Sand adding concentration optimization method for acid fracturing and sand adding composite transformation process

Abstract

The invention discloses a sand adding concentration optimization method for an acid fracturing sand adding composite transformation process, and belongs to the technical field of oil and gas field development. The invention discloses a method. Comprising the following steps of (1) manufacturing an acid etching and sand adding composite fracture physical model, (2) testing seepage parameters of the acid fracturing and sand adding composite fracture, (3) establishing a permeability and composite fracture width prediction formula, drawing a relation curve of the permeability, the composite fracture width and the closing pressure, and obtaining an index relation expression of the permeability, the composite fracture width and the closing pressure under different sand adding concentrations; and (4) optimizing the sand adding concentration of the acid fracturing sand adding composite transformation process. The method considers the influence of the complex accumulation form of the proppant in the acid etching seam on the change rule of the seam width and the permeability in the production process, obtains the empirical relation of the permeability and the seam width along with the change of the sand adding concentration by fitting the relation curve of the permeability, the seam width and the closing pressure under different sand adding concentrations, and predicts the productivity of the process well. And the method has great significance for improving single-well yield.

Description

A method for optimizing the sand concentration in an acid fracturing and sand addition composite modification process Technical Field

[0001] This invention belongs to the field of oil and gas field development technology, and specifically relates to a method for optimizing the sand concentration in an acid fracturing and sand addition composite modification process. Background Technology

[0002] To meet the strategic needs of deep development of tight carbonate oil and gas reservoirs in China, various reservoir stimulation technologies and production enhancement methods have been explored and applied in the field. Among them, the acid fracturing combined with proppant fracturing technology has been successfully applied with significant production enhancement effects. This technology aims to first improve the matrix permeability of the near-wellbore zone and increase the effective stimulation volume by pre-applying acid, and then combine it with proppant fracturing technology to break through the acid etching limit distance, thereby improving the conductivity at the far end of the fracture while slowing down the long-term decline rate of conductivity.

[0003] Tight carbonate reservoirs exhibit strong heterogeneity. The discontinuous distribution of different minerals leads to greater non-uniformity in the etching of fracture walls by pre-fracturing acid, resulting in more complex proppant deposition within the fractures and making it difficult to achieve the densest packing found in conventional proppant-fracturing. Consequently, the prediction of the initial fracture width of the composite fracture is poor. Furthermore, the high tortuosity of acid-etched fractures results in more contact points and a larger contact area between the fracture and proppant. Under closure stress, the embedding, deformation, and breakage of the proppant cause more complex fracture width variations, increasing the difficulty of predicting post-fracturing productivity decline. Therefore, based on the heterogeneous characteristics of carbonate reservoirs, it is necessary to clarify the impact of proppant concentration on the width and permeability decline of composite fractures under different closure pressures, and to optimize post-fracturing productivity prediction and proppant concentration based on the analysis results. This necessitates establishing a method for optimizing proppant concentration in acid fracturing and proppant-fracturing composite stimulation processes to guide the production enhancement of carbonate reservoirs using this method.

[0004] The existing methods for optimizing the sand concentration in acid fracturing and sand addition composite stimulation processes are usually based on empirical design of sand concentration parameters in test wells, construction pressure, and ease of sand addition. Without conducting simulation and experimental research on sand concentration in test wells, problems such as unreasonable sand concentration design often lead to low construction success rates. Summary of the Invention

[0005] The purpose of this invention is to provide a sand concentration for an acid fracturing and sand addition composite modification process, in order to solve the problem of unreasonable sand concentration parameters designed based on construction experience in existing technologies.

[0006] To achieve the above objectives, the present invention employs the following technical solution: The present invention discloses a method for optimizing the sand concentration in an acid-etched sand-added composite modification process, comprising the following steps: fabricating a physical model of an acid-etched sand-added composite crack; obtaining seepage parameters of the acid-etched sand-added composite crack based on the physical model; establishing permeability prediction formulas and composite crack width prediction formulas based on the seepage parameters; and optimizing the sand concentration in the acid-etched sand-added composite modification process by combining the permeability prediction formulas and composite crack width prediction formulas.

[0007] Furthermore, the physical model of the acid-etched and sand-injected composite fracture includes the following steps: S1: Based on the block geological data, well logging data, and previous fracturing construction data, calculate the acid volume and injection rate of the acid etching on the rock plate using the Reynolds similarity criterion; S2: Design the experimental parameters for the acid-fracturing and sand-injected composite flow conduction test; S3: Subsequently, take a full-diameter core of the target layer to conduct the acid-fracturing and sand-injected composite flow conduction test, and complete the preparation of the physical model of the acid-etched and sand-injected composite fracture.

[0008] Furthermore, in S1, the calculation of the acid volume and acid injection rate for acid etching of the rock slab is performed using the following formula: ;in, For indoor simulation experiments, the acid discharge rate is m 3 / min; For engineering-scale acid injection discharge, m 3 / min; Width of the rock slab, in meters; The height of the hydraulic fracture in the reservoir is given in meters (m). The width of the hydraulic fracture in the reservoir is given in meters (m). The crack width formed in the experimental rock slab is in meters (m); it is generally taken as 1 mm; N is the power law exponent, dimensionless, and is taken as 1.446. ;in, The volume of acid injected for the indoor simulation experiment is m. 3 ; The volume of acid injected is measured in meters (m). 3 In S2, the experimental parameters for acid fracturing and sand diversion include acid type, acid volume, acid concentration, acid injection rate, reservoir temperature, proppant type, proppant particle size, and closure pressure.

[0009] Further, the specific steps for obtaining the seepage parameters of the acid-etched sand-coated composite fracture based on the physical model of the acid-etched sand-coated composite fracture are as follows: S11: Place the physical model of the acid-etched sand-coated composite fracture into the flow chamber and set the experimental closure pressure; S22: Then inject water and test the permeability under the closure pressure for 50 hours, collecting permeability data every hour; S33: Then perform in-situ CT scanning under the same closure pressure conditions, and combine it with digital image processing technology to construct a 3D model of the composite fracture; place the 3D model of the composite fracture in the XYZ three-dimensional coordinate system, and divide it into N along the X-axis and Y-axis directions respectively. I Segment and N J The segment, containing two crack surfaces, is also divided into N sections. I ×N J Each crack surface element; then extracted at the same coordinate (X I ,Y J The heights of the two crack surface elements at point X are represented as a set, denoted as Z(X). I ,Y J Then, based on the positions above and below the crack surface, a set Z is constructed. u (X I ,Y J ) and Z d (X I ,Y J ), and calculate different coordinates (X) i ,Y j At point X, the height difference H(X) between the upper and lower crack surface elements. i ,Y j ),Right now: Calculate and record the crack width of the composite crack model, which is the height difference H(X) at different locations on the crack surface. i ,Y j The average value of ) is expressed as: ;in, This represents the crack width in the composite crack model.

[0010] Furthermore, the closing pressure is P B ;P B Set to 10 MPa; the injection water flow rate is 2 mL / min to 4 mL / min; put the acid-etched sand-coated composite crack physical model back into the flow chamber, increase the closing pressure in the flow chamber by 10 MPa, and repeat steps S22 and S33.

[0011] Furthermore, the specific steps for establishing permeability prediction formulas and composite fracture width prediction formulas based on the seepage parameters of acid-fusing sand-addition composite fractures are as follows: Based on the obtained permeability and fracture width, plot the relationship curves between permeability, fracture width, and closure pressure to obtain exponential relationships between permeability, fracture width, and closure pressure under different sand concentrations; statistically analyze the coefficients in the exponential relationships with the sand concentration data, fit the mathematical relationships, and if there is no obvious relationship between the data, take the average; substitute the relationships between each coefficient and the sand concentration into the exponential relationships to establish the permeability prediction formula. Composite seam width prediction formula .

[0012] Furthermore, the optimization of sand concentration in the acid fracturing and sand-addition composite modification process, combining the permeability prediction formula and the composite fracture width prediction formula, specifically includes the following steps: Based on the acid fracturing and sand-addition construction design scheme, the acid fracturing model is used to simulate and calculate the composite fracture half-length, composite fracture width distribution, composite fracture porosity distribution, composite fracture pressure distribution, matrix porosity distribution, matrix gas phase pressure distribution, and matrix gas phase saturation distribution parameters at the end of construction; the acid fracturing model uses the xy rectangular coordinate system to represent the reservoir length... and reservoir width Divided into Duan He Segment, formed The structured reservoir grid; the parameters calculated above are used as the initial values ​​for gas well production simulation, and the gas-water two-phase flow differential equation during the production process is: ; ; ; ; ; ; ; ;in, The unit conversion factor is 10. -3 ; For the closing pressure P B Absolute permeability of the composite fracture, mD; ρ represents the viscosity of the liquid phase in the reservoir matrix, in mPa·s; ρ represents the viscosity of the gas phase in the reservoir matrix, in mPa·s; is the volume factor of the liquid phase in the reservoir matrix, dimensionless; is the volume factor of the gas phase in the reservoir matrix, dimensionless; The pressure of the liquid phase in the reservoir matrix. ; This refers to the pressure of the gas phase in the reservoir matrix. ; The fluid pressure in the composite fracture. ; The relative permeability of the liquid phase in the composite fracture is dimensionless. The relative permeability of the gas phase in the composite fracture is dimensionless. For the source and sink terms of the liquid phase in the composite fracture, m 3 / s; For the source and sink terms of the gas phase in the composite fracture, m 3 / s; This refers to the liquid flow rate between the main fracture and the matrix during gas well production, expressed in m. 3 / s; This refers to the gas cross-flow rate between the main fracture and the matrix during gas well production, expressed in m. 3 / s; The liquid saturation in the composite crack is dimensionless. The gas saturation in the composite fracture is dimensionless. The time for gas well production, in seconds; When there are composite cracks penetrating the matrix mesh, When no composite cracks penetrate the matrix mesh, ; For the volume of a composite crack element, m 3 ; The length of the composite crack element is in meters (m). The effective closing stress within the composite crack is expressed in MPa. For the closing pressure P B Width of the lower composite crack, in meters; Let be the minimum horizontal principal stress of the reservoir, MPa; the initial conditions for the model solution are:

[0013] in, The initial pressure distribution of composite fractures in gas well production simulation. ; is the original formation pressure of the gas reservoir, MPa; i,j are the position coordinates of the grid, dimensionless; ;in, The initial gas phase saturation of the composite fracture in gas well production simulation is dimensionless. The initial liquid saturation of the composite fracture in the gas well production simulation is dimensionless; the inner boundary conditions are: ;in, , represents the x and y coordinates of the grid cell containing the gas well, in meters (m). For production time is Bottom-hole flowing pressure The outer boundary conditions are: ;in, The length of the target reservoir is in meters (m). The width of the target reservoir, in meters; the initial production time, i.e. At that time, the initial values ​​of the model are substituted into the calculation to obtain a time step. Fluid pressure inside the suture after completion Thus, the closing stress in the composite crack at this time can be calculated. ; then Substitute into the penetration rate prediction formula Composite seam width prediction formula In the middle, the next time step, i.e., production, can be obtained. The input values ​​for the fracture width and permeability at any given time are used to calculate the value for any production time t. s Porosity of each composite fracture unit in the lower reservoir Gas saturation of each composite fracture element Porosity of each matrix grid and the gas phase saturation of each matrix grid Finally, an evaluation of the production capacity effect is conducted.

[0014] Furthermore, the step of evaluating production capacity effectiveness is as follows: based on production time t m The porosity and gas saturation of each composite fracture element and the porosity and gas saturation of each matrix mesh were calculated to obtain the production time t. s Cumulative gas well production Q s : ; ;in, ; ; The cumulative production of the gas well at any given time, m 3 ; Let m be the cumulative production of the gas well in the m-th time step after production begins. 3 ; For a production time of t s The number of time steps, dimensionless; , This represents the total number of grid cells in the structured reservoir grid along both the x and y directions. The total number of acid-etched crack units at the end of the acid-fusing sand composite renovation construction is dimensionless. , Let i be the length and width of the matrix mesh at positions i,j, in meters. Gas well production time Porosity of the matrix mesh at positions i,j, dimensionless; Gas well production time Porosity of the Lth acid-etched crack unit, dimensionless; Gas well production time Liquid saturation of the matrix mesh at positions i,j, dimensionless; Gas well production time The liquid phase saturation of the Lth acid-etched crack unit is dimensionless.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a method for optimizing the sand concentration in the acid-curing sand-addition composite modification process. It considers the influence of the complex stacking form of the proppant in the acid-etched joint on the changes in joint width and permeability during the production process. By fitting the relationship curves of permeability, joint width and closing pressure under different sand concentrations, the empirical relationship between permeability and joint width and sand concentration is obtained, which provides a basis for subsequent post-curing capacity prediction and sand concentration optimization, and makes up for the shortcomings of the prior art.

[0016] Furthermore, the method of the present invention optimizes the sand concentration for construction based on the variation law of seepage parameters of acid-etched sand-added composite cracks under different closure stresses. This can effectively ensure the production increase effect of acid-fed sand-added composite modification process while optimizing construction costs and reducing construction difficulty. In addition, based on experimental methods, a permeability and crack width prediction formula is established and combined with the existing production capacity prediction model, which can effectively reduce the error of simulation results and further improve the credibility of the construction parameter optimization results, which has certain reference significance for on-site construction. Attached Figure Description

[0017] Figure 1 is a schematic diagram of the 3D model of the composite fracture; Figure 2 is the fitting curve of the composite fracture permeability; Figure 3 is the fitting curve of the composite fracture width; Figure 4 is the cumulative production curve of well A under different sand concentrations; Figure 5 is a comparison of the actual production and simulated production effects of well A. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0019] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0020] This invention discloses a method for optimizing the sand concentration in an acid fracturing and sand-addition composite fracturing process, comprising the following steps: (1) Creating a physical model of the acid-etching and sand-addition composite fracture ① Based on the geological data of the block, well logging data, and previous fracturing construction data, the acid volume and injection rate for acid etching of the rock plate are calculated using the Reynolds similarity criterion: (1); where, —Indoor simulation experiment acid injection discharge rate, m 3 / min; —Engineering-scale acid injection discharge, m 3 / min; — Width of the slab, in meters; —Reservoir hydraulic fracture height, m; — Width of hydraulic fracture in reservoir, in meters; — Crack width formed in the experimental rock slab, m; generally taken as 1 mm; n — Power law exponent, dimensionless, generally taken as 1.446; (2); where, —Acid injection volume in indoor simulation experiment, m 3 ; —Acid injection volume (m) for engineering purposes 3 ② Design an acid fracturing and proppant-assisted flow experimental scheme. Experimental parameters include acid type, acid volume, acid concentration, injection / discharge rate, reservoir temperature, proppant type, proppant particle size, and closure pressure. Among these, there are n horizontal values ​​for proppant concentration, with the maximum proppant concentration being the limit of the on-site equipment. The remaining parameters are consistent with the fracturing construction design scheme. ③ Take a full-diameter core from the target formation, process 2n rock plates, and group them in pairs, numbered sequentially as S... 1-1 S 1-2 ... S n-1 S n-2The dimensions of the rock slab are: length 17.74cm ± 0.04cm, width 3.76cm ± 0.05cm, and thickness > 0.9cm; ④ Acid fracturing and sand-based composite flow-guiding experiment was conducted using a flow-guiding capacity testing system: first, the flow-guiding chamber was cleaned and the rock slab S was installed and fixed. i-1 and S i-2 (i=1, 2, ..., n), place a 1mm thick stainless steel gasket between the ends of each of the two types of rock slabs; then pressurize the test system to 3MPa, pump in clean water to check for leaks, and turn on the system heating device. Set the system temperature according to the experimental plan and prepare the required acid solution. After the system temperature reaches the set temperature, inject acid according to the experimental discharge rate and acid volume to etch the rock slabs; after the acid injection is completed, switch the flow valve to a clean water flow, turn on the pump to replace the remaining acid in the flow, and stop the pump when clean water is discharged from the outlet. Disassemble the flow chamber, remove the rock slab, and rinse the flow chamber with clean water; finally, dry the rock slabs, apply sand based on the proppant specified in the experimental plan, and seal along the long side of the rock slab to prevent proppant leakage during the flow capacity test. This completes the physical model of the acid-etched and sand-coated composite crack model S. i Preparation of .

[0021] (2) Test the seepage parameters of the acid-etched sand-coated composite fracture based on the physical model of the acid-etched sand-coated composite fracture ① Place the physical model of the acid-etched sand-coated composite fracture into the flow chamber and set the experimental closure pressure P B ① The pressure is 10 MPa; ② Inject clean water at a flow rate of 2 mL / min to 4 mL / min and test its pressure at the closing pressure P. B The permeability was measured over a period of 50 hours, with permeability data collected every hour. After the permeability data collection was completed, the composite fracture model was removed. First, an in-situ CT scan was performed under the same closure pressure conditions. Then, combined with digital image processing technology, a 3D model of the composite fracture was constructed. Next, the 3D model of the composite fracture was placed in the XYZ three-dimensional coordinate system, and its dimensions were divided into N sections along the X and Y axes. I Segment and N J The model, containing two crack surfaces, is also divided into N segments. I ×N J Each crack surface element; then extract the elements at the same coordinate (X). I ,Y J The heights of the two crack surface elements at point X are represented as a set, denoted as Z(X). I ,Y J Then, based on the positions above and below the crack surface, a set Z is constructed. u (X I ,Y J ) and Z d (X I ,Y J ), and calculate different coordinates (X)i ,Y j At point X, the height difference H(X) between the upper and lower crack surface elements. i ,Y j ),Right now: (3); Finally, calculate and record the crack width of the composite crack model. That is, the height difference H(X) at different locations on the crack surface. i ,Y j The average value of ) is expressed as: (4); ④ The composite crack model is placed back into the flow chamber, and the closing pressure inside the flow chamber is increased by 10 MPa, i.e., P s =P s +10MPa, repeat steps ② and ③; (3) Based on the obtained permeability and crack width data, establish a prediction formula for the composite crack width of permeability and crack width ① Plot the relationship curve between permeability, composite crack width and closure pressure, and obtain the exponential relationship between permeability, composite crack width and closure pressure under different sand concentrations; ② Statistically analyze the coefficients in the exponential relationship and the sand concentration data, fit the mathematical relationship, and if there is no obvious relationship between the data, take the average; ③ Substitute the relationship between each coefficient and the sand concentration into the exponential formula to establish a permeability prediction formula. Composite seam width prediction formula (4) Combining the permeability prediction formula and the composite fracture width prediction formula, the sand concentration of the acid fracturing and sand addition composite modification process is optimized. ① According to the acid fracturing and sand addition construction design scheme, the acid fracturing model is used to simulate and calculate the seepage-related parameters such as the half-length of the composite fracture, the width distribution of the composite fracture, the porosity distribution within the composite fracture, the pressure distribution within the composite fracture, the matrix porosity distribution, the matrix gas phase pressure distribution, and the matrix gas phase saturation distribution at the end of the construction. The model calculates the reservoir length in the xy rectangular coordinate system. and reservoir width Divided into Duan He Segment, formed The structured reservoir grid is used. Simultaneously, an embedded discrete model is employed to handle the flow exchange between the composite fractures and the matrix grid, where the composite fractures are segmented by the structured reservoir grid. There are three composite crack elements, and the crack element number is... The length of each composite crack element is ② Using the calculation results from the previous step as the initial values ​​for the gas well production simulation, the differential equation for the gas-water two-phase flow during the production process is: (5); (6); (7); (8); (9); (10); (11); (12); where: —Unit conversion factor, 10 -3 ; — Closing pressure P B Absolute permeability of the composite fracture, mD; — Viscosity of the liquid phase in the reservoir matrix, mPa·s.

[0022] — Viscosity of the gas phase in the reservoir matrix, mPa·s.

[0023] —The volume factor of the liquid phase in the reservoir matrix, dimensionless; —Volume factor of the gas phase in the reservoir matrix, dimensionless; —The pressure of the liquid phase in the reservoir matrix, ; —Pressure of the gas phase in the reservoir matrix, ; —Fluid pressure in composite fractures, ; —Relative permeability of the liquid phase in the composite fracture, dimensionless; —Relative permeability of the gas phase in the composite fracture, dimensionless; —Source and sink terms of liquid phase in composite fractures, m 3 / s; —Source and sink terms of the gas phase in composite fractures, m 3 / s; —Liquid flow rate between the main fracture and the matrix during gas well production, in m³ 3 / s; —Gas flow rate between the main fracture and the matrix during gas well production, in m³ 3 / s; —Liquid saturation in composite cracks, dimensionless; —Gas saturation in composite cracks, dimensionless; —The time it takes for a gas well to produce gas, in seconds; —When composite cracks penetrate the matrix mesh, When no composite cracks penetrate the matrix mesh, ; —Volume of a composite crack element, m 3 ; — Length of composite crack element, m; —Effective closing stress within the composite crack, MPa; — Closing pressure P B Width of the lower composite crack, in meters; —Minimum horizontal principal stress of the reservoir, MPa; the initial conditions for model solution are: (13); where: —Initial pressure distribution of composite fractures in gas well production simulation ; —Original formation pressure of the gas reservoir, MPa; i,j —Grid position coordinates, dimensionless; (14); where: —Initial gas phase saturation of composite fractures in gas well production simulation, dimensionless; —Initial liquid saturation of composite fractures in gas well production simulation, dimensionless; internal boundary conditions are: (15); where, , —The horizontal and vertical coordinates of the grid cell containing the gas well, in meters; Production time is Bottom-hole flowing pressure The outer boundary conditions are: (16); where, — Length of the target reservoir, in meters; — Width of the target reservoir, in meters; the initial moment of production. When the initial values ​​of the model are substituted into the calculation, a time step can be obtained. Fluid pressure inside the suture after completion Then, the closing stress in the composite crack at this time is calculated using formula (12). Then The permeability prediction formula obtained by substituting the experimental results Seam width prediction formula In the middle, the next time step, i.e., production, can be obtained. The input values ​​are the fracture width and permeability of the composite fracture at any given time. Similarly, the values ​​for any production time t can be calculated. s Porosity of each composite fracture unit in the lower reservoir Gas saturation of each composite fracture element Porosity of each matrix grid and the gas phase saturation of each matrix grid ③ The capacity efficiency evaluation will include production time t m Substituting the porosity and gas saturation of each composite fracture element and each matrix mesh into equations (17) to (20), the production time t can be calculated. s Cumulative gas well production Q s : (17); (18); where: (19); (20); where: —Cumulative gas well production at any given time, in m 3 ; —The cumulative production of the gas well within the m-th time step after production begins, m 3 ; —Production time is t s The number of time steps, dimensionless; , —The total number of grid cells in the structured reservoir grid along the x and y directions; —The total number of acid-etched crack units at the end of the acid-fusing sand composite renovation construction, dimensionless; , —The length and width of the matrix mesh at positions i,j, in meters; —Time from gas well production Porosity of the matrix mesh at positions i,j, dimensionless; —Time from gas well production Porosity of the Lth acid-etched crack unit, dimensionless; —Time from gas well production Liquid saturation of the matrix mesh at positions i,j, dimensionless; —Time from gas well production The liquid phase saturation of the Lth acid-etched crack unit is dimensionless.

[0024] Plot the cumulative production curves of gas wells under different sand addition concentrations, compare and evaluate the production enhancement effect, and optimize the sand addition concentration for acid fracturing and sand addition composite stimulation based on construction cost and construction difficulty.

[0025] Example 1: The target gas reservoir has a burial depth of 2865m~3018m, a temperature of 89~101℃, and a formation pressure coefficient of 0.92. The main development interval is 3012.0-3018m. Electrical logging interpretation results show an apparent porosity of 1.32%, an apparent gas saturation of 43.19%, and a density of 2.69 g / cm³. 3 Well A is a vertical well with a length of 3435m. The proposed combined acid fracturing and proppant fracturing system will utilize Φ139.7mm casing and Φ60.3mm thickened 80S tubing for annular injection. In the fracturing design, the hydrochloric acid concentration in the single-phase acid system is 20%, and the pre-flush acid volume is 50m³. 3 The construction discharge volume is 2-3m 3 / min; the proppant-carrying fluid used is a guar gum fracturing fluid system with a base fluid viscosity greater than 45 mPa·s, for a total of 400 m 3 The construction discharge volume is 6-8m³. 3 / min; proppant is 20.5m of 40-70 mesh ceramsite. 3+70-140 mesh ceramsite 21.3m 3 The designed sand concentration is 5 kg / m³. 2 .

[0026] First, take a full-diameter core sample from the 3012.0-3018m well section, process 8 rock plates, group them in pairs, and number them sequentially as S. 1-1 S 1-2 S 2-1 S 2-2 S 3-1 S 3-2 S 4-1 S 4-2 Based on the block's geological data, well logging data, and previous fracturing data, the acid volume and injection rate for acid etching of the rock slab were calculated using the Reynolds similarity criterion, resulting in values ​​of 0.5 L and 20-30 mL / min, respectively. Experimental parameters were then set to etch the rock slab, and after etching, a sand concentration of 4 kg / m³ was added. 2 5kg / m 2 6kg / m 2 7kg / m 2 Composite fracture models S1, S2, S3, and S4 were prepared, and the permeability of each composite fracture under different closure pressures was tested and recorded using a conductivity testing system. After data acquisition, the composite crack model was retrieved and subjected to in-situ CT scanning under the same closure pressure conditions. A 3D model of the composite crack was then constructed using digital image processing technology, as shown in Figure 1. The 3D model of the composite crack was then placed in the XYZ three-dimensional coordinate system to calculate and record the crack width. The crack width of each composite crack model under different closure pressures was statistically analyzed. and penetration rate As shown in Table 1.

[0027] Table 1. Permeability and fracture width of various composite fracture models under different closure pressures.

[0028] Then, the relationship curves between permeability, composite joint width, and closure pressure were plotted, as shown in Figures 2 and 3. An exponential relationship between permeability, composite joint width, and closure pressure under different sand concentrations was fitted to establish a permeability prediction formula. Composite seam width prediction formula Finally, the acid fracturing model was used to simulate the seepage-related parameters at the end of construction, including the half-length of the composite fracture, the width distribution of the composite fracture, the porosity distribution within the composite fracture, the pressure distribution within the composite fracture, the matrix porosity distribution, the matrix gas phase pressure distribution, and the matrix gas phase saturation distribution. These parameters were used as the initial conditions for the production simulation of Well A. Combining the permeability and fracture width prediction formulas, the porosity and gas phase saturation parameters of each composite fracture unit and each matrix unit at different production times were iteratively calculated using the finite difference method. Then, the cumulative production of Well A under different sand addition concentrations was calculated, and the production prediction curve of Well A was plotted, as shown in Figure 4. Figure 4 shows that the sand addition concentration increased from 2 kg / m³... 2 Increased to 6kg / m 2 The cumulative output shows an upward trend because, as the closing stress continues to rise, the support points of the acid-etched trenches break, and the conductivity of the composite cracks is positively correlated with the width of the ceramic aggregate support joints; when the sand concentration is 8 kg / m³ 2 Although the width of the composite fracture was increased, the proportion of surface-to-point contact mode provided by the acid-etched wall decreased, while the proportion of point-to-point contact mode of the proppant increased. This led to increased proppant breakage under medium and high closure stress, resulting in greater damage to the permeability of the composite fracture. This manifested as a higher initial cumulative production value, but the growth rate of cumulative gas production after 720 days of production was related to the sand concentration of 6 kg / m³. 2 The corresponding cumulative gas production growth rates are almost the same.

[0029] From the perspective of reducing construction costs and difficulty, 6kg / m² was selected. 2 Figure 5 shows a comparison of the actual and simulated production effects of Well A to determine the optimal sand concentration for construction. The initial daily gas production of Well A after compression was 3.46 × 10⁻⁶. 4 m 3 / d, the average error between the cumulative yield predicted by the conventional model and the actual cumulative yield over 720 days is 16.15%, while the average error between the cumulative yield simulated by the formula combining the slit width and permeability and the actual cumulative yield is 6.03%, indicating a significant improvement in the model's prediction accuracy.

[0030] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process, characterized in that, Includes the following steps: A physical model of the acid-etched and sand-added composite crack was constructed; the seepage parameters of the acid-pressurized and sand-added composite crack were obtained by testing the physical model; permeability prediction formula and composite crack width prediction formula were established based on the seepage parameters; and the sand concentration of the acid-pressurized and sand-added composite modification process was optimized by combining the permeability prediction formula and the composite crack width prediction formula.

2. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 1, characterized in that, The steps for creating a physical model of an acid-etched and sand-coated composite crack are as follows: S1: Based on the block's geological data, well logging data, and previous fracturing construction data, calculate the acid volume and injection rate for acid etching of the rock plate using the Reynolds similarity criterion; S2: Design the experimental parameters for acid fracturing combined with sand control; S3: Subsequently, take full-diameter core samples from the target stratum to conduct acid fracturing combined with sand control experiments, and complete the preparation of the physical model of the acid etching combined with sand fracture.

3. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 2, characterized in that, In S1, the calculation of the acid volume and acid injection rate for acid etching of the rock slab is performed using the following formula: ;in, For indoor simulation experiments, the acid discharge rate is m 3 / min; For engineering-scale acid injection discharge, m 3 / min; Width of the rock slab, in meters; The height of the hydraulic fracture in the reservoir is given in meters (m). The width of the hydraulic fracture in the reservoir is given in meters (m). The crack width formed in the experimental rock slab is in meters (m); it is generally taken as 1 mm; N is the power law exponent, dimensionless, and is taken as 1.

446. ;in, The volume of acid injected for the indoor simulation experiment is m. 3 ; The volume of acid injected is measured in meters (m). 3 In S2, the experimental parameters for acid fracturing and sand diversion include acid type, acid volume, acid concentration, acid injection rate, reservoir temperature, proppant type, proppant particle size, and closure pressure.

4. The method for optimizing the sand concentration in the acid fracturing and sand addition composite modification process according to claim 1, characterized in that, The specific steps for obtaining the seepage parameters of the acid-etched sand-coated composite fracture based on the physical model of the acid-etched sand-coated composite fracture are as follows: S11: Place the physical model of the acid-etched sand-coated composite fracture into the flow chamber and set the experimental closure pressure; S22: Then inject water and test the permeability under the closure pressure for 50 hours, collecting permeability data every hour; S33: Then perform in-situ CT scanning under the same closure pressure conditions, and combine it with digital image processing technology to construct a 3D model of the composite fracture; Place the 3D model of the composite fracture in the XYZ three-dimensional coordinate system, and divide it into N along the X-axis and Y-axis directions respectively. I Segment and N J The segment, containing two crack surfaces, is also divided into N sections. I ×N J Each crack surface element; then extracted at the same coordinate (X I ,Y J The heights of the two crack surface elements at point X are represented as a set, denoted as Z(X). I ,Y J Then, based on the positions above and below the crack surface, a set Z is constructed. u (X I ,Y J ) and Z d (X I ,Y J ), and calculate different coordinates (X) i ,Y j At point X, the height difference H(X) between the upper and lower crack surface elements. i ,Y j ),Right now: Calculate and record the crack width of the composite crack model, which is the height difference H(X) at different locations on the crack surface. i ,Y j The average value of ) is expressed as: ;in, This represents the crack width in the composite crack model.

5. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 4, characterized in that, The closing pressure is P. B ;P B The pressure is set to 10 MPa; the flow rate of the injected water is 2 mL / min to 4 mL / min; after the steps described in claim 5 are completed, the physical model of the acid-etched and sand-added composite crack is placed back into the flow chamber, the closing pressure in the flow chamber is increased by 10 MPa, and steps S22 and S33 are repeated.

6. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 5, characterized in that, The specific steps for establishing permeability prediction formulas and composite fracture width prediction formulas based on the seepage parameters of acid-fracturing and sand-added composite fractures are as follows: Based on the obtained permeability and fracture width, plot the relationship curves between permeability, fracture width, and closure pressure to obtain exponential relationships between permeability, fracture width, and closure pressure under different sand concentrations; statistically analyze the coefficients in the exponential relationships with the sand concentration data, fit the mathematical relationships, and if there is no obvious relationship between the data, take the average; substitute the relationships between each coefficient and the sand concentration into the exponential relationships to establish the permeability prediction formula. Composite seam width prediction formula 。 7. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 6, characterized in that, The optimization of sand concentration in acid fracturing and sand addition composite modification process, combining permeability prediction formulas and composite fracture width prediction formulas, specifically includes the following steps: Based on the acid fracturing and sand addition construction design scheme, the acid fracturing model is used to simulate and calculate the composite fracture half-length, composite fracture width distribution, composite fracture porosity distribution, composite fracture pressure distribution, matrix porosity distribution, matrix gas phase pressure distribution, and matrix gas phase saturation distribution parameters at the end of construction; the acid fracturing model uses the xy rectangular coordinate system to represent the reservoir length... and reservoir width Divided into Duan He Segment, formed The structured reservoir grid; the parameters calculated above are used as the initial values ​​for gas well production simulation, and the gas-water two-phase flow differential equation during the production process is: ； ； ； ； ； ； ； ;in, The unit conversion factor is 10. -3 ; For the closing pressure P B Absolute permeability of the composite fracture, mD; ρ represents the viscosity of the liquid phase in the reservoir matrix, in mPa·s; ρ represents the viscosity of the gas phase in the reservoir matrix, in mPa·s; is the volume factor of the liquid phase in the reservoir matrix, dimensionless; is the volume factor of the gas phase in the reservoir matrix, dimensionless; The pressure of the liquid phase in the reservoir matrix. ； This refers to the pressure of the gas phase in the reservoir matrix. ； The fluid pressure in the composite fracture. ； The relative permeability of the liquid phase in the composite fracture is dimensionless. The relative permeability of the gas phase in the composite fracture is dimensionless. For the source and sink terms of the liquid phase in the composite fracture, m 3 / s; For the source and sink terms of the gas phase in the composite fracture, m 3 / s; This refers to the liquid flow rate between the main fracture and the matrix during gas well production, expressed in m. 3 / s; This refers to the gas cross-flow rate between the main fracture and the matrix during gas well production, expressed in m. 3 / s; The liquid saturation in the composite crack is dimensionless. The gas saturation in the composite fracture is dimensionless. The time taken for the gas well to produce gas, in seconds; When composite cracks penetrate the matrix mesh, When no composite cracks penetrate the matrix mesh, ； For the volume of a composite crack element, m 3 ; The length of the composite crack element is in meters (m). The effective closing stress within the composite crack is expressed in MPa. For the closing pressure P B Width of the lower composite crack, in meters (m); The minimum horizontal principal stress of the reservoir is given in MPa; subsequently, Substitute into the penetration rate prediction formula Composite seam width prediction formula In the middle, the next time step, i.e., production, can be obtained. The input values ​​for the fracture width and permeability at any given time are used to calculate the value for any production time t. s Porosity of each composite fracture unit in the lower reservoir Gas saturation of each composite fracture element Porosity of each matrix grid and the gas phase saturation of each matrix grid Finally, an evaluation of the production capacity effect is conducted.

8. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 7, characterized in that, The initial conditions for solving the model are: ;in, The initial pressure distribution of composite fractures in gas well production simulation. ； is the original formation pressure of the gas reservoir, MPa; i,j are the position coordinates of the grid, dimensionless; ;in, The initial gas phase saturation of the composite fracture in gas well production simulation is dimensionless. The initial liquid phase saturation of the composite fracture in gas well production simulation is dimensionless.

9. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 8, characterized in that, The internal boundary conditions for solving the model are: ;in, 、 represents the x and y coordinates of the grid cell containing the gas well, in meters (m). For production time is Bottom-hole flowing pressure The outer boundary conditions are: ;in, Let the length of the target reservoir be m; The width of the target reservoir, in meters; the initial production time, i.e. At that time, the initial values ​​of the model are substituted into the calculation to obtain a time step. Fluid pressure inside the seam after completion Thus, the closing stress in the composite crack at this time can be calculated. 。 10. The method for optimizing the sand concentration in an acid fracturing and sand-addition composite modification process according to claim 9, characterized in that, The steps for evaluating production capacity effectiveness are as follows: based on production time t m The porosity and gas saturation of each composite fracture element and the porosity and gas saturation of each matrix mesh were calculated to obtain the production time t. s Cumulative gas well production Q s : ； ;in, ； ； The cumulative production of the gas well at any given time, m 3 ; Let m be the cumulative production of the gas well in the m-th time step after production begins. 3 ; For a production time of t s The number of time steps in time, dimensionless; 、 This represents the total number of grid cells in the structured reservoir grid along both the x and y directions. The total number of acid-etched crack units at the end of the acid-fusing sand composite renovation construction is dimensionless. 、 Let i be the length and width of the matrix mesh at positions i,j, in meters. Gas well production time Porosity of the matrix mesh at positions i,j, dimensionless; Gas well production time Porosity of the Lth acid-etched crack unit, dimensionless; Gas well production time Liquid saturation of the matrix mesh at positions i,j, dimensionless; Gas well production time The liquid phase saturation of the Lth acid-etched crack unit is dimensionless.

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