A calculation method for the maximum horizontal principal stress applicable to strata in a tectonic extrusion state

Abstract

The present invention relates to a method for calculating the maximum horizontal principal stress of a formation suitable for a structural compression state, and belongs to the field of oil and gas field exploration technology, in particular to the field of new energy exploration such as shale gas exploration and geothermal exploration. The method analyzes the geostress of multiple shale gas exploration areas and geothermal exploration areas with structural compression states. Based on the Huang Rongzun model for conventional geostress calculation, the method discovers the intrinsic correlation between the maximum differential stress and the compression structure, introduces a correction term related to the maximum differential stress, and proposes an improved calculation formula, thereby improving the accuracy and adaptability of the Huang Rongzun model and enabling it to more accurately reflect the maximum horizontal principal stress field under the compression state.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field exploration technology, in particular to the field of new energy exploration such as shale gas exploration and geothermal exploration, and specifically to a method for calculating the maximum horizontal principal stress of a formation applicable to a structural compression state. Background Art

[0002] Calculation of in-situ stress is a crucial component of geophysical exploration, crucial for revealing the stress distribution of underground rock masses and the patterns of geological tectonic activity. With the continuous advancement of geophysical exploration technology and numerical simulation methods, in-situ stress research has gradually evolved from qualitative analysis to quantitative calculation, which can more accurately reflect the stress state under complex geological conditions. Current research focuses on combining various exploration methods with theoretical models to systematically analyze the stress field deep within the Earth's crust, thereby providing a scientific basis for oil and gas exploration, shale gas exploration, geothermal development, and earthquake prediction.

[0003] Research on geostress calculation methods aims to gain a deeper understanding of the stress state in rock masses within the Earth's crust, which is crucial for fields such as engineering construction, mineral extraction, and earthquake prediction. Currently, commonly used geostress calculation models include the Huang Rongzun model (Huang's model), the Anderson model, and the combined spring model. The Huang model, based on the principle of static equilibrium, proposes a method for calculating stress distribution in the Earth's crust that effectively accounts for variations in principal stress directions and stress gradients, possessing high application value in practical engineering. The Anderson model, by analyzing the three main stress states in the Earth's crust, reveals the stress mechanisms under different tectonic environments, demonstrating particular applicability in areas of frequent tectonic activity. The combined spring model, drawing on elasticity theory, analogizes the stress transfer and deformation processes in rock masses to the combined action of spring elements, enabling it to adapt to complex geological conditions and heterogeneous strata.

[0004] The introduction and continuous improvement of these models not only provide important theoretical support for the calculation and analysis of geostress, but also promote the development of the field of geological engineering and provide more scientific tools for stress prediction and prevention in complex geological environments. Summary of the Invention

[0005] (1) Analysis of the advantages of the present invention over conventional technologies

[0006] The present invention improves the conventional Huang Rongzun model (Huang's model) for geostress calculation by adding a correction term, which increases the accuracy of the calculation of the maximum horizontal principal stress of strata in a tectonic compression state; and designs a calculation process suitable for exploration applications in new energy fields such as shale gas and geothermal exploration.

[0007] (2) Core content of the present invention

[0008] This paper proposes a method for calculating the maximum horizontal principal stress of formations under tectonic compression conditions. The original Huang model fails to fully account for the influence of factors such as pore pressure and tectonic stress in compression environments, often leading to deviations and large errors in the calculated maximum horizontal principal stress. To address this issue, the present invention improves the accuracy and adaptability of the original Huang model by introducing a maximum differential stress correction term, enabling it to more accurately reflect the stress field under compression conditions.

[0009] The derivation process of the core calculation formula of a method for calculating the maximum horizontal principal stress of a formation suitable for tectonic compression state is as follows:

[0010] According to the effective stress principle, the relationship between the effective stress of rock and the overlying stratum pressure and formation pressure is:

[0011] (1)

[0012] Where, is the overlying formation pressure, unit: ; is the formation pore fluid pressure, unit: ; is the rock skeleton stress of the formation, unit: .

[0013] For the stress model of the stratum under compression, the maximum horizontal principal stress is greater than the vertical stress, so the maximum differential stress can be obtained is the difference between the maximum horizontal principal stress and the vertical stress, that is:

[0014] (2)

[0015] Where, is the maximum horizontal principal stress, in units of: ; is the vertical stress, in units of: .

[0016] Based on Fan Taoyuan et al. (Analysis of Geostress in Tectonic Mineralization: Research Status and Thinking, Modern Geology, 2024) “In a compression environment, when the pore fluid is in a hydrostatic pressure state, the maximum differential stress at 5 km is about 160 ", substituting into formula 2, we can get the maximum horizontal principal stress at this time is 285 .

[0017] Substitute the parameters into the Huang model proposed by Huang Rongzun:

[0018] (3)

[0019] (4)

[0020] In the formula and are the minimum and maximum horizontal principal stresses, ; is the vertical ground stress, ; is the Biot coefficient; is the pore pressure, ; is Poisson's ratio, dimensionless; and is the tectonic stress coefficient in the direction of maximum and minimum horizontal principal stresses, dimensionless.

[0021] Taking the Poisson's ratio as 0.25, the tectonic stress coefficient in the direction of the maximum horizontal principal stress is is a constant, and its value varies in different regions. The present invention obtains is 0.283, and when it is substituted into formula 4, the maximum horizontal principal stress is about 96 .

[0022] In an overpressure environment, based on Fan Taoyuan et al. (Analysis of Geostress in Tectonic Mineralization: Research Status and Thinking, Modern Geology, 2024), "When there is overpressure fluid inside the rock, in a compression environment, as the pore overpressure increases and the maximum differential stress decreases, the maximum differential stress decreases by about 2.12 times the increase in pore fluid pressure," that is:

[0023] (5)

[0024] In formula 5 is the reduction of the maximum differential stress, is the increase in pore fluid pressure.

[0025] The decrease in maximum differential stress with increasing pore overpressure can be expressed as:

[0026] (6)

[0027] The maximum differential stress at this time 55 , substitute into formula 2, Replace with , we can get the maximum horizontal principal stress at this time is 180 Similarly, substituting the parameters into formula 4, the maximum horizontal principal stress can be calculated to be approximately 115 The Huang model is inconsistent with the calculation results of the effective stress principle.

[0028] This calculation example uses publicly available data and is only used to illustrate that the Huang model has large errors in calculating the ground stress of strata under tectonic compression.

[0029] Therefore, the present invention analyzes the ground stress of multiple shale gas exploration areas and geothermal exploration areas with structural compression, and on the basis of Huang's model, finds the intrinsic correlation between the maximum differential stress and the compression structure, and introduces the correction term related to the maximum differential stress. , where k is an adjustment coefficient, which is obtained by fitting experimental data. An improved calculation formula is proposed. The improved Huang model can be written as:

[0030] (7)

[0031] Formula 7 is the core calculation formula for the maximum horizontal principal stress of a formation applicable to a tectonic compression state proposed in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a technical flow chart of the present invention;

[0033] Figure 2 This is a maximum horizontal principal stress prediction map of a shale gas formation in a certain area of Sichuan, calculated based on well logging data using the method of the present invention. DETAILED DESCRIPTION

[0034] Example 1

[0035] A method for calculating the maximum horizontal principal stress of a formation in a tectonic compression state, wherein the specific steps for calculating the maximum horizontal principal stress of a target layer based on well logging data include:

[0036] Step 1: Input the well data of the study area, the depth of the top and bottom interfaces of the target layer, and the average P-wave velocity of the target layer. , average shear wave velocity , average density , average formation pressure of target layer , the average maximum horizontal principal stress , average Biot coefficient , the average density of the strata above the target layer ;

[0037] Step 2: Calculate the maximum differential stress in the target layer of the well ,

[0038] (8)

[0039] (9)

[0040] Where, The average density of the stratum above the target layer, in kg / m 3 , is the depth of the target layer, unit: m, is the acceleration due to gravity, unit: m / s²;

[0041] Step 3: Establish the maximum differential stress based on the measured data of the drilled well The average P-wave velocity of the target layer The quadratic fitting relationship between:

[0042] (10)

[0043] Where, 、 、 is the fitting coefficient;

[0044] Step 4: Calculate the structural stress coefficient of the target layer in the drilled well ,

[0045] (11)

[0046] (12)

[0047] Step 5, calculate the coefficient ,

[0048] (13)

[0049] (14)

[0050] Where, is the maximum horizontal principal stress calculated by Huang’s model;

[0051] Step 6: Enter the depth of the target layer of the target well to start calculation and extract the P-wave velocity at the current depth. , shear wave velocity :

[0052] Step 6-1: Set the P-wave velocity at the current depth of the target layer to Substitute into the relationship obtained in step 3 to calculate the maximum differential stress at the current depth point of the target layer ;

[0053] Step 6-2: Calculate the Poisson's ratio of the rock physical parameter at the current depth ,

[0054] (15)

[0055] Step 6-3, calculate the maximum horizontal principal stress of the target layer at the current depth;

[0056] (16)

[0057] Step 7: Change the calculated depth of the target layer and repeat step 6;

[0058] Step 8: Output the maximum horizontal principal stress of the target layer of the target well.

[0059] Example 2

[0060] In order to illustrate the effectiveness and advancement of the core formula of the present invention, a well with measured formation pore pressure and ground stress data is used for analysis and explanation.

[0061] Figure 2 The first column is the natural gamma, unit: API; the second column is the longitudinal wave velocity, unit: m / s; the third column is the density, unit: Kg / m 3 ; The fourth column is the calculated maximum horizontal principal stress value, unit: MPa; there are two measured pressure points in the target layer, and the prediction errors are 0.4MPa and 0.3MPa respectively, both less than 0.5MPa, which is consistent with the prediction accuracy.

[0062] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for calculating the maximum horizontal principal stress of a stratum applicable to a tectonic compression state includes the following specific steps: Step 1: Input the well data of the study area, the depth of the top and bottom interfaces of the target layer, and the average P-wave velocity of the target layer. , average shear wave velocity , average density , average formation pressure of target layer , the average maximum horizontal principal stress , average Biot coefficient , the average density of the strata above the target layer ; Step 2: Calculate the maximum differential stress in the target layer of the well , Where, The average density of the stratum above the target layer, unit Kg / m 3 , is the depth of the target layer, unit: m, is the acceleration due to gravity, unit: m / s²; Step 3: Establish the maximum differential stress based on the measured data of the drilled well The average P-wave velocity of the target layer The quadratic fitting relationship between: Where, 、 、 is the fitting coefficient; Step 4: Calculate the structural stress coefficient of the target layer in the drilled well , Step 5, calculate the coefficient , Where, is the maximum horizontal principal stress calculated by Huang’s model; Step 6: Enter the depth of the target layer of the target well to start calculation and extract the P-wave velocity at the current depth. , shear wave velocity : Step 6-1: Set the P-wave velocity at the current depth of the target layer to Substitute into the relationship obtained in step 3 to calculate the maximum differential stress at the current depth point of the target layer ; Step 6-2: Calculate the Poisson's ratio of the rock physical parameter at the current depth , Step 6-3, calculate the maximum horizontal principal stress of the target layer at the current depth; Step 7: Change the calculated depth of the target layer and repeat step 6; Step 8: Output the maximum horizontal principal stress of the target layer of the target well.

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