An acid fracturing parameter optimization method based on full-slit acid etching fracture conductivity simulation

Abstract

The present application relates to the technical field of petroleum engineering, and particularly relates to an acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation, comprising the following steps: S1: obtaining basic parameters, and establishing a single-well geological feature model of a target area; S2: combining the construction parameters with the single-well geological feature model, and performing acid fracturing simulation calculation to obtain fracture dynamic environment parameters; S3: calibrating a reference position in the acid fracturing fracture, and obtaining dynamic changes of the environment parameters at different positions; S4: using a multi-field coupling acid etching model to carry out local acid etching simulation at different positions; S5: based on the simulation results in S4, using an acid etching fracture conductivity calculation model to obtain the full-domain acid etching fracture conductivity distribution characteristics; and S6: based on the acid etching fracture conductivity in S5, judging whether the acid etching fracture parameter requirements are met according to the target reservoir requirements. The present application can carry out acid etching simulation calculation at different positions in the fracture domain, so as to obtain the full-domain acid etching fracture conductivity distribution characteristics.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of petroleum engineering, and particularly relates to an acid fracturing parameter optimization method based on full-fracture acid-etched fracture conductivity simulation. BACKGROUND

[0002] The carbonate rock exploration area in China reaches 600,000 square kilometers, and the oil and gas reserves are large and widely distributed, especially in the three large basins of Tarim, Sichuan and Ordos, the proven reserves of oil and gas resources are not more than 15%, and the resource potential is huge. The super-deep carbonate rock stratum in China is ancient in age, is affected by multi-period accumulation reconstruction, has multiple types of vertically superimposed reservoirs, and has extremely strong heterogeneity, and as a whole has characteristics of super-high temperature (160-220℃), super-high closure stress (80-120MPa), low-permeability tightness (porosity is between 2-5%, and permeability is between 0.01-10mD). Due to the poor matrix physical property of the carbonate rock reservoir and the discontinuous geological characteristics of the rich oil and gas reservoir, more than 80% of the oil and gas wells need to be reconstructed by acid fracturing technology to realize effective development of oil and gas.

[0003] Numerical simulation is an important means for optimizing acid fracturing process parameters, and the acid fracturing numerical simulation research develops towards a global multi-field coupling model, aiming to realize complete reproduction of the acid fracturing fracture expansion-etching process under the mine field conditions. At present, there are many large-scale acid fracturing models for the mine field, and the local acid fracturing model focusing on the local etching characteristics of the fracture surface is relatively less. In the actual acid fracturing process, the flow rate, temperature, fracture width, acid concentration and other parameters at each position in the fracture are in a dynamic change process, resulting in great differences in etching morphology and acid-etched fracture conductivity at each position in the fracture.

[0004] When calculating the acid-etched fracture conductivity, the complex geological bodies such as natural fractures and caves have heterogeneity and anisotropy, so there is a great difference between the calculation results considering the complex medium and not considering the complex medium. The commonly used acid fracturing model can only calculate the average dissolution amount in a single grid, and uses the N-K model and other empirical models to calculate the acid-etched fracture conductivity, although the distribution characteristics of the acid-etched fracture conductivity in the 100m fracture under the oilfield scale can be obtained, but it is difficult to reflect the influence of the rough morphology and the change of the environmental parameters on the etching morphology and the conductivity in the process of calculating the conductivity, and the influence of the complex geological bodies on the acid-etched fracture conductivity is ignored, so the acid fracturing parameters cannot be optimized accurately. SUMMARY

[0005] The technical scheme adopted by the application is as follows:

[0006] An acid fracturing parameter optimization method based on full-fracture acid-etched fracture conductivity simulation, characterized in that the method comprises the following steps:

[0007] Step S1: Obtain basic parameters, and establish a single-well geological feature model of a target area;

[0008] Step S2: combine the construction parameters with the single-well geological feature model, perform acid fracturing simulation calculation to obtain fracture dynamic environment parameters;

[0009] Step S3: calibrate the reference position in the acid fracturing fracture, and obtain the dynamic change of the environment parameters at different positions;

[0010] Step S4: use a multi-field coupled acid etching model to carry out local acid etching simulation at different positions;

[0011] Step S5: based on the simulation results in step S4, use an acid-etched fracture conductivity calculation model to obtain the global acid-etched fracture conductivity distribution characteristics;

[0012] Step S6: based on the acid-etched fracture conductivity in step S5, determine whether the acid-etched fracture parameter requirements are met according to the target reservoir requirements.

[0013] Further, in step S2, when performing acid fracturing simulation calculation, it further includes:

[0014] Based on the multi-field coupled acid fracturing model, the fluid mass conservation equation, flow equation, convection-diffusion heat conduction, concentration field calculation equation, and dissolution width calculation equation in the fracture domain are established.

[0015] Further, the fracture dynamic environment parameters include:

[0016] Fracture width, flow field, temperature field, and concentration field parameters.

[0017] Further, step S3 further includes:

[0018] The central axis of the fracture length direction is selected as the reference line for conductivity calculation, a first reference point sequence is taken at intervals on the central axis, and a second reference point sequence is taken in the fracture height direction parallel to the central axis of the fracture length. The first reference point sequence and the second reference point sequence can cover the fracture surface. For complex geological body regions, a smaller interval and more reference point sequences are used to cover the entire region, so that the conductivity of the acid-etched fracture in the complex geological body can be calculated more finely.

[0019] Further, step S4 further includes:

[0020] The environment parameters of the first reference point sequence and the second reference point sequence in step S3 are used as the boundary conditions of the acid etching model. The etching model of the first reference point sequence and the second reference point sequence is solved, and an interpolation algorithm is used for fracture surface coarsening, so as to reproduce the acid liquid dynamic etching process in the fracture.

[0021] Further, step S4 further includes:

[0022] According to the simulation condition in step S3, the acid concentration-time curve is obtained;

[0023] When the acid concentration increases to 10% of the highest acid concentration, the corresponding time is determined as the reference point acid etching start time;

[0024] When the acid concentration decreases to 10% of the highest acid concentration, the corresponding time is determined as the reference point acid etching end time;

[0025] The acid etching time of the calculation point is determined.

[0026] Further, step S5 further includes:

[0027] A three-dimensional coordinate system is established by using the rough point cloud data obtained by acid etching the surface of the rock plate;

[0028] The acid etching rock plate is sectioned, and the length of the tortuous line of the profile top edge of the rock plate on the sectioned surface, the lateral tortuosity ratio of the rock sample surface, and the longitudinal tortuosity ratio of the rock sample surface are obtained, so as to calculate the acid etching crack conductivity.

[0029] Further, step S6 further includes:

[0030] The acid etching crack conductivity under different construction parameters is calculated, so as to optimize the construction parameters.

[0031] The setting of construction parameters such as acid amount and displacement is a key parameter for optimizing construction effect, the acid amount distribution is dynamically adjusted according to real-time monitoring data (such as pressure and temperature), the acid amount is preferentially reduced in high permeability areas, the acid amount is increased in low permeability areas, the dimensionless productivity index under different displacement and acid amount is calculated, the dimensionless productivity index and acid injection amount relationship curve is drawn, and the best treatment amount is determined.

[0032] In summary, the technical scheme provided by the present application can carry out acid etching simulation calculation at different positions in the crack domain, so as to obtain the global acid etching crack conductivity distribution characteristics, and the calculation result is more accurate than the simulation method in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical scheme in the examples of the present application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0034] Figure 1 The method flowchart of the present application is shown in the figure;

[0035] Figure 2 The acid etching rock plate surface structure schematic diagram in the embodiment of the present application is shown in the figure;

[0036] Figure 3 Point taking schematic view of reference point in embodiment of the present application;

[0037] Figure 4 Acid concentration-time change relation diagram of certain reference point in embodiment of the present application;

[0038] Figure 5 Flow conductivity distribution diagram in embodiment of the present application;

[0039] Figure 6 Acid etching fracture flow conductivity prediction result schematic diagram under different construction parameters in embodiment of the present application. DETAILED DESCRIPTION

[0040] The present application will be described in detail below with reference to the drawings.

[0041] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0042] Generally, the present application provides an acid fracturing parameter optimization method based on full-fracture acid etching fracture flow conductivity simulation. By obtaining the temperature, concentration, fracture width, flow rate and other environmental parameters of the calibration reference point in the fracture during acid fracturing and their changes over time, the environmental parameters are used as boundary conditions for the acid etching and flow conductivity numerical model calculation, and the full-fracture acid etching fracture flow conductivity is calculated by numerical simulation.

[0043] As shown in Figure 1 The present application provides an acid fracturing parameter optimization method based on full-fracture acid etching fracture flow conductivity simulation, which includes the following steps:

[0044] Step S1: Obtain basic parameters and establish a single-well geological feature model of the target area;

[0045] Specifically, when establishing the single-well geological feature model of the target area, the basic mechanical parameters and stress profile can be calculated through logging parameters to realize the stress profile gridding and calculate the ground stress.

[0046] In one embodiment, different processing methods can be selected for fractures of different scales, for example, for small-scale (opening ≤ 50 μm, density ≥ 5 / m) natural fractures, permeability tensor can be used for characterization, for large-scale natural fractures, DFM and EDFM display characterization can be used, and modeling can be based on the EDFM method. For far-well complex reservoirs, the heterogeneity feature model of the formation can be constructed based on far-probing acoustic waves.

[0047] Step S2: combine the construction parameters and the single well geological feature model, perform acid fracturing simulation calculation to obtain fracture dynamic environment parameters;

[0048] Specifically, the acid volume and the displacement can be set according to the construction parameter of the previous experience of the block, and the acid fracturing simulation can be carried out by selecting the average construction parameter of the block. When performing the acid fracturing simulation calculation, the multi-field simulation model in the prior art can be used for calculation.

[0049] In one embodiment, the multi-scale multi-field coupling acid fracturing model described in the specification is used, and the model can realize three-dimensional dynamic calculation of the fracture width, flow field, temperature and concentration field.

[0050] The fluid mass conservation equation in the fracture domain is:

[0051]

[0052] The flow momentum equation in the fracture domain is:

[0053]

[0054] The convection-diffusion heat conduction of the acid liquid in the fracture domain is described by using a three-dimensional convection-diffusion equation:

[0055]

[0056] The concentration field equation in the fracture is:

[0057]

[0058] The fracture wall surface dissolution width calculation is:

[0059] In one embodiment of the present application, it is assumed that the acid liquid that leaks from the hydraulic fracture wall surface into the matrix rock mass all participates in the dissolution of the rock and forms acid-etched wormholes, and does not participate in the dissolution of the hydraulic fracture wall surface, so the dissolution width calculation equation of the hydraulic wall surface is:

[0060]

[0061] In the formula, u x , u y , u z are the fluid flow rates in the length, width and height directions in the fracture domain, m / s; ψ is the flow velocity component in the length, width and height directions in the fracture domain, m / s; p hf is the fluid pressure, Pa; ρ l is the fluid density, kg / m 3 ; i is the x, y, z direction coordinate, dimensionless; μ is the fluid viscosity, Pa·s; c1 is the specific heat capacity of the fluid, J / (kg·K); T hfλ is the fluid temperature within the hydraulic fracture, K; λ1 is the fluid thermal conductivity, J / (m·K·s); C hf D represents the acid concentration within the hydraulic fracture, in mol / m³. e m is the effective mass transfer coefficient of the acid solution. 2 / s;;w hf ρ is the width of the hydraulic fracture, in meters; β is the solubility of the acid in the rock minerals, in kilograms; M is the molar mass of the rock, in kilograms per mol; φ is the porosity of the rock mass, dimensionless; ρ s Density of the rock skeleton, kg / m³ 3 ;k c denoted as the acid-rock reaction rate, in m / s.

[0062] Step S3: Calibrate reference positions within the acid-poured fracture and obtain dynamic changes in environmental parameters at different locations;

[0063] In step S3, by calibrating a reference location within the acid-plaque fracture, the dynamic changes in its environmental parameters are obtained, providing conditions for the acid etching model. This invention employs a multi-field coupled acid etching model to simulate acid etching at different locations within the fracture domain. Based on the acid-plaque simulation results, the conductivity of the acid-etched fracture is predicted, thereby obtaining the distribution characteristics of the conductivity of the acid-etched fracture throughout the entire fracture domain. For example... Figure 2 As shown, taking an acid fracturing fracture with a length of 100m and a width of 28m as an example, the central axis of the fracture length (y=0) is selected as the reference line for calculating the conductivity. Reference points are taken at 10m intervals along the central axis (the specific interval can be selected according to actual needs). Then, reference points are taken at 10m intervals along the fracture height parallel to the central axis of the fracture length, ensuring that the reference points cover the fracture surface. Due to the strong filtration effect of the complex medium, the length of the hydraulic fracture after encountering natural fractures is shortened, thus affecting the propagation of the acid-etched fracture. Therefore, reference points with 1m intervals need to be set at natural fractures within the acid fracturing fracture to more accurately characterize the influence of natural fractures on acid etching, and the conductivity of the acid-etched fracture at each reference point is calculated. S2 can be used to obtain environmental parameters such as fracture width, temperature, flow velocity, and acid concentration at any point within the hydraulic fracture during the entire acid fracturing process, as well as their changes over time.

[0064] Step S4: Simulate local acid etching at different locations using a multi-field coupled acid etching model;

[0065] In this step, a model of the local acid etching crack conductivity can be adopted based on the solid-fluid-temperature-reaction multi-field coupled acid non-uniform etching mechanism. By establishing flow coupling algorithms between different physical and chemical fields, the model can be numerically discretized and solved based on the finite volume method.

[0066] Specifically, the environmental parameters of each reference point in S3 are used as the boundary conditions of the acid etching model. By solving the etching model of each reference point, the required crack surface roughening is performed using the Kriging interpolation algorithm, thereby reproducing the dynamic acid etching process in the rough crack under in-situ conditions.

[0067] Because acid propagation within fractures takes time, the acid initially applied at the fracture opening reacts with the rock first, resulting in different onset times for acid etching at different locations. Therefore, it is necessary to calibrate the acid etching time at different locations. During acid fracturing, as the acid reacts with the rock, the acid-rock reaction consumes hydrogen ions, leading to a decrease in acid concentration. A significant change in acid concentration indicates the onset of a chemical reaction between the acid and the rock. A multi-field coupled acid fracturing model is solved using numerical calculation methods. The numerical algorithm is implemented through computer programming, and by inputting rock and acid parameters, boundary conditions, and initial conditions, the acid concentration over time can be calculated. By monitoring the acid concentration over time in real time, when a significant decrease in acid concentration is observed, it can be considered that the acid has begun to react with the rock; the corresponding time is the onset time of the acid reaction within the fracture.

[0068] like Figure 3 As shown, the steps are as follows: As the acid is injected and the reaction proceeds, the acid concentration is recorded every 1 minute until a period of time after the acid injection is completed, ensuring the reaction is essentially finished. Based on the recorded acid concentration and time data, a curve showing the change in acid concentration versus time is plotted.

[0069] By observing the acid concentration versus time curves at each reference point, the time when the acid concentration increases to 10% of the maximum concentration is determined as the start time of acid etching at that reference point. The time when the acid concentration decreases to 10% of the maximum concentration is determined as the end time of acid etching at that reference point. Since it takes time to reach the designated reference point after acid injection begins, the acid begins to react upon contact with the fracture wall, and acid injection continues during this time. Therefore, the acid concentration within the fracture generally increases. After acid injection stops, the acid-rock reaction continues, the acid solution changes from active acid to residual acid, and the acid concentration decreases. When the acid concentration begins to decrease to 10% of the maximum concentration, i.e., the acid concentration-time curve flattens out, the time at which this occurs is recorded as the end time of the acid reaction. In this way, the acid etching time at each reference point can be calculated.

[0070] Step S5: Based on the simulation results in Step S4, the conductivity calculation model of acid-etched cracks is used to obtain the distribution characteristics of conductivity of acid-etched cracks in the whole domain.

[0071] To accurately calculate the conductivity considering the surface geometry of the crack, this study investigates the surface geometry characterization and conductivity of acid-etched cracks. By obtaining surface characterization parameters of acid-fracturing cracks, a method for calculating the contact area ratio of rough cracks under closure pressure is derived based on experimental methods and theoretical derivation. The crack width variation law of rough cracks with closure pressure is studied using the contact ratio. Combined with the classical cubic law of cracks, a calculation model for the conductivity of acid-fracturing cracks is obtained, thus obtaining the conductivity of the entire domain.

[0072] Specifically, a laser 3D scanner is used to scan the surface of an acid-etched rock slab to obtain rough point cloud data, such as... Figure 4 Establish a three-dimensional XYZ coordinate system. Along any plane in the X direction (y = y... j The rock slab is cut, and the top edge of the profile on the cut surface is a zigzag line. The length of this zigzag line is defined as L. cc Similarly, for any plane of the rock slab along the Y direction (x = x j After cutting, the length of the bend at the top edge of the profile of the rock slab on the cut surface is defined as L. cl Take step size L x 0.3mm, defined The lateral tortuosity of the rock sample surface. Let be the longitudinal tortuosity ratio of the rock sample surface. Then, the following relevant calculation formulas can be obtained:

[0073]

[0074]

[0075]

[0076]

[0077] In the formula, columns X and Y are the planar coordinates of each point on the rock slab surface, column Z is the height of each point, and N is the height of each point. x N is the number of scan steps in the X direction. y It represents the number of scan steps in the Y direction.

[0078] Acid fracturing fracture conductivity calculation model:

[0079]

[0080]

[0081] In the formula, k f w represents the crack under pressure P. c The guiding capacity under action; (k f w) i This represents the initial conductivity of the crack; The transverse tortuosity ratio of the crack surface; is the longitudinal tortuosity of the fracture surface; m, n are parameters obtained by experimental fitting; β is a fitting coefficient.

[0082] By the above steps, the global acid-etched fracture conductivity distribution can be obtained, Figure 5 is the fracture conductivity distribution in an embodiment.

[0083] Step S6: Based on the acid-etched fracture conductivity in step S5, it is judged whether the acid-etched fracture parameter requirement is met according to the target reservoir requirement.

[0084] In combination with the above steps S1-S6, the acid-etched fracture conductivity under different construction parameters is calculated, so as to optimize the construction parameters.

[0085] In an embodiment, given the acid fracturing requirement of a target block, the lower limit value of the acid-etched fracture conductivity is 0.3Dc·cm. Considering that the acid-etched fracture conductivity change between reference points is linear, the required acid-etched fracture length and acid-etched fracture conductivity under different acid liquid dosages are simulated to determine the optimal acid liquid dosage. As shown in Figure 6 , all reference points in the longitudinal direction at x=0 are selected as the research object, and the conductivity of the farthest reference point is the lower limit value of the acid-etched fracture conductivity. From the simulation results under different acid conditions, it can be seen that when the acid liquid dosage is 400-500m 3 , the lower limit value of the acid-etched fracture conductivity is 0.38Dc·cm, which can meet the acid fracturing requirement. 3

[0086] Similarly, through similar multiple simulation experiments, other optimal construction parameters can be obtained.

[0087] As can be seen, the method provided by the present application can determine the lower limit of the conductivity according to the target reservoir requirement when applied in the field, calculate the average acid-etched fracture conductivity by the method provided in the present application, and judge whether the acid-etched fracture parameter requirement is met according to the productivity optimization requirement. If not, change the acid injection displacement, acid injection amount and other parameters, recalculate the global fracture conductivity distribution, and if it is met, the optimization is ended, at which time the preset parameters are the optimal construction parameters. Compared with the prior art, the present application can simulate the acid etching morphology of the fracture more realistically by combining the heterogeneous conditions of the formation, and more accurately calculate the acid-etched fracture conductivity under global conditions by calibrating the specified reference points.

[0088] The above only describes the preferred embodiments of the present application and should not be used to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.​

Claims

1. An acid fracturing parameter optimization method based on full-slug acid etched fracture conductivity simulation, characterized in that, The method comprises the following steps: Step S1: Obtain basic parameters and establish a single-well geological feature model of a target area; Step S2: Combine the construction parameters with the single-well geological feature model, and perform acid fracturing simulation calculation to obtain fracture dynamic environment parameters; Step S3: Calibrate a reference position in the acid fracturing fracture, and obtain the dynamic changes of the environment parameters at different positions; In step S3, the following steps are further included: selecting a central axis in the fracture length direction as a reference line for calculating the conductivity, taking a first reference point sequence at intervals on the central axis, and taking a second reference point sequence in the fracture height direction and parallel to the central axis of the fracture length at intervals, so that the first reference point sequence and the second reference point sequence can cover the fracture surface; Step S4: Perform local acid etching simulation at different positions by using a multi-field coupled acid etching model; In step S4, the following steps are further included: taking the environment parameters of the first reference point sequence and the second reference point sequence in step S3 as boundary conditions of the acid etching model, solving the acid etching model of the first reference point sequence and the second reference point sequence, and performing fracture surface coarsening by using an interpolation algorithm, so as to reproduce the acid liquid dynamic etching process in the fracture; Step S5: Based on the simulation results in step S4, the acid etching fracture conductivity calculation model is used to obtain the global acid etching fracture conductivity distribution characteristics; Step S6: Based on the acid etching fracture conductivity in step S5, it is judged whether the acid etching fracture parameter requirements are met according to the requirements of the target reservoir.

2. The acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation according to claim 1, wherein the acid fracturing simulation calculation in step S2 further comprises: Based on the multi-field coupled acid fracturing model, the fluid mass conservation equation, the flow quantity equation, the convection-diffusion heat conduction equation, the concentration field calculation equation, and the dissolution width calculation equation in the fracture domain are established.

3. The acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation according to claim 1, wherein the fracture dynamic environment parameters comprise: Fracture width, flow field, temperature field, and concentration field parameters.

4. The acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation according to claim 1, wherein step S4 further comprises: According to the simulation in step S3, a curve of acid concentration change with time is obtained for each reference point; When the acid concentration increases to 10% of the highest acid concentration, the corresponding time is judged as the acid etching start time of the reference point; When the acid concentration decreases to 10% of the highest acid concentration, the corresponding time is judged as the acid etching end time of the reference point; The acid etching time of the calculation point is determined.

5. The acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation according to claim 1, wherein step S4 further comprises: A three-dimensional coordinate system is established by using the rough point cloud data obtained by acid etching the rock plate surface; The acid etching rock plate is sectioned, the length of the tortuous line of the profile top edge of the rock plate on the sectioned surface, the rock sample surface transverse tortuosity ratio, and the rock sample surface longitudinal tortuosity ratio are obtained, and acid etching fracture conductivity calculation is performed.

6. The acid fracturing parameter optimization method based on full-fracture acid etching fracture conductivity simulation according to claim 1, wherein step S6 further comprises: Based on the construction parameters of the multivariate combination of acid etching fracture conductivity results, through the dimensionless productivity index analysis of acid fracturing parameters.

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