Method for obtaining electrochemical properties during hydrocarbon exploitation

Abstract

The present application relates to the field of oil and gas exploitation, and in particular, to a method for obtaining electrochemical properties in the process of oil and gas exploitation, comprising: constructing a chemical reaction network between shale and external fluid; constructing a surface complexation model and a diffusion double layer model, setting initial values of thermodynamic equilibrium constant and initial values of charge density of shale; obtaining theoretical values of Zeta potential through the surface complexation model and the diffusion double layer model; when the theoretical values of Zeta potential and actual values of Zeta potential meet the convergence condition, obtaining electrochemical properties of shale after contacting with external fluid; wherein the actual values of Zeta potential are measured through laboratory experiments. The theoretical values of Zeta potential obtained by the above method are relatively accurate, and multiple electrochemical properties of shale after contacting with external fluid can be efficiently and conveniently obtained based on the theoretical values of Zeta potential, which provides analysis data and scientific guidance for subsequent efficient development of shale oil reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction, and more specifically relates to a method for obtaining electrochemical properties during the oil and gas extraction process. Background Technology

[0002] In the petroleum industry, the exploration and development of shale oil and gas resources is particularly important. Shale oil refers to the petroleum resources contained in shale formations, which are mainly composed of shale, including petroleum in shale pores and fractures, as well as petroleum resources in adjacent sandstone or carbonate rock layers within the shale formation.

[0003] Because shale reservoirs are mostly terrestrial sediments, their mineral composition is complex, typically containing a large amount of clay minerals and high organic matter content. This results in extremely low porosity and permeability, making crude oil extraction extremely difficult and necessitating large-scale hydraulic fracturing technology for effective development. The specific principle is as follows: During hydraulic fracturing, a large amount of fracturing fluid (such as slickwater, carbon dioxide, low-salt water, nanoemulsions, etc., also known as exogenous fluids) enters the shale reservoir, inducing a series of chemical reactions, such as the hydration and expansion of clay minerals, the dissolution / precipitation of soluble minerals like halite and calcite, and ion adsorption in aqueous solutions. These chemical reactions can alter the porosity, permeability, and pore-throat connectivity of the shale reservoir, as well as the electrical properties of the shale surface (such as increasing the negative charge density on the shale surface), changing the wettability of the shale reservoir, thereby improving the reservoir's ability to flow crude oil and promoting the rate and efficiency of crude oil absorption.

[0004] However, the changes in porosity, permeability, and electrochemical properties of shale reservoirs during contact with different exogenous fluids remain unknown. Furthermore, studying the interaction between shale reservoirs and exogenous fluids experimentally is extremely difficult. For example, carbon dioxide fracturing in shale reservoirs requires high temperature and pressure conditions, and the corrosive nature of carbon dioxide imposes strict requirements on experimental equipment and operational safety. In addition, due to the high density of shale samples, it is difficult to saturate them with formation water or crude oil, resulting in experiments that can take several months. Therefore, efficiently and conveniently obtaining information on the interaction between shale reservoirs and exogenous fluids, and determining the electrochemical properties of the shale reservoir after contact with these fluids, is of great significance for shale oil extraction.

[0005] To date, no research has been conducted on the electrochemical reactions between shale reservoirs and external fluids, either domestically or internationally. Furthermore, there are no quantitative methods or methods for obtaining information on the changes in the electrochemical properties of shale reservoirs after contact with external fluids. Summary of the Invention

[0006] In view of the above-mentioned defects and problems of the prior art, the present invention develops a method for obtaining electrochemical properties during oil and gas extraction.

[0007] The first aspect of this invention provides a method for obtaining electrochemical properties during oil and gas extraction, comprising the following steps:

[0008] A chemical reaction network is constructed between shale and external fluids, the chemical reaction network including at least one of the following: dissolution reaction of shale minerals, precipitation reaction of shale minerals, adsorption reaction of various ions in external fluids on the mineral surface, lattice ion exchange reaction, and reaction between various ions of external fluids and formation fluids.

[0009] A surface complexation model is constructed, and the initial values ​​of the chemical reactions occurring in the diffused double layer and the thermodynamic equilibrium constants of each reaction are set. The chemical reaction network includes the chemical reactions.

[0010] A diffused double-layer model was constructed, and the initial value of the charge density of shale was set;

[0011] Based on the initial values ​​of the thermodynamic equilibrium constant and the initial values ​​of the charge density, the theoretical value of the Zeta potential on the shale surface is obtained through the surface complexation model and the diffuse double layer model.

[0012] When the theoretical value of the Zeta potential and the actual value of the Zeta potential meet the convergence condition, the electrochemical properties of the shale after contact with the external fluid are obtained; wherein, the actual value of the Zeta potential is measured by indoor experiments;

[0013] The electrochemical properties include at least one of the following: theoretical composition and theoretical pH of the formation fluid, theoretical surface potential of the shale surface, theoretical charge density, theoretical electrostatic force between shale and crude oil, and theoretical composition and theoretical concentration of each component of the external fluid.

[0014] The inventors unexpectedly discovered that by simulating the chemical reactions that occur in shale reservoirs after contact with external fluids using the method described in this application, the theoretical value of the Zeta potential can be accurately obtained. Furthermore, experiments conducted using this method show that the error between the theoretical Zeta potential obtained and the experimental data (actual Zeta potential value) is less than 5%. The electrochemical properties obtained from the theoretical Zeta potential values ​​after contact between shale and external fluids (such as the theoretical composition and pH of the formation fluid, the theoretical surface potential and charge density of the shale surface, the theoretical electrostatic force between shale and crude oil, and the theoretical composition and concentration of each component of the external fluid) are also relatively accurate. Therefore, the composition and concentration of each component of the external fluid can be optimized to increase the connectivity of the pore throats in the shale reservoir, promote the permeation rate and efficiency of crude oil, and thus provide strong guidance for the efficient development of shale oil reservoirs. Moreover, the above method can reduce experimental operating costs and obtain many electrochemical property data that cannot be obtained experimentally, which is of great significance to the field of shale oil extraction.

[0015] In another aspect, the present invention provides the application of the above-mentioned method for obtaining electrochemical properties during oil and gas extraction in the chemical contact process between shale and formation fluids.

[0016] In another aspect, the present invention provides the application of the method for obtaining electrochemical properties during the oil and gas extraction process in the process of gas dissolving shale minerals; preferably, the method for obtaining electrochemical properties during the oil and gas extraction process is applied in the process of carbon dioxide dissolving shale minerals.

[0017] In summary, this application has at least one of the following beneficial effects:

[0018] The method for obtaining electrochemical properties during oil and gas extraction in this invention can accurately simulate the chemical reaction between shale reservoirs and external fluids, and accurately obtain the theoretical value of the Zeta potential on the shale surface. Based on the theoretical value of the Zeta potential, it can also efficiently and conveniently obtain multiple electrochemical properties after contact between shale and external fluids, providing analytical data and scientific guidance for the efficient development of subsequent shale oil reservoirs. For example, it can predict whether external fluids, such as fracturing fluids, tend to cause precipitation and block pore throats, or tend to dissolve shale reservoir minerals and enlarge pore throats. It can also optimize the composition of fracturing fluids to increase the connectivity of shale reservoir pore throats and promote the permeation of crude oil by external fluids, which is of great significance in the field of shale oil and gas extraction. Furthermore, by implementing the above method steps, the drawbacks of experimental methods, such as high cost, long time consumption, and inaccurate results, can be overcome. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0020] Figure 1 A schematic diagram of a diffused double layer;

[0021] Figure 2 This is a flowchart illustrating a method for obtaining electrochemical properties during oil and gas extraction in one embodiment of this application.

[0022] Figure 3 This is a flowchart illustrating a method for obtaining electrochemical properties during oil and gas extraction in another embodiment of this application.

[0023] Figure 4 This is a schematic diagram showing the relationship between the saturation index of various minerals in formation fluids and the salinity of formation fluids.

[0024] Figure 5 This diagram illustrates the relationship between the surface potential and surface charge density at the shale-formation fluid interface and the formation fluid salinity.

[0025] Figure 6This diagram illustrates the relationship between electrostatic pressure between shale and crude oil and formation fluid salinity.

[0026] Figure 7 A schematic diagram showing the amount of carbon dioxide dissolved in water under different temperature and pressure conditions;

[0027] Figure 8 This diagram illustrates the relationship between carbon dioxide concentration in formation fluids and formation fluid pH with carbon dioxide pressure.

[0028] Figure 9 This is a schematic diagram illustrating the relationship between the saturation index of shale minerals and the pH of formation fluids. Detailed Implementation

[0029] The present invention will be further described below with reference to embodiments. It should be understood that the embodiments are only used to further illustrate and explain the present invention, and are not intended to limit the present invention.

[0030] Unless otherwise defined, the technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art. Although similar or identical methods and materials may be used in experimental or practical applications, materials and methods are described below. In case of conflict, the definitions included in this specification shall prevail. Furthermore, materials, methods, and examples are for illustrative purposes only and are not restrictive.

[0031] Unless otherwise specified, the experimental methods, testing methods, and conventional reagent preparation methods used in the embodiments of this invention are all in accordance with conventional practices in the field.

[0032] Before proceeding with the embodiments, in order to facilitate understanding of the technical solutions in this application, the relevant technologies are described in detail below:

[0033] (1) Chemical reaction network between shale and external fluids.

[0034] The chemical reaction network between shale and external fluids includes chemical reactions occurring in the shale within the external fluid, such as mineral dissolution; chemical reactions occurring when the external fluid contacts the shale, such as carbon dioxide dissolution and ion precipitation in solution; and chemical reactions occurring at the interface between the shale and the external fluid, such as mineral surface dehydrogenation, mineral surface hydrogenation, cation adsorption, anion adsorption, and lattice ion exchange. As shown in Table 1, a chemical reaction network suitable for the target scenario can be constructed based on Table 1 during actual calculations. It should be noted that the Zeta potential obtained after all the above chemical reactions reach equilibrium is the theoretical value of the Zeta potential on the shale surface.

[0035] Table 1. Chemical Reaction Network Between Shale and External Fluids

[0036]

[0037]

[0038] Note: X represents shale minerals, >X represents shale surface complexes, C represents cations, M represents cations participating in lattice ion exchange, and A represents anions.

[0039] (2) The initial values ​​of the thermodynamic equilibrium constants of each chemical reaction in the chemical reaction network can be measured experimentally or by referring to the results in the literature. Since the measurement of thermodynamic equilibrium constants is relatively complicated and the chemical reactions involved in this invention are complex, the results in the literature are preferred as a reference in this invention.

[0040] (3) The surface complexation reaction model in this application can be either a 1pK or a 2pK state model. Since the 1pK state model is more complex than the 2pK state model, it will increase the simulation time and difficulty. Therefore, the 2pK state model is preferred in this invention.

[0041] (4) The diffused double layer model in this application can be selected from the Helmholtz model, the Gouy-Chapman model, and the Gouy-Chapman-Stern model. Since the Helmholtz model is relatively simple and commonly used, and other models will increase the running time and difficulty, the Helmholtz model is preferred in this invention.

[0042] (5) Next, the principle behind why this application can perform simulation steps based on the initial value of the thermodynamic equilibrium constant, the 2pK state model, and the Helmholtz model will be explained. Due to the diversity of shale minerals and external fluids, the chemical reactions between shale and external fluids are complex and cannot be described in detail. Here, we will take the dehydrogenation and hydrogenation reactions, where the chemical reaction network only includes surface complexes (denoted by >X), as examples. The dehydrogenation reaction is as follows:

[0043] The hydrogenation reaction is as follows:

[0044] First, according to the 2pK state model, the thermodynamic equilibrium constant (Keq1) of the above dehydrogenation reaction and the thermodynamic equilibrium constant (Keq2) of the hydrogenation reaction can be expressed by the following formulas (1) and (2):

[0045]

[0046]

[0047] In the formula, [X] is the surface complex concentration (mol / L); F is the Faraday constant (96485C / mol); ψ0 is the surface potential (V); R is the molar gas constant (8.314J / (mol·K)); and T is the Fahrenheit temperature (K).

[0048] It should be noted that when performing the simulation step, the values ​​of Keq1 and Keq2 are input into the system as known initial values ​​for the simulation calculation, and ψ0 in the formulas needs to be calculated according to formulas (1) and (2). The values ​​of Keq1 and Keq2 can be obtained either through experimental measurement or by consulting references.

[0049] Understandably, when performing the simulation steps, it is necessary to calculate each of the above chemical reactions separately, and then base the total diffusion potential ψ on the equilibrium of each chemical reaction. d Obtain the theoretical value of the Zeta potential. For clarity, the entire simulation process will be described in detail here using only the hydrogenation reaction as an example. That is, the hydrogenation reaction is the first chemical reaction, and the following will only be described according to equation (2):

[0050] To calculate the surface potential ψ0 according to formula (2), the initial concentration [X] of the surface complex in formula (2) needs to be known. The principle for calculating the initial concentration [X] of the surface complex is as follows:

[0051] Because shale reacts with external fluids to form various surface complexes, these complexes may be positively or negatively charged. The sum of the charges of all charged surface complexes is equal to the surface charge density σ0 (C / m²). 2 As shown in equation (3):

[0052] σ0=eN A V∑ i [X] i Z i (3)

[0053] Where V is the volume of the external fluid (L), Z i Let σ0 represent the valence of each ion in the first chemical reaction, and i represent the i-th ion in the first chemical reaction. Once the value of σ0 is known, the preliminary concentration [X] of the surface complex can be calculated according to formula (3), and the surface potential ψ0 in the first chemical reaction can be calculated according to formula (2). The charge density σ0 on the shale surface can be estimated by analyzing the mineral composition and content of each component of the shale using X-ray diffraction.

[0054] After obtaining the surface potential ψ0, the theoretical value of the Zeta potential of the shale surface in the first chemical reaction should be obtained according to the Helmholtz model.

[0055] For the Helmholtz model, such as Figure 1 As shown, according to the principles of colloid and interface chemistry, a diffuse double layer will form at the interface between shale and external fluid. The negatively charged shale surface attracts cations to form a Stern layer, while the outer cations attract anions to form a diffuse layer. The Stern layer and the diffuse layer together are called the diffuse double layer. In this model, the potential of the Stern layer remains constant, as shown in equation (4), while the potential of the diffuse layer decreases exponentially with distance, as shown in equation (5).

[0056] ψ0=ψ d (4)

[0057] In equation (4), ψ0 is the surface potential (V); ψ d Given the diffusion potential (V), according to equation (4), when ψ0 is obtained, ψ can be determined. d Then, the theoretical value of the Zeta potential ζ is obtained according to the relationship between the theoretical value of the Zeta potential and the diffusion potential (Equation (5)). m .

[0058]

[0059] In equation (5), according to ψ d The potential ψ(x) at each point within the slip surface of the diffusion layer can be calculated. The unit of ψ(x) is V, and x is the position of the slip surface within the diffusion layer, with the unit being meters (m). ψ(x) is the theoretical value of the Zeta potential. (Unit: V), where κ -1 Let x be the Debye length (m), calculated as shown in equation (6). In solutions with an ion concentration greater than 0.1 mol / L, x = 0.5κ. -1 :

[0060]

[0061] In equation (6), ε0 is the vacuum permittivity (8.854 × 10⁻⁶). -12 C·m -2 ); ε is the relative permittivity of the formation fluid (78.65); k B Boltzmann constant (1.381 × 10⁻⁶) -23 J / K); T is temperature in Fahrenheit (K); e is charge (1.602 × 10⁻⁶). - 19 C); N A For Avogadro's constant (6.022 × 10⁻⁶), 23 mol -1 IS is the ionic strength, which can be calculated according to equation (7):

[0062]

[0063] In equation (7), C i Z represents the concentration (mol / L) of each ion in the external fluid; i The valences of the ions in the foreign fluid are represented.

[0064] The theoretical value of the Zeta potential ζ for the first chemical reaction was calculated. m Then, the convergence condition between the theoretical value and the actual value of the Zeta potential needs to be determined through the following steps: Step (1): Obtain the actual value of the Zeta potential ζ for the first chemical reaction by electrophoresis experiment. e Step (2): Compare the deviation between the theoretical and actual Zeta potential values ​​obtained from the simulation calculation. If the deviation is large, i.e., the deviation is not within a reasonable range, it is necessary to adjust the initial values ​​of the thermodynamic equilibrium constant and the initial values ​​of the shale charge density based on experience, and re-execute the simulation calculation steps to perform iterative calculations until the deviation between the theoretical and actual Zeta potential values ​​is within a reasonable range, thus obtaining the electrochemical properties of the shale after contact with the external fluid. At this point, the theoretical and actual Zeta potential values ​​meet the convergence condition.

[0065] It is understandable that when the theoretical value of the Zeta potential and the actual value of the Zeta potential meet the convergence condition, the deviation between them is less than the first threshold, which is generally in the range of 5-10%. As an example, when the standard deviation between the theoretical value of the Zeta potential and the actual value of the Zeta potential is less than 10%, the obtained theoretical value of the Zeta potential can be considered reliable, and subsequent steps can be performed.

[0066] It should be noted that electrochemical properties include at least one of the following: theoretical composition and theoretical pH of formation fluids, theoretical surface potential of shale, theoretical charge density, theoretical electrostatic force between shale and crude oil, and theoretical composition and theoretical concentration of each component of the introduced fluid. The theoretical charge density includes the theoretical charge density of the diffusion layer and the theoretical charge density of the Stern layer. The process of obtaining the theoretical charge density includes the following steps:

[0067] The theoretical surface potential is obtained based on the theoretical value of the Zeta potential;

[0068] The theoretical charge density is obtained based on capacitance, charge density, and theoretical surface potential. Wherein, ζ m The corresponding diffusion potential ψ d That is, the theoretical surface potential, because ψ d =ψ0.

[0069] Specifically:

[0070] ① If the theoretical charge density of the Stern layer is obtained, then according to ψ d The theoretical surface charge density σ0 of the Stern layer is calculated using Equation (8).

[0071]

[0072] In equation (8), σ0 is the theoretical surface charge density (C / m²). 2 C is the Stern layer capacitance (999F); S is the specific surface area (m²). 2 / g); ρ is the density of the external fluid (kg / m³). 3 Both S and ρ are input as known parameters because S and ρ can be measured experimentally; for example, the specific surface area S of shale can be obtained by nitrogen isothermal adsorption-desorption; and the density ρ of the external fluid can be obtained by weighing.

[0073] It is understandable that the theoretical concentrations [X] of reactants and products in the first chemical reaction can be calculated based on equation (3) and the theoretical surface charge density σ0, and then the theoretical composition and theoretical pH of the formation fluid, as well as the theoretical composition and theoretical concentration of each component of the external fluid, can be obtained.

[0074] ②If the theoretical charge density of the diffusion layer is obtained, then according to ψ d Calculate the theoretical charge density σ of the diffusion layer d It can be calculated according to the Gouy-Chapman equation (i.e., equation (9)):

[0075]

[0076] In equation (9), ε0 is the vacuum permittivity (8.854 × 10⁻⁶). -12 C·m -2 ); ε is the relative permittivity of the formation fluid (78.65); k B Boltzmann constant (1.381 × 10⁻⁶) -23 J / K); T is temperature in Fahrenheit (K); e is charge (1.602 × 10⁻⁶). - 19 C); C i Z represents the concentration (mol / L) of each ion in the external fluid; i Let σ represent the valences of the ions in the external fluid; F is the Faraday constant (96485.33 C / mol); and R is the molar gas constant (8.314 J / (mol·K)). It is understandable that σ... d It can be used to calculate the theoretical electrostatic force of the shale-exogenous fluid-crude oil system.

[0077] ③ If the theoretical electrostatic force between shale and crude oil is obtained, it can be based on ζ mThe corresponding surface potential ψ0 is used to calculate the theoretical electrostatic force of the shale-external fluid-crude oil system using equation (10), namely:

[0078]

[0079] Where ψ0 is the surface potential of the shale, in V; ψ oo ε0 is the surface potential of crude oil, in V. ε0 is the vacuum permittivity (8.854 × 102). -12 C·m -2 ); ε is the relative permittivity of the formation fluid (78.65); k B Boltzmann constant (1.381 × 10⁻⁶) -23 J / K); T is temperature in Fahrenheit (K); e is charge (1.602 × 10⁻⁶). -19 C).

[0080] The following is a brief description of methods for obtaining electrochemical properties during oil and gas extraction, such as... Figure 2 As shown, the method includes:

[0081] (1) Construct a chemical reaction network between shale and external fluids. The chemical reaction network includes at least one of the following: dissolution reaction of shale minerals, precipitation reaction of shale minerals, adsorption reaction of various ions in the external fluid on the mineral surface, lattice ion exchange reaction, and reaction between ions in the external fluid and formation fluid. Write the code for each chemical reaction in the chemical reaction network using the open-source software PHREEQC;

[0082] (2) Construct a surface complexation model and set the initial values ​​of the chemical reactions that occur in the diffuse double layer and the thermodynamic equilibrium constants of each reaction; wherein, the diffuse double layer includes the Stern layer and the diffusion layer; the surface complexation model is a mathematical model that describes the chemical reactions in the diffuse double layer between shale and external fluid.

[0083] (3) Construct a diffusion double layer model and set the initial value of the charge density of shale; wherein, the diffusion double layer model is a mathematical model describing the electric potential and charge density in the diffusion double layer between shale and external fluid;

[0084] (4) Based on the initial values ​​of the thermodynamic equilibrium constant and the initial values ​​of the charge density of shale, the theoretical value of the Zeta potential on the shale surface is obtained through the surface complexation model and the diffuse double electric layer model.

[0085] Specifically, input the initial value of the thermodynamic equilibrium constant and the initial values ​​of each parameter involved in the above formulas (3) to (7) (such as the initial value of the surface potential and charge density of shale), and according to the surface complexation model, based on the surface charge density of shale, the composition and concentration of external fluid ions in the first chemical reaction, obtain the preliminary concentration of reactants and products in the first chemical reaction; the chemical reaction network includes the first chemical reaction;

[0086] The surface potential of the first chemical reaction is obtained based on the initial value of the thermodynamic equilibrium constant and the initial concentration.

[0087] Based on the diffused double layer model, the theoretical value of the Zeta potential on the shale surface is obtained based on the surface potential. When the diffused double layer model is the Helmholtz model, the diffusion potential is obtained based on the surface potential, and the theoretical value of the Zeta potential is obtained based on the relationship between the theoretical value of the Zeta potential and the diffusion potential. The relationship between the theoretical value of the Zeta potential and the diffusion potential is given by equation (5) above.

[0088] (5) When the theoretical value of Zeta potential and the actual value of Zeta potential meet the convergence condition, the electrochemical properties of shale after contact with external fluid are obtained; wherein, the actual value of Zeta potential is measured by indoor experiment.

[0089] It should be noted that when the deviation between the theoretical and actual Zeta potential values ​​is less than a first threshold (the first threshold ranges from 5-10%), the theoretical and actual Zeta potential values ​​meet the convergence condition. Electrochemical properties include at least one of the following: theoretical composition and theoretical pH of the formation fluid; theoretical surface potential and theoretical charge density of the shale surface; theoretical electrostatic force between shale and crude oil; and theoretical composition and theoretical concentration of each component of the external fluid.

[0090] In addition, methods for obtaining electrochemical properties during oil and gas extraction can also include, for example... Figure 3As shown, the specific steps include: constructing a chemical reaction network between shale and external fluid, and constructing a surface complexation model, setting initial values ​​for the chemical reactions occurring in the diffuse double layer and the thermodynamic equilibrium constants of each reaction; constructing a diffuse double layer model and setting an initial value for the charge density of the shale; then, based on the initial values ​​of the thermodynamic equilibrium constant and charge density, obtaining the theoretical value of the Zeta potential on the shale surface through the surface complexation model and the diffuse double layer model; then, determining whether the theoretical value of the Zeta potential and the actual value of the Zeta potential satisfy the convergence condition; when the convergence condition is satisfied, obtaining the electrochemical properties of the shale after contact with the external fluid; when the convergence condition is not satisfied, replacing at least one of the initial values ​​of the thermodynamic equilibrium constant and the initial value of the charge density, and recalculating the theoretical value of the Zeta potential.

[0091] The following two specific embodiments illustrate the method for obtaining electrochemical properties during oil and gas extraction and its specific applications.

[0092] Example 1: Acquisition of electrochemical properties during shale-formation fluid contact.

[0093] External fluids include formation fluids and surface fluids. During actual contact between shale and formation fluids, when the formation fluid salinity reaches 60,000 mg / L, it may cause degradation of various chemical agents (such as polymers and surfactants) in the fracturing fluid, thereby reducing the hydraulic fracturing effect. Surface fluid salinity is generally around 10,000 mg / L, lower than that of formation fluids. Therefore, pre-flushing the shale reservoir with surface fluids can continuously reduce the salinity of the formation fluids as surface fluids are continuously injected.

[0094] During pre-flushing, the different ion concentrations in formation fluids and surface fluids can lead to precipitation between ions in the formation fluids, hydration and expansion of clay minerals (i.e., lattice exchange), and dissolution of shale minerals. These changes alter the electrochemical properties of the shale surface, affecting the porosity and permeability characteristics and pore-throat connectivity of the shale reservoir, as well as its wettability, ultimately impacting the effectiveness of crude oil recovery. Therefore, determining the optimal injection volume of surface fluids and the optimal salinity of formation fluids, and predicting the impact of injected surface fluids on the pore-throat structure and surface wettability of shale reservoirs, is crucial.

[0095] This embodiment obtains the electrochemical properties of the chemical contact process between shale and formation fluids, providing scientific guidance for key technical indicators in the aforementioned shale oil extraction process. The specific implementation process of this embodiment is as follows:

[0096] (I) The process of obtaining electrochemical properties during the contact between shale and formation fluids

[0097] Step S1: Construct a chemical reaction network between shale and external fluids. The chemical reaction network includes at least one of the following: dissolution reaction of shale minerals, precipitation reaction of shale minerals, adsorption reaction of various ions in external fluids on the mineral surface, lattice exchange reaction between clay minerals and various ions in external fluids, and reaction between external fluids and various ions in formation water.

[0098] It is important to note that before constructing the chemical reaction network between shale and the external fluid, it is necessary to determine the main components of the shale to be simulated and the ionic composition of the external fluid to ensure the accuracy of the constructed chemical reaction network. The main components of the shale and the ionic composition of the external fluid can be obtained through the following methods:

[0099] First, it should be noted that the shale to be analyzed in this embodiment is taken from the shale reservoir of Changqing Oilfield; the formation water (denoted as FW) is taken from the formation water of Changqing Oilfield; and the surface water (denoted as AW) is taken from the surface water of Changqing Oilfield.

[0100] (1) Obtaining the main components of the shale to be simulated

[0101] 1) Crush the blocky shale to be simulated into small pieces, and put the small pieces of shale into a ball mill for grinding for 10-20 minutes to obtain shale powder;

[0102] 2) Take 0.5g of shale powder and put it into an X-ray diffractometer to analyze the mineral composition of the shale. As an example, the mineral composition results are shown in Table 2.

[0103] Table 2. Shale Composition and Content

[0104] Mineral types quartz plagioclase Pyrite illite Content (wt.%) 39.2 9.2 27.0 24.6

[0105] As shown in Table 2, the shale used in this embodiment is mainly composed of quartz, plagioclase, pyrite, and illite. It is understood that since the shale contains very little organic matter, and the organic matter is primarily organic carbon, it is unlikely to chemically react with formation fluids. Therefore, the interaction between organic matter and formation fluids can be disregarded.

[0106] (2) Obtaining the composition of the foreign fluid to be simulated

[0107] When the external fluid includes formation water and surface water, the ionic composition of formation water and surface water can be measured by ion chromatography, and the pH of formation water and surface water can be measured by pH meter. The results are shown in Table 3.

[0108] Table 3. Ionic composition of formation water and surface water

[0109]

[0110] As shown in Table 3, the total salinity (TDS) of the formation water used in this embodiment is 60278 mg / L, which is classified as high-salinity formation water. In contrast, the total salinity of the surface water is approximately 11194 mg / L, significantly lower than that of the formation water. Furthermore, both formation water and surface water contain substantial amounts of anions and metal cations. It is also worth noting that the formation water contains a relatively high amount of calcium ions (1802 mg / L), while the surface water contains a large amount of sulfate ions (5678 mg / L). The mixture of these two types can produce calcium sulfate precipitate, which may affect reservoir permeability.

[0111] Based on the determination of the ionic composition of the external fluid and the main components in the shale matrix, the chemical reaction network between the shale and the external fluid is shown in Table 4. It should be noted that the thermodynamic equilibrium constants for each reaction in Table 4 were obtained by consulting the literature.

[0112] Table 4. Chemical Reaction Network and Thermodynamic Equilibrium Constants between Shale and Formation Fluids

[0113]

[0114] Table 4 shows that the chemical reaction network between shale and formation water includes: ① dissolution of pyrite in formation fluids; ② dissolution of carbon dioxide in formation fluids; ③ dehydrogenation of quartz / illite in formation fluids; ④ adsorption of cations in formation fluids on the surface of quartz / illite; ⑤ dehydrogenation and hydrogenation of illite in formation fluids; ⑥ adsorption of cations and anions in formation fluids on the surface of illite; and ⑦ ion exchange between cations in formation fluids and the illite lattice.

[0115] Step S2: Construct a surface complexation model and set the initial values ​​of the chemical reactions occurring in the diffused double layer and the thermodynamic equilibrium constants of each chemical reaction; that is, the thermodynamic equilibrium constants of each reaction formula in Table 4.

[0116] Step S3: Construct a diffuse double electric layer model and set an initial value for the charge density of the shale. The initial value for the charge density is obtained experimentally.

[0117] Step S4: Based on the initial values ​​of the thermodynamic equilibrium constant and charge density, the theoretical value of the Zeta potential (denoted as ζ) of the shale surface is obtained through the surface complexation model and the diffuse double layer model. m The theoretical values ​​of the Zeta potential obtained are shown in Table 5.

[0118] Step S5: When the theoretical value of the Zeta potential and the actual value of the Zeta potential satisfy the convergence condition, obtain the electrochemical properties of the shale after contact with the external fluid; wherein, the actual value of the Zeta potential (ζ) e The result was obtained through indoor experiments.

[0119] It should be noted that the entire process described above was performed under conditions of 298K and 0.1MPa.

[0120] (II) Experimental Operation Procedure

[0121] (1) Preparation of formation water and surface water

[0122] 1) Based on the test results in Table 2, prepare 50 mL each of formation water and surface water containing the same ionic components and concentrations using the corresponding inorganic salts and distilled water.

[0123] 2) Surface water and formation water were mixed at volume ratios of 1:1, 1:2, 1:5, 1:10 and 1:20 (denoted as LS1, LS2, LS5, LS10 and LS20, respectively) to simulate the process of surface water continuously diluting formation water after being injected into shale reservoirs. The salinity of the mixed formation fluids was 36000, 28000, 20000, 16000 and 14000 mg / L, respectively.

[0124] 3) Dilute the above-mixed water by 100 times to obtain diluted water. Measure the pH of the diluted water with a pH meter and measure the density ρ of the diluted water by weighing.

[0125] (2) Obtaining the specific surface area of ​​shale:

[0126] 1) Take 0.5g of shale powder, vacuum dry for 12h to remove moisture and volatile gases from the shale;

[0127] 2) Obtain nitrogen isothermal adsorption-desorption curves at relative pressures of 0.05–0.35;

[0128] 3) The specific surface area of ​​the shale was estimated using the BET specific surface area method (experimentally obtained as 97.3 m²). 2 / g).

[0129] (3) Measurement of the actual value of shale zeta potential

[0130] 1) Take 0.05g of the above-ground shale powder and put it into 50mL of dilution water with different volume ratios obtained in step (1). Use an ultrasonic disperser to evenly disperse the shale powder into the dilution water and sonicate for 10min to make a suspension (the obtained suspensions are respectively LS1*, LS2*, LS5*, LS10* and LS20*).

[0131] 2) Take 1 mL of the suspension and place it into the Zeta potential sample cell. Measure the Zeta potential (i.e., ζ) using a Zeta potential meter. e A total of 5 measurements were taken, and the average value was used as the final result. The results are shown in Table 5.

[0132] Table 5. Data from experimental and simulation processes after shale comes into contact with external fluids.

[0133]

[0134] According to the data in Table 5, the ζ for each chemical reaction included in the constructed chemical reaction network is... e With ζ m The relative errors between them are all less than 4%, which is within a reasonable range. Therefore, it can be concluded that the scheme of this application can obtain accurate ζ. m Based on these simulation values, the required electrochemical properties can be simulated and calculated.

[0135] This embodiment is based on the theoretical value of the Zeta potential ζ. m The obtained electrochemical properties are as follows:

[0136] (1) The trend of interionic chemical reactions with the change of formation water salinity.

[0137] It should be noted that due to the different concentrations of calcium, sulfate, and bicarbonate ions in formation water and surface water, precipitates such as anhydrite, gypsum, calcite, and dolomite will form. The saturation index (SI) is typically used to indicate whether a given mineral is saturated, unsaturated, or supersaturated in water: SI = 0, saturated; SI < 0, unsaturated; SI > 0, supersaturated.

[0138] Based on the theoretical Zeta potential values ​​obtained in this application, the theoretical concentrations of reactants and products in each chemical reaction are calculated, thereby determining the saturation index SI corresponding to each chemical reaction. The relationships between the saturation indices of anhydrite, gypsum, calcite, and dolomite and formation water salinity are as follows: Figure 4 As shown.

[0139] like Figure 4 As shown, anhydrite is unsaturated in formation fluids and exists in a dissolved state; calcite and dolomite are supersaturated in formation fluids and exist in a precipitated state; gypsum is unsaturated when the formation fluid salinity is greater than 20,000 mg / L and saturated when the formation fluid salinity is less than 20,000 mg / L. Therefore, there exists an optimal salinity of 20,000 mg / L (i.e., LS5), resulting in a smaller amount of precipitation and less impact on reservoir properties.

[0140] (2) Obtain the surface potential (denoted as ψ0) and surface charge density (denoted as σ0) at the interface between shale and formation fluid.

[0141] ψ0 and σ0 are calculated according to equations (4) and (8) respectively. The relationship between shale surface potential and surface charge density and formation water salinity is as follows: Figure 5 As shown. According to Figure 5It can be seen that the surface electric potential of shale gradually decreases with decreasing formation fluid salinity, while the surface negative charge density of shale gradually increases with decreasing formation fluid salinity, indicating that the negative charge on the shale surface increases with decreasing salinity. However, when the formation fluid salinity is below 20000 mg / L, the decrease in surface electric potential and surface charge density of shale decreases.

[0142] (3) Predict the theoretical electrostatic force between shale, external fluid and crude oil.

[0143] The theoretical electrostatic repulsion between shale, formation fluid, and crude oil is calculated according to equation (10), where the surface potential of crude oil (ψ) is assumed to be... oo The electrostatic repulsion between shale and crude oil is -20mV, and the surface potential of crude oil is the same in formation fluids with different salinities. The relationship between the electrostatic repulsion between shale and crude oil and the salinity of the formation fluid is as follows: Figure 6 As shown, Figure 6 In this context, FW represents formation water; AW represents surface water. According to... Figure 6 It can be seen that as the formation fluid salinity decreases, the electrostatic repulsion between shale and crude oil gradually increases. This indicates that the lower the formation fluid salinity, the less likely crude oil is to wet the shale surface, and the more hydrophilic the shale surface, the easier it is for crude oil to detach from the shale surface. However, when the formation fluid salinity is below 20,000 mg / L, the change in electrostatic repulsion between shale and crude oil is small, indicating that the optimal salinity is reached when the volume ratio of surface water to formation water is 1:5 (LS5). However, continuing to inject surface water not only has little effect on improving the recovery rate but also increases the injection cost.

[0144] A comprehensive analysis of the above electrochemical properties will help to optimize the composition and concentration of external fluid ions, directing the electrochemical reaction toward mineral dissolution to increase the connectivity of pore throats in shale reservoirs, and simultaneously increasing the hydrophilicity of the shale surface to promote the absorption rate and efficiency of crude oil, thus providing strong guidance for the efficient development of shale oil reservoirs.

[0145] Example 2. Acquisition of electrochemical properties during the contact process between shale and carbon dioxide system

[0146] Carbon dioxide fracturing in shale reservoirs is currently the main development method for shale oil. It's important to note that carbon dioxide fracturing is generally conducted under high temperature and high pressure conditions. The solubility of carbon dioxide in formation fluids is significant, and dissolved carbon dioxide can lower pH, produce precipitation, and dissolve shale minerals. Due to the tightness of shale reservoirs, the corrosiveness of carbon dioxide, and the high temperature and high pressure conditions, laboratory experiments cannot accurately simulate the real reservoir environment, making it difficult to accurately evaluate the amount of carbon dioxide dissolved in formation fluids, the impact of carbon dioxide on formation fluid pH and ionic composition, and the dissolving effect of carbon dioxide on shale minerals. Therefore, this application aims to obtain the electrochemical properties of the shale-carbon dioxide contact process to provide scientific guidance for key technical indicators in the aforementioned carbon dioxide fracturing process of shale reservoirs. The specific implementation process of this embodiment is as follows:

[0147] It is understood that the oil recovery environment simulated in this embodiment is the same as that simulated in Example 1. The chemical reaction network involved in the entire oil recovery process remains unchanged, except that the carbon dioxide content increases. Therefore, the chemical reaction network in Example 1 can be used in this embodiment. Example 1 has verified the accuracy of the model. Therefore, after obtaining the theoretical value of the Zeta potential in this embodiment, the required electrochemical properties can be directly obtained based on the obtained theoretical value of the Zeta potential. The specific process for obtaining the electrochemical properties is as follows:

[0148] (1) The amount of carbon dioxide dissolved in formation fluids under different temperature and pressure conditions during carbon dioxide fracturing.

[0149] This embodiment obtained the solubility of carbon dioxide in water under different temperature (25-100℃) and pressure (0-50MPa) conditions, and the results are as follows: Figure 7 As shown by the solid line in the image. Figure 7 It can be seen that the solubility of carbon dioxide in water decreases with increasing temperature and increases with increasing pressure.

[0150] It is worth mentioning that, Figure 7 The data points are experimental data, taken from the literature "An improved model for the calculation of CO2 solubility in aqueous solutions containing Na". + ,K + Ca 2+ Mg 2+ ,Cl - and SO4 2-The invention relates to the methods described in the paper "CO2-H2O Mixtures in the Geological Sequestration of CO2. II. Partitioning in Chloride Brines at 12-100℃ and up to 600 bar". This demonstrates that the carbon dioxide dissolution rate obtained using the method described in this invention for acquiring the electrochemical properties between shale and external fluids is consistent with experimental data, with an error of less than 5%, further indicating that the method described in this invention for acquiring the electrochemical properties between shale and external fluids is relatively reliable.

[0151] (2) The relationship between the dissolution effect of carbon dioxide on shale and the pH of formation water during carbon dioxide fracturing.

[0152] As shown in Example 1, pre-flushing shale reservoirs with surface water can reduce formation fluid salinity, and when the formation water to formation fluid volume ratio is 1:5 (i.e., LS5), it can effectively reduce the formation of precipitates between ions. Therefore, Example 2 simulated the process of injecting carbon dioxide into a shale reservoir at 100°C using the method of obtaining the electrochemical properties between shale and external fluids in this application, and obtained the amount of carbon dioxide dissolved in LS5 and its effect on its pH and ionic composition. The results are shown in […]. Figure 8 .

[0153] like Figure 8 As shown, the amount of carbon dioxide dissolved in formation fluid LS5 increases with increasing formation pressure. At a formation pressure of 40 MPa, the amount of carbon dioxide dissolved in each cubic meter of formation fluid is approximately 0.034 L. Furthermore, as the amount of carbon dioxide dissolved in the formation fluid increases, the pH value of the formation fluid continuously decreases. When the formation pressure is less than 4 MPa, the pH value of the formation fluid rapidly decreases from 5.5 to 3.5; when the formation pressure is greater than 4 MPa, the pH value of the formation fluid only decreases from 3.5 to 3.0. Clearly, the dissolution of carbon dioxide in the formation fluid significantly reduces the pH value. This acidic formation fluid corrodes shale minerals (such as pyrite and clay minerals), which helps increase the connectivity of shale pore throats.

[0154] (3) The relationship between shale mineral saturation index and formation fluid pH.

[0155] Due to carbon dioxide dissolution, the pH of the formation fluid decreases. To better understand the impact of carbon dioxide dissolution on the solubility of shale minerals, Example 2 used the method described in this application for obtaining the electrochemical properties between shale and external fluids to determine the relationship between the saturation index of calcite, dolomite, and gypsum and the pH of the formation fluid. The results are as follows: Figure 9 As shown. Figure 9In the study, the pH of the formation fluid without carbon dioxide injection was approximately 7.23, while carbon dioxide injection reduced the pH of the formation fluid to 5.62 and 3.00.

[0156] Depend on Figure 9 It can be seen that as pH decreases, the saturation index of anhydrite decreases, indicating that the soluble content of anhydrite increases. The saturation indices of calcite, dolomite, and gypsum change from positive to negative, indicating that these three minerals change from precipitation to dissolution, and the lower the pH, the greater the soluble content. Obviously, during carbon dioxide fracturing, the lower the pH, the greater the dissolution of the above four minerals, which will expand the shale porosity and increase the connectivity of shale pore throats, thereby promoting the production enhancement effect of carbon dioxide fracturing.

[0157] The aforementioned electrochemical properties help to optimize the pH value of the external fluid, directing the electrochemical reaction toward mineral dissolution, thereby increasing the connectivity of pore throats in shale reservoirs, promoting the permeation rate and efficiency of crude oil, and thus providing strong guidance for the efficient development of shale oil reservoirs.

[0158] It should be noted that after obtaining the theoretical value of the Zeta potential through the method for obtaining electrochemical properties in the oil and gas extraction process of this application, the required electrochemical properties can be obtained according to actual needs. By comprehensively analyzing the obtained electrochemical properties, the ease or difficulty of reservoir development can be determined. Furthermore, by optimizing the composition and pH of the external fluid, the electrochemical reaction can be directed towards mineral dissolution, thereby increasing the connectivity of pore throats in shale reservoirs and promoting the permeation rate and efficiency of crude oil.

[0159] In summary, as demonstrated by Examples 1 and 2 above, the method for obtaining electrochemical properties during oil and gas extraction in this application has good accuracy, can efficiently simulate actual processes, and can conveniently obtain various electrochemical properties after shale comes into contact with external fluids, especially electrochemical properties that cannot be obtained experimentally or require harsh operating environments. This allows staff to effectively analyze the most suitable extraction conditions during oil and gas extraction based on electrochemical properties, thus making a certain contribution to the field of oil and gas extraction.

[0160] It should be understood that the disclosed invention is not limited to the specific methods, schemes, and substances described, as these are all subject to variation. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of the invention, which is limited only by the appended claims.

[0161] Those skilled in the art will also recognize, or be able to identify, many equivalents of the specific embodiments of the invention described herein using no more than conventional experiments. These equivalents are also included in the appended claims.

Claims

1. A method for obtaining electrochemical properties during oil and gas extraction, characterized in that, Includes the following steps: A chemical reaction network is constructed between shale and external fluids, the chemical reaction network including at least one of the following: dissolution reaction of shale minerals, precipitation reaction of shale minerals, adsorption reaction of various ions in external fluids on the mineral surface, lattice ion exchange reaction, and reaction between various ions of external fluids and formation fluids. A surface complexation model is constructed, and the initial values ​​of the chemical reactions occurring in the diffused double layer and the thermodynamic equilibrium constants of each reaction are set. The chemical reaction network includes the chemical reactions. A diffused double-layer model was constructed, and the initial value of the charge density of shale was set; Based on the initial values ​​of the thermodynamic equilibrium constant and the initial values ​​of the charge density, the theoretical value of the Zeta potential on the shale surface is obtained through the surface complexation model and the diffuse double layer model. When the theoretical value of the Zeta potential and the actual value of the Zeta potential meet the convergence condition, the electrochemical properties of the shale after contact with the external fluid are obtained; wherein, the actual value of the Zeta potential is measured by indoor experiments; The electrochemical properties include at least one of the following: theoretical composition and theoretical pH of the formation fluid, theoretical surface potential of the shale surface, theoretical charge density, theoretical electrostatic force between shale and crude oil, and theoretical composition and theoretical concentration of each component of the external fluid; The theoretical value of the Zeta potential on the shale surface is obtained based on the initial values ​​of the thermodynamic equilibrium constant and the initial values ​​of the charge density, using the surface complexation model and the diffuse double layer model, including: Based on the surface complexation model, and taking into account the surface charge density of the shale, the composition and concentration of the foreign fluid ions, the preliminary concentrations of reactants and products in the first chemical reaction are obtained; the chemical reaction network includes the first chemical reaction. The surface potential of the first chemical reaction is obtained based on the initial value of the thermodynamic equilibrium constant and the initial concentration. Based on the diffused double layer model, the theoretical value of the Zeta potential is obtained based on the surface potential.

2. The method for obtaining electrochemical properties during oil and gas extraction according to claim 1, characterized in that, When the deviation between the theoretical value of the Zeta potential and the actual value of the Zeta potential is less than the first threshold, the theoretical value of the Zeta potential and the actual value of the Zeta potential satisfy the convergence condition.

3. The method for obtaining electrochemical properties during oil and gas extraction according to claim 2, characterized in that, The first threshold value ranges from 5 to 10%.

4. The method for obtaining electrochemical properties during oil and gas extraction according to claim 1, characterized in that, The surface complexation reaction model can be selected as a 1pK or 2pK state model.

5. The method for obtaining electrochemical properties during oil and gas extraction according to claim 4, characterized in that, The diffused double layer model can be selected from the Helmholtz model, the Gouy-Chapman model, or the Gouy-Chapman-Stern model.

6. The method for obtaining electrochemical properties during oil and gas extraction according to claim 1, characterized in that, When the diffused double layer model is a Helmholtz model, obtaining the theoretical value of the Zeta potential based on the surface potential includes: Based on the surface potential, the diffusion potential is obtained; Based on the relationship between the theoretical value of the Zeta potential and the diffusion potential, the theoretical value of the Zeta potential is obtained. The relation is as follows: in, ζ m The given Zeta potential is a theoretical value, in V. x This represents the location of the slip surface within the diffusion layer, in meters (m). κ -1 This is the length of the Debye, in meters.

7. The method for obtaining electrochemical properties during oil and gas extraction according to claim 1, characterized in that, Obtaining the theoretical charge density includes: Based on the theoretical value of the Zeta potential, the theoretical surface potential is obtained; The theoretical charge density is obtained based on the theoretical surface potential; The theoretical charge density includes the theoretical charge density of the diffusion layer and the theoretical charge density of the Stern layer.

8. The method for obtaining electrochemical properties during oil and gas extraction as described in claim 1 is applied to the chemical contact process between shale and formation fluids.

9. The application of the method for obtaining electrochemical properties during oil and gas extraction as described in claim 1 in the process of gas dissolving shale minerals.

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