effect curve based on the solved wellbore-fracture water hammer model and the fracture network model;

[0008] S4, comparing the simulated water hammer effect curve with the actual wellhead pump stop pressure drop curve, if the error e between the two is greater than the set threshold, adjusting the fracture network model and the wellbore-fracture water hammer model until the error e is no greater than the set threshold;

[0009] S5. Based on the fracture network model, two evaluation parameters, fracture length index and branch index, are introduced to establish a dual-parameter evaluation chart based on the fracture network geometry. Based on the wellbore-fracture water hammer model, two evaluation parameters, boundary index and attenuation coefficient, are introduced to establish a dual-parameter evaluation chart based on the water hammer effect. The four evaluation parameters are calculated to obtain the fracture evaluation results.

[0010] Furthermore, the fracture network model includes: a linear elastic fracture model, a hydraulic fracture and a natural fracture intersection model;

[0011] The equilibrium equations and boundary conditions of the linear elastic crack model are as follows:

[0012]

[0013] in, is the gradient operator, σ is the stress tensor corresponding to the displacement field u, f is the given volume force, Ω is the calculation area, n represents the unit vector of the boundary normal, g represents the surface force, Γ g represents the boundary where the surface force is applied, p represents the internal pressure on the crack surface, Γ c represents edge cracks, u 0 represents a fixed displacement, Γ u represents the boundary to which a fixed displacement is applied, λ represents the first parameter of the Lame constant, μ represents the second parameter of the Lame constant, trace is the trace of the tensor, I is the identity matrix, and ε represents the strain tensor;

[0014] In the intersection model of hydraulic fractures and natural fractures, the stress state at the intersection of hydraulic fractures and natural fractures includes:

[0015] Stop extension: p 1 +σ n <T coh ,σ 1 <T rock ;

[0016] Through natural cracks: p 1 +σ n <T coh ,σ 1 ≥T rock ;

[0017] Opening natural fractures and penetrating: σ 1 ≥T rock ;

[0018] Opening of natural fractures: σ 1 <T rock ;

[0019] Among them, p 1 is the water pressure in the crack, σ n represents the normal stress, T cohrepresents the surface strength of natural cracks, σ 1 represents the maximum principal stress, T rock Represents the tensile strength of rock.

[0020] Furthermore, the crack rupture satisfies the maximum circumferential stress criterion, which is expressed as:

[0021]

[0022] Where θ is the crack deflection angle, K I and K II is the first and second type stress intensity factor, K IC Represents the fracture toughness of rock.

[0023] Furthermore, in the wellbore-fracture water hammer model, transient flow theory is used to describe the propagation of pressure waves generated by water hammer in the wellbore and fractures, and the control equation is obtained:

[0024]

[0025] Where p is the fluid pressure, ρ is the fluid density, a is the pressure wave velocity, V is the fluid velocity, |V| represents the absolute value of the velocity, t represents the time, x represents the one-dimensional coordinate along the pipeline, g is the gravitational acceleration, θ represents the angle between the pipeline and the horizontal plane, f represents the pipeline friction coefficient, and D is the pipeline diameter.

[0026] Furthermore, in the wellbore-fracture water hammer model, the flow rate at the i-th fracture outlet is obtained according to the Bernoulli principle:

[0027]

[0028] Among them, Q i represents the flow rate at the i-th crack outlet, Q 0 represents the flow rate in the wellbore, d i and l i represent the pipe diameter and length at the i-th crack outlet, f represents the pipe friction coefficient, and f j , l ij , d j represent the friction coefficient, pipe length and pipe diameter of the jth pipe on the i-th pipe flow path respectively.

[0029] Furthermore, the boundaries of the wellbore-fracture water hammer model include: a flow boundary at the wellhead, a closed boundary at the bottom of the well, and a pressure boundary at the fracture front;

[0030] The flow boundary at the wellhead location satisfies: Q 0 (t) = Q 0 f(t), where Q 0 (t) represents the flow rate at the wellhead at time t, Q0 is the wellhead flow rate at the initial moment of pump stop, f(t) represents the function determined by the flow rate change process monitored at the wellhead, satisfying f(t)∈[0,1];

[0031] In the closed boundary at the bottom of the well, the flow rate is always 0 and the pressure is unknown;

[0032] In the pressure boundary at the fracture front, the pressure is known but the flow rate is unknown.

[0033] Furthermore, the characteristic line method is used to discretize the wellbore-fracture water hammer model, specifically:

[0034] The water head height H and pipe flow rate Q are used to describe the transient flow in the pipe:

[0035]

[0036] Among them, H represents the water head height, p represents the fluid pressure, ρ represents the fluid density, g represents the acceleration of gravity, z represents the pipeline height, Q represents the pipeline flow rate, V represents the fluid velocity, and A represents the pipeline cross-sectional area;

[0037] The control equation is rewritten as follows:

[0038]

[0039] Among them, L 1 and L 2 Used to express two formulas, t represents time, a represents the flow velocity in the pipeline, x is the one-dimensional coordinate along the pipeline, f is the friction coefficient, and D is the pipeline diameter;

[0040] Expand the rewritten control equation along the characteristic line direction to obtain the control iterative solution format along the characteristic line:

[0041]

[0042] in, represents the water head height of the ith node at time t+Δt, Δt represents the time step interval, represents the flow of the i-th node at time t+Δt, represents the water head height of the i-1th node at time t, represents the flow of the i-1th node at time t, represents the water head height of the i+1th node at time t, represents the flow of the i+1th node at time t, C P , B M , C M , B P They represent the intermediate parameters of the calculation process, B = a / (gA), R = fΔx / (2gDA2 ), Δx represents the grid size along the pipeline.

[0043] Further,

[0044] The seam length index is defined as:

[0045]

[0046] Among them, f l is the seam length index; l total is the total length of the fracture trajectory; d is the characteristic length of the perforation area;

[0047] The branch index is defined as:

[0048]

[0049] Among them, f n is the branch index; n b is the number of branch fractures with average fracture length; n perf is the number of perforation clusters;

[0050] The boundary index is defined as:

[0051]

[0052] In the formula, f b represents the boundary index, a is the propagation speed of pressure fluctuation; T is the water hammer effect period; H is the depth of the perforation position;

[0053] The attenuation coefficient α is defined as:

[0054] p=p A e -αt ,

[0055] Where p represents water hammer fracturing, p A represents the water hammer fracturing amplitude, e represents the natural index, t represents the time, and α represents the attenuation coefficient.

[0056] The present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the above-mentioned pump-off pressure drop fracture evaluation method is implemented.

[0057] The present invention also proposes an electronic device, comprising a processor and a memory, wherein the processor and the memory are interconnected, wherein the memory is used to store a computer program, the computer program comprises computer-readable instructions, and the processor is configured to call the computer-readable instructions to execute the above-mentioned pump-off pressure drop fracture evaluation method.

[0058] The beneficial effects brought by the technical solution provided by the present invention are:

[0059] The present invention first constructs a complex fracture network model based on a linear elastic fracture model, a hydraulic fracture and a natural fracture intersection model, and calculates the fracture network morphology, taking into account the influence of the complex fracture network structure on the water hammer effect, and based on the transient flow theory, establishes a wellbore-fracture water hammer model according to the fracture network morphology, and uses the characteristic line method for numerical solution. By analyzing the characteristics of the water hammer curve, a dual-parameter evaluation chart based on the fracture network geometry and the water hammer effect curve is established, forming a rapid evaluation method for the complexity of the hydraulic fracture network. The present invention can effectively diagnose the pump-off water hammer phenomenon under complex fracture network conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 is a flow chart of a method for evaluating cracks during pump stop pressure drop according to an embodiment of the present invention;

[0061] Figure 2 is a schematic diagram of a two-dimensional crack model according to an embodiment of the present invention;

[0062] Figure 3 is a physical model of a wellbore-fracture coupling system according to an embodiment of the present invention;

[0063] Figure 4 A dual-parameter evaluation chart based on seam network geometry established for an embodiment of the present invention;

[0064] Figure 5 It is a dual-parameter evaluation chart based on water hammer effect established in an embodiment of the present invention;

[0065] Figure 6 is a block diagram of an electronic device in an exemplary embodiment of the present invention;

[0066] Figure 7 is the fracture network morphology obtained by simulation in an embodiment of the present invention;

[0067] Figure 8 is a comparison curve diagram of a simulated water hammer curve and a measured water hammer curve according to an embodiment of the present invention;

[0068] Fig. 9 It is a schematic diagram of the result of the dual-parameter evaluation chart based on the crack geometry according to an embodiment of the present invention;

[0069] Fig.10 It is a schematic diagram of the double-parameter evaluation chart result based on the water hammer effect according to an embodiment of the present invention;

[0070] Fig.11 : is a distribution diagram of water hammer boundary index and attenuation coefficient of different fracturing stages of 5 wells in an embodiment of the present invention;

[0071] Fig.12 It is a schematic diagram of the result of projecting the boundary index and attenuation coefficient onto the evaluation plate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0072] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0073] The flowchart of the method for evaluating cracks by stopping pump pressure drop according to an embodiment of the present invention is as follows: Figure 1 , specifically including the following steps:

[0074] S1. Obtain the magnitude of geostress, distribution of natural fractures and fracturing construction parameters in the study area, and establish a fracture network model. Geostress is the natural stress existing in the earth's crust that is not disturbed by engineering. Fracturing construction parameters include construction material properties, structural stress after construction, temperature stress, and artificial fracture parameters.

[0075] Based on the extended finite element method, the influence of ground stress, injection flow rate and natural fractures is considered to simulate the fracture network morphology generated by the perforation section. The fracture network model includes: linear elastic fracture model, hydraulic fracture and natural fracture intersection model.

[0076] The linear elastic crack model takes into account the problem of elastic static cracks. Please refer to Figure 2 , Figure 2 Schematic diagram of a two-dimensional crack model according to an embodiment of the present invention. Contains an edge crack Γ c , where the internal pressure on the crack surface is p; the boundary Γ u A fixed displacement u0 is applied to the other boundaries. The surface force g is applied at the crack. The unit of the direction normal to the boundary is n. The surface forces on both sides of the crack are and to differentiate and The normal unit vectors of are denoted by n+ and n- respectively. Define n - =-n + =n,p + =-p - =p, where p + =-pn + , p - =-pn - The equilibrium equations and boundary conditions of the linear elastic crack model are as follows:

[0077]

[0078] in, is the gradient operator, σ is the stress tensor corresponding to the displacement field u, f is the given object force, Ω is the calculation area, n represents the unit vector of the boundary normal, g represents the surface force, Γ grepresents the boundary where the surface force is applied, p represents the internal pressure on the crack surface, Γ c represents edge cracks, u 0 represents a fixed displacement, Γ u represents the boundary to which a fixed displacement is applied, λ represents the first parameter of the Lame constant, μ represents the second parameter of the Lame constant, trace is the trace of the tensor, I is the identity matrix, and ε represents the strain tensor;

[0079] In the intersection model of hydraulic fractures and natural fractures, the intersection of hydraulic fractures and natural fractures may experience penetration, stop, capture, capture and turn, etc. The stress state at the intersection of hydraulic fractures and natural fractures includes:

[0080] Stop extension: p 1 +σ n <T coh ,σ 1 <T rock ;

[0081] Through natural cracks: p 1 +σ n <T coh ,σ 1 ≥T rock ;

[0082] Opening natural fractures and penetrating: σ 1 ≥T rock ;

[0083] Opening of natural fractures: σ 1 <T rock .

[0084] Among them, p 1 is the water pressure in the crack, σ n represents the normal stress, T coh represents the surface strength of natural cracks, σ 1 represents the maximum principal stress, T rock Represents the tensile strength of rock.

[0085] S2. Calculate the fracture rupture and expansion of fractures based on the fracture network model, obtain the fracture network morphology, establish a wellbore-fracture water hammer model according to the fracture network morphology, and use the characteristic line method to discretely solve the wellbore-fracture water hammer model.

[0086] The crack rupture satisfies the maximum circumferential stress criterion, which is expressed as:

[0087]

[0088] Where θ is the crack deflection angle, K I and K II is the first and second type stress intensity factor, K IC Represents the fracture toughness of rock.

[0089] In the wellbore-fracture water hammer model, based on the wellbore-fracture coupling system, the transient flow theory is used to describe the propagation of the pressure wave generated by the water hammer phenomenon in the wellbore and fractures, and the control equation is obtained. The physical model of the wellbore-fracture coupling system in the embodiment of the present invention is referenced Figure 3 , the control equation is:

[0090]

[0091] Where p is the fluid pressure, ρ is the fluid density, a is the pressure wave velocity, V is the fluid velocity, |V| represents the absolute value of the velocity, t represents the time, x represents the one-dimensional coordinate along the pipeline, g is the gravitational acceleration, θ represents the angle between the pipeline and the horizontal plane, f represents the pipeline friction coefficient, and D is the pipeline diameter.

[0092] In the wellbore-fracture water hammer model, the flow rate at the i-th outlet of the fracture is obtained according to the Bernoulli principle:

[0093]

[0094] Among them, Q i represents the flow rate at the i-th crack outlet, Q 0 represents the flow rate in the wellbore, d i and l i represent the pipe diameter and length at the i-th crack outlet, f represents the pipe friction coefficient, and f j , l ij , d j represent the friction coefficient, pipe length and pipe diameter of the jth pipe on the i-th pipe flow path respectively.

[0095] The boundaries of the wellbore-fracture water hammer model include: the flow boundary at the wellhead, the closed boundary at the bottom of the well, and the pressure boundary at the fracture front.

[0096] The flow boundary at the wellhead location satisfies: Q 0 (t) = Q 0 f(t), where Q 0 (t) represents the flow rate at the wellhead at time t, Q 0 is the wellhead flow rate at the initial moment of pump shutdown, f(t) represents the function determined by the flow change process monitored at the wellhead, satisfying f(t)∈[0,1]; in the closed boundary at the bottom of the well, the flow rate is always 0 and the pressure is unknown; in the pressure boundary at the fracture front, the pressure is known and the flow rate is unknown.

[0097] For complex seam network pipeline structures, one intersection may be connected to n branch pipelines. The pressure at the pipeline intersection and the flow rate Q of each branch pipeline can be obtained by solving the following linear equations with (n+1) unknown variables: i distributed:

[0098]

[0099] Among them, p n Indicates the pressure at the junction of the nth branch pipe, Q i Indicates the flow rate of the nth branch pipeline.

[0100] The characteristic line method is used to discretize the wellbore-fracture water hammer model, specifically:

[0101] The water head height H and pipe flow rate Q are used to describe the transient flow in the pipe:

[0102]

[0103] Among them, H represents the head height, p represents the fluid pressure, ρ represents the fluid density, g represents the acceleration of gravity, z represents the pipeline height, Q represents the pipeline flow rate, V represents the fluid velocity, and A represents the pipeline cross-sectional area.

[0104] The control equation is rewritten as follows:

[0105]

[0106] Among them, L 1 and L 2 They are used to represent the following formulas respectively, and have no specific meanings. t represents time, a represents the flow velocity in the pipeline, x is the one-dimensional coordinate along the pipeline, f is the friction coefficient, and D is the pipeline diameter;

[0107] Expand the rewritten control equation along the characteristic line direction to obtain the control iterative solution format along the characteristic line:

[0108]

[0109] in, represents the water head height of the ith node at time t+Δt, Δt represents the time step interval, represents the flow of the i-th node at time t+Δt, represents the water head height of the i-1th node at time t, represents the flow of the i-1th node at time t, represents the water head height of the i+1th node at time t, represents the flow of the i+1th node at time t, C P , B M , C M , B P They represent the intermediate parameters of the calculation process, B = a / (gA), R = fΔx / (2gDA 2 ), Δx represents the grid size along the pipeline.

[0110] S3. Based on the solved wellbore-fracture water hammer model and fracture network model, a simulated water hammer effect curve is obtained.

[0111] S4. Compare the simulated water hammer effect curve with the actual wellhead pump stop pressure drop curve, and compare the amplitude, frequency and attenuation amplitude of the two curves. If the error e between the two is greater than the set threshold, adjust the fracture network model and the wellbore-fracture water hammer model until the error e is no greater than the set threshold.

[0112] S5. Based on the fracture network model, two evaluation parameters, fracture length index and branch index, are introduced to establish a dual-parameter evaluation chart based on fracture network geometry; based on the wellbore-fracture water hammer model, two evaluation parameters, boundary index and attenuation coefficient, are introduced to establish a dual-parameter evaluation chart based on water hammer effect, and four evaluation parameters are calculated to obtain fracture evaluation results. According to the complexity of the fracture network system, it is divided into 4 levels from high to low: Zone I to Zone IV.

[0113] The fracture length index reflects the average fracture volume of the fracture network; the branching index reflects the complexity of the fracture network branching structure. Figure 4 , Figure 4 The dual parameter evaluation chart based on the fracture network geometry established for the embodiment of the present invention shows that the larger the fracture length index and branch index are, the more complex the fracture network system is. Complex fractures with large fracture volume, complex fractures with moderate fracture volume, and relatively complex fracture morphology with large fracture volume are divided into Zone I; complex fractures with small fracture volume and relatively complex fracture morphology with moderate fracture volume are divided into Zone II; relatively complex fracture morphology with small fracture volume, single fracture morphology with moderate fracture volume, and single fracture morphology with large-scale dominant fractures are divided into Zone III; and single fracture morphology with small fracture volume is divided into Zone IV.

[0114] The seam length index is defined as:

[0115]

[0116] Among them, f l is the seam length index; l total is the total length of the fracture trajectory; d is the characteristic length of the perforation area;

[0117] The branch index is defined as:

[0118]

[0119] Among them, f n is the branch index; n b is the number of branch fractures with average fracture length; n perf is the number of perforation clusters.

[0120] The boundary index reflects the periodic characteristics of the water hammer curve and represents different flow boundaries at the bottom of the well. The attenuation coefficient is obtained by exponential fitting the peak envelope of the water hammer curve, reflecting the dissipation effect of artificial fractures on water hammer pressure fluctuations. Figure 5 , Figure 5 It is a dual parameter evaluation chart based on water hammer effect established by the embodiment of the present invention. In this chart, the larger the boundary index and the attenuation coefficient, the higher the complexity of the fracture network system. Among them, the fracture volume is large and the fracture is complex, the fracture volume is moderate and the complexity is high, and the fracture volume is large and the fracture is complex. It is divided into Zone I; the fracture volume is large and the fracture is simple, and the fracture volume is moderate and the complexity is moderate. It is divided into Zone II; the fracture volume is moderate and the form is simple, the fracture volume is limited and the complexity is moderate, the fracture volume is limited and there are large-scale dominant fractures, and the formation filtration is large. It is divided into Zone III; the fracture volume is limited and the fracture is simple. It is divided into Zone IV.

[0121] The boundary index is defined as:

[0122]

[0123] In the formula, f b represents the boundary index, a is the propagation speed of pressure fluctuation; T is the water hammer effect period; H is the depth of the perforation position.

[0124] The attenuation coefficient α is defined as:

[0125] p=p A e -αt ,

[0126] Where p represents water hammer fracturing, p A represents the amplitude of water hammer fracturing, e represents the natural index, t represents the time, and α represents the attenuation coefficient. The water hammer pressure here is the measured pump stop pressure at the wellhead minus the natural pressure drop of the reservoir, eliminating the influence of the natural pressure drop process of the reservoir on water hammer diagnosis.

[0127] In an exemplary embodiment, a computer-readable storage medium is included, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the above-mentioned pump-off pressure drop fracture evaluation method is implemented.

[0128] See also Figure 6 In an exemplary embodiment, an electronic device is also included, including at least one processor, at least one memory, and at least one communication bus.

[0129] Wherein, a computer program is stored in the memory, and the computer program includes computer-readable instructions. The processor calls the computer-readable instructions stored in the memory through the communication bus to execute the above-mentioned pump-off pressure drop fracture evaluation method.

[0130] In order to verify the effectiveness of the method of the present invention, the first stage of fracturing of a shale gas well in the study area was selected. The basic parameters of the target well and target reservoir are shown in Table 1. The actual shape of the well is given by the well logging inclination data, and the perforation position and the number of perforation clusters are determined by the well fracturing design plan. The first stage of the fracturing section of the well is 75 meters long, with 8 clusters and 8 holes, and the hole diameter is 10 mm. From the injection flow rate of 3.0m 3 The pump is stopped at 1000 rpm and the flow rate drops to 0 during the pump stopping process. The water hammer effect simulation time is 320 seconds.

[0131] Table 1

[0132]

[0133] First, a fracture network fracturing model was established based on the geostress, natural fracture distribution and fracturing construction parameters of the block to simulate the fracture network morphology produced by the perforation section. The simulation program is based on the extended finite element method, taking into account the influence of geostress, injection flow and natural fractures. Figure 7 , Figure 7 This is the fracture network morphology obtained by simulation in the embodiment of the present invention. It can be seen that the artificial fractures near each perforation cluster communicate with the natural fractures of the reservoir during the extension process, forming a complex fracture network. At the same time, the perforation clusters on both sides have a stress interference effect on the artificial fractures of the middle perforation cluster, partially limiting the expansion process of the artificial fractures in the middle cluster.

[0134] based on Figure 7 The seam network morphology shown in the figure is simulated by using the water hammer effect model established by the present invention to simulate the water hammer curve after the pump is stopped, as shown in FIG. Figure 8 As shown, Figure 8 It is a comparison curve diagram of the simulated water hammer curve and the measured water hammer curve of the embodiment of the present invention. It can be seen that the simulated water hammer curve and the measured water hammer curve are in good agreement, which verifies the accuracy of the seam network model. The seam length index and branch index of the seam network morphology are calculated based on the seam network complexity evaluation method, and the boundary index and attenuation coefficient are calculated based on the pump-off water hammer curve, and projected onto Figure 4 and Figure 5 In the evaluation chart in the figure, the dual parameter evaluation results are shown in Fig. 9 and Fig.10 , Fig. 9 It is a schematic diagram of the result of the dual-parameter evaluation chart based on the crack geometry according to an embodiment of the present invention; Fig.10 It is a schematic diagram of the results of the dual-parameter evaluation chart based on the water hammer effect of the embodiment of the present invention. From the crack morphology evaluation template, it can be seen that the number of crack network branches is large and the equivalent crack length is large. From the dual-parameter chart of the water hammer curve, it can be seen that the attenuation coefficient of the water hammer curve is large, and the crack has a significant attenuation effect on water hammer. The evaluation conclusions of the two crack complexity evaluation templates are consistent, which verifies the accuracy of the water hammer effect model proposed in this article.

[0135] The present invention also conducted a pump-off water hammer analysis on a total of 102 fracturing stages in 5 shale gas wells in the study area. All fracturing stages of the 5 wells began to stop pumping after the displacement stabilized at 3.0 m3 / min. After the pump was stopped for 10 seconds, the flow rate dropped to 0. A high-frequency pressure monitor was used at the wellhead to measure the pressure and flow data at the wellhead. The geomechanical parameters of the 5 wells are shown in Table 2, where σ hmax is the maximum horizontal principal stress, σ hmin is the minimum horizontal principal stress, σ H As for the vertical principal stress, it can be seen that the Young's modulus of the reservoir rock in Well 3 is the smallest, and the minimum horizontal principal stress is also small, so the required fracturing is the smallest among the 5 wells. The original geostress distribution of the reservoir in Well 1 is the largest among the 5 wells, so the required fracturing is also the largest.

[0136] Table 2

[0137]

[0138] Using the pump-off water hammer effect model proposed in the present invention, pump-off water hammer simulation is performed on all fracturing sections, and the simulated pump-off water hammer duration is 320 seconds. The boundary index and attenuation coefficient of all fracturing sections are calculated based on the simulated water hammer curve. The calculation results are distributed as follows: Fig.11 As shown, Fig.11 The water hammer boundary index and attenuation coefficient distribution diagram of different fracturing stages of 5 wells in the study area in the embodiment of the present invention are shown in Figure 2. The calculated attenuation coefficient and boundary index are projected onto the dual parameter evaluation chart to obtain the following: Fig.12 The evaluation results shown are Fig.12 It is a schematic diagram of the result of projecting the boundary index and attenuation coefficient onto the evaluation chart of the embodiment of the present invention. It can be seen from the evaluation chart that the boundary index and attenuation coefficient corresponding to the fracturing section of wells 1 and 2 are smaller than those of wells 3 / 4 / 5, and the overall fracturing effect is poor.

[0139] Through the field construction data, it is known that the natural fracture azimuth of Well 1 and Well 2 in the block is small, which is easy to form pressure channeling phenomenon. The fracturing is mainly based on the dominant fracture, and the fracturing transformation effect is relatively poor compared with Wells 3 / 4 / 5. This is consistent with the conclusion obtained from the water hammer effect simulation analysis in this paper, thus verifying the effectiveness and accuracy of the water hammer model. At the same time, the water hammer effect model proposed in this paper only needs to use the wellhead pressure monitoring data to obtain the evaluation of the fracturing construction effect. It is simple to implement and has low cost, providing a new way for on-site fracturing diagnosis.

[0140] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for evaluating cracks during pump stop pressure drop, characterized in that: The following steps are involved: S1. Obtain the ground stress magnitude, natural fracture distribution and fracturing construction parameters in the study area, and establish a fracture network model; S2. Calculate the fracture and expansion of fractures based on the fracture network model, obtain the fracture network morphology, establish a wellbore-fracture water hammer model according to the fracture network morphology, and use the characteristic line method to perform discrete solution on the wellbore-fracture water hammer model; S3, obtaining a simulated water hammer effect curve based on the solved wellbore-fracture water hammer model and the fracture network model; S4, comparing the simulated water hammer effect curve with the actual wellhead pump stop pressure drop curve, if the error e between the two is greater than the set threshold, adjusting the fracture network model and the wellbore-fracture water hammer model until the error e is no greater than the set threshold; S5. Based on the fracture network model, two evaluation parameters, fracture length index and branch index, are introduced to establish a dual-parameter evaluation chart based on the fracture network geometry. Based on the wellbore-fracture water hammer model, two evaluation parameters, boundary index and attenuation coefficient, are introduced to establish a dual-parameter evaluation chart based on the water hammer effect. The four evaluation parameters are calculated to obtain the fracture evaluation results.

2. A method for evaluating cracks during pump stop pressure drop according to claim 1, characterized in that: The fracture network model includes: linear elastic fracture model, hydraulic fracture and natural fracture intersection model; The equilibrium equations and boundary conditions of the linear elastic crack model are as follows: in, is the gradient operator, σ is the stress tensor corresponding to the displacement field u, f is the given volume force, Ω is the calculation area, n represents the unit vector of the boundary normal, g represents the surface force, Γ g represents the boundary where the surface force is applied, p represents the internal pressure on the crack surface, Γ c represents edge crack, u0 represents fixed displacement, Γ u represents the boundary to which a fixed displacement is applied, λ represents the first parameter of the Lame constant, μ represents the second parameter of the Lame constant, trace is the trace of the tensor, I is the identity matrix, and ε represents the strain tensor; In the intersection model of hydraulic fractures and natural fractures, the stress state at the intersection of hydraulic fractures and natural fractures includes: Stop expansion: p1+σ n <T coh ,σ1 <T rock ; Passing through natural fractures: p1+σ n <T coh ,σ1≥T rock ; Opening and penetrating natural fractures: σ1 ≥ T rock ; Opening of natural fractures: σ1 <T rock ; Where p1 is the water pressure in the crack, σ n represents the normal stress, T coh represents the surface strength of the natural crack, σ1 represents the maximum principal stress, T rock Represents the tensile strength of rock.

3. A method for evaluating cracks during pump-off pressure drop according to claim 1, characterized in that: The crack rupture satisfies the maximum circumferential stress criterion, which is expressed as: Where θ is the crack deflection angle, K I and K II is the first and second type stress intensity factor, K IC Represents the fracture toughness of rock.

4. A method for evaluating cracks during pump-off pressure drop according to claim 1, characterized in that: In the wellbore-fracture water hammer model, transient flow theory is used to describe the propagation of pressure waves generated by water hammer in the wellbore and fractures, and the control equation is obtained: Where p is the fluid pressure, ρ is the fluid density, a is the pressure wave velocity, V is the fluid velocity, |V| represents the absolute value of the velocity, t represents the time, x represents the one-dimensional coordinate along the pipeline, g is the gravitational acceleration, θ represents the angle between the pipeline and the horizontal plane, f represents the pipeline friction coefficient, and D is the pipeline diameter.

5. A method for evaluating cracks during pump-off pressure drop according to claim 1, characterized in that: In the wellbore-fracture water hammer model, the flow rate at the i-th outlet of the fracture is obtained according to the Bernoulli principle: Among them, Q i represents the flow rate at the i-th outlet of the fracture, Q0 represents the flow rate in the wellbore, and d i and l i represent the pipe diameter and length at the i-th crack outlet, f represents the pipe friction coefficient, and f j , l ij , d j represent the friction coefficient, pipe length and pipe diameter of the jth pipe on the i-th pipe flow path respectively.

6. A method for evaluating cracks during pump-off pressure drop according to claim 1, characterized in that: The boundaries of the wellbore-fracture water hammer model include: the flow boundary at the wellhead, the closed boundary at the bottom of the well, and the pressure boundary at the fracture front; The flow boundary at the wellhead position satisfies: Q0(t) = Q0f(t), where Q0(t) represents the flow at the wellhead position at time t, Q0 is the wellhead flow at the initial moment of pump shutdown, and f(t) represents a function determined by the flow change process monitored at the wellhead, satisfying f(t)∈[0,1]; In the closed boundary at the bottom of the well, the flow rate is always 0 and the pressure is unknown; In the pressure boundary at the fracture front, the pressure is known but the flow rate is unknown.

7. A method for evaluating cracks during pump-off pressure drop according to claim 4, characterized in that: The characteristic line method is used to discretize the wellbore-fracture water hammer model, specifically: The water head height H and pipe flow rate Q are used to describe the transient flow in the pipe: Among them, H represents the water head height, p represents the fluid pressure, ρ represents the fluid density, g represents the acceleration of gravity, z represents the pipeline height, Q represents the pipeline flow rate, V represents the fluid velocity, and A represents the pipeline cross-sectional area; The control equation is rewritten as follows: Among them, L1 and L2 are used to represent two formulas, t represents time, a represents the flow velocity in the pipeline, x is the one-dimensional coordinate along the pipeline, f is the friction coefficient, and D is the pipeline diameter; Expand the rewritten control equation along the characteristic line direction to obtain the control iterative solution format along the characteristic line: in, represents the water head height of the ith node at time t+Δt, Δt represents the time step interval, represents the flow of the i-th node at time t+Δt, represents the water head height of the i-1th node at time t, represents the flow of the i-1th node at time t, represents the water head height of the i+1th node at time t, represents the flow of the i+1th node at time t, C P , B M , C M , B P They represent the intermediate parameters of the calculation process, B = a / (gA), R = fΔx / (2gDA 2 ), Δx represents the grid size along the pipeline.

8. A method for evaluating cracks during pump-off pressure drop according to claim 1, characterized in that: The seam length index is defined as: Among them, f l is the seam length index; l total is the total length of the fracture trajectory; d is the characteristic length of the perforation area; The branch index is defined as: Among them, f n is the branch index; n b is the number of branch fractures with average fracture length; n perf is the number of perforation clusters; The boundary index is defined as: In the formula, f b represents the boundary index, a is the propagation speed of pressure fluctuation; T is the water hammer effect period; H is the depth of the perforation position; The attenuation coefficient α is defined as: p=p A e -αt , Where, p represents water hammer fracturing, p A represents the water hammer fracturing amplitude, e represents the natural index, t represents the time, and α represents the attenuation coefficient.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the method according to any one of claims 1 to 8 is implemented.

10. An electronic device, characterized in that: The method comprises a processor and a memory, wherein the processor and the memory are interconnected, wherein the memory is used to store a computer program, the computer program comprises computer-readable instructions, and the processor is configured to call the computer-readable instructions to execute the method according to any one of claims 1 to 8.

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