A method for simulating nuclear magnetic resonance in rocks considering multiphase flow and mixed-wetting characteristics

Abstract

The application discloses a rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics, and belongs to the field of rock wettability, and specifically comprises the following steps: step 1, constructing a three-dimensional digital rock model; step 2, determining the pore size of the three-dimensional digital rock model; step 3, constructing a rock model with mixed wetting characteristics; step 4, using a Shan-Chen LBM method to simulate the multiphase flow of the rock model, so that a mixed wetting rock model with oil-water distribution is obtained; step 5, using a random walk method to simulate the mixed wetting rock model under different wettability conditions, collecting a nuclear magnetic resonance CPMG pulse sequence of the mixed wetting rock model, and recording a total magnetization vector signal; and step 6, inverting the recorded total magnetization vector signal, so that the distribution of the mixed wetting characteristic rock under a certain wettability condition is obtained. T 2The application provides a solution for the research on the nuclear magnetic resonance response of rock wettability.

Description

Technical Field

[0001] This invention belongs to the field of rock wettability, and in particular to a nuclear magnetic resonance simulation method for rocks that takes into account multiphase flow and mixed wetting characteristics. Background Technology

[0002] Wettability refers to the tendency of a fluid to spread or adsorb on a solid surface in the presence of an immiscible fluid phase. Oil reservoir rock wettability is one of the characteristic parameters of rock physics. It affects the distribution of pore fluids, determines the microscopic displacement efficiency of the rock, restricts the formulation and implementation of recovery strategies, and plays an important role in evaluating parameters such as permeability and saturation.

[0003] Most reservoir rocks are neither completely hydrophilic nor completely oleophilic, exhibiting heterogeneous wetting characteristics, which can be specifically divided into spotted wetting and mixed wetting. Spotted wetting is mainly caused by the different wettabilities of the minerals that make up the rock. Mixed wetting is related to the distribution of rock pore size. When crude oil enters an initially hydrophilic reservoir, after undergoing "aging," the wettability of large pores changes from hydrophilic to oleophilic due to the adsorption and deposition of polar compounds from the crude oil, while the surface of small pores remains hydrophilic. Therefore, most tight reservoirs are dominated by mixed wetting. In the past decade of oil exploration and development, more and more oil-wetted and mixed-wetted reservoirs have been discovered. The complexity of rock wettability significantly affects rock physical response; therefore, clarifying how mixed wetting affects reservoir rock fluid distribution and rock physical response, and further quantifying mixed wetting, has become an important scientific problem.

[0004] Nuclear magnetic resonance (NMR) is an essential analytical testing technique for exploring complex oil and gas reservoirs. It obtains rock physical parameters such as porosity, permeability, pore size distribution, and bound water saturation by utilizing information like relaxation time, and also observes and analyzes the dynamic behavior of fluid molecules; it is an effective tool for studying rock wettability. Since wettability factors are difficult to completely control in rock samples, simulation techniques offer a solution.

[0005] However, existing techniques for studying wettability using rock models in NMR response analysis either simply define pores as completely hydrophilic or completely oleophilic, or use the Kovscek model to explain variations in rock pore wettability. Therefore, existing rock models have the following drawbacks: (1) they assume uniform wettability across all pore surfaces; (2) they do not distinguish between local and apparent wettability; (3) they simplify pore fluid distribution based on wetting film thickness; and (5) they do not consider moderate wetting conditions. These drawbacks lead to significant biases in NMR response studies based on these rock models. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention discloses a nuclear magnetic resonance simulation method for rocks that considers multiphase flow and mixed wetting characteristics, providing a solution for the study of nuclear magnetic resonance response of rock wettability.

[0007] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0008] A nuclear magnetic resonance simulation method for rocks that considers multiphase flow and mixed wetting characteristics, specifically including:

[0009] Step 1: Construct a three-dimensional digital rock model using random stacking and sedimentary process-based reconstruction methods;

[0010] Step 2: Determine the aperture size of the 3D digital rock model based on the average distance from the pixels on the surface of the spherical particles to the surrounding hole walls;

[0011] Step 3: Determine whether the particle surface is hydrophilic or oleophilic based on the aging characteristics of the reservoir rocks. If the pore size R calculated in Step 2 is less than the pore size cutoff value R... cutoff If the rock surface is set to hydrophilic, and the pore size R calculated in step 2 is greater than or equal to the pore size cutoff value R... cutoff The rock surface is then set to be oleophilic, thus constructing a rock model with mixed wetting characteristics;

[0012] Step 4: The Shan-ChenLBM method is used to simulate multiphase flow in the rock model constructed in Step 3. The pore oil-water distribution of the rock model is calculated based on the input adjustable parameters, thereby obtaining a stable mixed-wetting rock model with oil-water saturation. The adjustable parameters include oil saturation, contact angle of the hydrophilic surface, contact angle of the oleophilic surface, and evolution number.

[0013] Step 5: The random walk method was used to simulate the mixed wettability rock model under different wettability conditions, and its nuclear magnetic resonance CPMG pulse sequence was collected to record the total magnetization vector signal;

[0014] Step 6: Invert the total magnetization vector signal recorded in Step 5 to obtain the T2 distribution of mixed wetting characteristic rocks under certain wetting conditions, which can be used for nuclear magnetic resonance studies of rocks with different wetting properties.

[0015] Preferably, in step 1, the spherical particles in the three-dimensional digital rock model are randomly stacked, and the particle size of the spherical particles is consistent or exhibits a Gaussian distribution.

[0016] Preferably, in step 2, the method for calculating the aperture size of the three-dimensional digital rock model is as follows:

[0017] With a pixel on the surface of the spherical particle as the center, rays are emitted in N directions around it, denoted as l.n The process continues until each ray encounters the boundary of another particle, n = 1, 2, ..., N, and stops. The distance from each ray to the particle boundary is then obtained and denoted as Dl. n The aperture R is calculated as follows: n = 1, 2, ..., N, where n is the distance from the pixel point on the surface of the spherical particle to the boundary.

[0018]

[0019] Preferably, in step 4, the Shan-ChenLBM method uses the D3Q19 model for multiphase flow simulation, and N is set to 18.

[0020] Preferably, in step 4, the contact angle of the hydrophilic interface and the contact angle of the oleophilic interface are both 0°-90°.

[0021] Preferably, in step 5, the wettability conditions are determined based on the contact angle of the hydrophilic surface, the contact angle of the oleophilic surface, and the wettability area ratio.

[0022] Beneficial effects: This invention discloses a nuclear magnetic resonance simulation method for rocks that considers multiphase flow and mixed wetting characteristics, and has the following advantages:

[0023] (1) This invention utilizes multiphase flow simulation technology to provide the oil-water distribution within rock pores, thereby improving the accuracy of pore fluid distribution simulation;

[0024] (2) This invention establishes a mixed wetting rock model related to pore size and improves the heterogeneous wetting rock model;

[0025] (3) The present invention provides the response of rock local wettability (contact angle) to nuclear magnetic resonance T2, which is beneficial for analyzing the nuclear magnetic resonance response characteristics of micro wettability. Attached Figure Description

[0026] Figure 1 This is a flowchart illustrating the implementation of the present invention.

[0027] Figure 2 This refers to the rock model considering mixed wetting characteristics obtained in step 3 of Embodiment 1 of the present invention;

[0028] Figure 3 The rock model considering multiphase flow obtained in step 4 of embodiment 1 of the present invention;

[0029] Figure 4 When the oil saturation is 50%, the oil wetting area ratio is 0.41, and the T2 distribution of the actual contact angle of the fixed hydrophilic surface is determined.

[0030] Figure 5When the oil saturation is 50%, the oil wetting area ratio is 0.41, and the T2 distribution of the actual contact angle of the fixed oleophilic surface is determined.

[0031] Figure 6 The T2 distribution is for different wetting area ratios when the oil saturation is 50%. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. The description of the specific embodiments below is merely exemplary and should be understood as being used only to explain the invention, and not in any way to limit the invention or its applications or uses.

[0033] Example 1

[0034] A nuclear magnetic resonance simulation method for rocks that considers multiphase flow and mixed wetting characteristics, such as Figure 1 As shown, the specific methods include:

[0035] Step 1: Construct a three-dimensional digital rock model using random stacking and sedimentary process-based reconstruction methods, such as... Figure 2 As shown, the spherical particles in the three-dimensional digital rock model are randomly stacked, and the particle size of the spherical particles exhibits a Gaussian distribution.

[0036] Step 2: Determine the aperture size of the 3D digital rock model based on the average distance from the pixels on the surface of the spherical particles to the surrounding pore walls. The specific calculation method is as follows:

[0037] Radiation is emitted in 18 directions from the pixel on the surface of the spherical particle, denoted as l. n The process continues until each ray encounters the boundary of another particle, n = 1, 2, ..., 18. The distance from each ray to the particle boundary is then obtained and denoted as Dl. n n=1,2…18, the average distance from the pixel point on the surface of the spherical particle to the boundary is the aperture R, as shown in formula (1):

[0038]

[0039] The value of N in this invention is determined based on the model in the Shan-ChenLBM method. Since the D3Q19 model is used in this embodiment, the value of N is selected as 18.

[0040] Step 3: Determine whether the particle surface is hydrophilic or oleophilic based on the aging characteristics of the reservoir rocks. If the pore size R calculated in Step 2 is less than the pore size cutoff value R... cutoff If the rock surface is set to hydrophilic, and the pore size R calculated in step 2 is greater than or equal to the pore size cutoff value R... cutoffThe rock surface is then set to be oleophilic, thus constructing a rock model with mixed wetting characteristics, such as... Figure 2 As shown.

[0041] Step 4: Using the D3Q19 model based on the Shan-ChenLBM method, multiphase flow simulation is performed to obtain the pore oil-water distribution. Based on the input adjustable parameters (including oil saturation, contact angle of the hydrophilic surface, contact angle of the oleophilic surface, and evolution number), the pore oil-water distribution of the rock model is calculated, thus obtaining a stable mixed-wetting rock model with saturated oil-water distribution, such as... Figure 3 As shown. In this invention, step 4 uses the Shan-ChenLBM method to obtain the pore oil-water distribution. Generally, more than 1000 evolutions are needed to obtain a stable oil-water distribution. In this embodiment 1, the number of evolutions is 1000.

[0042] Step 5: The random walk method was used to simulate the mixed wettability rock model under different wettability conditions, and its nuclear magnetic resonance CPMG pulse sequence was collected to record the total magnetization vector signal;

[0043] Step 6: Invert the total magnetization vector signal recorded in Step 5 to obtain the T2 distribution of mixed wetting characteristic rocks under certain wetting conditions, which can be used for nuclear magnetic resonance studies of rocks with different wetting properties.

[0044] like Figure 4 The figure shows the T2 distribution of mixed-wetting characteristic rocks under conditions of 50% oil saturation, an oil-wetting area ratio of 0.41, a fixed hydrophilic surface contact angle of 0°, and different oleophilic surface contact angles (0°, 30°, 50°, 80°, 90°). The oil-wetting area ratio is equal to (greater than) the pore size cutoff value R. cutoff (pixels) / total pixels. As can be seen from the figure, when the oil saturation, hydrophilic surface contact angle and wetting area ratio are fixed, changing the oleophilic surface contact angle will affect the T2 distribution of rock nuclear magnetic resonance.

[0045] like Figure 5 The figure shows the T2 distribution of mixed-wetting characteristics rocks under the conditions of 50% oil saturation, oil-wetting area ratio of 0.41, and a fixed oleophilic surface contact angle of 0°, with different hydrophilic surface contact angles (0°, 30°, 50°, 80°, 90°). As can be seen from the figure, when the oil saturation, oleophilic surface contact angle, and wetting area ratio are fixed, changing the hydrophilic surface contact angle will affect the T2 distribution of the rock nuclear magnetic resonance.

[0046] like Figure 6 The figure shows the T² results for different wetting area ratios of hydrophilic and oleophilic surfaces under conditions of 50% oil saturation and a hydrophilic / oleophilic contact angle of 30°. It can be seen from the figure that as the pore size cutoff value R... cutoffThe increase in the wetted area, or the decrease in the oil-wetting area, causes the right peak of T2 (considered to be the aqueous phase) to shift to the left, and the left peak of T2 (considered to be the oil phase) to shift to the right. This is because the surface relaxation of water is enhanced, while the surface relaxation of oil is weakened. The figure also shows that when the oil saturation, hydrophilic surface contact angle, and oleophilic surface contact angle are fixed, changing the wetted area ratio affects the T2 distribution of the rock's nuclear magnetic resonance.

[0047] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A method for simulating rock nuclear magnetic resonance considering multiphase flow and mixing wetting characteristics, characterized in that, Specific methods include: Step 1: Construct a three-dimensional digital rock model using random stacking and sedimentary process-based reconstruction methods; Step 2: Determine the aperture size of the 3D digital rock model based on the average distance from the pixels on the surface of the spherical particles to the surrounding hole walls; Step 3: Determine whether the particle surface is hydrophilic or oleophilic based on the aging characteristics of the reservoir rocks. If the pore size R calculated in Step 2 is less than the pore size cutoff value R... cutoff If the rock surface is set to hydrophilic, and the pore size R calculated in step 2 is greater than or equal to the pore size cutoff value R... cutoff The rock surface is then set to be oleophilic, thus constructing a rock model with mixed wetting characteristics; Step 4: The Shan-ChenLBM method is used to simulate multiphase flow in the rock model constructed in Step 3. The pore oil-water distribution of the rock model is calculated based on the input adjustable parameters, thereby obtaining a stable mixed-wetting rock model with oil-water saturation. The adjustable parameters include oil saturation, contact angle of the hydrophilic surface, contact angle of the oleophilic surface, and evolution number. Step 5: The random walk method was used to simulate the mixed wettability rock model under different wettability conditions, and its nuclear magnetic resonance CPMG pulse sequence was collected to record the total magnetization vector signal; Step 6: Invert the total magnetization vector signal recorded in Step 5 to obtain the T2 distribution of mixed wetting characteristic rocks under certain wetting conditions, which can be used for nuclear magnetic resonance studies of rocks with different wetting properties.

2. The rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics according to claim 1, characterized in that, In step 1, the spherical particles in the three-dimensional digital rock model are randomly stacked, and the particle size of the spherical particles is consistent or exhibits a Gaussian distribution.

3. The rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics according to claim 1, characterized in that, In step 2, the method for calculating the aperture size of the three-dimensional digital rock model is as follows: With a pixel on the surface of the spherical particle as the center, rays are emitted in N directions around it, denoted as l. n The process continues until each ray encounters the boundary of another particle, n = 1, 2, ..., N, and stops. The distance from each ray to the particle boundary is then obtained and denoted as Dl. n The aperture R is calculated as follows: n = 1, 2, ..., N, where n is the distance from the pixel point on the surface of the spherical particle to the boundary.

4. The rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics according to claim 3, characterized in that, In step 4, the Shan-ChenLBM method uses the D3Q19 model for multiphase flow simulation, and N is set to 18.

5. The rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics according to claim 1, characterized in that, In step 4, the contact angle of the hydrophilic interface and the contact angle of the oleophilic interface are both 0°-90°.

6. The rock nuclear magnetic resonance simulation method considering multiphase flow and mixed wetting characteristics according to claim 1, characterized in that, In step 5, the wettability conditions are determined based on the contact angle of the hydrophilic surface, the contact angle of the oleophilic surface, and the ratio of wetted area.

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