CO2 acid fracturing full-seam flow conductivity distribution calculation method

Abstract

The invention discloses a CO2 acid fracturing full-seam flow conductivity distribution calculation method which comprises the following steps: S1, collecting basic parameters including reservoir parameters, acid liquor parameters and construction parameters, carrying out a pure acid liquor etching rock plate experiment, and testing the acid rock reaction rate Rs of a pure acid liquor system; s2, carrying out an experiment of etching a rock plate after mixing supercritical CO2 with an acid solution, testing an acid rock reaction rate Rs'after adding CO2, and fitting a relational expression between Rs' and Rs; s3, establishing a CO2 and acid liquor mixed etching model, substituting Rs', solving the model by adopting a pressure-velocity coupling algorithm, and calculating the acid etching seam width distribution; and S4, constructing a flow conductivity model, and calculating CO2 acid fracturing full-seam flow conductivity distribution according to the acid etching seam width. Based on the supercritical CO2 acid etching experiment and multi-physics field coupling numerical simulation, the CO2 acid fracturing full-seam flow conductivity distribution calculation method is innovatively provided, and an effective means is provided for optimizing CO2 acid fracturing construction parameters.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction technology, and specifically relates to a method for calculating the conductivity distribution of CO2 acid fracturing full fracture. Background Technology

[0002] Acid fracturing is a key technology for the efficient development of tight carbonate oil and gas reservoirs. However, tight carbonate reservoirs suffer from poor physical properties, high temperatures, and insufficient formation pressure. Conventional acid fracturing results in rapid acid-rock reaction rates and insufficient acid etching depth, leading to poor production enhancement. CO2 acid fracturing, due to its ability to effectively reduce acid-rock reaction rates, enhance acid penetration, and improve fracture conductivity, has been applied in medium-deep carbonate reservoirs in the Middle East and tight carbonate reservoirs in the Ordos Basin of my country, achieving significant results in replenishing formation energy and increasing oil and gas production. The CO2 acid fracturing process typically involves simultaneously pumping liquid CO2 and acid into the wellbore at a certain flow rate using high-pressure and acid pumps. The two fluids begin to mix within the wellbore, reaching the CO2 critical condition before reaching the bottom of the well, forming a supercritical CO2 + acid multiphase fluid. This fluid then reacts slowly along hydraulic fractures, forming acid-etched fractures.

[0003] However, current research on CO2 acid fracturing process design and effect prediction still faces the following challenges: the synergistic etching mechanism of supercritical CO2 and acid under multiphase flow and mass transfer conditions is extremely complex. Existing acid-rock reaction kinetic models based on pure acid cannot accurately describe this process, leading to significant deviations in the prediction of fracture width distribution. Consequently, it is difficult to quantitatively assess the conductivity of fractures after acid fracturing and optimize construction parameters. Therefore, it is urgent to establish a calculation method that can reflect the synergistic etching effect of CO2 and acid and can be used to predict the conductivity distribution of the entire fracture under CO2 acid fracturing, providing a reliable theoretical basis and technical support for the optimized design of CO2 acid fracturing schemes. Summary of the Invention

[0004] To address the above problems, the technical solution of the present invention is as follows:

[0005] In a first aspect, the present invention provides a method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam, comprising the following steps:

[0006] S1: Collect basic parameters, including reservoir parameters, acid parameters, and construction parameters; conduct pure acid etching experiments on rock slabs to test the acid-rock reaction rate R of the pure acid system. s ;

[0007] S2: Conducting experiments to etch rock slabs after mixing supercritical CO2 with acid, and testing the acid-rock reaction rate R after adding CO2. s ’ Fitting R s ’ With Rs Relationships;

[0008] S3: Establish a CO2 and acid mixed etching model and substitute it into R. s ’ The pressure-velocity coupling algorithm was used to solve the model and calculate the acid etching crack width distribution;

[0009] S4: Construct a conductivity model and calculate the conductivity distribution of the entire CO2 acid-etched crack based on the crack width distribution.

[0010] Furthermore, step S1 includes:

[0011] S11: Collect reservoir parameters, acid parameters, and construction parameters. The reservoir parameters include reservoir temperature and formation pressure. The acid parameters include acid viscosity and acid concentration. The construction parameters include CO2 discharge rate and acid discharge rate.

[0012] S12: Design experimental conditions for etching rock slabs with pure acid, and convert the acid discharge rate into experimental scale using the Reynolds number similarity criterion;

[0013] S13: Conduct an experiment to etch a rock slab with pure acid, measure the rock slab mass m1 and m2 before and after the experiment, and calculate the acid-rock reaction rate R of pure acid. s :

[0014]

[0015] In the formula, R s The acid-rock reaction rate of pure acid solution, mol·m −2 ·s −1 m1 is the mass of the rock slab before the experiment, in g; m2 is the mass of the rock slab after the experiment, in g; M i A is the average molar mass of the rock slab, in g / mol; A is the area of ​​the crack surface of the rock slab, in m². 2 t represents the acid injection time, in seconds.

[0016] S14: Based on the acid-rock reaction rate R of pure acid solution s The acid-rock reaction constant k was obtained by fitting the Arrhenius formula. a and reaction order n a The fitting formula is:

[0017]

[0018] In the formula; k a n is the reaction rate constant. a C is the acid-rock reaction order; C is the acid concentration, mol·m −3 .

[0019] Furthermore, step S2 includes:

[0020] S21: Collect n API rock plates prepared from downhole cores;

[0021] S22: Design experimental conditions for etching rock slabs using a mixture of supercritical CO2 and acid. Convert CO2 and acid emissions into experimental emissions using similarity criteria.

[0022]

[0023] In the formula, h f The design joint height is in meters (m); w is the width of the slab (m); b f The average seam width is designed for the site, in meters (m); b e n is the experimental slit width, m; f As a two-phase index, in CO2 acid pressure, the acid solution is the continuous phase. Therefore, the non-Newtonian characteristics of the system are determined by the acid solution, i.e., satisfying n f = n l Q represents the on-site design mixed displacement, in meters. 3 / s, calculated by the following formula:

[0024]

[0025] In the formula, Q g Designed CO2 emission capacity, m 3 / s;Q l Designed displacement for acid, m 3 / s.

[0026] S23: Conduct an experiment to etch a rock slab using a mixture of supercritical CO2 and acid, and measure the mass m1 of the rock slab before and after the experiment. ’ m2 ’ Calculate the acid-rock reaction rate R s ’ ;

[0027] S24: Based on experimental testing R s ’ Data were used to establish a modified acid-rock reaction rate calculation equation and determine a modified reaction rate correction function based on CO2 content.

[0028] The modified equation for calculating the acid-rock reaction rate is as follows:

[0029]

[0030] In the formula, R s ’ The acid-rock reaction rate with added CO2, mol·m−2 ·s −1 ; For correction functions; α CO2 This refers to the CO2 injection ratio.

[0031] Furthermore, step S3 includes:

[0032] S31: The initial hydraulic gap width b is generated using a random function. h ;

[0033] S32: Construct the control equations for the CO2 acid pressure flow field based on the laws of mass and momentum conservation in two-phase flow;

[0034] S33: Construct the CO2 acid pressure convection diffusion equation based on the two-phase flow material conservation law;

[0035] S34: Calculate the width of the acid-etched fracture based on the amount of acid-rock reaction dissolution;

[0036] S35: Determine the simulation area, divide the grid along the seam length and seam height, discretize the control equations established in steps S32-S34, substitute the input parameters in step S1, and use the pressure-flow velocity coupled solution algorithm to solve the model, finally obtaining the acid etching seam width distribution.

[0037] Furthermore, step S4 includes:

[0038] S41: Following step S23, a flow conductivity test was conducted on the rock slab etched by CO2 + acid solution. The flow conductivity of the rock slab after adding CO2 was obtained as a function of the closing pressure σ. c Changes;

[0039] S42: Obtain the mass m1 of the rock slab before and after etching following the addition of CO2 using step S23. ’ m2 ’ Calculate the injection ratio of different CO2 (α) CO2 The average acid etching crevice width b under the following conditions a The specific calculation formula is as follows:

[0040]

[0041] In the formula, l is the length of the rock slab, in meters; w is the width of the rock slab, in meters; ρ r Density of rock slab, kg / m³ 3 .

[0042] S43: Based on the flow capacity data obtained in step S41 and the average acid etching width b under different CO2 injection ratios calculated in step S42 a Establish a local diversion capacity prediction model;

[0043] S44: Based on the local conductivity prediction model established in step S43 and the final acid-etched seam width obtained in step S35, calculate the conductivity distribution of the entire seam.

[0044] Furthermore, the mass conservation equation for the two-phase flow in step S32 is:

[0045]

[0046]

[0047] In the formula, S g S l ρ represents the saturation of CO2 and acid solution, dimensionless; g ρ l Densities of CO2 and acid solution, kg / m³ 3 ;u g u l ν represents the velocities of CO2 and acid, respectively, in m / s; t represents time, in seconds.

[0048] The momentum conservation equation is:

[0049]

[0050]

[0051] In the formula, p is the common pressure field of the two phases, MPa; For Hamiltonian operator notation; The symbol for the Kronecker product;

[0052] S g and S l satisfy:

[0053]

[0054] Furthermore, step S33 includes: constructing the CO2 acid pressure convection-diffusion equation based on the two-phase flow material conservation law, the convection-diffusion equation being:

[0055]

[0056] In the formula, C is the acid concentration, mol·m −3 ;D eff For the effective diffusion coefficient, m 2 ·s −1 Calculated by the following formula:

[0057]

[0058] In the formula, D l For H + The diffusion coefficient in the liquid phase, m 2 ·s −1 ;D g For H + The diffusion coefficient in the gas phase, m 2 ·s −1 .

[0059] Furthermore, step S34 includes: calculating the width of the acid-etched fracture based on the acid-rock reaction dissolution amount, with the governing equation being:

[0060]

[0061] In the formula, b a ρ is the width of the acid-etched crevices, in meters (m); β is the acid's solubility, dimensionless; ρ r Density of rock, kg / m³ 3 φ represents rock porosity, which is dimensionless.

[0062] Furthermore, the boundary conditions for solving equation S35 in step S35 include:

[0063]

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071] In the formula, x and y represent the fracture length and fracture height directions, respectively; p is the pressure of the two-phase fluid shared in the fracture, in MPa; u l u g V represents the flow velocities of acid and CO2 along the slit length, respectively, in m / s; v l vg The flow velocities of acid and CO2 along the slit height are respectively, in m / s; p f Formation pressure, MPa; C ini The initial concentration of the injected acid solution, mol / m 3 ;l f h represents the location of the maximum suture length in the simulated region. f The location of the maximum suture height in the simulated area; u l,inj u g,inj Let be the injection rates of acid and CO2, respectively, in m / s, calculated by the following formulas:

[0072]

[0073]

[0074] In the formula, Δy is the mesh size along the seam height direction, in meters; b| x=0 Q is the seam width at the seam opening, in meters; Q is the on-site design mixed displacement, in meters. 3 / s;S l,inj S g,inj The saturation levels at the bottom of the well for acid and CO2 are respectively calculated using the following formulas:

[0075]

[0076]

[0077] Furthermore, the initial conditions for solving equation S35 in step S35 include:

[0078]

[0079] Furthermore, in step S35, when using the pressure-velocity coupled solution model, the predicted velocity u is first obtained by solving equations (9) and (10) using the predicted pressure p*. l *、u g Then, solve the pressure correction equation to obtain the pressure correction p', and calculate the velocity correction u. l '、u g If the convergence condition is met at this time, the corrected velocity is the final solution result at this time step; if the convergence condition is not met, the corrected pressure and velocity are substituted into equations (9) and (10) to resolve the predicted velocity u. l *、u g * Repeat the above steps until the convergence condition is met;

[0080] The pressure correction equation is as follows:

[0081]

[0082] In the formula, a p,l and a p,g These are the diagonal terms of the linear equations after discretization of equations (9) and (10), respectively; p' is the pressure correction in MPa; R l and R g The mass residuals of acid and CO2 are respectively calculated using the following formulas:

[0083]

[0084]

[0085] In the formula, u g *、u l *Predicted velocities of CO2 and acid, respectively, in m / s.

[0086] Furthermore, the velocity correction equation in step S35, the pressure-velocity coupled solution process, is as follows:

[0087]

[0088]

[0089] In the formula, u l '、u g 'These are the velocity corrections for CO2 and acid, respectively, in m / s.

[0090] The corrected pressure and flow rate satisfy:

[0091]

[0092]

[0093] Furthermore, the convergence condition for the pressure-velocity coupled solution process in step S35 is:

[0094]

[0095] Furthermore, in step S43, the local diversion capacity prediction model is selected as the modified NK model, with the specific formula as follows:

[0096]

[0097] In the formula, a i The fitting parameters are σ (i = 1, 2, 3, 4). c The closing pressure is in MPa.

[0098] In a second aspect, the present invention provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement any of the methods described above.

[0099] Thirdly, the present invention provides a computer-readable storage medium, comprising:

[0100] The computer-readable storage medium stores a computer program that, when executed by a processor, implements any of the methods described above.

[0101] Fourthly, the present invention also provides a computer program product, the computer program product comprising a computer program that, when executed by a processor, implements any of the methods described above.

[0102] Compared with the prior art, the present invention has the following beneficial technical effects:

[0103] This invention provides a method for calculating the conductivity distribution of a full fracture under CO2 acid fracturing, which can comprehensively reflect the synergistic etching effect of CO2 and acid, and overcomes the defect in the existing technology that the acid rock reaction kinetic model of pure acid liquid cannot accurately describe the synergistic etching mechanism under CO2 acid fracturing conditions. Attached Figure Description

[0104] Figure 1 The acid-rock reaction rate R measured in this embodiment of the invention under different CO2 injection ratios s ’ ;

[0105] Figure 2 This is the fitted formula for the acid-rock reaction rate correction function in the embodiments of the present invention;

[0106] Figure 3 This refers to the acid etching crack width distribution calculated in the embodiments of the present invention;

[0107] Figure 4 This describes the flow-guiding capacity under different CO2 injection ratios in the embodiments of the present invention;

[0108] Figure 5 This is the predicted result of the full-slit flow capacity distribution in an embodiment of the present invention. Detailed Implementation

[0109] The core of this invention is to provide a method for calculating the conductivity distribution of CO2 acid fracturing in the entire fracture, which integrates acid-rock reaction kinetics experiments under supercritical CO2 environment with multi-physics field coupled numerical simulation, and realizes the description of the CO2-acid liquid synergistic etching mechanism.

[0110] Terminology Explanation:

[0111] Acid-rock reaction rate: The acid-rock reaction rate refers to the rate at which a chemical reaction occurs between acid and rock during acidification operations, and is the amount of rock dissolved by acid per unit time.

[0112] Supercritical CO2: Supercritical CO2 refers to CO2 in a supercritical state. When CO2 is above its critical temperature (30.98℃) and critical pressure (7.375MPa), the phase interface between the gas and liquid phases disappears, and CO2 is in a state that is neither gaseous nor liquid, but has some characteristics of both gas and liquid. This state is called the supercritical state.

[0113] Acid-rock reaction constant: The acid-rock reaction constant is a coefficient in the acid-rock reaction kinetic equation that reflects the proportional relationship between the reaction rate and the reactant concentration.

[0114] Reaction order: A parameter used to describe the relationship between the rate of a chemical reaction and the concentration of reactants. It represents the sum of the exponents of the concentrations of each reactant in the chemical reaction rate equation.

[0115] Diffusion coefficient: A parameter used to describe the diffusion ability of a substance, representing the amount of substance passing through a unit area per unit time under a unit concentration gradient. In real systems, due to the influence of various complex factors, such as the obstruction of porous media and the interaction between substances, the diffusion behavior of substances often cannot be described by a simple diffusion coefficient. The effective diffusion coefficient is a parameter that takes these practical factors into account and provides an equivalent description of the diffusion process.

[0116] This invention provides a method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam, comprising the following steps:

[0117] S1: Collect basic parameters, including reservoir parameters (temperature, formation pressure, etc.), acid parameters (acid viscosity, rheological index, etc.), and construction parameters (CO2 discharge, acid discharge, etc.). Conduct pure acid etching experiments on rock plates to test the acid-rock reaction constant and reaction order of the acid system.

[0118] S11: Collect reservoir parameters, acid parameters, and construction parameters. In this embodiment, the acid is selected by default as 20wt% hydrochloric acid. Other basic parameters are shown in Table 1.

[0119] Table 1. Basic Model Parameters

[0120] parameter numerical values parameter numerical values Dimension of the longitudinal grid of the rock fissure, Δx, m 1 Rock fissure vertical grid size Δy, m 1 <![CDATA[Acid solution density ρ f , kg / m 3 > 1000 <![CDATA[Acid injection time t a , min]]> 30 <![CDATA[Rock density ρ r , kg / m 3 > 2870 <![CDATA[CO2 displacement Q g , m 3 / min]]> 0.6 Acid solubility β 1.07 <![CDATA[Acid solution displacement Q l , m 3 / min]]> 2.4 <![CDATA[Acid viscosity μ l , mPa·s]]> 1.1 <![CDATA[CO2 density ρ g , g / cm³]]> 0.7 Porosity φ, % 8 <![CDATA[CO2 viscosity μ g , mPa·s]]> 0.095 <![CDATA[Initial acid concentration C ini , wt%]]> 20 <![CDATA[Formation pressure P f , MPa]]> 70 Formation temperature, °C 80 <![CDATA[Diffusion coefficient D in the liquid phase l , m 2 / s]]> <![CDATA[5×10 -9 ]]> <![CDATA[Diffusion coefficient D in the gas phase g > <![CDATA[5×10 -9 ]]> <![CDATA[Rock molar mass M i > 100 <![CDATA[Saturation S of CO2 at the bottom of the well g,inj > 0.2 <![CDATA[Saturation S at the bottom of the acid wellbore l,inj > 0.8 Time step Δt, s 10 <![CDATA[Closing pressure σ c , MPa]]> 40MPa

[0121] S12: Design the experimental conditions for etching the rock slab with pure acid. In this step, pure acid refers to acid that does not contain CO2. Specifically, the experimental temperature, acid system, and acid injection time remain consistent with step S11, while the acid displacement is converted to experimental scale using the dimensionless similarity criterion. In this embodiment, the Reynolds number similarity criterion is used, with the formula:

[0122]

[0123] In the formula, h f The design joint height is in meters (m); w is the width of the slab (m); b f The average seam width is designed for the site, in meters (m); b e n is the experimental slit width, m; l Q is the flow index of the acid solution; Q is the on-site design discharge capacity, in meters. 3 / min. The parameters used for displacement conversion are shown in Table 2. The final displacement for the experimental scale obtained by conversion was 260 ml / min.

[0124] Table 2 Displacement Conversion Parameters

[0125] parameter numerical values parameter numerical values Slab width w, m 0.038 <![CDATA[Experimental slit width b e , m]]> 0.001 <![CDATA[Reservoir fracture height h f , m]]> 88.0 <![CDATA[Field design average joint width b f , m]]> 0.0027 <![CDATA[Flow state index n l > 0.146 <![CDATA[Experimental displacement q l , ml / min]]> 260 <![CDATA[On-site design displacement Q, m 3 / min]]> 3

[0126] S13: Based on the parameters of step S12, conduct acid etching experiments, test the rock slab masses m1 and m2 before and after the experiment, and calculate the acid-rock reaction rate R of pure acid solution. s :

[0127]

[0128] In the formula, m1 and m2 are the masses of the rock slab before and after the experiment, respectively, in grams; M i A is the average molar mass of the rock slab, in g / mol; A is the area of ​​the crack surface of the rock slab, in cm². 2 t represents the acid injection time, in seconds.

[0129] S14: Based on the experimentally obtained acid-rock reaction rate R s Data, fitted to the acid-rock reaction constant k according to the Arrhenius formula. a and reaction order n a The specific formula is as follows:

[0130]

[0131] In the formula, k a n is the reaction rate constant. a C is the acid-rock reaction order; C is the acid concentration, mol·m −3 .

[0132] In this step, the acid-rock reaction constant k is obtained through fitting.a and reaction order n a Then, this fitting formula can be used to determine the acid-rock reaction rate R under different acid concentration conditions. s .

[0133] In this embodiment, the acid-rock reaction rate R of the 20wt% hydrochloric acid system without the addition of CO2 is... s 3.33×10 -6 mol / (cm 2 ·s).

[0134] S2: Conducting experiments to etch rock slabs after mixing supercritical CO2 with acid, and testing the acid-rock reaction rate R after adding CO2. s ’ Specifically, it includes the following steps:

[0135] S21: Collect 4 pure limestone API rock plates from the well;

[0136] S22: Conduct an experiment to etch a rock slab using a mixture of supercritical CO2 and acid. Specifically, the experimental temperature, acid system, acid injection time, and CO2 injection time remain consistent with step S11, while the CO2 displacement and acid displacement are converted into experimental displacement using the Reynolds number similarity criterion.

[0137] In this embodiment, Q g Q l 0.6m respectively 3 / min, 2.4m 3 / min, according to equation (4), the on-site design mixed displacement Q is 3.0m³. 3 / min.

[0138] In CO2 acid pressure testing, the acid solution is a continuous phase; therefore, the non-Newtonian characteristics of the system are determined by the acid solution. It is assumed that the rheological index of the CO2 + acid solution system is the same as that of the pure acid solution (i.e., n...). f = n l According to the Reynolds number similarity criterion, the experimental displacement at this time is still 260 ml / min.

[0139] S23: Using the experimental apparatus disclosed in patent CN202010120301.3 to simulate the CO2 acid fracturing and etching process of cracks, a supercritical CO2 and acid solution mixed etching experiment was conducted on a rock slab, and the mass m1 of the rock slab before and after the experiment was measured. ’ m2 ’ The acid-rock reaction rate R of the mixed system after adding CO2 was calculated using the method in equation (1). s ’ , Figure 1 The acid rock reaction rate R under different CO2 injection ratios for four rock slabs s ’ ;

[0140] S24: Based on experimental testing R s ’ Based on the data, a modified equation for calculating the reaction rate of acid rocks was established, and a correction function for the reaction rate based on CO2 content was determined.

[0141] In this embodiment, such as Figure 2 As shown, the fitted R s ’ The calculation formula is:

[0142]

[0143] S3: Construct a CO2 and acid mixed etching model, solve the model using a pressure-velocity coupled algorithm, and calculate the acid etching groove width distribution. Specifically, this includes the following steps:

[0144] S31: The initial hydraulic gap width b is generated using a random function. h ;

[0145] S32: Construct the governing equations for the CO2 acid pressure flow field based on the laws of mass and momentum conservation in two-phase flow. According to the law of mass conservation, the flow of CO2 + acid in a rough crack satisfies:

[0146]

[0147]

[0148] In the formula, S g S l ρ represents the saturation of CO2 and acid solution, dimensionless; g ρ l Densities of CO2 and acid solution, kg / m³ 3 ;u g u l denoted by CO2 and acid velocity, respectively, in m / s; t represents time, in seconds.

[0149] According to the law of conservation of momentum, the flow of CO2+ acid in the rough crack satisfies:

[0150]

[0151]

[0152] In the formula, p represents the common pressure field, in MPa; For Hamiltonian operator notation; This is the Kronecker product symbol.

[0153] S g and S l satisfy:

[0154]

[0155] S33: Construct the CO2 acid pressure convection-diffusion equation based on the two-phase flow material conservation law. The convection-diffusion equation is as follows:

[0156]

[0157] In the formula, C is the acid concentration, mol·m −3 ;D eff For the effective diffusion coefficient, m 2 ·s −1 Calculated by the following formula:

[0158]

[0159] In the formula, D l For H + The diffusion coefficient in the liquid phase, m 2 ·s −1 ;D g For H + The diffusion coefficient in the gas phase, m 2 ·s −1 .

[0160] S34: The width of the acid-etched fracture is calculated based on the acid-rock reaction dissolution amount. The governing equation is:

[0161]

[0162] In the formula, b a ρ is the width of the acid-etched crevices, in cm; β is the dissolving power of the acid, dimensionless; r Density of rock, g / cm³ 3 φ represents rock porosity, which is dimensionless.

[0163] S35: Define the simulation region as 120m × 88m, and divide the crack surface into a two-dimensional structured mesh along the crack length and height directions, with mesh size Δx = Δy = 1m. Discretize the control equations established in steps S32-S34 using the finite volume method (FVM), substitute the input parameters from step S1, and solve the model using the SIMPLE pressure-velocity coupling algorithm.

[0164] When using the pressure-velocity coupled solution model, the predicted velocity u is first obtained by solving equations (9) and (10) using the predicted pressure p*. l *、u g Then, solve the pressure correction equation to obtain the pressure correction p', and calculate the velocity correction u. l '、u gIf the convergence condition is met at this time, the corrected velocity is the final solution result at this time step. If the convergence condition is not met, the corrected pressure and velocity are substituted into equations (9) and (10) to resolve for the predicted velocity u. l *、u g Repeat the above steps until the convergence condition is met.

[0165] In this embodiment, by substituting the input parameters in Table 1, the final acid etching crack width distribution is obtained as follows: Figure 3 As shown.

[0166] S4: Construct a conductivity model and calculate the average conductivity of the CO2 acid fracturing fractures based on the fracture width distribution. Specifically, this includes the following steps:

[0167] S41: Following step S23, a flow conductivity test was conducted on the rock slab etched by CO2 + acid solution. The flow conductivity of the rock slab after adding CO2 was obtained as a function of the closing pressure σ. c The changes. In this embodiment, the flow guidance capacity test results are as follows: Figure 4 As shown.

[0168] S42: Obtain the mass m1 of the rock slab before and after etching following the addition of CO2 using step S23. ’ m2 ’ Calculate the injection ratio of different CO2 (α) CO2 The average acid etching crevice width w under ( ) a The specific calculation formula is as follows:

[0169]

[0170] In the formula, l is the length of the rock slab, in cm; ρ r Density of rock slab, g / cm³ 3 In this embodiment, the average acid-etched crevices width b a The calculation results are shown in Table 3:

[0171] Table 3. Average acid etching width under different CO2 injection ratios

[0172] <![CDATA[CO2 injection ratio α CO2 , %]]> 5 10 15 20 <![CDATA[Average width b of etched fissures a , cm]]> 0.1957 0.1512 0.04327 0.02847

[0173] S43: Based on the flow capacity data obtained in step S41 and the average acid etching width b under different CO2 injection ratios calculated in step S42 a A local diversion capacity prediction model is established. In this embodiment, the modified NK model is selected as the local diversion capacity prediction model, and the specific formula is as follows:

[0174]

[0175] In the formula, a i(i = 1, 2, 3, 4) are the fitting parameters; σ c The closing pressure is MPa. Based on the conductivity test results of step S41, the parameter fitting results are shown in Table 4:

[0176] Table 4. Fitting Results of Flow Guiding Capacity Model Parameters

[0177] parameter value parameter value <![CDATA[a1]]> 889.22 <![CDATA[a3]]> 0.18 <![CDATA[a2]]> 0.69 <![CDATA[a4]]> 0.03

[0178] S44: Based on the model established in step S43 and the final acid-etched fracture width obtained in step S35, calculate the overall fracture conductivity distribution. In this embodiment, the calculated overall fracture conductivity distribution is as follows: Figure 5 As shown.

[0179] This invention provides a method for calculating the conductivity distribution of CO2-fracturing fractures. Through experiments involving etching rock slabs with pure acid and etching rock slabs with a mixture of supercritical CO2 and acid, the acid-rock reaction rate R after adding CO2 is obtained. s ’ A theoretical model for CO2 and acid mixed etching was established, and a pressure-velocity coupling algorithm was used to solve the theoretical model, thereby calculating the acid etching crack width distribution and the full crack conductivity distribution, providing an effective means for optimizing CO2 acid fracturing construction parameters.

[0180] 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 calculating the conductivity distribution of a CO2 acid-pressurized seam, characterized in that, Includes the following steps: S1: Collect basic parameters, including reservoir parameters, acid parameters, and construction parameters; conduct pure acid etching experiments on rock slabs to test and obtain the acid-rock reaction rate R. s ; S2: Conduct experiments to etch rock slabs after mixing supercritical CO2 with acid, and test the acid-rock reaction rate R after adding CO2. s ’ Fitting R s ’ With R s Relationships; S3: Establish a CO2 and acid mixed etching model and substitute it into R. s ’ The pressure-velocity coupled algorithm was used to solve the model and calculate the acid etching crack width distribution. S4: Construct a conductivity model and calculate the conductivity distribution of the entire CO2 acid-etched crack based on the crack width distribution.

2. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 1, characterized in that, Step S1 includes: S11: Collect reservoir parameters, acid parameters, and construction parameters. The reservoir parameters include reservoir temperature and formation pressure. The acid parameters include acid viscosity and acid concentration. The construction parameters include CO2 discharge rate and acid discharge rate. S12: Design experimental conditions for etching rock slabs with pure acid, and convert the acid discharge rate into experimental scale using the dimensionless similarity criterion; S13: Conduct an experiment to etch a rock slab with pure acid, measure the rock slab mass m1 and m2 before and after the experiment, and calculate the acid-rock reaction rate R of pure acid. s : In the formula, m1 is the mass of the rock slab before the experiment, in g; m2 is the mass of the rock slab after the experiment, in g; M i A is the average molar mass of the rock slab, in g / mol; A is the area of ​​the crack surface of the rock slab, in cm². 2 t represents the acid injection time, in seconds. S14: Based on the acid-rock reaction rate R of pure acid solution s The acid-rock reaction constant k was obtained by fitting the Arrhenius formula. a and reaction order n a The fitting formula is: In the formula, k a n is the reaction rate constant. a C is the acid-rock reaction order; C is the acid concentration, mol·m −3 .

3. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 1, characterized in that, Step S2 includes: S21: Collect n API rock plates prepared from downhole cores; S22: Design experimental conditions for etching rock slabs with a mixture of supercritical CO2 and acid, and convert the injection rate into experimental scale using the dimensionless similarity criterion. S23: Conduct an experiment to etch a rock slab using a mixture of supercritical CO2 and acid, and measure the mass m1 of the rock slab before and after the experiment. ’ m2 ’ Calculate the acid-rock reaction rate R after adding CO2. s ’ ; S24: Based on experimental testing R s ’ Based on the data, a modified acid-rock reaction rate calculation equation was established, and a correction function for the reaction rate based on CO2 content was determined. The modified acid-rock reaction rate calculation equation is as follows: In the formula, R s ’ The acid-rock reaction rate with added CO2, mol·m −2 ·s −1 ; α is the fitting function; CO2 This refers to the CO2 injection ratio.

4. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 1, characterized in that, Step S3 includes: S31: The initial hydraulic gap width b is generated using a random function. h ; S32: Construct the control equations for the CO2 acid pressure flow field within rough cracks based on the laws of mass and momentum conservation in two-phase flow; S33: Construct the CO2 acid pressure convection and diffusion equation in rough cracks based on the material conservation law of two-phase flow; S34: Calculate the width of the acid-etched fracture based on the amount of acid-rock reaction dissolution; S35: Determine the simulation area, divide the grid along the seam length and seam height, discretize the control equations established in steps S32-S34, substitute the input parameters in step S1, and use the pressure-flow velocity coupled solution algorithm to solve the model, finally obtaining the acid etching seam width distribution.

5. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 4, characterized in that, The mass conservation equation for the two-phase flow in step S32 is: In the formula, g , l ρ represents the saturation of CO2 and acid solution, dimensionless; g ρ l Densities of CO2 and acid solution, kg / m³ 3 ;u g u l ν represents the velocities of CO2 and acid, respectively, in m / s; t represents time, in seconds. The momentum conservation equation is: In the formula, p is the shared pressure field of the two phases, in MPa; For the Hamiltonian operator notation; This is the Kronecker product symbol. S g and S l satisfy: 。 6. The method for calculating the conductivity distribution of the entire seam under CO2 acid fracturing according to claim 4, characterized in that, Step S33 includes: constructing the CO2 acid pressure convection-diffusion equation based on the two-phase flow material conservation law. The convection-diffusion equation is as follows: In the formula, C is the acid concentration, mol·m −3 ;D eff m is the effective diffusion coefficient of hydrogen ions. 2 ·s −1 Calculated by the following formula: In the formula, D l For H + The diffusion coefficient in the liquid phase, m 2 ·s −1 ;D g For H + The diffusion coefficient in the gas phase, m 2 ·s −1 .

7. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 4, characterized in that, Step S34 includes: calculating the width of the acid-etched fracture based on the acid-rock reaction dissolution amount, with the governing equation being: In the formula, b a ρ is the width of the acid-etched crevices, in meters (m); β is the acid's solubility, dimensionless; ρ r Density of rock, kg / m³ 3 φ represents rock porosity, which is dimensionless.

8. The method for calculating the conductivity distribution of CO2 acid fracturing throughout the seam according to claim 1, characterized in that, Step S4 includes: S41: Using step S23, a rock slab etched with a mixture of supercritical CO2 and acid was used to conduct a conductivity test experiment to obtain the conductivity of the rock slab after adding CO2 as a function of the closing pressure σ. c Changes; S42: Obtain the mass m1 of the rock slab before and after etching with supercritical CO2 and acid solution using step S23. ’ m2 ’ Calculate the injection ratio α for different CO2 levels. CO2 The average acid etching crevice width b a The calculation formula is: In the formula, w is the width of the rock slab, in meters; l is the length of the rock slab, in meters; ρ r Density of rock slab, kg / m³ 3 . S43: Based on the flow capacity data obtained in step S41 and the average acid etching width b under different CO2 injection ratios calculated in step S42 a Establish a local diversion capacity prediction model; S44: Based on the model established in step S43 and the final acid-etched seam width obtained in step S35, calculate the distribution of the conductivity of the entire seam.

9. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the method of any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the method of any one of claims 1 to 8.

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