A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameters

Abstract

The present invention discloses a method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter, which comprises the following steps: S1, generating particles by using the PFC software and assembling the particles to simulate the rock sample; S2, setting the opening degree of the fluid domain channel, generating a fluid domain network in the pore throats of the rock sample by using the PFC fluid-solid coupling algorithm, and setting the injection and outflow conditions of the fluid at the inlet and outlet ends of the rock sample by using the fluid domain network; S3, monitoring the flow rates at the inlet and outlet ends of the rock sample based on the numerical simulation process of simulating the fluid displacement through the core, and simultaneously measuring the magnitude of the rock permeability; when the flow rates at the inlet and outlet ends of the rock sample are stable, the permeability of the rock sample is measured and calculated at this time; S4, if the permeability does not meet the requirements of subsequent numerical simulations, then the opening degree of the fluid domain channel is modified again, and S2-S3 are repeated until the permeability of the rock sample measured and calculated meets the requirements of subsequent numerical simulations, and the calibration work is completed.

Description

Technical Field

[0001] The present invention relates to the field of rock physical property analysis in petroleum engineering, and particularly to a method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter. Background Art

[0002] Rock permeability is one of the very important rock physical property parameters. The magnitude of permeability is directly related to the seepage capacity of oil and gas in underground rocks, and directly affects the recovery rates of oil and gas. Evaluating and measuring the magnitude of rock permeability is the first rock physical property parameter to be measured in the design of oil and gas field development plans, the selection of drilling locations, the optimization of water injection horizons, the configuration of water shutoff and profile control agents, the optimal design of hydraulic fracturing, etc. in the field of petroleum engineering.

[0003] In the field of petroleum engineering, the magnitude of rock permeability is usually measured based on the Darcy seepage experiment, and then the measured permeability parameter is input into numerical simulation software for oil and gas production simulation and the optimal design of engineering plans. However, the discrete element numerical simulation software PFC cannot directly set the magnitude of rock permeability during numerical simulation. Its rock macroscopic physical property parameters are all calibrated and obtained based on the adjustment of microscopic parameters. So far, there are few reports on the method for simulating the Darcy seepage steady-state experiment and calibrating the permeability parameter using the PFC software, which severely restricts the numerical simulation work of engineering plans by oilfield engineering designers using this software. Summary of the Invention

[0004] In order to overcome the problem that the discrete element numerical simulation software PFC cannot measure rock permeability, the present invention provides a method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter. Using the method of the present invention, the simulation of the steady-state Darcy seepage experiment of the core can be carried out in the PFC software, and then the core permeability can be calibrated, providing reliable core physical property parameters for the next numerical simulation.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter, comprising the following steps:

[0007] S1, generating particles using the PFC software and assembling the particles to simulate a rock sample;

[0008] S2, setting the opening degree of the fluid domain channel, generating a fluid domain network in the pore throats of the rock sample using the PFC fluid-solid coupling algorithm, and setting the injection condition of the fluid at the inlet end of the rock sample and the outflow condition of the fluid at the outlet end of the rock sample using the fluid domain network to simulate the process of fluid displacement through the core.

[0009] S3. Based on the numerical simulation process of simulating fluid displacement through the core, monitor the flow rates at the inlet and outlet ends of the rock sample, and simultaneously measure the rock permeability using Darcy's permeability formula; when the flow rates at the inlet and outlet ends of the rock sample are stable, it indicates that the requirements of the Darcy steady-state experiment are met at this time, and the permeability of the rock sample is measured and calculated at this time;

[0010] S4. If the permeability does not meet the requirements of subsequent numerical simulations, then re-modify the opening degree of the fluid domain channels in the fluid domain network, and repeat S2 to S3 until the measured and calculated permeability of the rock sample meets the requirements of subsequent numerical simulations, completing the calibration work.

[0011] Preferably, the said S1 includes the following steps:

[0012] S1.1. According to the type of reservoir rock to be simulated, select the value range of the particle size of the PFC software and the size of the rock porosity, and set the particle size grading in a linear distribution manner;

[0013] S1.2. Use the selected core particle size and porosity values to establish the rock sample required for the rock Darcy seepage experiment, and use the cementation model of the PFC software to cement the rock sample.

[0014] Preferably, the value range of the particle size and the size of the rock porosity meet the following requirements:

[0015] R min and R max are the minimum and maximum values of the particle size respectively, R max / R min = 1.6; the number of particle size levels is n levels, and the size of the i-th level particle size is ; the value range of the porosity of high-permeability reservoir rocks is ≥ 30%, and the value range of the particle size is ≥ 5 mm; the value range of the porosity Φ1 of medium-permeability reservoir rocks is 15% ≤ Φ < 30%, and the value range of the particle size R1 is 1 mm ≤ R < 5 mm; the value range of the porosity Φ2 of low-permeability reservoir rocks is 5% ≤ Φ < 15%, and the value range of the particle size R2 is 0.5 mm ≤ R < 1 mm; the value range of the porosity Φ3 of extra-low-permeability reservoir rocks is < 5%, and the value range of the particle size R3 is < 0.5 mm.

[0016] Preferably, the size of the rock sample is as follows: the length L is 8 cm to 10 cm, and the width W is 5 cm to 6.5 cm; the PFC rock sample cementation model uses the Linear Parallel Bond model.

[0017] Preferably, in S2, the opening degree of the fluid domain channel is set as: , where represents a dimensionless opening degree control coefficient; the value range of α is between 0 and 1; when using the fluid-solid coupling algorithm of PFC software to conduct Darcy seepage steady-state experimental simulation, the gap_mul function is set to 0.

[0018] Preferably, in S2, the pressures at the inlet end and outlet end of the rock sample are set to conduct numerical simulation of Darcy seepage experiment;

[0019] When conducting numerical simulation of Darcy seepage experiment, water with a viscosity of 1 mPa·s and a compression modulus of 2.0*10 9 Pa is selected as the displacement fluid; a region with a length L1 = 2 cm to 4 cm is selected on the left side of the core as the injection point of the fluid domain network. The injection point is used as the inlet end of the fluid, and the displacement pressure at the injection point is set to P w , P w the value range is 1 MPa to 2.5 MPa; a region with a length L2 = 1.5 cm to 3 cm is selected on the right side of the core as the outlet end of the fluid, and the pressure at the outlet end is set to a constant pressure of 0 MPa, and then the displacement process of Darcy seepage experiment is simulated with a constant pressure at the inlet end.

[0020] Preferably, in S3, a script file is written using the fish language of PFC software to monitor the flow rates at the inlet end and outlet end of the rock sample. The basic logic of the script file is to first determine whether the DOM_FIX of all fluid domains is 1. If it is 1, then further determine the magnitude between DOM_VSUM and the numerical value 0. The flow rates of all fluid domains greater than 0 are accumulated as the fluid flow rate out_flow at the outlet end of the core, and the flow rates of all fluid domains less than 0 are accumulated as the fluid flow rate in_flow at the inlet end of the core. Finally, during the numerical simulation of Darcy seepage experiment, the history function is used to monitor the curve corresponding to the fluid flow rate in real time. If the curve no longer fluctuates, it is considered at this time that the flow rates at the inlet end and outlet end of the rock sample are stable.

[0021] Preferably, during the numerical simulation of Darcy seepage experiment, the calculation formula for permeability is , where, is the permeability, L is the length of the rock sample, L1 is the distance between the inlet end of the rock sample and the left end of the rock sample, L2 is the distance between the outlet end of the rock sample and the right end of the rock sample, W is the width of the rock sample, P w is the displacement pressure at the inlet end of the rock sample.

[0022] The present invention has the following beneficial effects:

[0023] The present invention establishes a method for simulating the steady-state experiment of core Darcy seepage using PFC software. Based on this method, research work on discrete element Darcy seepage experiments can be carried out. A permeability measurement and calibration method is established using the established numerical simulation method for steady-state Darcy seepage experiments. Using this method, accurate measurement of rock permeability in the discrete element numerical simulation software PFC can be achieved, thus providing reliable basic rock physical property parameters for the next numerical simulation work. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Flow chart of the method for simulating core Darcy seepage experiment and calibrating permeability parameters of the present invention.

[0025] Figure 2 Schematic diagram of the Darcy seepage experiment rock sample established based on the PFC discrete element method in the embodiment of the present invention.

[0026] Figure 3 Flow rate monitoring curve of the inlet and outlet of the core in Embodiment 1 of the present invention.

[0027] Figure 4 Monitoring result curve of rock Darcy permeability in Embodiment 1 of the present invention.

[0028] Figure 5 Flow rate monitoring curve of the inlet and outlet of the core in Embodiment 2 of the present invention.

[0029] Figure 6 Monitoring result curve of rock Darcy permeability in Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following further describes the present invention with reference to the drawings and embodiments.

[0031] The principle of the present invention is as follows: Particles are generated using PFC software, and the particles are assembled to simulate the rock sample. Then, a fluid domain network is generated in the pore throats of the rock sample (or called core) using the PFC fluid-solid coupling algorithm. The injection and outflow conditions of the fluid at the inlet and outlet ends of the rock sample are set using the fluid domain network, thereby simulating the process of fluid displacement through the core, which realizes the numerical simulation of the Darcy seepage experiment process. Based on the numerical simulation process of this experiment, the flow rates at the inlet and outlet ends of the rock sample are monitored by writing a script file. When the flow rates at the inlet and outlet ends of the rock sample are stable, the requirements of the Darcy steady-state experiment are met, and at this time, the permeability of the rock can be measured and calculated.

[0032] Specifically, referring to Figure 1 and Figure 2 , the method for simulating core Darcy seepage experiment and calibrating permeability parameters of the present invention includes the following steps:

[0033] Step 1: Setting of core particle size and porosity: Select the value range of particle size of PFC software and the size of rock porosity according to the simulated reservoir rock type, and set the particle size grading in a linear distribution manner; specifically, assuming that the minimum and maximum values of the particle size are R min and R max , then it should satisfy R max / R min = 1.6; the number of particle size levels is n levels, then the size of the i-th level particle size is . The value range of high-permeability reservoir rock porosity is ≥ 30%, and the value range of particle size is ≥ 5 mm; the value range of medium-permeability reservoir rock porosity is 15% ≤ Φ < 30%, and the value range of particle size is 1 mm ≤ R < 5 mm; the value range of low-permeability reservoir rock porosity is 5% ≤ Φ < 15%, and the value range of particle size is 0.5 mm ≤ R < 1 mm; the value range of extra-low-permeability reservoir rock porosity is < 5%, and the value range of particle size is < 0.5 mm;

[0034] Step 2: Sampling and cementing of the core: Use the selected core particle size and porosity values to establish the rock sample required for the rock Darcy seepage experiment, and use the cementing model of PFC software to cement the rock sample; among them, the length of the rock sample size L is 8 cm to 10 cm, and the width W is 5 cm to 6.5 cm; the PFC rock sample cementing model adopts the Linear Parallel Bond model;

[0035] Step 3: Simulating the process of core Darcy seepage experiment: Set the opening of the fluid domain channel to , where represents the dimensionless opening control coefficient, and the value range of α is between 0 and 1; then use the fluid-solid coupling algorithm of PFC software to generate a fluid domain network in the rock sample, and then set the pressures at the inlet and outlet ends to conduct numerical simulation of the Darcy seepage experiment; when simulating, the gap_mul function is set to 0 to avoid the influence of water pressure on the steady-state seepage simulation of rock particles; in addition, when simulating, water with a viscosity of 1 mPa·s and a compression modulus of 2.0×10 9 Pa is selected as the displacement fluid; taking the orientation shown in Figure 2 as an example, a region with a length L1 = 2 cm to 4 cm on the left side of the core is selected as the injection point of the fluid domain network, that is, the inlet end of the fluid, and the injection point displacement pressure is set to P w , and the value range is 1 MPa to 2.5 MPa; taking the orientation shown in Figure 2 as an example, a length is selected on the right side of the coreL2 The area of 1.5 cm to 3 cm is the outlet end of the fluid. Set the pressure at the outlet end to a constant pressure of 0 MPa, and then conduct a constant-pressure displacement simulation at the inlet end to simulate the Darcy seepage experiment process.

[0036] Step 4: Monitoring of core flow rate and measurement of permeability: Monitor the flow rates at the inlet end and outlet end of the core, and simultaneously measure the rock permeability using the Darcy permeability formula. After the flow rates at the inlet end and outlet end are stable and their monitoring curves no longer fluctuate, the rock permeability measured at this time is the final permeability value. The calculation formula for permeability is ; Specifically, write a script file using the fish language of the PFC software to monitor the fluid flow rates at the inlet end and outlet end. The basic logic of the script file is to first determine whether the DOM_FIX of all fluid domains is 1. If it is 1, then further determine the magnitude between DOM_VSUM and the value 0. Accumulate the flow rates of all fluid domains greater than 0 to obtain the outlet-end fluid flow rate out_flow, and accumulate the flow rates of all fluid domains less than 0 to obtain the inlet-end fluid flow rate in_flow. Finally, use the history function to monitor the curves corresponding to the fluid flow rates in real time during the numerical simulation of the Darcy seepage experiment.

[0037] Step 5: Calibration of core permeability: After the curve is stable and the rock permeability is measured, if the permeability does not meet the requirements of subsequent numerical simulations, then re-modify the dimensionless control coefficient α of the fluid domain channel opening, and repeat Steps 3 and 4 until the measured rock permeability meets the requirements of subsequent numerical simulations, thus completing the calibration work.

[0038] Example 1:

[0039] In this example, a high-permeability reservoir is selected for Darcy seepage simulation and permeability measurement. Its particle size distribution has a total of five levels, and the particle sizes are: 6 mm, 6.9 mm, 7.8 mm, 8.7 mm, 9.6 mm; the porosity is 10%; the length of the rock sample is 10 cm, and the width is 5 cm; the length of the fluid inlet end is 4 cm, and the length of the fluid outlet end is 2 cm; the dimensionless control coefficient α of the fluid domain channel opening is set to 0.025.

[0040] Set the displacement pressure at the inlet end to 1.0 MPa, use the fluid-solid coupling algorithm of the PFC software to generate the fluid domain network, and then conduct a constant-pressure Darcy seepage experiment simulation. Monitor the flow rates at the inlet and outlet in real time during the simulation. The results are as Figure 3 、 Figure 4As shown. It can be seen from the simulation results that when the flow rate curves at the inlet and outlet of the core are stable and close, the monitoring results of the permeability also tend to be stable, indicating that the Darcy seepage experiment simulation of the rock has reached a stable state at this time. The finally measured rock permeability is 210 mD, meeting the numerical simulation requirements of high-permeability reservoirs.

[0041] Example 2

[0042] In this example, a low-permeability reservoir was selected for Darcy seepage simulation and permeability measurement. Its particle size distribution has five grades, and the particle sizes are: 0.5 mm, 0.575 mm, 0.65 mm, 0.725 mm, 0.8 mm; the porosity is 8%; the length of the rock sample is 10 cm and the width is 5 cm; the length of the fluid inlet end is 4 cm, and the length of the fluid outlet end is 2 cm; the dimensionless control coefficient α of the channel opening of the fluid domain is set to 0.0025.

[0043] The displacement pressure at the inlet end was set to 1.0 MPa, and the fluid domain network was generated using the fluid-solid coupling algorithm of the PFC software. Then, a Darcy seepage experiment simulation under constant pressure was carried out. The flow rates at the inlet and outlet were monitored in real time during the simulation, and the results are as Figure 5 、 Figure 6 shown. It can be seen from the simulation results that when the flow rate curves at the inlet and outlet of the core are stable and close, the monitoring results of the permeability also tend to be stable, indicating that the Darcy seepage experiment simulation of the rock has reached a stable state at this time. The finally measured rock permeability is 0.198 mD, meeting the numerical simulation requirements of low-permeability reservoirs.

Claims

1. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameters, characterized in that, It includes the following steps: S1. Generate particles using PFC software and assemble the particles to simulate rock samples. S2. Set the opening degree of the fluid domain channel, use the PFC fluid-solid coupling algorithm to generate a fluid domain network in the pore throats of the rock sample, and use the fluid domain network to set the injection conditions of the fluid at the inlet end of the rock sample and the outflow conditions of the fluid at the outlet end of the rock sample to simulate the process of fluid displacement through the core. S3. Monitor the flow rates at the inlet and outlet ends of the rock sample based on the numerical simulation process of simulating fluid displacement through the core, and simultaneously measure the magnitude of the rock permeability using Darcy's permeability formula; when the flow rates at the inlet and outlet ends of the rock sample are stable, it indicates that the requirements of the Darcy steady-state experiment are met at this time, and the permeability of the rock sample is obtained through measurement and calculation at this time. S4. If the permeability does not meet the requirements of subsequent numerical simulations, then re-modify the opening degree of the fluid domain channel in the fluid domain network, and repeat S2 to S3 until the permeability of the rock sample obtained through measurement and calculation meets the requirements of subsequent numerical simulations, and the calibration work is completed.

2. The method for simulating a core Darcy seepage experiment and calibrating permeability parameters according to claim 1, wherein The S1 includes the following steps: S1.

1. Select the value range of the particle size of the PFC software and the magnitude of the rock porosity according to the reservoir rock type to be simulated, and set the particle size grading in a linear distribution manner. S1.

2. Use the selected core particle size and porosity values to establish the rock sample required for the rock Darcy seepage experiment, and use the cementation model of the PFC software to cement the rock sample.

3. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter according to claim 2, characterized in that, The value range of the particle size and the magnitude of the rock porosity meet the following requirements: R min and R max are the minimum and maximum values of the particle size respectively, R max / R min = 1.6; the number of grades of the particle size is n grades, and the size of the particle size of the i-th grade is ; the value range of the porosity of the high-permeability reservoir rock is ≥ 30%, and the value range of the particle size is ≥ 5 mm; the value range of the porosity Φ1 of the medium-permeability reservoir rock is 15% ≤ Φ < 30%, and the value range of the particle size R1 is 1 mm ≤ R < 5 mm; the value range of the porosity Φ2 of the low-permeability reservoir rock is 5% ≤ Φ < 15%, and the value range of the particle size R2 is 0.5 mm ≤ R < 1 mm; the value range of the porosity Φ3 of the extra-low permeability reservoir rock is < 5%, and the value range of the particle size R3 is < 0.5 mm.

4. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter according to claim 2, characterized in that, The dimensions of the rock sample are as follows: length L is 8 cm to 10 cm, width W is 5 cm to 6.5 cm; the LinearParallel Bond model is used for the PFC rock sample cementation model.

5. A method for simulating a core Darcy seepage experiment and calibrating permeability parameters according to claim 1, characterized in that, In S2, the opening degree of the fluid domain channel is set as: , where represents a dimensionless opening degree control coefficient; the value range of α is between 0 and 1; when performing Darcy seepage steady-state experiment simulation using the fluid-structure interaction algorithm of PFC software, the gap_mul function is set to 0.

6. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter according to claim 1, characterized in that In S2, set the pressures at the inlet and outlet ends of the rock sample and conduct numerical simulation of the Darcy seepage experiment. When conducting numerical simulation of Darcy flow experiments, water with a viscosity of 1 mPa·s and a compression modulus of 2.0×10 9 Pa is selected as the displacement fluid; a region with a length L1 = 2 cm to 4 cm is selected on the left side of the core as the injection point of the fluid domain network. The injection point serves as the inlet end of the fluid, and the displacement pressure at the injection point is set to P w , P w with a value range of 1 MPa to 2.5 MPa; a region with a length L2 = 1.5 cm to 3 cm is selected on the right side of the core as the outlet end of the fluid, and the pressure at the outlet end is set to a constant pressure of 0 MPa. Then, the process of the Darcy flow experiment is simulated by displacement with a constant pressure at the inlet end.

7. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter according to claim 1, characterized in that In S3, use the fish language of the PFC software to write a script file to monitor the flow rates at the inlet and outlet ends of the rock sample. The basic logic of the script file is to first determine whether the DOM_FIX of all fluid domains is 1. If it is 1, then further determine the magnitude between DOM_VSUM and the numerical value 0. Accumulate the flow rates of all fluid domains greater than 0 as the fluid flow rate out_flow at the outlet end of the core, and accumulate the flow rates of all fluid domains less than 0 as the fluid flow rate in_flow at the inlet end of the core. Finally, use the history function to monitor the curve corresponding to the fluid flow rate in real time during the numerical simulation of the Darcy seepage experiment. If the curve no longer fluctuates, it is considered that the flow rates at the inlet and outlet ends of the rock sample are stable at this time.

8. A method for simulating the Darcy seepage experiment of a core and calibrating the permeability parameter according to claim 7, characterized in that, During the numerical simulation of the Darcy seepage experiment, the calculation formula for permeability is , where is the permeability, L is the length of the rock sample, L1 is the distance between the inlet end of the rock sample and the left end of the rock sample, L2 is the distance between the outlet end of the rock sample and the right end of the rock sample, W is the width of the rock sample, P w is the displacement pressure at the inlet end of the rock sample.

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