A method for predicting real-time permeability of in-situ rock mass under thermal disturbance conditions

Abstract

A method for predicting the real-time permeability of an in-situ rock mass under thermal disturbance conditions belongs to the field of high-temperature rock mechanics and engineering research, and includes the following steps: 1) determining the thermal shock factor ω of the rock; 2) establishing an empirical formula for rock permeability using the thermal shock factor ω and the average effective stress p of the rock as physical parameters characterizing the thermal shock effect and burial depth; 3) optimizing the empirical formula for rock permeability; 4) substituting the average effective stress of the rock and the thermal shock factor to which the in-situ rock is subjected through the optimized empirical formula for permeability to predict the real-time evolution results of the in-situ rock mass permeability under thermal disturbance. The permeability coefficient can be derived using the real-time evolution results of the rock mass permeability, and a comprehensive, reasonable and effective evaluation can be made on the permeability of the in-situ rock mass. The present invention can better predict the effect of increasing the permeability after the thermal shock fracture of the rock, and provide reasonable and effective guidance for the development of geothermal resources.

Description

Technical Field

[0001] The invention belongs to the field of high-temperature rock mechanics and engineering research, and specifically relates to a method for predicting the real-time permeability of an in-situ rock mass under thermal disturbance conditions. Background Art

[0002] In recent years, with the changes in the national energy structure and the formulation of the "dual carbon" goals, the utilization and development of new energy sources have gradually entered the "fast lane". Geothermal energy has been identified by countries around the world as a "green energy" to maintain sustainable social development due to its characteristics of "large resources, continuous heat energy, clean and pollution-free", and has been well developed and utilized in countries rich in geothermal resources. Geothermal energy resources are divided into shallow geothermal energy, hydrothermal geothermal energy and hot dry rock geothermal energy. Hot dry rock refers to a high-temperature rock mass with a temperature greater than 180°C, a burial depth of several thousand meters, no fluid inside or only a small amount of underground fluid. The rock type is mainly granite, which has the characteristics of large burial depth, high temperature, high density and low permeability. In the process of artificial heat storage construction and geothermal extraction, the injection of low-temperature fluid causes the reservoir rock mass to be thermally disturbed and thermally shocked. The thermal shock causes the reservoir rock mass to produce thermal shock fractures, enhance permeability, and improve mining efficiency and development. Therefore, predicting the evolution of reservoir rock permeability can well guide the development of geothermal resources. However, the reservoir rock mass in situ is subject to complex THMC coupling, and due to the limitation of engineering scale, it is difficult for engineers to measure the permeability of the target reservoir on site. Therefore, how to obtain the real-time permeability of the reservoir rock mass under thermal disturbance has become one of the problems to be solved urgently in the development of geothermal resources.

[0003] Thermal shock fractures occur during heat transfer between high-temperature rocks and low-temperature fluids. Under thermal shock, rocks will be damaged at a microscopic level, generating a large number of pores and cracks. As thermal shock continues, the pores and cracks in the rocks continue to develop, expand, and overlap to form interconnected seepage channels, which enhances the permeability of the rocks. Therefore, the focus of studying the evolution of in-situ rock permeability in geothermal mining is to analyze the thermal shock damage mechanism of the rock mass.

[0004] The essence of rock thermal shock damage is that when the rock is rapidly heated or cooled, a drastic temperature gradient occurs inside the rock, which in turn generates shock thermal stress. When the shock thermal stress exceeds the rock's destructive strength, thermal shock damage occurs. In order to quantitatively characterize the destructive power of thermal shock, the physical quantity "thermal shock factor" is introduced. That is, the rate of change of the temperature gradient inside the rock per unit time during the thermal shock process, expressed as Where dT / dx is the temperature gradient and τ is the time. The thermal shock factor can quantitatively characterize the destructive capacity of different thermal shock levels and the changes in macroscopic mechanical properties and permeability. Therefore, studying the thermal shock factor provides a way to obtain the permeability of reservoir rock during thermal shock destruction. At the same time, it is of great significance to promote the theoretical development of high-temperature rock mechanics and engineering research. Summary of the invention

[0005] The main purpose of the present invention is to solve the deficiencies in the prior art and provide a method for predicting the real-time permeability of in-situ rock mass under thermal disturbance, which uses the thermal shock factor ω that characterizes the thermal shock destructive capacity and the average effective stress p of the rock borne by the in-situ rock mass to predict and evaluate the permeability of the in-situ rock mass under thermal disturbance from the perspective of heat transfer, thereby effectively guiding the development and utilization of geothermal resources.

[0006] The design concept of the present invention is: based on the engineering geological conditions, the average effective stress of the in-situ rock mass is calculated, and the real-time thermal shock factor of the in-situ rock mass when it is thermally disturbed is obtained through numerical simulation; then, the thermal shock factor of the in-situ rock mass when it is thermally disturbed and the average effective stress of the rock are substituted into the permeability empirical formula, and the evolution result of the in-situ rock mass permeability caused by thermal disturbance can be predicted. In order to solve the above problems, the technical solution of the present invention is:

[0007] A method for predicting the real-time permeability of an in-situ rock mass under thermal disturbance conditions comprises the following steps:

[0008] S1. Determination of thermal shock factor ω of rock:

[0009]

[0010] In formula ①, dT / dx is the temperature gradient and τ is the time;

[0011] The temperature gradient change rate per unit time inside the rock during the thermal shock process is defined as the thermal shock factor. The thermal shock factor is a physical parameter used to characterize the thermal shock destructive ability. The method for determining the thermal shock factor ω of the rock is described in "A method for determining the thermal shock factor during the thermal shock fracture of the rock" (Announcement No.: CN 109709135 A), which will not be described in detail here.

[0012] S2. The reasons for the evolution of the in-situ rock permeability due to thermal disturbance are analyzed. It is believed that thermal shock and the burial depth of the in-situ rock mass are the main influencing factors. Therefore, the thermal shock factor ω and the average effective stress p of the rock are used as the physical parameters representing the thermal shock and the burial depth of the in-situ rock mass. The poly model in the form of polynomials is used to establish the empirical formula of the rock permeability κ with respect to the thermal shock factor ω and the average effective stress p of the rock, as shown in Formula ②:

[0013] κ=f(ω,p)=a+∑ i＝1,2,… b i *ω i +∑ j＝1,2,… c j *p j +f*ω*p; ②

[0014] In formula ②, κ is the rock permeability, ω is the thermal shock factor, p is the average effective stress of the rock, and a, b, c, and f are polynomial parameters;

[0015] S3. Optimize formula ②: Since the high-order power values ​​of the thermal shock factor ω and the average effective stress p of the rock are small and have a weak effect on the permeability, in order to simplify the calculation, the thermal shock factor ω and the third and higher powers of the average effective stress p of the rock are omitted, and the empirical formula of rock permeability is optimized as shown in formula ③:

[0016] κ=f(ω,p)=a+b1*ω 1 +c1*p 1 +b2*ω 2 +c2*p 2 +f*ω*p; ③

[0017] S4. Conduct rock permeability test in the laboratory, obtain multiple groups of rock permeability measured parameters corresponding to different thermal shock factors ω and rock average effective stress p through multiple groups of test conditions, and substitute multiple groups of rock permeability measured parameters and corresponding thermal shock factors ω and rock average effective stress p into formula ③, determine the values ​​of polynomial parameters: a, b1, c1, b2, c2, f, and obtain the empirical formula of rock permeability;

[0018] S5. Substitute the actual thermal shock factor and the actual average effective stress of the rock when the in-situ rock mass is subjected to thermal disturbance into the empirical formula of permeability to predict the evolution of the permeability of the in-situ rock mass caused by thermal disturbance.

[0019] Furthermore, in step S2, the expression of the average effective stress p of rock is:

[0020]

[0021] Where p is the average effective stress of rock, in MPa; θ is the volume stress of rock, in MPa; p p is the pore pressure of the rock, in MPa; α is the biot coefficient, the value of α is determined by the physical and mechanical properties of the rock itself, and the value range of α is 0~1.

[0022] Furthermore, in step S4, in addition to using laboratory measurement methods to conduct rock permeability measurement tests, numerical simulation techniques can also be used, namely: a surface diagram of the relationship between rock permeability and thermal shock factor ω and rock average effective stress p is drawn through software to determine the values ​​of polynomial parameters in the rock permeability empirical formula: a, b1, c1, b2, c2, f.

[0023] Furthermore, the real-time rock permeability coefficient k is determined according to the rock permeability empirical formula:

[0024] k=κ·ρ·g / η; ⑤

[0025] In formula ⑤, ρ is the density of the fluid passing through the rock, g is the acceleration of gravity, and η is the dynamic viscosity coefficient of the fluid. The real-time permeability coefficient k of the rock can further predict and evaluate the real-time permeability of the in-situ rock mass, thereby effectively guiding the development and utilization of geothermal resources. Through important physical parameters such as permeability and permeability coefficient, the permeability of reservoir rocks in the process of geothermal resource exploitation can be comprehensively, reasonably and effectively predicted and evaluated.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The concept of thermal shock factor is introduced to quantitatively characterize the destructive ability of thermal shock on rocks, which expands the research ideas of high-temperature rock mechanics and provides a theoretical basis for the thermal shock damage mechanism of rocks from the perspective of heat transfer;

[0028] 2. The empirical formula of rock permeability is constructed by using thermal shock factor and effective stress. The permeability results calculated by the empirical formula of rock permeability are highly consistent with the laboratory test results (the correlation coefficient is more than 90%), which can better predict the increase effect of permeability after rock thermal shock fracture;

[0029] 3. Using the improved rock permeability empirical formula combined with the thermal shock factor and effective stress value of the in-situ reservoir rock mass to predict and evaluate permeability can provide reasonable and effective guidance for geothermal resource development. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 :Design process flow chart;

[0031] Figure 2 :Physical model diagram of numerical simulation of granite geothermal mining

[0032] Figure 3 : Surface diagram of the relationship between granite permeability, thermal shock factor and effective stress;

[0033] Figure 4: Temperature field contour map of thermally stimulated permeability zone of granite artificial thermal reservoir (diagonal longitudinal section of two wells at 160 days);

[0034] Figure 5 : Contour map of thermal shock factor in the thermally stimulated permeability zone of granite artificial thermal reservoir (diagonal longitudinal section of two wells at 160 days);

[0035] Figure 6 : Permeability contour map of thermally stimulated permeability enhancement zone of granite artificial thermal reservoir (diagonal longitudinal section of two wells at 160 days);

[0036] Figure 7 : Contour map of permeability coefficient in the thermally stimulated permeability zone of granite artificial thermal reservoir (diagonal longitudinal section of the two wells at 160 days). DETAILED DESCRIPTION

[0037] Combine the following Figures 1 to 7 The present invention is further described in detail with engineering examples.

[0038] There is a geothermal development demonstration project with a reservoir depth of 4,000 meters, a geostress gradient of 25 MPa / km, and a reservoir temperature of 300°C. A 120m×120m×80m rectangular area in the reservoir was selected to establish a physical model for numerical simulation. The coordinates of the two wells are: injection well (25m, 25m, 80m), production well (95m, 95m, 80m), and the borehole diameter is 0.6m. The physical model is as follows: Figure 2 To explore the evolution of the permeability of the granite reservoir during thermal disturbance, the specific steps are as follows:

[0039] S1. Obtain the granite reservoir core drilled from the geothermal exploration well for subsequent rock thermal shock measurement test;

[0040] S2. Determine the thermal shock factor ω of the rock: The temperature gradient change rate per unit time inside the rock during the thermal shock process is defined as the thermal shock factor ω. The thermal shock factor ω is a physical parameter used to characterize the destructive capacity of thermal shock, and its expression is:

[0041]

[0042] Where dT / dx is the temperature gradient and τ is the time;

[0043] S3. The method for determining the thermal shock factor ω of rock is described in “A method for determining the thermal shock factor in the process of thermal shock fracture of rock” (Announcement No.: CN 109709135 A). The method is not described here. The thermal shock factor determination test of granite is carried out by using the method. The value of the thermal shock factor ω is calculated by the test data. In this specific implementation method, the thermal shock of 20°C cold water is applied to the granite reservoir at 300°C. The calculated value of the thermal shock factor is 16026°C·(m·s) -1 ;

[0044] S4. Analyze the reasons for the change of granite reservoir permeability due to thermal disturbance, and consider thermal shock and burial depth as the main influencing factors, and use thermal shock factor ω and rock average effective stress p as physical parameters to characterize thermal shock and burial depth respectively. Use the poly model in polynomial form to establish the empirical formula of rock permeability with respect to thermal shock factor ω and rock average effective stress p, as shown in formula ②:

[0045] κ=f(ω,p)=a+∑ i＝1,2,… b i *ω i +∑ j＝1,2,… c j *p j +f*ω*p; ②

[0046] In formula ②, κ is the granite permeability, ω is the thermal shock factor, and p is the average effective stress of the rock;

[0047] Formula ② is further optimized, ignoring the influence of thermal shock factor ω and rock average effective stress p3 and above on permeability, and formula ② is optimized as shown in formula ③:

[0048] κ=f(ω,p)=a+b1*ω 1 +c1*p 1 +b2*ω 2 +c2*p 2 +f*ω*p; ③

[0049] S5. Carry out granite permeability test in the laboratory, change the test conditions, obtain multiple sets of permeability values ​​under different thermal shock factors ω and average effective stress p of rock, substitute multiple sets of permeability values ​​and corresponding thermal shock factors ω and effective stress p into the optimized empirical formula, and draw the relationship surface diagram of key parameters (permeability, thermal shock factor, effective stress), such as Figure 3 As shown. Determine the parameters in the permeability empirical formula: a = 0.6, b1 = -7.473 × 10 -5 , c1=2.598×10 -2 b2=2.434×10 -9、c2=8×10 -4 f = -2.236 × 10 -6 , we get the empirical formula of granite permeability as follows:

[0050] κ=0.6-7.473×10 -5 *ω+2.598×10 -2 *P+2.434×10 -9 *ω 2 +8×10 -4 *P 2 -2.236×10 -6 *ω*P;

[0051] In the formula, κ is the permeability of granite, P is the average effective stress of rock, and ω is the thermal shock factor;

[0052] S6. Using the empirical formula of granite permeability, combined with the actual thermal shock factor (obtained by numerical simulation) and the actual average effective stress of the in-situ granite reservoir (here, the maximum and minimum horizontal stresses of the in-situ reservoir are taken as 1.2 and 0.8 times the overburden load pressure, respectively, and the overburden load is 100MPa, so the volume stress of the reservoir is Θ = 120 + 100 + 80 = 300MPa, the pore pressure of the reservoir at a burial depth of 4000m is 50MPa, and α is taken as 1, the average effective stress of the granite reservoir is obtained. Substituting the values ​​of various physical parameters, we can obtain the change in permeability of the granite reservoir after thermal shock rupture under thermal disturbance (the permeability of the thermally stimulated permeability zone of the reservoir is about k = 3.899Md after 160 days of heat transfer). The contour map of the temperature field of the diagonal longitudinal section of the two wells, the contour map of the thermal shock factor, the contour map of the predicted permeability, and the contour map of the predicted permeability coefficient after 160 days of heat transfer in the thermally stimulated permeability zone of the reservoir are shown in the figure below. Figure 4 to Figure 7 shown.

[0053] S7. Using the granite permeability prediction results, other permeability parameters can be derived, such as the permeability coefficient k = κ·ρ·g / η. Where ρ is the fluid density, g is the gravitational acceleration, and η is the dynamic viscosity coefficient (in this example, ρ = 1.5×10 3 Kg / m 3 ,g=9.8m / s 2 ,η=6×10 -4 Pa·s. The permeability coefficient of the 160-day artificial thermal reservoir in this project example can be obtained as k = 8.254×10 -3 m / a, the permeability of unfractured igneous or metamorphic rocks ranges from 2.6×10 -9 ~2.6×10 -5m / a, it can be seen that the geothermal reservoir has a significant permeability enhancement effect and good permeability). Through important physical parameters such as permeability and permeability coefficient, the real-time evolution results of the permeability of the granite reservoir during the geothermal resource exploitation process can be reasonably and effectively predicted and evaluated.

[0054] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for predicting the real-time permeability of in-situ rock mass under thermal disturbance conditions, characterized by: The following steps are included: S1. Determination of thermal shock factor ω of rock: In formula ①, dT / dx is the temperature gradient and τ is the time; S2. The thermal shock factor ω and the average effective stress p of the rock are used as the physical parameters to characterize the thermal shock effect and the burial depth of the in-situ rock mass. The rate of change of the temperature gradient per unit time inside the rock during the thermal shock process is defined as the thermal shock factor. The thermal shock factor is a physical parameter used to characterize the destructive capacity of thermal shock. The expression of the average effective stress p of rock is: Where, p is the average effective stress of rock, in MPa; Θ is the volume stress of rock, in MPa; p p is the pore pressure of the rock, in MPa; α is the biot coefficient, and the value range of α is 0 to 1; The empirical formula of rock permeability κ with respect to thermal shock factor ω and average effective stress p of rock is established by using the poly model in polynomial form as shown in formula ②: κ=f(ω,p)=a+∑ i＝1,2，... b i *oh i +S j＝1,2，... c j *p j +f*ω*p; ② In formula ②, κ is the rock permeability, ω is the thermal shock factor, p is the average effective stress of the rock, and a, b, c, and f are polynomial parameters; S3. Optimize formula ②: Omit the thermal shock factor ω and the third power or above of the average effective stress p of the rock, and optimize the empirical formula of rock permeability to be as shown in formula ③: K=f(ω,p)=a+b1*ω 1 +c1*p 1 +b2*ω 2 +c2*p 2 +f*ω*p; ③ S4. Conduct rock permeability test in the laboratory, obtain multiple groups of rock permeability measured parameters corresponding to different thermal shock factors ω and rock average effective stress p through multiple groups of test conditions, and substitute multiple groups of rock permeability measured parameters and corresponding thermal shock factors ω and rock average effective stress p into formula ③, determine the values ​​of polynomial parameters: a, b1, c1, b2, c2, f, and obtain the empirical formula of rock permeability; According to the empirical formula of rock permeability, the real-time rock permeability coefficient k is determined as: k = κ·ρ·g / η; ⑤ In formula ⑤, ρ is the density of the fluid passing through the rock, g is the acceleration of gravity, and η is the dynamic viscosity coefficient of the fluid; S5. Substitute the actual thermal shock factor and the actual average effective stress of the rock when the in-situ rock mass is subjected to thermal disturbance into the empirical formula of permeability to predict the evolution of the permeability of the in-situ rock mass caused by thermal disturbance.

2. The method for predicting the real-time permeability of an in-situ rock mass under thermal disturbance conditions according to claim 1, characterized in that: In step S4, a surface diagram of the relationship between rock permeability, thermal shock factor ω and rock average effective stress p is drawn by software to determine the values ​​of polynomial parameters a, b1, c1, b2, c2 and f in the empirical formula of rock permeability.

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