Permeation characteristic parameter inversion method suitable for high-permeability rock

Abstract

The invention discloses a percolation characteristic parameter inversion method suitable for high-permeability rock, which comprises the following steps: S1, building a one-dimensional porous medium unsaturated two-phase percolation model, taking a porous medium, setting a continuous water supply boundary, corresponding to a one-dimensional percolation experiment, setting the permeability and porosity of a sample according to a water physical characteristic parameter test result of the sample, the inlet capillary pressure pec and the pore distribution index lambda are set as typical values, and simulation calculation of the seepage field of the sample is carried out; s2, obtaining the inlet capillary pressure pec and the pore distribution index lambda of the sandstone and the granite through calculation inversion; s3, comparing seepage field calculation results of different samples with seepage simulation experiment results, and carrying out simulation calculation to obtain pore size and distribution index constitutive characteristic parameters of the rock on the basis of rock permeability and porosity in combination with rock one-dimensional seepage experiment results; according to the invention, the seepage characteristic parameters of the high-permeability rock can be inverted with high precision.

Description

Technical Field

[0001] The invention relates to the technical field of geotechnical engineering, and in particular to a seepage characteristic parameter inversion method suitable for high-permeability rocks. Background Art

[0002] Rocks are a hard substance as a result of various geological processes in the development of the earth. They are mineral aggregates composed of one or more rock-forming minerals, which mainly constitute the crust and mantle of the earth. These natural mineral aggregates include not only solid minerals, but also pores between mineral particles, showing unique physical properties, such as specific gravity, porosity and compressive strength. Rocks are a special porous medium, in which fluids usually exist, and solid minerals and pore fluids form a multiphase body. After the pumped storage power station reservoir is filled with water, due to the characteristics of the porous medium of the rock and the effect of external water pressure, the reservoir may cause leakage problems. Specifically, this leakage phenomenon refers to the infiltration of the reservoir water after the water storage into the low-lying valley area outside the watershed of the reservoir bank through the pores, cracks, faults and caves of the rock. Reservoir leakage reduces the benefits of the reservoir and sometimes causes salinization, swamping and other phenomena. The Shisanling Pumped Storage Power Station and Tai'an Pumped Storage Power Station have adopted measures to prevent the whole reservoir basin from seepage due to the problem of reservoir leakage. In addition to the reservoir leakage problem, some pumped storage power stations also have water seepage problems in underground powerhouses and other parts. In addition to the leakage problem in the reservoir, there are also rock concrete seepage problems in tunnels, highways and other construction facilities. Therefore, it is very necessary to study the leakage characteristics of rocks.

[0003] Since rock is a special porous medium with developed and complex fractures, the movement of fracture water is very complex due to many factors. If the model is completely based on the geometric characteristics, hydraulic characteristics and mechanical evolution laws of fractured rock mass, the resulting model will be extremely complex. Therefore, on the basis of simplification, there are currently several fractured medium seepage models: equivalent continuous medium model, dual medium model and discontinuous network fractured medium model. The equivalent continuous medium model regards the complex rock block-fracture system as a continuous whole and uses the continuous medium theory to express the seepage equation. This model assumes that the fluid fills the entire seepage area uniformly and directionally, takes the permeability tensor as the core, and uses the classic Biot porous medium seepage analysis method to describe the seepage phenomenon in rock. Its main advantage is that it can directly apply the more mature porous medium seepage theory, but its disadvantage is that not all fractured rock masses can be equivalent to continuous media, so there are certain applicability limitations. On the other hand, the dual medium model regards rock as a medium that continuously fills the entire domain with two parts, the pore system and the fracture system, in which the pore system is mainly responsible for water storage, and the fracture system is mainly responsible for water conduction. The model fully considers the actual material exchange between the rock mass and the fractures, and has a high degree of simulation authenticity. However, it cannot fully reflect the heterogeneity of the fracture network system and the versatility of water flow, and its scope of application is limited. Summary of the invention

[0004] The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a method for inverting the seepage characteristic parameters of high permeability rocks, which can invert the seepage characteristic parameters of high permeability rocks with high precision.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for inverting seepage characteristic parameters applicable to high permeability rocks, comprising the following steps: S1. Based on the one-dimensional rock simulation seepage experiment, a one-dimensional porous medium unsaturated two-phase seepage model is constructed. The porous medium is taken and a continuous water supply boundary is set to correspond to the one-dimensional seepage experiment. Combined with the experimental results, the rock permeability coefficient is inverted according to the seepage formula as a known input parameter of the model. The inlet capillary pressure p ec and pore distribution index λ are set to typical values, and simulation calculation of the seepage field of the sample is carried out; S2, inlet capillary pressure p ec The initial infiltration rate of the sample is determined by the pore distribution index λ, which determines the inflection point of the sample coefficient infiltration rate. The inlet capillary pressure p of sandstone and granite is obtained by calculation and inversion. ec and pore distribution index λ; S3. The calculated seepage field results of different samples are compared with the seepage simulation experimental results. Assuming that the calculated rock seepage field results based on two-phase flow in porous media are consistent with the measured results, based on the rock permeability and porosity, combined with the one-dimensional seepage experimental results of rocks, simulation calculations are carried out to obtain the pore size and distribution index constitutive characteristic parameters of the rock.

[0006] Furthermore, in S1, the one-dimensional rock simulation seepage experiment includes the following steps: The rock section at one end is fastened by a clamp and placed flat on an epoxy resin bracket. The water spray system controls the nozzle to spray water evenly to the rock sample. The rock sample is fully wrapped with water-blocking tape and polyvinyl chloride film to prevent the flow and evaporation of surface water. Only the section at one end of the clamp is exposed to the humidification range of the nozzle. The rock humidification end is covered with an epoxy resin perforated partition with a hole smaller than the rock sample section to prevent the nozzle humidification water from splashing to the side of the sample and affecting the sample weight test; The process of the one-dimensional rock simulation seepage experiment is as follows: (1) The rock sample was dried and cooled, and the surface was wrapped with water-blocking tape and polyvinyl chloride film, and a clamp was installed. The weight of the treated dry rock sample was recorded as m0, and the start time of the experiment was recorded as 0s; (2) The experiment adopted the method of timed water spraying and weighing to record the water absorption of the rock samples. The rock samples were sprayed with water for the first time at 0s. The water spraying system was humidified regularly to ensure that the humidification end of the sample was evenly moistened and there was a layer of water film. The thickness of the water film h was less than the length of the sample L. Different weighing time intervals were taken depending on the difference in water absorption of the rock samples. (3) For green, dark red and light red sandstone, a weighing interval of 10 min is used, for granite, a weighing interval of 10-15 min is used, and for yellow sandstone, a weighing interval of 7-10 min is used. The state of the water film on the surface of the sample is observed within one weighing interval. If the water film on the cross section of the sample is incomplete, water is sprayed once until the cross section of the sample is wiped dry after one weighing interval. The weighing is recorded as Δm1=m1-m0; (4) Repeat the water spraying 5 times and record m0, Δm1, Δm2, Δm3, Δm4, and Δm5 respectively; where Δm1=m1-m 0； Δm2=m2-m 1； Δm3=m3-m 2； Δm4=m4-m 3； Δm5=m5-m 4； m1, m2, m3, m4, and m5 are the weights of the specimens when sprayed with water for the 1st to 5th time respectively; Simulated seepage experiments were carried out on the above rock samples to obtain the change curve of the infiltration capacity of the porous medium with time.

[0007] Furthermore, in S1, the one-dimensional porous medium unsaturated two-phase seepage model is that the soil area above the wetting front reaches the saturated water content, while below the wetting front, the water content of the soil remains at the initial water content. The model is: ; In the formula, f —Infiltration rate; K s —Permeability coefficient; S f —Average suction at the wetting front; f —generalized depth of wetting front; h—total water potential at the surface; When the water film thickness on the rock sample surface is less than 2 mm, it can be rewritten as: .

[0008] Furthermore, the process of inverting the rock permeability coefficient according to the seepage formula in S1 is as follows: In the one-dimensional infiltration process of rock, the thickness of the unsaturated zone is considered to be 0, the initial water content of the sample is 0, and the cumulative infiltration amount I is expressed as: ; Based on the weight gain Δm and porosity θ of the sample in the one-dimensional simulated seepage experiment of rock s The wetting peak depth z of the sample during the seepage process can be calculated f ,have to: ; Assume that the infiltration rate of the rock sample at each time interval during the seepage process is f Constant, record the sample seepage test process f and I , draw the f =F( I ), the intercept K of the function segment is obtained by fitting the curve s , that is, the permeability coefficient of the sample.

[0009] Furthermore, in S2, the inlet capillary pressure p of sandstone and granite is obtained by calculation inversion. ec The specific process of calculating the pore distribution index λ includes: S2.1. Seepage characteristic parameters: inlet capillary pressure p ec and the pore distribution index λ are model coefficients of the capillary pressure-saturation constitutive relationship; the capillary pressure-saturation constitutive relationship is as follows: According to the static equilibrium condition in capillary phenomena, the sum of the pressure p at the top of the capillary liquid surface and the water pressure ρgH of the water column is equal to the atmospheric pressure p0: ; p is the difference between atmospheric pressure p0 and surface tension pressure 2α / R, where α is the surface tension coefficient of the liquid surface, and R represents the force of the liquid surface being tightened inward perpendicular to the liquid surface. From this, the capillary pressure on the water surface can be obtained as: ; According to the above formula, we can know that: ; In the formula, r is the capillary radius, and φ is the angle between the liquid surface bending tangent and the tube wall. It can be seen that the capillary pressure is inversely proportional to the capillary radius. In porous media, the finer the pores, the greater the capillary force. The soil-water characteristic curve fitting model is used to describe the capillary pressure-saturation constitutive relationship, as shown in the following formula. As the water phase saturation increases, the absolute value of the capillary pressure decreases, which is consistent with the actual situation. ; In the formula, S e is the effective saturation, S w is the volume fraction of saturated water phase in porous media, S wr is the residual water phase volume fraction in porous media, p c is the capillary pressure, p ec is the inlet capillary pressure, and λ is the pore distribution index.

[0010] Furthermore, in S2, the inlet capillary pressure p of sandstone and granite is obtained by calculation inversion. ec The specific process of pore distribution index λ also includes: S2.2, the characteristic parameter inlet capillary pressure p ec and the pore distribution index λ can form the relative permeability-saturation constitutive relationship, which is the key to describing the seepage characteristics of high permeability rocks; the relative permeability-saturation relationship is as follows: When the water phase saturation in the porous medium reaches saturation, the water phase channel is the shortest. When the water phase saturation decreases, the water flow length increases due to the presence of gas phase in the pores. This phenomenon is expressed by the tortuosity Г as: ; Where Г is the tortuosity, L is the length of the porous medium, and L e is the length of the water flow; is the water saturation, τ is the model parameter related to the pore tortuosity of porous media; Substituting the soil-water characteristic curve fitting model formula into the relative permeability-saturation constitutive relationship model can obtain the relative permeability-saturation constitutive relationship model in porous media two-phase flow, namely the VG-M model: ; in: k is the relative permeability of rock at different water saturations, is the water saturation, λ is the pore distribution index, is the effective water saturation; In the description of two-phase transport in rock porous media, the VG-M model is used to describe the variation characteristics of relative permeability and capillary pressure with saturation.

[0011] Beneficial effects of the present invention: By combining experimental data and numerical simulation technology, the present invention can invert the seepage characteristic parameters of high permeability rocks with high precision, such as inlet capillary pressure (pec) and pore distribution index (λ), and the accuracy of these parameters is crucial to understanding the seepage mechanism of rocks and predicting seepage behavior; by comparing and verifying the simulation results with the experimental results, the accuracy and reliability of the inversion results can be ensured, thereby improving the accuracy of the entire research process; compared with the traditional method of directly measuring the seepage characteristics of rocks, the scheme adopts a combination of numerical simulation and experimental verification to avoid direct damage to rock samples; this helps to protect precious rock samples and allows multiple tests and analyses of the same sample; the numerical simulation method is more efficient than the traditional experimental method, and through computer simulation, a large number of seepage simulations under different conditions can be completed in a short time, thereby quickly obtaining the required seepage characteristic parameters; the scheme also reduces the experimental cost because it does not require a large amount of experimental equipment and materials, while reducing the errors and uncertainties that may occur during the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a schematic diagram of the rock seepage experimental platform; Figure 2 It is the water phase infiltration curve of porous media; Figure 3 It is a schematic diagram of capillary phenomenon; Figure 4 shows the simulation and experimental control curves of sandstone and granite samples, including: (a) simulation and experimental control curves of dark red sandstone; (b) simulation and experimental control curves of light red sandstone; (c) simulation and experimental control curves of green sandstone; (d) simulation and experimental control curves of yellow sandstone; and (e) simulation and experimental control curves of granite. DETAILED DESCRIPTION

[0013] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0014] Embodiment 1: This embodiment provides a method for inverting seepage characteristic parameters applicable to high permeability rocks, including: Based on the one-dimensional rock simulation seepage experiment, a one-dimensional porous medium unsaturated two-phase seepage model was built. The porous medium was taken and the continuous water supply boundary was set. Corresponding to the one-dimensional seepage experiment, the sample permeability and porosity were set according to the test results of the sample water characteristic parameters. The inlet capillary pressure p ec and pore distribution index λ are set to typical values, and simulation calculation of the seepage field of the sample is carried out; Inlet capillary pressure p ec The initial infiltration rate of the sample is determined by the pore distribution index λ, which determines the inflection point of the sample coefficient infiltration rate. The inlet capillary pressure p of sandstone and granite is obtained by calculation and inversion. ec and pore distribution index λ; The calculated seepage field results of different samples were compared with the results of seepage simulation experiments. The calculated rock seepage field based on two-phase flow in porous media was consistent with the measured results. Based on the rock permeability and porosity, combined with the results of one-dimensional rock seepage experiments, simulation calculations were carried out to obtain the constitutive characteristic parameters of the rock, such as the pore size and distribution index.

[0015] The present invention carries out experimental preparation in the early stage: based on the one-dimensional rock simulation seepage experiment, prepare the corresponding experimental equipment and materials, build a one-dimensional porous medium unsaturated two-phase seepage model, and ensure that the model can accurately simulate the seepage process in the rock.

[0016] The specific requirements for model setting are: take porous media samples and set continuous water supply boundaries to simulate actual seepage conditions. According to the test results of the hydraulic characteristic parameters of the samples, set the permeability and porosity of the samples to ensure that the model parameters are consistent with the actual conditions.

[0017] The parameter setting requirements are: set the inlet capillary pressure (pec) and pore distribution index (λ) to a specific range of values. These parameters have an important influence on the seepage characteristics. The inlet capillary pressure pec determines the initial infiltration rate of the sample, while the pore distribution index λ determines the inflection point of the sample coefficient infiltration rate.

[0018] Simulation calculation: Carry out simulation calculation of the seepage field of the sample and predict the seepage behavior through numerical simulation method. During the calculation process, the values ​​of pec and λ are constantly adjusted to find the parameter combination that best matches the experimental results.

[0019] Result verification: The calculated seepage field results of different samples were compared with the results of seepage simulation experiments to verify the accuracy and reliability of the model. The calculated rock seepage field based on porous medium two-phase flow was compared with the measured results to ensure good consistency.

[0020] Parameter inversion: The inlet capillary pressure pec and pore distribution index λ of high permeability rocks such as sandstone and granite are obtained through computational inversion. These parameters are key indicators for describing the seepage characteristics of rocks and are of great significance for understanding the rock seepage mechanism and predicting seepage behavior.

[0021] Extraction of constitutive characteristic parameters: Combined with the results of one-dimensional rock seepage experiments, further simulation calculations are carried out to obtain the constitutive characteristic parameters of the rock, such as pore size and distribution index. These parameters help to gain a deeper understanding of the internal structure and seepage characteristics of the rock.

[0022] The principle of the present invention is: Selection of seepage model The one-dimensional porous medium unsaturated two-phase seepage model can better simulate the seepage process in rocks, especially considering the heterogeneity and porosity of rocks. Parameter sensitivity analysis: The inlet capillary pressure pec and the pore distribution index λ are key parameters affecting the seepage characteristics. By adjusting these parameters, significant changes in the seepage rate and seepage path can be observed. Numerical simulation and experimental verification: The numerical simulation method can provide a large amount of detailed seepage data, but its accuracy needs to be ensured by experimental verification. By comparing the simulation results with the experimental results, the reliability and accuracy of the model can be evaluated.

[0023] The seepage characteristic parameters of high permeability rocks obtained through parameter inversion not only help to understand the seepage mechanism of rocks, but also provide important references for engineering design and disaster prediction. This inversion method can efficiently obtain the seepage characteristic parameters of high permeability rocks by combining experimental data and numerical simulation technology, providing strong support for research and application in related fields.

[0024] The method of the present invention is described in detail below.

[0025] Inversion of rock permeability based on Greep-Ampt model; Carry out one-dimensional rock seepage experiment, rock unsaturated seepage experiment platform such as Figure 1 The rock seepage test platform consists of a water spray system, a perforated baffle, an epoxy resin bracket and a hoop, which is placed in a constant temperature and humidity laboratory. The rock section at one end is fastened by the hoop and placed flat on the epoxy resin bracket. The water spray system controls the nozzle to spray water evenly to the rock sample. The rock sample is fully wrapped with water-blocking tape and polyvinyl chloride film to prevent surface water flow and evaporation. Only the section at one end of the hoop is exposed to the humidification range of the nozzle. The rock humidification end is covered with an epoxy resin perforated baffle, and the plate hole is slightly smaller than the rock sample section to prevent the nozzle humidification water from splashing to the side of the sample and affecting the sample weight test.

[0026] The process of the one-dimensional rock simulation seepage experiment is as follows: (1) The rock sample was dried and cooled, and the surface was wrapped with water-blocking tape and polyvinyl chloride film, and a clamp was installed. The weight of the treated dry rock sample was recorded as m0, and the start time of the experiment was recorded as 0s; (2) The experiment adopted the method of timed water spraying and weighing to record the water absorption of the rock samples. The rock samples were sprayed with water for the first time at 0s. The water spraying system was humidified regularly to ensure that the humidification end of the sample was evenly moistened and a thin layer of water film existed. The thickness of the water film h was much smaller than the length of the sample L. Different weighing time intervals were taken depending on the difference in water absorption of the rock samples. (3) For green, dark red and light red sandstone, a weighing interval of 10 min is used, for granite, a weighing interval of 10-15 min is used, and for yellow sandstone, a weighing interval of 7-10 min is used. The state of the water film on the surface of the sample is observed within one weighing interval. If the water film on the cross section of the sample is incomplete, water is sprayed once until the cross section of the sample is wiped dry after one weighing interval. The weighing is recorded as Δm1=m1-m0; (4) Repeat the water spraying 5 times and record m0, Δm1, Δm2, Δm3, Δm4, and Δm5 respectively; where Δm1=m1-m 0； Δm2=m2-m 1； Δm3=m3-m 2； Δm4=m4-m 3； Δm5=m5-m 4； m1, m2, m3, m4, and m5 are the weights of the specimens when sprayed with water for the 1st to 5th time respectively; The simulated seepage experiments were carried out on the above rock samples. The experimental results are shown in Table 1. It can be seen from the table that the water absorption performance of granite is significantly lower than that of sandstone, and the water absorption performance of different sandstones is also slightly different. The yellow sandstone has the best water absorption performance, and the water absorption capacity is about ten times that of other samples. Observing the results of water absorption experiments of different types of rock samples, it can be seen that the water absorption rate of rocks gradually slows down and tends to be stable as its saturation increases.

[0027] Table 1 Results of simulated seepage test on sandstone and granite samples

[0028] From a microscopic perspective, the pores inside the porous medium are filled with water and gas. When a small amount of water drops onto the surface of the rock and soil medium, it will enter the rock and soil through the pores of the medium, which can be regarded as an infiltration process. During the infiltration process of the porous medium, the curve of the change of the infiltration capacity of the porous medium with time is as follows: Figure 2As shown in Figure 1, the change process can generally be divided into three stages: (1) Infiltration stage. In this stage, since the porous medium is in a relatively dry state, the capillary action and molecular force are strong, making the medium have a high water absorption capacity. As the water gradually fills the pores of the porous medium, the molecular force and capillary pressure in the medium decrease rapidly, which causes its infiltration capacity to decrease rapidly; (2) Leakage stage. When reaching this stage, a thin layer of water film has formed on the surface of the particles of the porous medium, making the molecular force close to zero. At the same time, due to the slow increase rate of capillary pressure, the rate of decrease of the permeability of the porous medium also slows down accordingly. The water movement in this stage is mainly driven by capillary pressure, but its influence gradually weakens; (3) Infiltration stage. When the water content of the porous medium is close to saturation, it enters the infiltration stage. At this time, the influence of capillary action and molecular force has become negligible. The water movement is mainly affected by gravity. Gravity is a relatively stable force. Therefore, in this stage, the permeability of the porous medium reaches a low and stable level (approximately the saturated permeability coefficient).

[0029] Based on the above porous media infiltration law, there are many different infiltration models such as Kostiakov model, Kostiakov-Leiws model, Parlange model, Philip model, Horton model, Greep-Ampt model and Smith model, etc. The earliest Greep-Ampt model is as follows (2), which assumes that the soil area above the wetting front (wetting peak) reaches the saturated water content, while below the wetting front, the water content of the soil remains at the initial water content.

[0030] (1) In the formula, f —Infiltration rate; K s —Soil saturated hydraulic conductivity (also known as permeability coefficient); S f —Average suction at the wetting front; f —Generalized depth of the wetting front; h—total water potential at the surface.

[0031] The changing trend of the one-dimensional seepage experiment of rock is also consistent with the infiltration characteristics of porous media. Its infiltration nature and process are similar to those of soil. Therefore, it is believed that the infiltration process of rock also conforms to the basic law of the three stages of water infiltration into porous media.

[0032] Based on the Green-Ampt model, when the water film thickness on the rock sample surface is very small, less than 2 mm, equation (1) can be rewritten as: (2) Based on the Green-Ampt model assumption, in the one-dimensional infiltration process of rock, the thickness of the unsaturated zone is considered to be 0, and the initial water content of the sample is 0, so the cumulative infiltration amount I can be expressed as: (3) Based on the weight gain Δm and porosity θ of the sample in the one-dimensional simulated seepage experiment of rock s The wetting peak depth z of the sample during the seepage process can be calculated f , combining equations (2) and (3) we can get: (4) Assume that the infiltration rate of the rock sample at each time interval during the seepage process is f Constant, can record the sample seepage test process f and I Based on formula (4), the f =F( I ), the intercept K of the function segment is obtained by fitting the curve s , that is, the permeability coefficient of the sample. The inversion results of the permeability coefficient of each sample are shown in Table 2: Table 2 Inversion results of permeability coefficients of sandstone and granite samples

[0033] Comparing the test and inversion results, it can be seen that the permeability coefficient inversion based on the one-dimensional seepage experiment of rock is suitable for the inversion of samples with large permeability coefficients. When the permeability coefficient of the sample is small, the weighing error of the sample leads to the intercept K s The inversion error overwhelms the true result. In addition, due to the initial infiltration of rock f The test time interval is difficult to meet the measurement of small time intervals, and the inversion is performed within each time interval. f The average value is used for calculation, and when the permeability coefficient of the sample is small, it will also cause the error to overwhelm the actual result. In summary, the rock permeability inversion method proposed based on the Greep-Ampt model is suitable for rocks with large permeability coefficients. Although it has limitations, it can greatly reduce the time consumption of rock permeability testing. Considering the integrity of the research, the present invention uses the permeability coefficient obtained by Darcy's law test as the seepage field calculation parameter.

[0034] The VG model parameter inversion based on rock seepage simulation is explained as follows: Since Darcy's law has a limited scope of application and is difficult to describe the saturated seepage flow with complex boundaries, in 1931, Richards made an important contribution to the water flow in porous media. He emphasized the essential difference between the saturated zone and the unsaturated zone in terms of water pressure and permeability. In the unsaturated zone, the water pressure is mainly determined by capillary action, while its permeability is related to the saturation of the medium. Based on this understanding, combined with Darcy's law, Richards proposed the unsaturated seepage control equation Richards equation. The slow flow of liquid in variable saturated porous media can be described by Richards equation, which is expressed as: (5) Where θ is the volumetric water content, t is the time, K is the permeability coefficient, ψ is the total soil water potential of the unsaturated soil, and x, y, and z represent the three-dimensional directions.

[0035] Although the Richards equation can describe the saturated-unsaturated seepage movement of porous media, from the perspective of physical mechanism, the Richards equation ignores the influence of pore gas inside the porous media on the seepage movement process, that is, it ignores the obstruction of gas to water flow and does not consider the interaction between the gas phase and the water phase. It is more suitable for unsaturated seepage containing a certain amount of rock and soil media. Therefore, the Richards equation is difficult to be used for the description of the seepage field under the natural infiltration conditions of rock humidification in the present invention.

[0036] Rocks are complex porous media, and their pores contain not only water and air, but also other substances such as soil. In rock formations without groundwater, most rocks are dry or unsaturated, which means that the water in the pores does not completely fill all available space, and coexists with gaseous fluids such as air. In such a multiphase system, fluids such as water and air are usually in different phases. Each fluid exists as an independent phase with a clear interface and is different from other phases in material properties. Since different phases are generally considered to be immiscible, there will be a certain interaction force between them. This interaction force is usually manifested as capillary pressure, which is caused by the difference in surface tension between fluid and solid. Generally speaking, in the seepage study of unsaturated rocks, it is assumed that the skeleton of the rock porous medium does not deform and the solid particles do not dissolve. The seepage study of unsaturated rocks focuses more on analyzing the movement characteristics of water and gas phases in the rock pores and the interaction between them.

[0037] When one fluid displaces another fluid in a porous medium, the pressure of the displacing phase needs to be greater than a certain value in order to displace the displaced phase. According to the porous medium water infiltration model, during the two-phase displacement process in porous media, each phase will be subject to three forces: the pressure of other phases, capillary pressure, and gravitational potential. When the sum of the pressures of the displacing phase is greater than the sum of the pressures of the displaced phase, phase transfer and multiphase flow inside the porous medium can be achieved.

[0038] Two-phase flow in porous media follows the wetting ( w ) and non-wet( nw ) fluid independent equations, as shown in equations (6) and (7): (6) (7) In the formula, θ s is the porosity; θ r is the residual volume fraction of porous media; S e is the effective saturation; k0 is the permeability of the porous medium, m 2 ; k(S e ) is the relative permeability, i.e., the permeability function that varies with fluid saturation; μ is the fluid dynamic viscosity, Pa·s; p is the pressure, Pa; ρ is the fluid density, kg / m 3 ; t is time; g is the acceleration due to gravity; z is the height in one-dimensional direction, m.

[0039] In a continuous flow field, the two phases will be displaced, so the volume fraction of the wet phase (water body) θ w will change from zero or a small residual value to a porosity θ s , effective saturation S e The volume fraction θ and effective saturation S vary between 0 and 1. e are functions of the pressures of the two fluids in the porous medium. The equilibrium equations followed by the water phase and the gas phase in the unsaturated porous medium include linear momentum balance, momentum balance and mass balance equations. According to the previous text, each phase in the porous medium will be affected by a variety of forces during movement. The above forces are not constant during the movement, but will change with the phase saturation. The changes in capillary pressure and relative permeability with saturation have a greater impact on the seepage. Based on the above functional relationship, two constitutive relationships need to be constructed according to the characteristics of the porous medium.

[0040] (1) Capillary pressure-saturation constitutive relationship Capillary action is a common physical phenomenon involving the movement of liquids in very small tubes or pores. The microscopic principle of this phenomenon is mainly the surface tension of the liquid. The surface tension makes the liquid surface behave like a stretched film, tending to reduce the surface area, thereby generating a force that makes the liquid surface tend to be flat. When the liquid comes into contact with a solid (such as the wall of a capillary tube), if the liquid can wet the solid, the liquid surface will be concave near the solid surface, forming a concave liquid surface, such as Figure 3 This is because the attraction between the liquid molecules and the molecules on the solid surface is greater than the mutual attraction between the liquid molecules, so the concave liquid surface exerts an upward pull on the liquid below, which causes the liquid to rise along the tube wall. The process of the liquid rising in the capillary tube continues until the upward force (capillary action caused by surface tension) and the downward force (mainly the weight of the liquid column) reach equilibrium. At this point, the liquid column stops rising and reaches a stable state at a certain height. This equilibrium height depends on many factors, including the surface tension of the liquid, the density of the liquid, the radius of the capillary tube, and the contact angle between the liquid and the solid material.

[0041] According to the static equilibrium condition in capillary phenomena, the sum of the pressure p at the top of the capillary liquid surface and the water pressure ρgH of the water column is equal to the atmospheric pressure p0: (8) p is the difference between atmospheric pressure p0 and surface tension pressure 2α / R, where α is the surface tension of the liquid. From this, the capillary pressure on the water surface is: (9) According to the above formulas (8) and (9), we can know that: (10) In the formula, r is the capillary radius, and φ is the angle between the liquid surface bending tangent and the tube wall. It can be seen that the capillary pressure is inversely proportional to the capillary radius. In porous media, the finer the pores, the greater the capillary force. For dense porous media such as rock, the pore radius is generally in the μm range, and the capillary force is relatively large.

[0042] From a macroscopic point of view, in the unsaturated gas-water phase seepage in porous media, as the wet water phase saturation increases, the pore radius decreases and the capillary pressure increases. In the slow seepage process, the influence of the capillary pressure changing with the water phase saturation cannot be ignored. At present, the capillary pressure-saturation relationship models commonly used in water-gas two-phase flow systems are Brooks and Corey (BC) model and Van Genuchten (VG) model. The BC model is discontinuous at the air intake value, while the soil-water characteristic curve fitting model (VG model) can not only reflect the relationship between capillary pressure and water saturation, but also has a smooth transition at the air intake value and residual water saturation. Therefore, the present invention adopts the VG model to describe the capillary pressure-saturation constitutive relationship, as shown in the following formula (11). As the water phase saturation increases, the absolute value of the capillary pressure decreases, which is consistent with the actual situation.

[0043] (11) In the formula, S e is the effective saturation, S w is the volume fraction of saturated water phase in porous media, S wr is the residual water phase volume fraction in porous media, p c is the capillary pressure, p ec is the inlet capillary pressure, and λ is the pore distribution index.

[0044] (2) Relative permeability-saturation constitutive relationship From a microscopic perspective, when two fluids exist in the pores of a porous medium, the movement of the two fluids will interfere with each other. For example, an increase in water phase saturation will lead to an increase in the curvature of the water phase channel. Therefore, the relative permeability of the porous medium will change with the saturation of different fluid phases. From a macroscopic perspective, relative permeability is related to saturation. In 1976, Mualem proposed a constitutive relationship model of relative permeability-saturation: (12) Where τ is a model parameter related to the pore tortuosity of porous media.

[0045] When the water phase saturation in the porous medium reaches saturation, the water phase channel is the shortest. When the water phase saturation decreases, the water flow length increases due to the presence of gas phase in the pores. This phenomenon can be expressed by the tortuosity Г: (13) Where Г is the tortuosity, L is the length of the porous medium, and L e is the length of the water flow.

[0046] Substituting the VG model formula into equation (12) can obtain the relative permeability-saturation constitutive relationship model in two-phase flow in porous media, namely the VG-M model: (14) In the description of two-phase transport in rock porous media, the VG-M model can be used to describe the variation characteristics of relative permeability and capillary pressure with saturation, where the inlet capillary pressure p ec and pore distribution index λ are key parameters of the VG model and are indispensable in the calculation of two-phase seepage field of rock. Conventional water-soil characteristic curve tests include moisture absorption curve and dehumidification curve. It is necessary to measure the matrix potential of the medium at different water contents and invert its constitutive parameters based on the VG model. At present, water-soil characteristic curve tests are mostly carried out on soil, and the test on rock medium is more difficult. Therefore, a VG model parameter inversion based on rock seepage simulation is proposed. Combined with the test results of porosity and permeability of rock samples mentioned above, one-dimensional seepage simulation calculation of rock is carried out, and compared with the results of one-dimensional simulated seepage experiment, the constitutive characteristic parameters of rock VG model are inverted.

[0047] From the analysis of the multiphase displacement process inside porous media, it can be seen that there is an inlet pressure at the interface between different fluid phases, and the pore inlet pressure is related to both the pore size and the phase pressure. During the water-gas two-phase displacement process inside the rock, since there is no water phase accumulation at the interface, it is believed that there is no water phase pressure at the water-gas interface. At this time, the inlet capillary pressure p ec It is only related to the pore size. The pore size of common sandstone is in the μm range. The contact angle of the two-phase interface is 0°, and the surface tension of water is 0.07 N / m. From formula (11), we can know that the inlet capillary pressure p ec The present invention conducts unsaturated seepage research on low, medium and high permeability rock media, the sandstone pore distribution index λ can be 0.1-0.88, and the permeability of high, medium and low permeability rock media is 100mD-1000mD, 3mD-100mD, 0.1mD-3mD respectively.

[0048] The multiphase transport in porous media in COMSOL can be used for the movement of gas-water two-phase fluid in the rock medium of the present invention. The porous media flow module includes tools that can be used to simulate multiphase flow in porous media, where the number of phases can be two or any number of mobile phases, and the properties of porous media can be specified to simulate wicking, water transport or other transport phenomena in porous media. The present invention intends to use the porous media phase transport module in COMSOL to simulate the simulated seepage process of cylindrical rock samples.

[0049] Based on the one-dimensional rock simulation seepage experiment, a one-dimensional porous medium unsaturated two-phase seepage model was built. The porous medium was taken as a circular surface with a length of 10 cm and a cross section of 2.5 cm in radius. The boundary was set as a continuous water supply boundary with a water phase volume fraction of 1. Corresponding to the one-dimensional seepage experiment, the sample permeability and porosity were set according to the test results of the sample hydraulic characteristic parameters, and the inlet capillary pressure p was set as ecThe pore distribution index λ is set to typical values, and the simulation calculation of the sample seepage field is carried out. ec Determines the initial infiltration rate of the sample, and the pore distribution index λ determines the inflection point of the sample coefficient infiltration rate. The inlet capillary pressure p of sandstone and granite can be inverted by calculation. ec and pore distribution index λ are shown in Table 3. The comparison between the seepage field calculation results and the seepage simulation experimental results of different samples is shown in Figure 4. From the comparison curves of different samples, it can be seen that the rock seepage field calculation based on porous medium two-phase flow is consistent with the measured results. Based on the rock permeability and porosity, combined with the one-dimensional seepage experimental results of rocks, simulation calculations can be carried out to obtain the constitutive characteristic parameters of rocks such as pore size and distribution index.

[0050] Table 3 Capillary pressure p at the inlet of sandstone and granite samples ec and the pore distribution index λ

[0051] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for inversion of seepage characteristic parameters applicable to high permeability rocks, characterized by: The following steps are involved: S1. Based on the one-dimensional rock simulation seepage experiment, a one-dimensional porous medium unsaturated two-phase seepage model is constructed. The porous medium is taken and a continuous water supply boundary is set to correspond to the one-dimensional seepage experiment. Combined with the experimental results, the rock permeability coefficient is inverted according to the seepage formula as a known input parameter of the model. The inlet capillary pressure p ec and pore distribution index λ are set to typical values, and simulation calculation of the seepage field of the sample is carried out; S2, inlet capillary pressure p ec The initial infiltration rate of the sample is determined by the pore distribution index λ, which determines the inflection point of the sample coefficient infiltration rate. The inlet capillary pressure p of sandstone and granite is obtained by calculation and inversion. ec and pore distribution index λ; S3. The calculated seepage field results of different samples are compared with the seepage simulation experimental results. Assuming that the calculated rock seepage field results based on two-phase flow in porous media are consistent with the measured results, based on the rock permeability and porosity, combined with the one-dimensional seepage experimental results of rocks, simulation calculations are carried out to obtain the pore size and distribution index constitutive characteristic parameters of the rock.

2. The method for inversion of seepage characteristic parameters applicable to high permeability rocks according to claim 1, characterized in that: In S1, the one-dimensional rock simulation seepage experiment includes the following steps: The rock section at one end is fastened by a clamp and placed flat on an epoxy resin bracket. The water spray system controls the nozzle to spray water evenly to the rock sample. The rock sample is fully wrapped with water-blocking tape and polyvinyl chloride film to prevent the flow and evaporation of surface water. Only the section at one end of the clamp is exposed to the humidification range of the nozzle. The rock humidification end is covered with an epoxy resin perforated partition with a hole smaller than the rock sample section to prevent the nozzle humidification water from splashing to the side of the sample and affecting the sample weight test; The process of the one-dimensional rock simulation seepage experiment is as follows: (1) The rock sample was dried and cooled, and the surface was wrapped with water-blocking tape and polyvinyl chloride film, and a clamp was installed. The weight of the treated dry rock sample was recorded as m0, and the start time of the experiment was recorded as 0s; (2) The experiment adopted the method of timed water spraying and weighing to record the water absorption of the rock samples. The rock samples were sprayed with water for the first time at 0s. The water spraying system was humidified regularly to ensure that the humidification end of the sample was evenly moistened and there was a layer of water film. The thickness of the water film h was less than the length of the sample L. Different weighing time intervals were taken depending on the difference in water absorption of the rock samples. (3) For green, dark red and light red sandstone, a weighing interval of 10 min is used, for granite, a weighing interval of 10-15 min is used, and for yellow sandstone, a weighing interval of 7-10 min is used. The state of the water film on the surface of the sample is observed within one weighing interval. If the water film on the cross section of the sample is incomplete, water is sprayed once until the cross section of the sample is wiped dry after one weighing interval. The weighing is recorded as Δm1=m1-m0; (4) Repeat the water spraying 5 times and record m0, Δm1, Δm2, Δm3, Δm4, and Δm5 respectively; where Δm1=m1-m 0； Δm2=m2-m 1； Δm3=m3-m 2； Δm4=m4-m 3； Δm5=m5-m 4； m1, m2, m3, m4, and m5 are the weights of the specimens when sprayed with water for the 1st to 5th time respectively; Simulated seepage experiments were carried out on the above rock samples to obtain the change curve of the infiltration capacity of the porous medium with time.

3. The method for inversion of seepage characteristic parameters applicable to high permeability rocks according to claim 1, characterized in that: In S1, the one-dimensional porous medium unsaturated two-phase seepage model is that the soil area above the wetting front reaches the saturated water content, while below the wetting front, the water content of the soil remains at the initial water content. The model is: ； In the formula, f —Infiltration rate; K s —Permeability coefficient; S f —Average suction at the wetting front; f —generalized depth of wetting front; h—total water potential at the surface; When the water film thickness on the rock sample surface is less than 2 mm, it can be rewritten as: 。 4. The method for inversion of seepage characteristic parameters applicable to high permeability rocks according to claim 1, characterized in that: The process of inverting the rock permeability coefficient according to the seepage formula in S1 is as follows: In the one-dimensional infiltration process of rock, the thickness of the unsaturated zone is considered to be 0, the initial water content of the sample is 0, and the cumulative infiltration amount I is expressed as: ； Based on the weight gain Δm and porosity θ of the sample in the one-dimensional simulated seepage experiment of rock s The wetting peak depth z of the sample during the seepage process can be calculated f ,have to: ； Assume that the infiltration rate of the rock sample at each time interval during the seepage process is f Constant, record the sample seepage test process f and I , draw the f =F( I ), the intercept K of the function segment is obtained by fitting the curve s , that is, the permeability coefficient of the sample.

5. The method for inversion of seepage characteristic parameters applicable to high permeability rocks according to claim 1, characterized in that: In S2, the inlet capillary pressure p of sandstone and granite is obtained by calculation inversion. ec The specific process of calculating the pore distribution index λ includes: S2.

1. Seepage characteristic parameters: inlet capillary pressure p ec and the pore distribution index λ are model coefficients of the capillary pressure-saturation constitutive relationship; the capillary pressure-saturation constitutive relationship is as follows: According to the static equilibrium condition in capillary phenomena, the sum of the pressure p at the top of the capillary liquid surface and the water pressure ρgH of the water column is equal to the atmospheric pressure p0: ； p is the difference between atmospheric pressure p0 and surface tension pressure 2α / R, where α is the surface tension coefficient of the liquid surface, and R represents the force of the liquid surface being tightened inward perpendicular to the liquid surface. From this, the capillary pressure on the water surface can be obtained as: ； According to the above formula, we can know that: ； In the formula, r is the capillary radius, and φ is the angle between the liquid surface bending tangent and the tube wall. It can be seen that the capillary pressure is inversely proportional to the capillary radius. In porous media, the finer the pores, the greater the capillary force. The soil-water characteristic curve fitting model is used to describe the capillary pressure-saturation constitutive relationship, as shown in the following formula. As the water phase saturation increases, the absolute value of the capillary pressure decreases, which is consistent with the actual situation. ； In the formula, S e is the effective saturation, S w is the volume fraction of saturated water phase in porous media, S wr is the volume fraction of residual water phase in porous media, p c is the capillary pressure, p ec is the inlet capillary pressure, and λ is the pore distribution index.

6. The method for inversion of seepage characteristic parameters applicable to high permeability rocks according to claim 5, characterized in that: In S2, the inlet capillary pressure p of sandstone and granite is obtained by calculation inversion. ec The specific process of pore distribution index λ also includes: S2.2, the characteristic parameter inlet capillary pressure p ec and the pore distribution index λ can form the relative permeability-saturation constitutive relationship, which is the key to describing the seepage characteristics of high permeability rocks; the relative permeability-saturation relationship is as follows: When the water phase saturation in the porous medium reaches saturation, the water phase channel is the shortest. When the water phase saturation decreases, the water flow length increases due to the presence of gas phase in the pores. This phenomenon is expressed by the tortuosity Г as: ； Where Г is the tortuosity, L is the length of the porous medium, and L e is the length of the water flow; is the water saturation, τ is the model parameter related to the pore tortuosity of porous media; Substituting the soil-water characteristic curve fitting model formula into the relative permeability-saturation constitutive relationship model can obtain the relative permeability-saturation constitutive relationship model in porous media two-phase flow, namely the VG-M model: ； in: k is the relative permeability of rock at different water saturations, is the water saturation, λ is the pore distribution index, is the effective water saturation; In the description of two-phase transport in rock porous media, the VG-M model is used to describe the variation characteristics of relative permeability and capillary pressure with saturation.

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