In-situ evaluation method for seepage evolution of engineering rock mass

Abstract

The invention discloses an engineering rock mass seepage evolution in-situ evaluation method. The method comprises the steps that a test hole of a rock mass to be tested is formed; injecting preset pressure gas into the test hole and performing sealing operation to obtain a test area; recording a pressure change condition in the test area, and obtaining a pressure gradient; based on the pressure gradient, obtaining the effective gas permeability of the rock mass to be measured; and evaluating the seepage of the engineering rock mass based on the effective gas permeability. Calculation is carried out based on the pressure gradient in the test hole through a pressure attenuation method, a high-precision flow control and pressure stabilizing device is not needed, the problem that the low-permeability medium test time is too long through an existing engineering scale steady-state test method is solved on the basis that a test system is remarkably simplified, meanwhile, a theoretical model does not depend on coal seam fracture seepage characteristics any more, and the test efficiency is greatly improved. The problem that an existing engineering scale transient method is difficult to be widely applied to general lithology is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of rock mass seepage evolution, and in particular relates to an in-situ evaluation method for engineering rock mass seepage evolution. Background Art

[0002] In various deep geological projects, accurate permeability measurement is a key task in evaluating the effects of surrounding rock grouting reinforcement, deep rock fracturing, and the sealing performance of deep chamber barrier systems. The more mature permeability tests currently available mainly use core-scale tests, where a certain number of standard samples are drilled on-site and brought back to the laboratory for permeability testing. There are two main permeability testing methods at the existing engineering scale. One is the steady-state test, which injects a fluid with a fixed flow rate or pressure into the test area and calculates the permeability by testing the pressure or flow after the seepage stabilizes. The other is the transient test, also known as the pressure pulse method, which applies pulse pressure to the test area and measures the permeability of the rock mass by testing the change in pressure difference over time.

[0003] Traditional core-scale testing methods, which do not preserve the in-situ environment, distort measurement parameters by obtaining samples through traditional coring techniques. This makes permeability test results less reflective of the actual seepage behavior of the rock mass under complex field conditions. Furthermore, the limited representational scale of core-scale samples makes their test results difficult to directly apply to engineering practice, limiting their applicability in complex site conditions.

[0004] As for the engineering-scale permeability test method, the existing steady-state method requires pressure and flow control, and the test equipment and test process are relatively complex. In addition, the test time for low-permeability media is too long.

[0005] Although the existing transient method has improved the testing efficiency, its theoretical model is highly dependent on the seepage characteristics of coal seam fractures, making it difficult to apply to general lithology and rock masses, and its application scenario is relatively single (only for coal seams). Summary of the Invention

[0006] In order to solve the above technical problems, the present invention proposes an in-situ evaluation method for seepage evolution of engineering rock masses, which solves the problem that the existing engineering-scale steady-state test method has too long a test time for low-permeability media.

[0007] To achieve the above-mentioned object, the present invention provides an in-situ evaluation method for seepage evolution of engineering rock mass, comprising:

[0008] Setting up test holes for the rock mass to be tested;

[0009] Injecting a preset pressure gas into the test hole and performing a sealing operation to obtain a test area;

[0010] Recording pressure changes in the test area to obtain a pressure gradient;

[0011] Based on the pressure gradient, obtaining the effective gas permeability of the rock mass to be measured;

[0012] Based on the effective gas permeability, the seepage of the engineering rock mass is evaluated.

[0013] Optionally, before setting the test hole for the rock mass to be tested, the method further includes: obtaining a calculation formula for effective gas permeability based on an engineering-scale in-situ gas permeability test principle.

[0014] Optionally, obtaining the calculation formula for the effective gas permeability includes:

[0015] Obtain the pressure distribution function of the gas;

[0016] Based on the pressure change, the average gas pressure is obtained;

[0017] Use Darcy's law to obtain the average gas flow rate;

[0018] Obtaining a gas mass conservation equation based on the average gas pressure and the average gas flow rate;

[0019] updating the average gas flow rate based on the pressure distribution function;

[0020] Based on the gas mass conservation equation and the updated average gas flow rate, the effective gas permeability calculation formula is obtained.

[0021] Optionally, obtaining the pressure distribution function includes:

[0022] Obtain gas density and gas seepage velocity;

[0023] The pressure distribution function is obtained according to the gas density and gas seepage velocity in combination with the gas mass conservation equation.

[0024] Optionally, the pressure distribution function method is:

[0025] P 2 =C1lnr+C2

[0026] Among them, P is the pressure between the rock mass, r is the distance from the center of the borehole, and C1 and C2 are calculation constants.

[0027] Optionally, obtaining the gas density includes:

[0028] Obtain the ideal gas state equation;

[0029] Obtain the first relationship between the amount, mass, and molar mass of a substance;

[0030] Obtain the second relationship between mass, volume, and density;

[0031] The gas density is obtained based on the ideal gas state equation, the first relationship and the second relationship.

[0032] Optionally, the gas density method is:

[0033]

[0034] Where ρ is the gas density, M is the relative molecular mass, R is the molar gas constant, and T is the temperature.

[0035] Alternatively, the method for obtaining the average gas pressure based on pressure changes is:

[0036] P mean =P1-ΔP / 2,

[0037] Among them, P mean is the average gas pressure, P1 is the injection pressure, and ΔP is the pressure change within Δt time;

[0038] The method to obtain the average gas flow rate using Darcy's law is:

[0039]

[0040] Among them, Q mean1 is the average gas flow rate, K is the effective permeability, A is the side area of ​​the test hole, μ is the gas dynamic viscosity coefficient, P is the rock mass pressure, r is the distance from the center of the borehole, L is the length of the test section, r in is the drilling radius, C1 and C2 are calculation constants;

[0041] Based on the average gas pressure and average gas flow, the method for obtaining the gas mass conservation equation is:

[0042] P mean Q mean Δt=ΔPV0

[0043] Among them, V0 is the effective volume of the test section;

[0044] Based on the pressure distribution function, the method for updating the average gas flow is:

[0045]

[0046] Among them, Q mean2 is the average gas flow rate, K is the effective permeability, A is the side area of ​​the test hole, μ is the gas dynamic viscosity coefficient, P is the rock mass pressure, r is the distance from the center of the borehole, L is the length of the test section, r in is the drilling radius, C1 and C2 are calculation constants;

[0047] Based on the gas mass conservation equation and the updated average gas flow rate, the method for obtaining the calculation formula of the effective gas permeability is:

[0048]

[0049] Where k is the permeability, P mean is the average pressure of the test section during Δt time, P0 is the atmospheric pressure, μ is the gas dynamic viscosity coefficient r out is the effective test radius, r in is the drilling radius, V0 is the effective test volume, and L is the length of the test section.

[0050] Compared with the prior art, the present invention has the following advantages and technical effects:

[0051] Compared with the core-scale test method, the engineering-scale in-situ gas permeability test of the present invention can more realistically reflect the permeability characteristics of the rock medium under actual engineering conditions, and significantly expand the representative scale of the test results, which has stronger guiding significance for engineering practice.

[0052] Compared with the existing steady-state method, the test method of the present invention only needs to monitor the pressure drop in the pore to calculate the pressure gradient and permeability, without the need for high-precision flow control and pressure stabilization devices, thereby significantly simplifying the test system; in low-permeability medium testing, compared with the flow signal, the pressure response is more sensitive, the calculation results are more stable, and to a certain extent, the test time is shortened and the test efficiency is improved.

[0053] Compared with the existing transient method, the test equation of the present invention is no longer highly dependent on the seepage characteristics of coal seam fractures. The test object is not limited to coal seams and can be widely applied to general lithology, providing new ideas for expanding the applicable scenarios of in-situ gas permeability testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0055] Figure 1 This is a flow chart of an in-situ evaluation method for seepage evolution of an engineering rock mass according to an embodiment of the present invention;

[0056] Figure 2 2. It is a schematic diagram of the principle of engineering-scale in-situ gas permeability test according to an embodiment of the present invention;

[0057] Figure 3 Schematic diagram of the pressure drop curve of the test hole in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0059] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0060] The present invention proposes an in-situ evaluation method for seepage evolution of engineering rock mass, such as Figure 1 As shown, the specific steps include:

[0061] Setting up test holes for the rock mass to be tested;

[0062] Injecting preset pressure gas into the test hole and performing a sealing operation to obtain the test area;

[0063] Record the pressure changes in the test area and obtain the pressure gradient;

[0064] Based on the pressure gradient, the effective gas permeability of the rock mass to be tested is obtained;

[0065] The seepage of engineering rock mass is evaluated based on the effective permeability.

[0066] Specifically, ① drilling the required test holes in the rock mass to be tested and completing the hole cleaning operation;

[0067] ②Install the gas injection tube and expansion plug into the predetermined position in the hole, and use the inflation device to inject gas of the required pressure into the expansion plug to complete the sealing operation of the test hole;

[0068] ③Inject gas with a pressure of P1 into the hole at one time through the gas injection pipe and record the pressure drop process in the test area.

[0069] ④Substitute the slope obtained by fitting the pressure drop curve recorded in ③, the average pressure of the test section, atmospheric pressure, test section length, borehole radius, effective test radius, gas dynamic viscosity coefficient and effective test volume into the gas effective permeability formula to measure the effective gas permeability of the rock mass to be tested. Figure 3 shown.

[0070] Furthermore, before setting the test hole of the rock mass to be tested, the method further includes: obtaining a calculation formula for effective gas permeability based on the engineering-scale in-situ gas permeability test principle.

[0071] Furthermore, the calculation formula for obtaining the effective gas permeability includes:

[0072] Obtain the pressure distribution function of the gas;

[0073] Based on the pressure change, the average gas pressure is obtained;

[0074] Use Darcy's law to obtain the average gas flow rate;

[0075] Obtain the gas mass conservation equation based on the average gas pressure and average gas flow rate;

[0076] Based on the pressure distribution function, the average gas flow rate is updated;

[0077] Based on the gas mass conservation equation and the updated average gas flow rate, the calculation formula for the effective gas permeability is obtained.

[0078] Furthermore, obtaining the pressure distribution function includes:

[0079] Obtain gas density and gas seepage velocity;

[0080] The pressure distribution function is obtained based on the gas density and gas seepage velocity combined with the gas mass conservation equation.

[0081] Furthermore, obtaining the gas density includes:

[0082] Obtain the ideal gas state equation;

[0083] Obtain the first relationship between the amount, mass, and molar mass of a substance;

[0084] Obtain the second relationship between mass, volume, and density;

[0085] The gas density is obtained based on the ideal gas state equation, the first relationship, and the second relationship.

[0086] The present embodiment is described in detail below with reference to the accompanying drawings:

[0087] Unlike the traditional in-situ gas permeability steady-state test method, gas is not continuously injected into the test hole at the same flow rate. This test method is to extend the steady-state method based on the inlet pressure gradient at the core scale to the engineering scale. At the beginning of the test, gas with a pressure of P1 is injected into the hole, and then the pressure change in the test hole is recorded, and the permeability is calculated based on the pressure gradient of the test hole. Under this test method, the engineering scale has similar pressure boundary conditions to the core scale. The derivation of the in-situ gas permeability test principle is carried out based on the following five assumptions: ① The gas seepage in the rock mass is laminar, that is, it conforms to Darcy's law; ② The gas satisfies the ideal gas state equation; ③ The temperature condition of the test is constant temperature; ④ The rock mass satisfies uniform isotropy; ⑤ The gas seepage in the rock mass is radial flow. The specific test principle is as follows: Figure 2 shown.

[0088] The gas mass conservation equation in porous media is:

[0089]

[0090] According to the assumption ②, there is an ideal gas state equation:

[0091] PV=nRT (2)

[0092] Based on the relationship between the amount of substance, mass and molar mass, and the relationship between mass, volume and density, we can get:

[0093] m=nM (3)

[0094] m=ρV (4)

[0095] Combining equations (2), (3) and (4) we can get the gas density:

[0096]

[0097] According to assumption ①, the gas seepage velocity can be obtained:

[0098]

[0099] According to assumption ⑤, the experimental model can be further simplified into a one-dimensional radial model in a cylindrical coordinate system. According to assumptions ③ and ④, the gas permeability k, gas dynamic viscosity coefficient μ, and temperature T are all constants. Substituting equations (5) and (6) into equation (1), (1) can be simplified to:

[0100] ΔP 2 =0 (7)

[0101] Where Δ is the Laplace operator.

[0102] Then we can get the one-dimensional radial expression of (7) in the cylindrical coordinate system:

[0103]

[0104] From formula (8), we can get P 2 The analytical solution is:

[0105] P 2 =C1lnr+C2 (9)

[0106] Since this test method is different from the traditional in-situ gas permeability steady-state test method, gas is not continuously injected into the test hole at the same flow rate. At the beginning of the test, gas with a pressure of P1 is injected into the hole. After Δt time, the gas pressure in the test section drops by ΔP. The average gas pressure in the test section can be obtained as:

[0107]

[0108] Therefore, the pressure boundary condition in the test hole is not the P1 mentioned in the existing in-situ gas permeability steady-state test method, but the average gas pressure P obtained by the steady-state method based on the inlet pressure gradient at the core scale. mean Assume that the hole wall of the test section is r in The gas pressure at P mean ; Distance from the axis of the test section r out The pressure at the point is P0; hence the boundary condition P│r in =P mean , P│r out =P0. Substituting it into (9) we can get:

[0109]

[0110] According to Darcy's law, the average flow rate is:

[0111]

[0112] According to the gas mass conservation equation, we can get:

[0113] P mean Q mean Δt=ΔPV0 (14)

[0114] Among them, V0 is the effective volume of the test section.

[0115] From formula (9), we can get:

[0116]

[0117] At the test section, r=r in , we can get:

[0118]

[0119] Substituting formula (16) into formula (13) yields:

[0120]

[0121] Combining equations (17) and (14), we can get the effective gas permeability:

[0122]

[0123] Where k is the permeability (m 2 );P mean is the average pressure of the test section during the Δt time (Pa); P0 is the atmospheric pressure, the specific value is 1×10 5 (Pa); μ is the gas dynamic viscosity coefficient (Pa·s); r out is the effective test radius (m); r inis the drilling radius (m); V0 is the effective test volume (m 3 ); L is the length of the test section (m).

[0124] The above are merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. An in-situ evaluation method for seepage evolution of engineering rock mass, characterized by: include: Setting up test holes for the rock mass to be tested; Injecting a preset pressure gas into the test hole and performing a sealing operation to obtain a test area; Recording pressure changes in the test area to obtain a pressure gradient; Based on the pressure gradient, obtaining the effective gas permeability of the rock mass to be measured; Based on the effective gas permeability, the seepage of the engineering rock mass is evaluated.

2. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 1, characterized in that: Before setting the test hole of the rock mass to be tested, the method further includes: obtaining an effective gas permeability calculation formula based on an engineering-scale in-situ gas permeability test principle.

3. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 2, characterized in that: The calculation formula for obtaining the effective gas permeability includes: Obtain the pressure distribution function of the gas; Based on the pressure change, the average gas pressure is obtained; Use Darcy's law to obtain the average gas flow rate; Obtaining a gas mass conservation equation based on the average gas pressure and the average gas flow rate; updating the average gas flow rate based on the pressure distribution function; Based on the gas mass conservation equation and the updated average gas flow rate, the effective gas permeability calculation formula is obtained.

4. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 3, characterized in that: Obtaining the pressure distribution function includes: Obtain gas density and gas seepage velocity; The pressure distribution function is obtained according to the gas density and gas seepage velocity in combination with the gas mass conservation equation.

5. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 4, characterized in that: The method of the pressure distribution function is: P 2 =C1lnr+C2 Among them, P is the pressure between the rock mass, r is the distance from the center of the borehole, and C1 and C2 are calculation constants.

6. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 4, characterized in that: Obtaining the gas density includes: Obtain the ideal gas state equation; Obtain the first relationship between the amount, mass, and molar mass of a substance; Obtain the second relationship between mass, volume, and density; The gas density is obtained based on the ideal gas state equation, the first relationship and the second relationship.

7. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 6, characterized in that: The gas density method is: Where ρ is the gas density, M is the relative molecular mass, R is the molar gas constant, and T is the temperature.

8. The in-situ evaluation method for seepage evolution of engineering rock mass according to claim 6, characterized in that: Based on the pressure change, the method to obtain the average gas pressure is: P mean =P1-ΔP / 2, Among them, P mean is the average gas pressure, P1 is the injection pressure, and ΔP is the pressure change within Δt time; The method to obtain the average gas flow rate using Darcy's law is: Among them, Q mean1 is the average gas flow rate; Based on the average gas pressure and average gas flow, the method for obtaining the gas mass conservation equation is: P mean Q mean Δt=ΔPV0 Among them, V0 is the effective volume of the test section; Based on the pressure distribution function, the method for updating the average gas flow is: Among them, Q mean2 is the average gas flow rate, K is the effective permeability, A is the side area of ​​the test hole, μ is the gas dynamic viscosity coefficient, P is the rock mass pressure, r is the distance from the center of the borehole, L is the length of the test section, r in is the drilling radius, C1 and C2 are calculation constants; Based on the gas mass conservation equation and the updated average gas flow rate, the method for obtaining the calculation formula of the effective gas permeability is: Where k is the permeability, P mean is the average pressure of the test section during Δt time, P0 is the atmospheric pressure, μ is the gas dynamic viscosity coefficient r out is the effective test radius, r in is the drilling radius, V0 is the effective test volume, and L is the length of the test section.

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