Method for Establishing Core Skeleton Model of Shale Multi-Mineral Composition Based on Molecular Simulation

Abstract

The present invention relates to a method for establishing a core framework model of shale multi-mineral components based on molecular simulation, including: (1) extracting formation shale samples, obtaining mineral composition information through X-ray diffraction experiments, and calculating the percentage content of each mineral component; (2) respectively constructing primitive unit cell models of each mineral; (3) cutting the primitive unit cell models to cut out their most easily exposed surfaces as the minimum units of the composite mineral model; (4) calculating the minimum integer ratio of each mineral molecule in the shale sample to obtain the number of each mineral molecule required for model establishment; (5) using the Amorphous Cell Calculation section of the molecular simulation software Materials Studio to randomly adsorb and fill the cut surface models of each mineral primitive unit cell to form a composite mineral model composed of different mineral unit cells; (6) optimizing the structure of the composite mineral model. The model established by the present invention can accurately calculate the shale oil and gas adsorption amount under real formation conditions, providing a theoretical basis for the exploration and development of shale oil and gas.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and particularly to a method for establishing a core skeleton model of shale multi-mineral composition based on molecular simulation. Background Art

[0002] The mineral composition of shale reservoirs is relatively complex, consisting of a variety of mixed minerals. After deposition, these minerals enter the compaction and cementation stages. Under the action of overlying rock pressure, the forces between sedimentary layers, and tectonic stress, a series of physical and chemical changes occur, which change the physical properties of the reservoir. Among them, compaction and cementation have the greatest impact on reservoir transformation. That is, compaction and cementation are the most important diagenetic types during the diagenetic process (Wang Zhao et al., Influence of Compaction and Cementation on the Chang-8 Reservoir in Ordos [J]. Journal of Southwest Petroleum University (Natural Science Edition), 2018, 40(06): 56-68). Therefore, the connection between different minerals mainly exists in physical cementation. Thus, physical combinations of multiple minerals can be used to restore the original shale reservoir mineral skeleton model.

[0003] Molecular simulation has been widely used in the petroleum field and has now become an effective means for simulating the adsorption on the surface of shale minerals. At present, domestic and foreign scholars' research on the adsorption simulation of shale minerals mostly stays on the study of single-mineral models (Huang Liang et al., Molecular Simulation of Methane Adsorption Characteristics in Deep Shale Illite [J]. Journal of Central South University (Natural Science Edition), 2022, 53(09): 3522-3531), such as montmorillonite, illite, quartz, etc., while ignoring the existence of other minor minerals in real shale. These minor components are distributed in the shale mineral skeleton, which will not only affect the porosity and permeability of the reservoir, but also affect the pore network structure and mechanical properties of the reservoir. The mineral composition of real shale should be a combination of multiple minerals. Therefore, using only a single mineral for simulation calculations cannot reflect the states of various shale oils and gases under heterogeneous shale reservoir conditions. To characterize the true physical and chemical properties of shale minerals under reservoir conditions, the existence of minor components in the structure cannot be ignored.

[0004] Currently, there is an urgent need for a method for establishing a core skeleton model of multi-mineral composition based on molecular simulation to directly and accurately predict the shale oil and gas adsorption amount under original formation conditions, more reliably characterize the microscopic mechanism of shale oil and gas adsorption, and solve the defects and deficiencies of single-mineral molecular simulation. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for establishing a core skeleton model of shale multi-mineral composition based on molecular simulation. The principle of this method is reliable, the operation is simple, and the established model can accurately calculate the shale oil and gas adsorption amount under real formation conditions, so as to study the laws and microscopic mechanisms of shale oil and gas adsorption, and provide a theoretical basis for the subsequent exploration and development of shale oil and gas.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solutions.

[0007] A method for establishing a core framework model of shale multi-mineral components based on molecular simulation successively includes the following steps:

[0008] (1) Extract formation shale samples, obtain information on the types of mineral components through X-ray diffraction experiments, calculate the percentage content of each mineral component (SY / T 5163-2010, X-ray diffraction analysis method for clay minerals and common non-clay minerals in sedimentary rocks), and obtain the unit cell information of each mineral by extracting the mineral crystal information in the X-ray diffraction pattern;

[0009] (2) According to the unit cell information of each mineral, input the corresponding unit cell parameters and respectively construct the unit cell models of each mineral's primitive cell;

[0010] (3) Cut the unit cell models of each mineral's primitive cell to cut out the most easily exposed surface as the minimum unit of the composite mineral model;

[0011] (4) Calculate the minimum integer ratio of the mass of each mineral according to the percentage content of each mineral component, then divide by the molar mass of each mineral's unit cell to obtain the minimum integer ratio of the molecules of each mineral in the shale sample. According to the size of the composite mineral model to be established, multiply the minimum integer ratio by the corresponding coefficient to obtain the number of molecules of each mineral required to establish this model;

[0012] (5) Use the Amorphous Cell Calculation module of the molecular simulation software Materials Studio, and randomly adsorb and fill the unit cell cross-section models of each mineral obtained in step (3) with reference to the number of molecules of each mineral obtained in step (4) to form a composite mineral model composed of different mineral unit cells;

[0013] (6) Optimize the structure of the composite mineral model until the total energy of the model reaches the minimum convergence value to obtain the core framework model of shale multi-mineral components.

[0014] Further, the step (1) includes the following contents:

[0015] S1. Wash, dry, and grind the shale sample into powder;

[0016] S2. Place the sample powder in the groove of the sample slide, make the measured surface of the sample rough, and keep it flat and uniform with the surface of the sample slide;

[0017] S3. Obtain the X-ray diffraction pattern and delimit the baseline by using analysis software or manually selecting baseline points;

[0018] S4. Calculate the X-ray diffraction peak intensity. The X-ray diffraction peak intensity uses the integrated intensity after background subtraction. Read the relevant data from the X-ray diffraction pattern of the sample, and then compare it with the standard X-ray diffraction data of the mineral to determine the type of mineral composition.

[0019] S5. Extract the mineral crystal information from the X-ray diffraction pattern to obtain the unit cell information of each mineral in the shale sample.

[0020] S6. Perform peak separation and fitting on the X-ray diffraction pattern to calculate the mass percentage content of various clay minerals and non-clay minerals.

[0021] Further, in step (2), unit cell models of each mineral are constructed respectively. The quartz uses the α-quartz unit cell model, the illite uses the illite type at the end of the transformation process, and the unit cell models of montmorillonite and dolomite both come from the American Mineralogist Crystal Structure Database.

[0022] Further, in step (3), the unit cell models of each mineral are sectioned, and the most easily exposed surface is cut out as the smallest unit of the composite mineral model. The (1 0 0) plane is cut from the quartz mineral unit cell, the (0 0 1) plane is cut from the illite unit cell, the (0 0 -1) plane is cut from the montmorillonite unit cell, and the (1 0 4) plane is cut from the dolomite unit cell.

[0023] Further, step (6) refers to using the Geometry Optimization task in the Forcite module of the molecular simulation software Materials Studio to optimize the structure of the composite mineral model to make it at the lowest energy state.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention adopts a hybrid model constructed by multiple mineral unit cells, avoiding the problem that the adsorption amount calculated using a single mineral model has a large gap with the experimental value.

[0026] (2) The present invention uses X-ray diffraction experiments to analyze the mineral composition and constructs a mineral model with the same composition, which more truly and effectively characterizes the composition, content, and structure of real shale minerals.

[0027] (3) The present invention uses the AC section of Materials Studio for random filling of minerals to avoid the uneven wettability caused by local accumulation of a single mineral.

[0028] (4) The shale multi-mineral composition core framework model constructed by the present invention provides a theoretical basis for subsequent research on the adsorption and migration laws of micro-nano pore oil and gas using molecular dynamics.

[0029] Since the present invention improves and innovates key steps in the modeling process, the model established by the present invention can directly and accurately predict the shale gas adsorption amount under real formation conditions, thereby exploring the microscopic mechanism of shale gas adsorption change. Description of the Drawings

[0030] Figure 1 Scanning electron microscope image (a) and energy spectrum diagram (b) of quartz in shale.

[0031] Figure 2 Relative content of each mineral in shale reservoir.

[0032] Figure 3 Schematic diagram of X-ray diffraction pattern of each mineral:

[0033] (a) Montmorillonite, (b) Dolomite, (c) Potassium feldspar, (d) Illite.

[0034] Figure 4 Initial unit cell diagram of each mineral:

[0035] (a) Quartz, (b) Illite, (c) Montmorillonite, (d) Dolomite.

[0036] Figure 5 Section view of unit cell of each mineral:

[0037] (a) 100 plane of quartz, (b) 001 plane of illite, (c) 00-1 plane of montmorillonite, (d) 104 plane of dolomite.

[0038] Figure 6 Initial model of core framework model of multi-mineral composition of shale.

[0039] Figure 7 Total energy change diagram during structure optimization process. Detailed Description of the Invention

[0040] The present invention will be further described in detail below in conjunction with the drawings and embodiments. However, the embodiments of the present invention are not limited thereto. Without departing from the above technical idea of the present invention, various substitutions, changes and arbitrary combinations, etc., made according to common general technical knowledge and conventional means in the art shall all be included within the protection scope of the present invention.

[0041] Silica, illite, dolomite, and montmorillonite are the main components of tight shale. The present invention uses these four minerals as the basic units for establishing the core framework model of multi-mineral composition of shale.

[0042] Embodiment

[0043] A method for establishing a core framework model of multi-mineral composition of shale based on molecular simulation, successively including the following steps:

[0044] (1) Extract the shale minerals required for the experiment. Taking the shale of the 12th - 13th sand member of the lower third sub - member of the Shahejie Formation as an example, its mineral composition mainly consists of brittle minerals such as clay minerals and quartz. Among them, the clay minerals are mainly montmorillonite and illite. Obtain the information on the types of mineral components in the tight shale through X - ray diffraction experiments. It is measured that the relatively large proportions in the mineral components are quartz ( Figure 1 ), montmorillonite, illite, and dolomite. The detailed mass fraction ratio information is as Figure 2 shown.

[0045] S1. Wash the rock samples and dry them at a temperature below 60 °C. After cooling, grind them until all particles are less than 40 μm or there is no particle feeling when rubbed between fingers;

[0046] S2. Place the sample powder in the groove of the sample slide, making the measured surface of the sample rough and keeping it flat and uniform with the surface of the sample slide to avoid preferred orientation of the crystal powder;

[0047] S3. Obtain the X - ray diffraction pattern of the sample, observe the diffraction peak intensity, diffraction peak shape, and d - value in it. According to the characteristics of each type of X - ray diffractometer and the characteristics of the sample, use analysis software or manually select baseline points to delimit the baseline;

[0048] S4. Calculate the X - ray diffraction peak intensity. The X - ray diffraction peak intensity uses the integrated intensity after subtracting the background. Read the relevant data according to the X - ray diffraction pattern of the sample, and then compare it with the standard X - ray diffraction data of the minerals to determine the types of mineral components. The X - ray diffraction patterns of montmorillonite, dolomite, potassium feldspar, and illite are as Figure 3 shown;

[0049] S5. Extract the mineral crystal information in the X - ray diffraction pattern, obtain the unit cell information of quartz, illite, montmorillonite, and dolomite respectively, and obtain the density information, diffraction angle, interplanar spacing d - value, measured peak intensity, corresponding phase, and crystal plane index of each clay mineral in the rock sample. At the same time, list the diffraction angle on the standard card and the difference between the standard diffraction angle and the measured value;

[0050] S6. After determining the types of each mineral component in the sample, perform peak separation and fitting processing on the diffraction pattern. Based on the content of each phase and combined with the peak intensity ratio of each clay mineral measured, use formula (1) to calculate the content of various clay minerals and non - clay minerals:

[0051]

[0052] In the formula, X i —The percentage content of the i - th mineral, expressed as a percentage;

[0053] I i —The selected diffraction integrated intensity of the i - th mineral;

[0054] K i —Reference intensity of the mineral.

[0055] (2) Respectively construct the unit cell models of each mineral measured in step 1. According to the characteristics of the material, input the corresponding unit cell parameters to construct the heterogeneous shale molecular model. For quartz, use the α - quartz unit cell model, as shown in Figure 4 (a). Its lattice parameters are: a = b = 0.491 nm, c = 0.54 nm, α = β = 90°, γ = 120°; for illite, use the illite type at the end of the transformation process, as shown in Figure 4 (b). Its lattice parameters are: a = 0.522 nm, b = 0.9 nm, c = 1.032 nm, α = 90.26°, β = 103.05°, γ = 89.97°; the unit cell models of montmorillonite and dolomite both come from the American Mineralogist Crystal Structure Database (AMCSD), as shown in Figure 4 (c) and 4(d) respectively. The lattice parameters of montmorillonite are: a = 0.518 nm, b = 0.898 nm, c = 1.5 nm, α = β = γ = 90°; the lattice parameters of dolomite are: a = b = 0.488 nm, c = 1.63 nm, α = β = 90°, γ = 120°.

[0056] (3) Cut the original unit cell models of each mineral, and cut out the most easily exposed surface as the smallest unit for synthesizing the composite model. Cut the (1 0 0) plane of the quartz mineral unit cell. The remaining number of atoms in the cut unit cell is 6 O atoms and 4 Si atoms, and the molar mass of the obtained unit cell is 208 g / mol; cut the (0 0 1) plane of the illite unit cell. The remaining number of atoms in the cut unit cell is 4 Si atoms, 12 O atoms, 2 Al atoms, and 1 K atom, and the molar mass of the obtained unit cell is 451 g / mol; cut the (0 0 -1) plane of the montmorillonite unit cell. The remaining number of atoms in the cut unit cell is 3 Ca atoms, 16 O atoms, 8 Si atoms, and 4 Al atoms, and the molar mass of the obtained unit cell is 708 g / mol; cut the (1 0 4) plane of the dolomite unit cell. The remaining number of atoms in the cut unit cell is 4 Ca atoms, 9 O atoms, 3 C atoms, and 2 Mg atoms, and the molar mass of the obtained unit cell is 388 g / mol. The cut unit models are as shown in Figure 5 as shown.

[0057] (4) Through the percentage content of each mineral, the smallest integer ratio of the molecular masses of each mineral is calculated as quartz: illite: montmorillonite: dolomite = 20:13:10:5. Divide it by the molar mass of the corresponding unit cell to obtain the smallest integer ratio of the number of molecules of each mineral as quartz: illite: montmorillonite: dolomite = 96:29:14:13. Subsequently, use the AC module of MS, input the number of adsorbed molecules of the corresponding mineral, and calculate the size of the core skeleton model. The size of the core skeleton model in this embodiment is The number of molecules of each required mineral is as follows: 96 for quartz, 29 for illite, 14 for montmorillonite, and 13 for dolomite.

[0058] (5) Select the unit models of quartz, illite, montmorillonite, and dolomite established in step (3), and construct a composite model through the AC module of MS. According to the number of molecules of each mineral calculated in step (4): 96 for quartz, 29 for illite, 14 for montmorillonite, and 13 for dolomite, make its composition exactly the same as the mineral composition ratio measured in step 1. Finally, obtain a composite mineral model composed of different mineral unit cells, as Figure 6 shown.

[0059] (6) Use the Geometry Optimization task in the Forcite module of MS to optimize the structure of the composite mineral model (as Figure 7 shown), so that it is in the lowest energy state, and obtain a core skeleton model of shale multi-mineral composition, thereby ensuring that the subsequent calculation results are more accurate.

[0060] The above is only a preferred embodiment of the present invention, and does not impose any form of limitation on the present invention. Any simple modification or equivalent change made to the above embodiments according to the technical methods of the present invention shall fall within the protection scope of the present invention.

Claims

1. Method for establishing core framework model of shale multi-mineral composition based on molecular simulation, successively including the following steps: (1) Extract formation shale samples, obtain mineral composition type information through X-ray diffraction experiments, calculate the percentage content of each mineral composition, and obtain the unit cell information of each mineral by extracting the mineral crystal information in the X-ray diffraction pattern; (2) According to the unit cell information of each mineral, input the corresponding unit cell parameters, and respectively construct the primitive unit cell models of each mineral; (3) Cut the primitive unit cell models of each mineral, cut out the most easily exposed surface as the minimum unit of the composite mineral model; (4) Calculate the minimum integer ratio of the mass of each mineral according to the percentage content of each mineral composition, and then divide by the molar mass of each mineral unit cell to obtain the minimum integer ratio of the molecules of each mineral in the shale sample. According to the size of the composite mineral model to be established, multiply the minimum integer ratio by the corresponding coefficient to obtain the number of molecules of each mineral required to establish the model; (5) Use the Amorphous Cell Calculation section of the molecular simulation software Materials Studio, and randomly adsorb and fill the primitive unit cell section models of each mineral obtained in step (3) with reference to the number of molecules of each mineral obtained in step (4) to form a composite mineral model composed of different mineral unit cells; (6) Optimize the structure of the composite mineral model until the total energy of the composite mineral model reaches the minimum convergence value to obtain the core framework model of shale multi-mineral composition.

2. The method for establishing a core framework model of shale multi-mineral composition based on molecular simulation according to claim 1, characterized in that, the step (1) includes the following contents: S1. Wash, dry and grind the shale sample into powder; S2. Place the sample powder in the groove of the sample slide, make the measured surface of the sample rough, and keep it flat and uniform with the surface of the sample slide; S3. Obtain the X-ray diffraction pattern, and use analysis software or manually select baseline points to delimit the baseline; S4. Read relevant data according to the X-ray diffraction pattern of the sample, and then compare it with the standard X-ray diffraction data of the mineral to determine the type of mineral composition; S5. Extract the mineral crystal information in the X-ray diffraction pattern to obtain the unit cell information of each mineral in the shale sample; S6. Perform peak separation and fitting processing on the X-ray diffraction pattern, and calculate the mass percentage content of various clay minerals and non-clay minerals.

3. The method for establishing a core framework model of shale multi-mineral composition based on molecular simulation according to claim 1, characterized in that, In step (2), the unit models of the primitive unit cells of each mineral are constructed respectively. For quartz, the α -quartz unit cell model is used. For illite, the illite type at the end of the transformation process is adopted. The unit cell models of montmorillonite and dolomite are both from the American Mineralogist Crystal Structure Database.

4. The method for establishing a core framework model of shale multi-mineral composition based on molecular simulation according to claim 1, characterized in that, the step (6) refers to optimizing the structure of the composite mineral model by using the Geometry Optimization task of the Forcite module in the molecular simulation software Materials Studio to make it in the lowest energy state.

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