Prediction and early warning method for cavity deformation and land subsidence of salt cavern gas storage

Abstract

The invention belongs to the technical field of safety evaluation analysis in the construction and operation process of an underground salt cavern gas storage, and particularly relates to a prediction and early warning method for cavity deformation and land subsidence of a salt cavern gas storage. Comprising the following steps: S1, carrying out a gas storage field creep parameter inversion test to obtain actual rock mechanical parameters of salt cavern gas storage cavity engineering; s2, constructing a land subsidence prediction model by using the rock mechanical parameters obtained in the step S1, and predicting land subsidence by using the model; and S3, determining a ground subsidence early warning threshold value, and performing salt cavern gas storage cavity safety early warning based on the threshold value and the ground subsidence prediction in the S2. According to the method, the obtained mechanical parameters are closer to the rock mechanical parameters of actual engineering, so that it is guaranteed that the rock mechanical properties at the salt bed of the actual engineering are accurately reflected, a prediction model established based on the mechanical parameters is more reliable, and land subsidence prediction and early warning are more accurately carried out.

Description

Technical Field

[0001] The invention relates to a method for predicting and early warning of cavity deformation and ground subsidence of a salt cavern gas storage, and belongs to the technical field of safety evaluation and analysis in the construction and operation of an underground salt cavern gas storage. Background Art

[0002] Natural gas is an important part of my country's energy system, and its consumption will continue to increase. In order to ensure a stable supply of natural gas, various gas storage methods have been developed, among which underground salt cavern gas storage is one of the main ways to store natural gas energy and an important part of my country's basic gas storage facilities.

[0003] Underground salt cavern gas storage is generally located in thick salt rock layers. Salt rock has low porosity and permeability and good creep characteristics, making it a good surrounding rock medium for gas storage. However, due to its good creep characteristics, creep will occur even under small deviatoric stress, causing the salt rock around the cavity to flow inward, causing the cavity to shrink and deform, thereby causing surface subsidence. When the subsidence is large or even the surface collapses, it will cause more serious accidents. Therefore, the prediction of cavity deformation and surface subsidence of gas storage and early warning are of great guiding significance for the stable operation of gas storage and the safety evaluation of the storage area.

[0004] At present, in order to timely grasp the relevant safety issues in the construction and operation of gas storage, salt cavity sonar detection technology is mainly used to detect the cavity morphology, so as to obtain the cavity morphology changes by comparing the results of sonar cavity measurement in a certain time interval; traditional leveling measurement or emerging technologies such as InSAR, GPS, PSI, etc. are used to monitor the ground subsidence of gas storage; in addition, three-dimensional numerical simulation based on geological data, cavity data, and rock mechanics parameters of the storage area is used to predict the deformation of the cavity under the current operating pressure to guide actual engineering and determine the operating pressure limit of the gas storage.

[0005] The prediction of cavity deformation and ground subsidence of salt cavern gas storage is generally obtained through numerical simulation. Numerical simulation is divided into two parts: modeling and calculation. Whether the calculation results of numerical simulation are accurate and can be used to guide actual engineering depends on the establishment of numerical models, the research and selection of calculation methods, and the appropriateness of the selection of calculation parameters.

[0006] Most of the existing numerical simulation modeling schemes can only reflect the approximate shape of the salt cavern cavity, which is far from the actual shape of the cavity.

[0007] In the existing numerical simulation calculation schemes, whether using ABAQUS, Adagio, or FLAC 3DAll numerical calculation software require the mechanical parameters of salt rock. At present, the mechanical parameters of salt rock are generally obtained by laboratory measurements. However, due to the characteristics of my country's multi-layered salt rock formations, the mechanical parameters obtained from laboratory tests may not accurately reflect the rock mechanical properties of the actual engineering salt layer. In the theoretical prediction of surface settlement of salt cavern gas storage, the influence function method is usually used. Its principle is simple and its applicability is strong. However, in its application, the research on the key parameters (volume transfer coefficient) is not in-depth. At present, its value is still based on empirical values.

[0008] On the other hand, the study of ground subsidence threshold setting is one of the key parts in the risk analysis of salt cavern gas storage, which can be directly applied to actual engineering guidance and is of great significance to ensuring the construction and operation safety of salt cavern gas storage. However, the criteria for judging the early warning threshold of surface subsidence of salt cavern gas storage are still unclear.

[0009] In addition, due to the mutual influence of the cavities, it is difficult for existing technologies to accurately calculate the ground subsidence of underground salt cavern gas storage.

[0010] As my country's gas storage projects are in a rapid development stage, safety issues during their construction and operation should be paid more attention. Therefore, a complete and accurate prediction and early warning method for gas storage cavity deformation and ground subsidence is urgently needed, which can be used for on-site guidance to ensure the safety of gas storage construction and operation. Summary of the invention

[0011] The purpose of the present invention is to provide a prediction and early warning method for cavity deformation and ground subsidence of a salt cavern gas storage reservoir, so as to solve the problem that the mechanical parameters of salt rock are generally obtained by laboratory measurements in the prior art, but for the characteristics of my country's multi-layered salt rock strata, the creep parameters obtained from laboratory tests may not accurately reflect the rock mechanical properties in the actual engineering salt layer.

[0012] To achieve the above purpose, the technical solution of the present invention is a method for predicting and warning the deformation of the cavity of a salt cavern gas storage and ground subsidence, comprising the following steps:

[0013] S1: Conduct creep parameter inversion test on site of gas storage to obtain the actual rock mechanics parameters of the salt cavern gas storage cavity engineering;

[0014] S2: constructing a ground subsidence prediction model using the rock mechanics parameters obtained in step S1, and applying the model to predict ground subsidence;

[0015] S3: Determine the ground subsidence warning threshold and perform a safety warning for the salt cavern gas storage cavity based on the threshold and the ground subsidence prediction in S2.

[0016] The present invention obtains the actual rock mechanical parameters of the project by conducting on-site creep parameter inversion tests of gas storage reservoirs. The obtained mechanical parameters are closer to the rock mechanical parameters of the actual project, so as to ensure that the rock mechanical properties of the salt layer in the actual project are accurately reflected, thereby making the prediction model established based on the mechanical parameters more reliable, thereby more accurately predicting and warning ground subsidence.

[0017] Furthermore, in step S1, the rock mechanics parameters consistent with the actual project are obtained by conducting an on-site creep parameter inversion test of the gas storage reservoir, including:

[0018] S11: Conduct a closed-well pressure-raising field test: Select a cavity that has been completed and has undergone sonar testing, measure the data of wellhead pressure changes over time, and draw a curve of wellhead pressure changes over time;

[0019] S12: Calculate the volume change of the cavity during the closed-well pressure test, that is, the actual cavity volume shrinkage rate;

[0020] S13: simulating and calculating the test cavity numerical model to obtain the cavity shrinkage rate occurring during the well shut-in and pressure-increasing test;

[0021] S14: Compare the actual cavity shrinkage rate in S12 with the cavity shrinkage rate that occurred during the closed-well pressure test in S13, and select the combined value of creep parameters when the two are closest as the rock mechanics parameters for subsequent numerical simulation analysis.

[0022] The present invention provides a specific preferred scheme for calculating the actual cavity shrinkage rate and the cavity shrinkage rate that occurs during the closed-well pressure-raising test. The preferred scheme can ensure that reliable actual cavity shrinkage rates and the cavity shrinkage rates that occur during the closed-well pressure-raising test are obtained, so that through comparison, rock mechanical parameters that can better reflect the rock properties in actual engineering are determined.

[0023] Furthermore, when measuring the data of the wellhead pressure changing with time, a pressure gauge capable of remote data transmission is installed at the wellhead to realize the wellhead pressure data collection and remote transmission.

[0024] A pressure gauge capable of remote data transmission is installed at the wellhead, and the collected pressure data at the inlet is remotely transmitted to the data processing end.

[0025] Furthermore, in step S2, constructing a land subsidence prediction model includes:

[0026] S21: Select the cavity of the salt cavern gas storage that has been put into production, has been subjected to sonar cavity measurement, and has the injection and production operation pressure data since the sonar cavity measurement. Combine the cavity sonar cavity measurement data and geological data to establish a three-dimensional numerical simulation analysis geological model, set boundary conditions, and use the creep parameter inversion test of the gas storage site in S1 to obtain the actual rock mechanics parameters of the project. The monitoring object is set to the cavity volume change and the displacement of the ground settlement monitoring point of the upper stratum of the cavity;

[0027] S22: Arrange the numerical calculation code of the cavity injection and production operation pressure data, expand the existing injection and production operation data to several years, perform numerical simulation, and obtain the cavity volume change and the monitoring data of the ground settlement monitoring point of the upper stratum of the cavity;

[0028] S23: sorting the data obtained in step S22 to obtain the ground settlement curve of the cavity at different volume shrinkage rates;

[0029] S24: calculating the corresponding volume transfer coefficient according to the land settlement curves at different volume shrinkage rates in S23, and obtaining a variation curve of the volume transfer coefficient at different volume shrinkage rates;

[0030] S25: Apply the above volume transfer coefficient to the ground settlement prediction model to obtain a settlement model for calculating the ground settlement of other cavities under corresponding volume shrinkage rates.

[0031] The present invention provides a solution for constructing a ground subsidence prediction model based on a system transfer coefficient obtained by theoretical derivation. Compared with the solution in the prior art that only relies on experience to determine the system transfer coefficient, this solution has higher reliability.

[0032] Furthermore, when determining the ground subsidence warning threshold, the ground subsidence warning threshold is determined based on the allowable deformation of the salt cavern cavity of 15-20%.

[0033] Based on the analysis of cavity shrinkage risk: when the cavity volume shrinkage rate reaches 20%, it will cause more serious risk consequences, the cavity will be seriously deformed and damaged, the main functions will be impaired, and it will be in a more dangerous state. When the cavity volume shrinkage rate reaches 30%, it will cause catastrophic consequences. Therefore, from a conservative point of view, the cavity volume shrinkage rate should not exceed 20%. The present invention provides an optional range of 15-20% allowable deformation in combination with specific engineering conditions to ensure the rationality of the determined warning threshold.

[0034] Furthermore, when determining the ground subsidence warning threshold, the surface subsidence of each cavity is calculated respectively, and then a correction coefficient is added to the surface subsidence of each cavity for linear superposition to obtain the overall surface subsidence.

[0035] The present invention takes into account the mutual influence effect between cavities, thereby obtaining a ground subsidence warning threshold that is more in line with actual conditions.

[0036] Furthermore, the determination of the ground subsidence warning threshold includes:

[0037] S31: Select a salt cavern gas storage area;

[0038] S32: Collect sonar test data and geological data of all cavities in the salt cavern gas storage area;

[0039] S33: using step S24 to verify the obtained volume transfer coefficient, influence angle and theoretical prediction model of ground settlement, and calculating the ground settlement amount caused by each cavity when the cavity shrinks to a set value;

[0040] S34: Determine the cavity that will produce the group cavity mutual influence effect, add a correction coefficient to the surface settlement of each cavity, perform linear superposition, and obtain the overall surface settlement prediction data;

[0041] S35: according to the above-mentioned overall ground subsidence prediction data, find the maximum cumulative value position of the ground subsidence in the selected area, and then set the warning threshold value in combination with the allowable deformation of the selected salt cavern gas storage area;

[0042] S36: Provide a safety warning based on the warning threshold in S35 and the ground subsidence predicted in S2.

[0043] The present invention provides a specific preferred solution for early warning of ground subsidence based on the allowable deformation of the salt cavern cavity and taking into account the mutual influence between cavern groups.

[0044] Furthermore, when finding the position of the maximum cumulative value of ground subsidence in the selected area according to the overall ground subsidence prediction data in S35, the following method is adopted: a corresponding ground subsidence cloud map is made according to the overall ground subsidence prediction data, and based on the ground subsidence cloud map, the position of the maximum cumulative value of ground subsidence in the selected area is found.

[0045] The ground subsidence cloud map can directly and vividly reflect the ground subsidence prediction data, so as to accurately and quickly find the location with the maximum accumulated value of ground subsidence. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a flow chart of the method of the present invention;

[0047] Figure 2 It is a schematic diagram of the creep shrinkage mechanism of the solution cavity in the present invention;

[0048] Figure 3 It is a schematic diagram of the surface settlement of the spherical cavity in the present invention;

[0049] Figure 4 is a flow chart of obtaining rock mechanical parameters according to the present invention;

[0050] Figure 5 is a flow chart of the present invention for calculating the ground settlement curve of the cavity under different volume shrinkage rates;

[0051] Figure 6 It is a flow chart of establishing the land subsidence prediction model of the present invention;

[0052] Figure 7 is a flow chart of determining a ground subsidence warning threshold value according to the present invention;

[0053] Figure 8 is a curve diagram of the change of wellhead pressure over time of the present invention;

[0054] Fig. 9 It is a schematic diagram of a three-dimensional model of a cavity established by the present invention using modeling software;

[0055] Fig.10 It is a schematic diagram of establishing a numerical calculation model based on cavity geological data in the present invention;

[0056] Fig.11 It is a comparison chart of the cavity shrinkage rate during the closed well pressure test and the actual cavity shrinkage rate;

[0057] Fig.12 It is a schematic diagram of the geological body model constructed by the present invention;

[0058] Fig.13 It is a numerical simulation result of the present invention - a curve diagram of the cavity volume shrinkage rate changing with time;

[0059] Fig.14 is a curve diagram showing the change of the volume transfer coefficient with the cavity volume shrinkage rate of the present invention;

[0060] Fig.15 It is a curve comparison diagram of the theoretical model and the numerical simulation when the cavity shrinks by 20% of the present invention;

[0061] Fig.16 This is the distribution map of the cavities in the Jintan salt cavern gas storage area;

[0062] Fig.17 It is a ground settlement cloud diagram when the volume shrinkage rate of the cavity in the present invention is 20%. DETAILED DESCRIPTION

[0063] In order to more clearly illustrate the technical route, implementation strategy and solution advantages of the invention, the invention will be analyzed and explained in depth below in combination with the accompanying drawings, but the implementation method of the invention is not limited to this.

[0064] Example 1

[0065] S1: Obtain the actual rock mechanics parameters of the project by conducting on-site creep parameter inversion tests at the gas storage facility:

[0066] S11: Conduct a closed-well pressure-raising field test: Select a cavity that has been completed and has undergone sonar testing.

[0067] Measure the data of wellhead pressure changing with time and draw the curve of wellhead pressure changing with time;

[0068] S12: Calculate the change in the cavity volume during the closed-well pressure test, that is, the actual cavity volume shrinkage rate.

[0069] The curve of the change of wellhead pressure over time is linearly fitted to obtain the wellhead pressure change rate, that is, the pressure growth rate dp / dt. The original rock stress p0 at the depth of the salt cavity and the brine pressure p in the cavity are calculated based on the change of wellhead pressure and the location of the cavity. i , combined with formula (3), the A value corresponding to different n values ​​is calculated. A and n are creep parameters. According to the original rock stress p0 at the depth of the salt cavity and the brine pressure p in the cavity, i , combined with equation (1) and equation (3), the volume change of the cavity during the closed-well pressure test can be calculated, that is, the actual cavity volume shrinkage rate;

[0070] S13: Simulate and calculate the test cavity numerical model to obtain the cavity shrinkage rate during the closed-well pressure test:

[0071] Simulate the cavity deformation from the start of the sonar cavity test to the start of the closed-well pressure test;

[0072] Calculate the cavity shrinkage rate during the closed-well pressure test under different combinations of creep parameters A and n obtained in S12;

[0073] S14: Compare the actual cavity shrinkage rate in S12 with the cavity shrinkage rate in S13 during the closed-well pressure test, and select the combined value of creep parameters A and n when the two are closest as the rock mechanics parameters for subsequent numerical simulation analysis;

[0074] Among them, the cavity creep shrinkage mechanism (see Figure 2 ), the entire cavity is considered to be under a confining pressure of p0-p i The brine body, assuming the bulk modulus of brine is K, then the brine body is subjected to the confining pressure p0-p i The volume strain per unit time ε v for:

[0075]

[0076] The formula for the volume shrinkage rate of the infinite body cavity is:

[0077]

[0078] Where V is the original volume of the salt cavity, represents the volume shrinkage of the salt cavity per unit time, A and n are creep parameters, p0 is the original rock stress at the depth of the salt cavity, and p i is the brine pressure in the cavity;

[0079] Since the increase in the internal pressure of the dissolution cavity is mainly due to the creep shrinkage of the confining pressure, the volume shrinkage rate is equal to the volume strain of the brine, that is:

[0080]

[0081] in, The pressure increase rate.

[0082] S2: Construct a ground subsidence prediction model and apply the model to predict ground subsidence.

[0083] S3: Determine the ground subsidence warning threshold and perform a safety warning for the salt cavern gas storage cavity based on the threshold and the ground subsidence prediction in S2.

[0084] Example 2

[0085] A method for predicting and warning of cavity deformation and ground subsidence of a salt cavern gas storage reservoir, comprising:

[0086] S1: Conducting creep parameter inversion tests on gas storage sites to obtain rock mechanics parameters that are more consistent with engineering practice, see Figure 4 :

[0087] S11: Conduct a closed-well pressure-raising field test: select a cavity that has been completed and has been subjected to sonar testing. The cavity is filled with saturated brine and no volume change caused by salt rock dissolution will occur. Install a pressure gauge capable of remote data transmission at the wellhead, and then close the wellhead valve. At this time, the pressure in the cavity will gradually rise due to the creep of the surrounding rock. Set the pressure data transmission interval to 2 hours, record the changes in the wellhead pressure value on a daily basis, and compile statistics on the pressure monitoring data of the wellhead pressure gauge for about 2 months, and draw a curve of the change in wellhead pressure over time.

[0088] S12: Calculate the volume change of the cavity during the closed-well pressure test, that is, the actual cavity volume shrinkage rate;

[0089] The curve of the change of wellhead pressure over time is linearly fitted to obtain the wellhead pressure change rate, that is, the pressure growth rate dp / dt in formula (3). The original rock stress p0 at the depth of the salt cavity and the brine pressure p in the cavity are calculated by combining the wellhead pressure change and the location of the cavity. i , combined with formula (3), the A value corresponding to n value of 1.5-4.0 is calculated (A and n are creep parameters), and different creep parameter combinations are obtained. According to the original rock stress p0 at the depth of the salt cavity and the brine pressure p i , combined with equation (1) and equation (3), the volume change of the cavity during the closed-well pressure test can be calculated, that is, the actual cavity volume shrinkage rate;

[0090] S13: Use numerical simulation software to simulate and calculate the numerical model of the test cavity.

[0091] Simulate the cavity deformation from the start of the sonar cavity test to the start of the closed-well pressure test;

[0092] The deformation of the cavity during the creep parameter inversion field test with the change of the cavity pressure over time is simulated, and the creep parameter combination value is changed. According to the creep parameter n value obtained from the indoor test, it is generally taken as 3.5. 11 specific values ​​of n are selected from 3.0-4.0. The cavity shrinkage rate of the cavity during the closed-well pressure test is calculated under the corresponding 11 groups of different creep parameter combination values ​​(n, A).

[0093] S14: Compare the actual cavity shrinkage rate with the cavity shrinkage rate that occurred during the closed-well pressure test in S13, and select the closest creep parameter A, n combination value between the two as the rock mechanics parameters for subsequent numerical simulation analysis;

[0094] The schematic diagram of the creep shrinkage mechanism of the cavity is shown in Figure 2 As shown, the entire cavity is considered to be under a confining pressure of p0-p i The brine body, assuming the bulk modulus of brine is K, then the brine body is subjected to the confining pressure p0-p i The volume strain per unit time ε v for:

[0095]

[0096] The formula for the volume shrinkage rate of the infinite body cavity is:

[0097]

[0098] Where V is the original volume of the salt cavity, represents the volume shrinkage of the salt cavity per unit time, A and n are material parameters, p0 is the original rock stress at the depth of the salt cavity, and p i is the brine pressure in the cavity;

[0099] Since the increase in the internal pressure of the dissolution cavity is mainly due to the creep shrinkage of the confining pressure, the volume shrinkage rate is equal to the volume strain of the brine, that is:

[0100]

[0101] in, The pressure increase rate.

[0102] S2: Use 3D numerical simulation software to predict the deformation of the selected cavity. Select a salt cavern gas storage that has been put into operation and calculate the ground settlement curve of its cavity under different volume shrinkage rates. Figure 5 :

[0103] S21: Select a cavernous gas storage facility that has been put into production and is required to have undergone sonar cavity detection and have injection and production pressure data since the sonar cavity detection. Combine the cavity sonar cavity detection data and geological data, use the power law creep constitutive model, establish a three-dimensional numerical simulation analysis geological model, set boundary conditions, use the salt rock creep parameters selected in S14, and set the monitoring objects to the cavity volume change and the displacement of the ground settlement monitoring point in the upper strata of the cavity;

[0104] S22: Write numerical calculation code for cavity injection and production operation pressure data, expand the existing injection and production operation data to 30 years, perform numerical simulation, and save the cavity volume change during the numerical simulation process and the monitoring data of the ground settlement monitoring point of the upper strata of the cavity;

[0105] S23: sorting the data obtained in step S22 to obtain the ground settlement curve of the cavity at different volume shrinkage rates;

[0106] See also Figure 6 , S24: determine the influence range R0 of the ground subsidence according to the ground subsidence curves when the volume shrinkage rates are 5%, 10%, 15%, 20% and 25% in S23;

[0107] The size of the impact angle γ is calculated based on the impact range R0 and the depth f of the cavity;

[0108] S25: Then calculate the volume of the ground subsidence basin;

[0109] S26: Calculate the volume transfer coefficient by α=Vs(t) / Vc(t), and obtain the variation curve of the volume transfer coefficient under different volume shrinkage rates, that is, obtain the medium parameter system transfer coefficient α of the land subsidence prediction model (4) under different volume shrinkage rates;

[0110] S27: Substituting the above parameters into (4) to establish a ground subsidence prediction model, the ground subsidence of other cavities under corresponding volume shrinkage rates can be simplified according to the ground subsidence model;

[0111] Schematic diagram of surface subsidence of a spherical cavity Figure 3 As shown in the figure, the prediction model of ground subsidence in salt cavern gas storage is expressed as:

[0112]

[0113] In the formula, α is the volume transfer coefficient, which is calculated as α = Vs(t) / Vc(t), Vs(t) is the ground subsidence volume, Vc(t) is the cavity contraction volume, γ is the influence angle, f is the depth of the cavity center, and R0 is the influence range of the ground subsidence;

[0114] Salt cavern gas storage generally exists in the form of cavity groups. When calculating the surface movement and deformation of the cavity group, by studying the ground settlement curves of single cavity and cavity group, it is concluded that when calculating the overall ground settlement of the salt cavern gas storage, the mutual influence effect of the cavity group can be ignored first, and the surface settlement of each cavity is calculated separately. Then, a correction coefficient is added to the surface settlement of each cavity for linear superposition, and the calculation formula for the overall surface settlement is obtained as follows:

[0115]

[0116] Where k is the surface settlement correction coefficient of the cavity group, which can be estimated by formula (6), and the recommended value range is 1.0-1.2, and n is the number of cavities;

[0117] Correction factor calculation formula:

[0118]

[0119] In the formula, k d is the surface settlement correction factor within 1000m radial distance above the double cavity, r / r* is the ratio of the surface settlement of the double cavity to the surface settlement of the linear superposition of two single cavities, r* is the unit length, l is the cavity center distance, and d is the cavity diameter.

[0120] S3: Figure 7 , based on the regional overall ground subsidence prediction data obtained by considering the mutual influence of the group cavities and the allowable deformation of the cavity of the salt cavern gas storage, the ground subsidence warning threshold is determined:

[0121] S31: Select a salt cavern gas storage area and make a cavity group position distribution map in the salt cavern gas storage area;

[0122] S32: Collect sonar test data and geological data (information such as the depth, volume, and geological conditions of each cavity) of all cavities in the salt cavern gas storage area;

[0123] S33: using step S24 to verify the obtained volume transfer coefficient and influence angle and theoretical prediction model of ground settlement, i.e., formula (4), to perform simplified calculation of ground settlement prediction for all cavities in the area, and obtain the ground settlement amount caused by each cavity when the cavity is allowed to deform;

[0124] S34: according to the cavity group position distribution map in S31, determine the cavity that will produce the cavity group mutual influence effect, and divide the cavity that has the cavity group mutual influence effect together (generally, when the positions of two cavities exceed the influence range R0 of the ground subsidence, no cavity group effect will be produced);

[0125] S35: Calculate the ground subsidence of the divided cavities using formula (5), where the correction coefficient is 1.2, and superimpose the ground subsidence in the salt cavern gas storage area to obtain the overall ground subsidence prediction data for the area;

[0126] S36: making a corresponding ground subsidence cloud map according to the overall ground subsidence prediction data of the salt cavern gas storage reservoir area obtained in step S35;

[0127] S37: Based on the above ground subsidence cloud map, find out the position with the maximum cumulative ground subsidence value in the selected area, and then analyze and study the warning threshold value in combination with the allowable deformation of the cavity in the selected salt cavern gas storage area. Based on the analysis of scholars on the risk of cavity shrinkage: when the cavity volume shrinkage rate reaches 20%, it will cause more serious risk consequences, the cavity will be severely deformed and damaged, the main functions will be impaired, and it will be in a more dangerous state. When the cavity volume shrinkage rate reaches 30%, it will cause catastrophic consequences. Therefore, from a conservative point of view, the cavity volume shrinkage rate should not exceed 20%.

[0128] Example 3

[0129] A method for predicting and warning of cavity deformation and ground subsidence of a salt cavern gas storage reservoir is provided, which is based on Example 2 and has the following adjustments:

[0130] When measuring the data of the wellhead pressure changing with time, the pressure data transmission interval is set to 1-3h, preferably 1h, 1.5h, 2.5h, 3h;

[0131] Record the changes in wellhead pressure values ​​on a daily basis and collect data on the pressure monitoring of the wellhead pressure gauge for a period of about 1-4 months, with the preferred option being to collect data for 1 month, 2 months, 3 months or 4 months;

[0132] The creep parameter n in S13 is selected from a plurality of different values ​​between 3.0 and 4.0, wherein n is preferably 8, 9, 10, 11, and 12, and the specific preferred values ​​include 3.0, 3.5, and 4.0;

[0133] The surface settlement correction factor of the cavity group is preferably 1.0 or 1.2;

[0134] The permissible deformation of the salt cavern cavity is preferably 15% or 20%.

[0135] Example 4

[0136] A method for predicting and warning of cavity deformation and ground subsidence of a salt cavern gas storage reservoir is based on Example 1 (i.e., all the steps in Example 1 can be applied to this example). It is a specific example of Example 1. For example, the salt layer section of the well where the experiment was conducted is 937.2m-1093.0m, with a thickness of 155.8m. There are 6 mudstone layers with a thickness greater than or close to 1.0m, 977.1-978.0m, 1029.0-1030.1m, 1037.6-1038.5m, 1074.3-1075.6m, 1081.6-1083.3m, 1087.6-1091.8m, and the cavity depth is -1029m--1062m. During the experiment, the statistical curve of the change in wellhead pressure over time is as follows: Figure 8 As shown, the curve is fitted and calculated, and its slope is 0.00105MPa / hour, which is the volume strain per unit time of Formula 2.

[0137] According to the depth range of the cavity, which is -1029m to -1062m, the average original rock stress P0 is taken as 24.05MPa. According to the wellhead pressure and the brine depth, the variation range of the internal pressure of the cavity is calculated to be 14.62MPa-15.01MPa (the wellhead pressure is 2.27MPa, and the brine pressure is 12.35MPa-12.74MPa). The brine pressure Pi in the cavity is taken as 14.81MPa. According to the cavity temperature, the bulk modulus K of the brine at 50 degrees is taken as 3.56GPa in the calculation. Substituting the above parameters into Formula 3, different combinations of A and n can be calculated, as shown in Table 1.

[0138] According to the volume shrinkage rate formula of the infinite body internal dissolution cavity and its derived formula (Formula 1, Formula 3), it can be obtained that the actual cavity volume shrinkage rate during the test period is 0.0467%.

[0139] Table 1 Correspondence table of different A and n values ​​calculated

[0140]

[0141]

[0142] Use modeling software to build a three-dimensional model of the cavity, such as Fig. 9 As shown in Figure 2, a numerical calculation model is established based on the cavity geological data, as shown in Figure 2. Fig.10As shown, combined with the cavity pressure change during the test, 11 groups of creep parameters in Tables 3.0-4.0 above were selected for simulation calculation, and the calculation results were compared with the actual cavity shrinkage rate, as shown in Figure 1. Fig.11 As shown, taking n as 3.6, the corresponding A value is 1.344×10 -5 MPa -n a -1 .

[0143] Fig.12 The geological model is shown in the figure, which is modeled according to the lithology sections of the ground profile, such as clay layer, mudstone layer, basalt layer, mudstone layer, and salt rock layer. The gas storage is located in the salt rock layer, and the power law creep constitutive model is adopted for the salt rock. The mechanical parameters used in the calculation model are shown in Table 2:

[0144] Table 2 Lithology and mechanical properties of the calculated model formation

[0145]

[0146] According to the creep parameters obtained in step S1 and the injection and production operating pressure, a three-dimensional numerical calculation software is used to perform simulation calculations. The numerical simulation results show that the cavity volume shrinkage rate changes with time as shown in the following figure: Fig.13 shown.

[0147] According to the numerical simulation results of step S2, by analyzing and calculating the ground settlement curves with cavity shrinkage rates of 5%, 10%, 15%, 20%, and 25%, a curve of the volume transfer coefficient changing with the cavity volume shrinkage rate is drawn, such as Fig.14 , taking the surface settlement corresponding to the cavity volume shrinkage rate of 20% as an example for analysis, the volume transfer coefficient and influence angle γ of the surface movement deformation when the cavity volume shrinkage rate is 20% are calculated, so as to calculate the influence range R0. Substituting the above calculated parameters into formula (4), the ground settlement prediction model when the cavity volume shrinkage rate is 20% can be obtained. The application of this model can simplify the calculation of ground settlement when the cavity volume shrinkage rate is 20% for other cases; Fig.15 The surface settlement curve corresponding to the cavity shrinkage of 20% is shown. The surface center point settlement is the largest, which is 17.5mm, and is roughly S-shaped. The surface settlement gradually decreases with the increase of the distance from the center point and slowly tends to zero, which is similar to the normal distribution curve. The volume transfer coefficient of the surface movement deformation within the cavity volume shrinkage rate of 20% is 0.75 and the influence angle is 34°, then the influence range is 1522m. Substituting the parameters into the theoretical model of the influence function, the curve comparison between the theoretical model and the numerical simulation can be obtained. Fig.15 shown.

[0148] The calculation area of ​​the gas storage reservoir (Jintan salt cavern gas storage reservoir area) is selected, and the cavity distribution in this area is as follows: Fig.16As shown in Figure 1, the theoretical prediction parameters and model of land subsidence obtained in step S3 are applied to calculate the land subsidence of the cavity in the region when the volume shrinkage rate is 20%. Considering the mutual influence of the surface subsidence of the cavity group, the k value is taken as 1.2, and the current superposition calculation of the land subsidence value of the region is performed, and the land subsidence cloud map of the cavity when the volume shrinkage rate is 20% is made, as shown in Figure 1. Fig.17 shown.

[0149] When the cavity volume shrinkage rate of the cavity group is 20%, the maximum surface settlement of the Jintan salt cavern gas storage area is 213.65mm. Considering a certain safety factor and combining the actual operation of the Jintan gas storage, the annual cavity volume shrinkage rate does not exceed 1.5%, and the cumulative amount does not exceed 20%. Based on the allowable cavity volume shrinkage, the surface movement deformation warning threshold of the salt cavern gas storage area can be determined as follows:

[0150] (1) Starting from the first year after the salt cavity is put into operation, the changes in surface settlement at the monitoring points shall be regularly observed each year. The annual volume shrinkage rate of the cavity shall not exceed 1.5%, and the surface settlement shall not exceed 15 mm / year;

[0151] (2) Starting from the first year after the salt cavity is put into injection and production operation, the change in the cumulative value of surface settlement at the monitoring point shall be calculated. The cumulative value of surface settlement shall not exceed 200 mm.

Claims

1. A method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage, characterized in that: The steps include: S1: Carry out on-site creep parameter inversion test of gas storage to obtain rock mechanics parameters that are consistent with the actual cavity engineering of salt cavern gas storage; S2: constructing a ground subsidence prediction model using the rock mechanics parameters obtained in step S1, and applying the model to predict ground subsidence; S3: Determine the ground subsidence warning threshold and perform a safety warning for the salt cavern gas storage cavity based on the threshold and the ground subsidence prediction in S2.

2. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 1 is characterized in that: In step S1, the rock mechanics parameters consistent with the actual project are obtained by conducting an on-site creep parameter inversion test of the gas storage reservoir, including: S11: Conduct a closed-well pressure-raising field test: Select a cavity that has been completed and has undergone sonar testing, measure the data of wellhead pressure changes over time, and draw a curve of wellhead pressure changes over time; S12: Calculate the volume change of the cavity during the closed-well pressure test, that is, the actual cavity volume shrinkage rate; S13: simulating and calculating the test cavity numerical model to obtain the cavity shrinkage rate occurring during the well shut-in and pressure-increasing test; S14: Compare the actual cavity shrinkage rate in S12 with the cavity shrinkage rate that occurred during the closed-well pressure test in S13, and select the combined value of creep parameters when the two are closest as the rock mechanics parameters for subsequent numerical simulation analysis.

3. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 2 is characterized in that: When measuring the data of the wellhead pressure changing with time, a pressure gauge capable of remote data transmission is installed at the wellhead to realize the wellhead pressure data collection and remote transmission.

4. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 1 is characterized in that: In step S2, the construction of a land subsidence prediction model includes: S21: Select the cavity of the salt cavern gas storage that has been put into production, has been subjected to sonar cavity measurement, and has the injection and production operation pressure data since the sonar cavity measurement. Combine the cavity sonar cavity measurement data and geological data to establish a three-dimensional numerical simulation analysis geological model, set boundary conditions, and use the creep parameter inversion test of the gas storage site in S1 to obtain the actual rock mechanics parameters of the project. The monitoring object is set to the cavity volume change and the displacement of the ground settlement monitoring point of the upper stratum of the cavity; S22: Arrange the numerical calculation code of the cavity injection and production operation pressure data, expand the existing injection and production operation data to several years, perform numerical simulation, and obtain the cavity volume change and the monitoring data of the ground settlement monitoring point of the upper stratum of the cavity; S23: sorting the data obtained in step S22 to obtain the ground settlement curve of the cavity at different volume shrinkage rates; S24: calculating the corresponding volume transfer coefficient according to the land settlement curves at different volume shrinkage rates in S23, and obtaining a variation curve of the volume transfer coefficient at different volume shrinkage rates; S25: Apply the above volume transfer coefficient to the ground settlement prediction model to obtain a settlement model for calculating the ground settlement of other cavities under corresponding volume shrinkage rates.

5. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 1 is characterized in that: When determining the ground subsidence warning threshold, the ground subsidence warning threshold is determined based on the allowable deformation of the salt cavern cavity of 15-20%.

6. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 1 is characterized in that: When determining the ground subsidence warning threshold, the surface subsidence of each cavity is calculated respectively, and then a correction coefficient is added to the surface subsidence of each cavity for linear superposition to obtain the overall surface subsidence.

7. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 4 is characterized in that: Determining the ground subsidence warning threshold includes: S31: Select a salt cavern gas storage area; S32: Collect sonar test data and geological data of all cavities in the salt cavern gas storage area; S33: using step S24 to verify the obtained volume transfer coefficient, influence angle and theoretical prediction model of ground settlement, and calculating the ground settlement amount caused by each cavity when the cavity shrinks to a set value; S34: Determine the cavity that will produce the group cavity mutual influence effect, add a correction coefficient to the surface settlement of each cavity, perform linear superposition, and obtain the overall surface settlement prediction data; S35: according to the above-mentioned overall ground subsidence prediction data, find the maximum cumulative value position of the ground subsidence in the selected area, and then set the warning threshold value in combination with the allowable deformation of the selected salt cavern gas storage area; S36: Provide a safety warning based on the warning threshold in S35 and the ground subsidence predicted in S2.

8. The method for predicting and warning of cavity deformation and ground subsidence of salt cavern gas storage according to claim 7 is characterized in that: When finding the maximum cumulative ground subsidence position in the selected area according to the overall ground subsidence prediction data in S35, the following method is adopted: a corresponding ground subsidence cloud map is made according to the overall ground subsidence prediction data, and based on the ground subsidence cloud map, the maximum cumulative ground subsidence position in the selected area is found.

Images