Grid-based method for calculating original volume factor of shale gas in complex structure area

Abstract

The present application relates to a high-precision calculation method for the original volume coefficient of shale gas in a complex area based on gridding, and the technical scheme comprises the following steps: step one: gridding the top / bottom surface structure map and the ground elevation map of shale; step two: calculating the middle buried depth map of the shale gas reservoir by curved surface operation; step three: calculating the formation temperature and the formation pressure; step four: calculating the pseudo-contrast temperature and the pseudo-contrast pressure; step five: calculating the deviation coefficient; and step six: calculating the volume coefficient.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of oil and gas exploration and development, and particularly relates to a grid-based calculation method for original volume factor of shale gas in a complex structure area. BACKGROUND

[0002] After years of continuous research, China has made significant progress in basic geological theory and engineering technology in the field of shale gas. Shale gas in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Sichuan Basin and its surrounding areas has realized commercial development. Fuling, Weiyuan, Changning, Zhaotong, Weirong, Yongchuan and other shale gas fields have been discovered. As of the end of 2020, the cumulative proven shale gas geological reserves exceeded 2.0×10 12 m 3 , the annual output was 200×10 8 m 3 , and it has gradually become an important part and core growth point of China's natural gas industry.

[0003] Many domestic and foreign institutions have carried out evaluation and prediction of China's shale gas resources. Due to differences in evaluation methods and understanding, the results of the evaluation are quite different. In 2012, the Ministry of Land and Resources of China estimated that the shale gas geological resources in China were 134.42×10 12 m 3 , and the recoverable resources were 25.08×10 12 m 3 ; in 2013, the U.S. Energy Information Administration (EIA) estimated that the geological resources were 134.40×10 12 m 3 , and the recoverable resources were 31.57×10 12 m 3 ; in 2015, the fourth latest resource evaluation results of China Petroleum and Natural Gas Corporation, the shale gas geological resources in China were 80.45×10 12 m 3 , and the recoverable resources were 12.85×10 12 m 3 ; according to the 2015 resource evaluation results of the Ministry of Land and Resources, the shale gas geological resources in China were 121.86×10 12 m 3 , and the recoverable resources were 21.81×10 12 m 3 , of which the marine recoverable resources were 13.00×10 12 m 3 .

[0004] The shale gas geological reserves are mainly calculated by using a static method, in which the adsorbed gas reserves are calculated by using a volume method, and the free gas reserves are calculated by using a volume method. The shale gas original volume coefficient is an important parameter for calculating the free gas reserves, and the parameter is obtained by using a high-pressure physical property analysis of shale gas samples. Because of high sampling and experimental cost, long cycle, and high requirements for a sampling well, sampling time, sampling point, sampling method, sampling process, and experimental analysis method, the number of analysis wells is limited. However, because the shale gas reservoir is strong in plane heterogeneity, especially in a complex structure area, the formation pressure coefficient of the gas reservoir is fast and greatly different in the plane, and the limited measured data points cannot objectively and truly reflect the change of the shale gas reservoir in the plane. The existing well point arithmetic average and well point area weighting shale gas reservoir volume coefficient calculation methods cannot meet the needs of the shale gas geological reserve calculation in the complex structure area. Therefore, an urgently needed shale gas volume coefficient calculation method with stronger adaptability and proven by production practice can ensure the accuracy of the shale gas geological reserve calculation in the complex structure area of a basin periphery, and provide guidance for determining a reasonable development index and preparing a scientific development plan. SUMMARY

[0005] The purpose of the present application is to solve the above-mentioned problems of the prior art, and to provide a grid-based shale gas original volume coefficient calculation method in a complex structure area, which can effectively improve the calculation accuracy of the shale gas original volume coefficient, so as to accurately calculate the shale gas free gas geological reserves.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] The grid-based shale gas original volume coefficient calculation method in a complex structure area comprises the following steps:

[0008] Step 1: Grid the shale top / bottom surface structure map and the ground elevation map.

[0009] The Kriging method is used to grid the shale top / bottom surface structure map and the ground elevation map, and the shale gas reservoir is divided into a plurality of calculation units, so as to provide a basis for accurately obtaining the middle buried depth map of the shale gas reservoir.

[0010] Step 2: Obtain the middle buried depth map of the shale gas reservoir by surface operation.

[0011] The middle buried depth map of the shale gas reservoir is determined by using the shale top surface structure map or the bottom surface structure map, the ground elevation map, and the gas reservoir thickness map, so as to provide a basis for calculating the temperature and pressure of the gas reservoir.

[0012] H=H 高 -H 顶 +h / 2

[0013] or

[0014] H=H高 -H 底 -h / 2

[0015] In the formula, H is the middle depth of shale gas reservoir, unit is m; H 高 is the surface elevation map of shale gas reservoir, unit is m; H 顶 is the shale top surface structure map, unit is m; H 底 is the shale bottom surface structure map, unit is m; h is the shale reservoir thickness, unit is m.

[0016] Step three: calculate the formation temperature and formation pressure.

[0017] Determine the formation temperature through the geothermal gradient and the middle depth map of shale gas reservoir, the geothermal gradient is a certain value in a certain area, generally obtained by the measured value average method; determine the formation pressure through the pressure coefficient and the depth map, the pressure coefficient can be obtained by micro-injection pressure drop test and seismic reservoir prediction.

[0018] T=(H-H0) / 100×t+T0

[0019] In the formula, T is the formation temperature, unit is K; H0 is the isothermal layer depth, unit is m; t is the geothermal gradient, unit is ℃ / 100m; T0 is the isothermal layer temperature, unit is K.

[0020] P=α p ×g×H / 1000

[0021] In the formula, P is the formation pressure, unit is MPa; α p is the formation pressure coefficient, dimensionless; g is the standard gravity acceleration, unit is m / s 2 .

[0022] Step four: calculate the pseudo-critical temperature and pseudo-critical pressure.

[0023] The pseudo-critical pressure is the ratio of the absolute pressure of the actual state of the gas to its critical pressure, and the pseudo-critical temperature is the ratio of the absolute temperature of the actual state of the gas to its critical temperature. For the same gas reservoir, it is considered that the gas components are consistent, and the critical temperature and critical pressure are consistent.

[0024] P r =P / P pc

[0025] T r =T / T pc

[0026] In the formula, P r is the pseudo-critical pressure, dimensionless; T r is the pseudo-critical temperature, dimensionless; P pc is the critical pressure, unit is MPa; Tpc Tc is the critical temperature, in K.

[0027] Step five: calculate the deviation factor.

[0028] The deviation factor of a gas varies with the gas component and the pressure and temperature, and can be determined by its relationship with the pseudo-comparison pressure and the pseudo-comparison temperature.

[0029] Z i =A×T r +B×P r +C

[0030] In the formula, Z i is the deviation factor; A, B and C are constants.

[0031] Step six: calculate the volume factor.

[0032] The volume factor can be determined by the deviation factor, the temperature and the pressure of the gas reservoir.

[0033] B gi =P sc ×Z i ×T / (T sc ×P)

[0034] In the formula, B gi is the volume factor, dimensionless; P sc is the standard surface pressure, in MPa; and T sc is the standard surface temperature, in K.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] The method can effectively improve the calculation precision of the original volume factor of a shale gas reservoir, thereby accurately calculating the geological reserves of shale gas free gas, and through production practice, it is proved to be feasible, and can ensure the accuracy of the calculation of the geological reserves of shale gas in a complex structure belt on the basin margin, so as to determine a reasonable development index and provide guidance for preparing a scientific development plan. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is a basic step schematic diagram of the present application;

[0038] Figure 2 is a calculation grid diagram of the middle burial depth of a gas reservoir in an embodiment of the present application;

[0039] Figure 3 is a calculation grid diagram of the formation pressure in an embodiment of the present application;

[0040] Figure 4 is a calculation grid diagram of the formation temperature in an embodiment of the present application;

[0041] Figure 5 The grid map for calculating the volume coefficient in the embodiment of the present application;

[0042] Figure 6 The grid map for calculating the volume coefficient in the embodiment of the present application. DETAILED DESCRIPTION

[0043] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined purposes, the specific embodiments, structures, features and effects according to the present application are described in detail below in combination with the drawings and preferred embodiments.

[0044] Step one: gridding the shale top / bottom surface structure map and the ground elevation map.

[0045] The shale top / bottom surface structure map and the ground elevation map are gridded by using the Kriging method, and the shale gas reservoir is divided into a plurality of calculation units, thereby providing a basis for accurately calculating the middle burial depth map of the shale gas reservoir.

[0046] Step two: calculating the middle burial depth map of the shale gas reservoir by surface operation.

[0047] The middle burial depth map of the shale gas reservoir is determined by the shale top surface structure map or the bottom surface structure map, the ground elevation map and the gas reservoir thickness map, thereby providing a basis for calculating the temperature and pressure of the gas reservoir.

[0048] H = H 高 -H 顶 +h / 2

[0049] or

[0050] H = H 高 -H 底 -h / 2

[0051] In the formula, H is the middle burial depth of the shale gas reservoir, and the unit is m; H 高 is the ground elevation map of the shale gas reservoir, and the unit is m; H 顶 is the shale top surface structure map, and the unit is m; H 底 is the shale bottom surface structure map, and the unit is m; and h is the shale reservoir thickness, and the unit is m.

[0052] Step three: calculating the formation temperature and the formation pressure.

[0053] The formation temperature is determined by the geothermal gradient and the middle burial depth map of the shale gas reservoir, and the geothermal gradient is a certain value in a certain area, which is generally obtained by the measured value averaging method; the formation pressure is determined by the pressure coefficient and the burial depth map, and the pressure coefficient can be obtained by the micro-pressurization drop test and the seismic reservoir prediction.

[0054] T = (H - H0) / 100 x t + T0

[0055] Where, T is formation temperature, K; H0 is the geothermal gradient, unit is m; t is geothermal gradient, unit is ℃ / 100 m; T0 is geothermal gradient temperature, unit is K.

[0056] P = a p × g × H / 1000

[0057] Where, P is formation pressure, unit is MPa; a p is formation pressure coefficient, dimensionless; g is standard gravity acceleration, unit is m / s 2 .

[0058] Step four: calculate pseudo-critical temperature and pseudo-critical pressure.

[0059] Pseudo-critical pressure is the ratio of the absolute pressure of the actual state of the gas to its critical pressure, and pseudo-critical temperature is the ratio of the absolute temperature of the actual state of the gas to its critical temperature. For the same gas reservoir, it is considered that the gas composition is consistent, and the critical temperature and critical pressure are consistent.

[0060] P r = P / P pc

[0061] T r = T / T pc

[0062] Where, P r is pseudo-critical pressure, dimensionless; T r is pseudo-critical temperature, dimensionless; P pc is critical pressure, unit is MPa; T pc is critical temperature, unit is K.

[0063] Step five: calculate the deviation factor.

[0064] The deviation factor of the gas changes with the change of the gas composition and the pressure and temperature, and it can be determined by its relationship with the pseudo-critical pressure and the pseudo-critical temperature.

[0065] Z i = A × T r + B × P r + C

[0066] Where, Z i is the deviation factor; A, B, C are constants.

[0067] Step six: calculate the volume factor.

[0068] The volume factor can be determined by the deviation factor, the temperature and the pressure of the gas reservoir.

[0069] B gi = Psc ×Z i ×T / (T sc ×P)

[0070] B gi is the volume factor, dimensionless; P sc is the standard pressure on the ground, in MPa; T sc is the standard temperature on the ground, in K.

[0071] Specific embodiments are shown in Figs. Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6 . The embodiments take the Nanchuan block in the southeast basin margin tectonic complex area as an example, the gas-bearing layer section is the shale of the Wufeng Formation-First Member of Longmaxi Formation, the gas reservoir burial depth is between 1800-4500m, the gas reservoir temperature is 89.8-122.0℃; the gas reservoir pressure coefficient is 1.05-1.55, it is a normal pressure-high pressure gas reservoir, the gas reservoir volume coefficient changes greatly, and it is typical.

[0072] Based on the characteristics of gas reservoir burial depth, temperature, pressure, etc., the gas reservoir gridding and fitting research are carried out, and finally the original volume coefficient of the gas reservoir is calculated and determined.

[0073] Firstly, the shale top / bottom surface structure map and the ground elevation map are gridded.

[0074] The Kriging method is used to grid the shale bottom surface structure map and the ground elevation map of the Wufeng Formation, and the grid density is controlled to be more than 500m×500m. This step aims to divide the shale gas reservoir into several calculation units, and to provide a basis for accurately calculating the middle burial depth map of the shale gas reservoir in the next step.

[0075] Secondly, the middle burial depth map of the shale gas reservoir is calculated by surface operation.

[0076] The middle burial depth map of the shale gas reservoir is determined by the shale bottom surface structure map, the ground elevation map and the gas reservoir thickness map, which provides a basis for the calculation of the temperature and pressure of the shale gas reservoir.

[0077] H=H 高 -H 底 -h / 2

[0078] In the formula, H is the middle burial depth of the shale gas reservoir, in m; H 高 is the ground elevation map of the shale gas reservoir, in m; H 底 is the shale top surface structure map, in m; h is the gas reservoir thickness, in m.

[0079] Thirdly, the formation temperature and the formation pressure are calculated.

[0080] Determine formation temperature by geothermal gradient and burial depth map of shale gas reservoir. For the example, the geothermal gradient is stable, and the average value is obtained by measurement; the formation pressure is determined by pressure coefficient and burial depth map, wherein the pressure coefficient is obtained by single well micro injection pressure drop test and seismic reservoir plane prediction.

[0081] The calculation formula is:

[0082] T=(H-20) / 100×2.45+289.75

[0083] In the formula, T is the formation temperature, K.

[0084] P=α p ×g×H / 1000

[0085] In the formula, P is the formation pressure, unit: MPa; α p is the formation pressure coefficient, dimensionless; g is the standard gravity acceleration, unit: m / s 2 .

[0086] Fourthly, calculate the pseudo-critical temperature and pseudo-critical pressure.

[0087] For the example gas reservoir, the gas components are consistent, so the critical temperature and critical pressure are also consistent. The calculation formula is:

[0088] Pr=P / 4.608

[0089] Tr=(T+273.15) / 191.06

[0090] In the formula, P r is the pseudo-critical pressure, dimensionless; T r is the pseudo-critical temperature, dimensionless.

[0091] Fifthly, calculate the deviation factor.

[0092] The deviation factor of the gas is fitted by the PVT experiment of the example well, and the relationship with the pseudo-critical pressure and the pseudo-critical temperature is established. The calculation formula is:

[0093] Z i =0.1553319×Tr+0.0369135×Pr+0.475631

[0094] In the formula, Z i is the deviation factor.

[0095] Sixthly, calculate the volume factor.

[0096] The volume factor is determined by the deviation factor, the temperature and pressure under the standard condition on the ground, and the temperature and pressure under the condition of the gas reservoir. The calculation formula is:

[0097] B gi =0.101 x Z i x (T+273.15) / (293.15 x P)

[0098] In the formula, B gi is a volume coefficient, dimensionless.

[0099] The beneficial effect of the embodiment has been verified in Nanchuan area of the complex tectonic belt in southeast basin margin of Chongqing. The volume coefficient calculated before the method is 0.00333, and the volume coefficient calculated by the method is 0.00309, which greatly improves the reserve calculation accuracy, promotes the evaluation and exploitation of shale gas resources in the complex tectonic belt of the basin margin, and adopts the technical scheme of the method. The reserve calculation and gas reservoir engineering deployment of shale gas in Nanchuan area of the complex tectonic belt in southeast basin margin of Chongqing, the proven reserves of the area have been accumulated to nearly 200 billion cubic meters, the built capacity is 21 billion cubic meters, and the cumulative production of shale gas is more than 36 billion cubic meters.

Claims

1. A method for calculating the original volume factor of shale gas in structurally complex areas based on gridding, characterized in that, Includes the following steps: Step 1: Gridded shale top / bottom structural map and ground elevation map The Kriging method was used to grid the top / bottom structural map and ground elevation map of the shale, dividing the shale gas reservoir into several calculation units; Step 2: Obtain the burial depth map of the middle part of the shale gas reservoir using surface calculations. Determine the burial depth of the middle part of the shale gas reservoir by using the top or bottom structural map of the shale, the ground elevation map, and the gas reservoir thickness map. Step 3: Calculate formation temperature and formation pressure Formation temperature was determined using geothermal gradients and burial depth maps of the central part of shale gas reservoirs; formation pressure was determined using pressure coefficients and burial depth maps. Step 4: Calculate the simulated temperature and simulated pressure. Calculate the relative temperature and relative pressure based on the absolute pressure and absolute temperature of the gas under its actual conditions, as well as its critical pressure and critical temperature. Step 5: Calculate the deviation coefficient The relationship between the deviation coefficient and the simulated comparative pressure and temperature was established through high-pressure physical property experiments. Step 6: Calculate the volume factor The volume factor is determined by the deviation factor, the temperature and pressure of the gas reservoir.

2. The method according to claim 1, characterized in that, The mathematical model for the burial depth of the middle part of the shale gas reservoir in step two is: H = H 高 -H 顶 +h / 2 In the formula, H represents the burial depth of the middle part of the shale gas reservoir, in meters; H 高 This is a surface elevation map of a shale gas reservoir, in meters (m); H 顶 The diagram shows the top surface structure of the shale formation, in meters (m); h represents the thickness of the shale reservoir, also in meters.

3. The method according to claim 1, characterized in that, The mathematical model for the burial depth of the middle part of the shale gas reservoir in step two is: H = H 高 -H 底 -h / 2 In the formula, H represents the burial depth of the middle part of the shale gas reservoir, in meters; H 高 This is a surface elevation map of a shale gas reservoir, in meters (m); H 底 The figure shows the structural diagram of the shale bottom surface, in meters (m); h represents the shale reservoir thickness, in meters (m).

4. The method according to claim 1, characterized in that, The mathematical model for formation temperature in step three is: T = (H - H0) / 100 × t + T0 In the formula, T is the formation temperature in K; H is the burial depth of the middle part of the shale gas reservoir in m; and H0 is the depth of the isothermal layer in m. t represents the geothermal gradient, in °C / 100m; T0 represents the temperature of the isothermal layer, in K.

5. The method according to claim 1, characterized in that, The mathematical model for formation pressure in step three is: P = α p ×g×H / 1000 In the formula, P is the formation pressure, in MPa; α p ρ is the formation pressure coefficient, dimensionless; g is the standard gravitational acceleration, in m / s². 2 H represents the burial depth of the middle part of the shale gas reservoir, in meters.

6. The method according to claim 1, characterized in that, The mathematical model for the simulated temperature in step four is: T r =T / T pc In the formula, T r The relative temperatures are dimensionless; T represents the formation temperature in Kelvin. pc This is the critical temperature, expressed in Kelvin (K).

7. The method according to claim 1, characterized in that, The mathematical model for the pressure comparison in step four is: P r =P / P pc In the formula, P r The pressure is dimensionless and used for comparison; P is the formation pressure in MPa. pc This is the critical pressure, measured in MPa.

8. The method according to claim 1, characterized in that, The mathematical model for the deviation coefficient in step five is: Z i =A×T r +B×P r +C In the formula, Z i T is the deviation coefficient; A, B, and C are constants; r For comparison temperature, dimensionless; P r For the purpose of comparing pressure, the value is dimensionless.

9. The method according to claim 1, characterized in that, The mathematical model for the volume coefficient in step six is: B gi =P sc ×Z i ×T / (T sc ×P) In the formula, B gi The volume factor is dimensionless. P sc Z represents the standard ground pressure, in MPa. i The deviation coefficient is T; T is the formation temperature in K. T sc ρ represents the standard surface temperature, in K; P represents the formation pressure, in MPa.

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