A method for calculating stress on bedding plane around wellbore in stratigraphic formation

Abstract

The present invention relates to a method for calculating the stress of the bedding plane around a well in a stratified formation, which comprises: step one, collecting field data to obtain actual stratum data and actual engineering data on site; step two, converting the ground stress to a wellbore coordinate system according to the actual ground stress of the formation, the well inclination angle and the wellbore azimuth; step three, assuming that the direction of the maximum principal stress around the well is 0° and the direction of the minimum principal stress is 90°, superimposing the ground stress component, pore pressure and fluid pressure in the wellbore in the wellbore coordinate system to obtain the rock stress around the well; step four, assuming that the inclination angle of the horizontal bedding plane is 0°, and calculating the normal stress on the bedding plane at different bedding plane inclination angles according to the rock stress around the well. The present invention fully considers the characteristics of the stratified formation and can accurately predict the bedding plane stress under different bedding plane inclination conditions.

Description

Technical field:

[0001] The invention relates to the technical field of mechanical analysis of oil and gas wells in stratified formations, and in particular to a method for calculating the stress of bedding surfaces around stratified formation wells. Background technology:

[0002] During the development of oil and gas wells, the borehole formed by drilling through the formation will destroy the original formation stress balance state and cause the redistribution of the surrounding rock stress. For stratified formations, the bedding plane is easier to destroy than the rock matrix. If the redistributed stress on the bedding plane exceeds the strength of the rock bedding plane, it may cause the wellbore instability earlier than the rock matrix. Therefore, it is particularly important to accurately calculate the stress of the bedding plane around the well after engineering disturbance. In addition, after in-situ formation coring, the difference between the surface environment and the underground overburden conditions makes the fracture width measurement results of the bedding plane different from the actual underground conditions, which needs to be corrected according to the stress conditions of the bedding plane. The stress state of the bedding plane has a decisive influence on the stability of oil and gas wells, the development of cracks, and the effect of fracturing operations. Although the importance of the stress state of the bedding plane around the well is self-evident, the existing methods for calculating the stress of the bedding plane around the well in stratified formations still face several key challenges.

[0003] 1. The currently commonly used wellbore bedding surface stress calculation model is mainly based on homogeneous and isotropic rock mechanics theory, and does not specifically calculate the actual stress state of the bedding surface in the stratigraphic formation, thus affecting the accuracy of engineering decision-making.

[0004] 2. The stress state of the bedding surface around the well in the stratified stratum is affected by many factors such as the wellbore azimuth, well inclination and bedding surface dip. It is often difficult to take these variables into account during the evaluation, especially under different bedding surface dip conditions, which makes the evaluation of the stress state of the bedding surface around the well lack accuracy and reliability.

[0005] During the drilling process of stratified formations, the existence of bedding planes has a significant impact on the strength of the surrounding rock. Accurately calculating the stress of the bedding planes around the well and judging the width of the bedding fractures under overburden conditions is the key to accurately designing plugging particles and ensuring safe and efficient development of stratified reservoirs. Therefore, a scientific method is urgently needed to solve the above problems, accurately and effectively calculate the stress of the bedding planes, and provide accurate guidance for the design of drilling fluid plugging particles. Summary of the invention:

[0006] The purpose of the present invention is to provide a method for calculating the stress of the bedding surface around a well in a stratigraphic formation. The method for calculating the stress of the bedding surface around a well in a stratigraphic formation is used to solve the problem that the stress calculation model of the bedding surface around a well in the prior art is mainly based on the homogeneous and isotropic rock mechanics theory, and does not specifically calculate the actual stress state of the bedding surface in the stratigraphic formation, and the existing calculation lacks accuracy and reliability.

[0007] The technical solution adopted by the present invention to solve the technical problem is: the method for calculating the stress of the bedding plane around the well in the bedding formation comprises the following steps:

[0008] Step 1: Collect field data to obtain actual field formation data and actual field engineering data;

[0009] Step 2: According to the actual formation in-situ stress, well inclination angle and wellbore azimuth, the in-situ stress is converted into the wellbore coordinate system;

[0010] Step 3: Assuming that the direction of the maximum principal stress around the well is 0° and the direction of the minimum principal stress is 90°, the ground stress component, pore pressure, and fluid pressure in the wellbore coordinate system are superimposed to obtain the rock stress around the well:

[0011]

[0012] Where: ij (i,j∈x,y,z b ) is [σ B ], MPa; σ r is the radial stress around the well caused by load imbalance; σ θ Circumferential stress caused by unbalanced load, MPa; A=(σ xx +σ yy ) / 2; B=(σ xx -σ yy ) / 2; C = r i 2 / r 2 ; D = r i 4 / r 4 ; θ is the angle between the radial direction of a point around the wellbore and the direction of the maximum horizontal principal stress, °; p i is the fluid pressure in the wellbore, MPa; p p is the formation pore pressure, MPa; r i is the borehole radius, m; r is the distance from the borehole center, m; α is the Biot coefficient, which is taken as 1;

[0013] Step 4: Assuming the horizontal bedding plane dip is 0°, calculate the normal stress on the bedding plane at different bedding plane dips based on the rock stress around the well:

[0014]

[0015] According to the rock stress around the well, the shear stress on the bedding plane at different bedding plane inclination angles is calculated:

[0016]

[0017] Where: β is the bedding plane dip; σw is the normal stress on the bedding plane, MPa; τ w is the shear stress on the bedding plane, MPa.

[0018] The actual on-site formation data in step 1 of the above scheme include the pressure stress of the overlying rock formation, the maximum horizontal stress, the minimum horizontal stress, the azimuth of the maximum horizontal stress, the pore pressure, and the inclination of the formation bedding surface; the actual on-site engineering data include the well inclination angle, the wellbore azimuth angle, the fluid pressure in the wellbore, and the wellbore radius.

[0019] The specific step 2 in the above scheme is: according to the actual formation in-situ stress, well inclination angle and wellbore azimuth, the in-situ stress is converted to the wellbore coordinate system:

[0020] [σ B ]=[R2][R1 T ][σ S ][R1][R2 T ]

[0021] in,

[0022]

[0023] Where: [σ s ] is the ground stress component in the ground stress coordinate system; [σ B ] is the component of the in-situ stress in the wellbore coordinate system; α1 is the azimuth of the maximum horizontal in-situ stress; β1 is the azimuth of the minimum horizontal in-situ stress; α2 is the wellbore azimuth and β2 is the well inclination; σ H is the maximum horizontal geostress in the geostress coordinate system; σ h is the minimum horizontal geostress in the geostress coordinate system; σ V is the overlying rock pressure in the geostress coordinate system.

[0024] Beneficial effects:

[0025] 1. The present invention takes into account the stress effects of geostress, pore pressure and fluid pressure in the wellbore on the bedding surface, and more truly and accurately reflects the stress conditions of the bedding surface around the well under different well inclination angles, azimuth angles and bedding surface inclination angles.

[0026] 2. The present invention can adapt to different geostress conditions and accurately calculate the stress of the bedding surface around the well, and is therefore suitable for stratified strata in different regions and diversified geostress environments.

[0027] 3. The present invention can accurately calculate the bedding surface stress around the well under different well inclination angles and wellbore azimuth conditions, and is therefore suitable for bedding formations under different working conditions.

[0028] 4. The present invention fully considers the characteristics of the layered formation and can accurately predict the bedding surface stress under different bedding surface inclination conditions, which provides a basis for calculating the bedding surface crack width around the well, and further provides important guidance for designing on-site drilling fluid plugging particles.

[0029] 5. The present invention takes into account the bedding characteristics of the stratigraphic strata in the drilling project, and proposes a calculation formula for the bedding surface stress around the well that takes into account factors such as ground stress, pore pressure and fluid pressure in the wellbore, providing effective data support for the design of drilling projects in stratigraphic strata and the calculation of geomechanical properties. Description of the drawings:

[0030] Figure 1 It is a flow chart of the present invention;

[0031] Figure 2 is the rock stress diagram around the well;

[0032] Figure 3 This is the stress distribution diagram of the bedding surface around the well;

[0033] Figure 4 This is a diagram showing the stress distribution of the bedding surface around the well under different well inclination conditions;

[0034] Figure 5 This is a diagram showing the stress distribution of the bedding surface around the well under different wellbore azimuth conditions;

[0035] Figure 6 This is a diagram showing the stress distribution of the bedding surface around the well under different bedding plane inclination conditions. Specific implementation method:

[0036] The present invention will be further described below:

[0037] The calculation method of the bedding stress around the well in this kind of layered formation is as follows:

[0038] Step 1: Collect field data and obtain actual formation data on site: overburden rock pressure stress, maximum horizontal stress, minimum horizontal stress, maximum horizontal stress azimuth, pore pressure, and formation bedding plane inclination; actual engineering data on site: well inclination angle, wellbore azimuth angle, fluid pressure in the wellbore, and wellbore radius.

[0039] Step 2: According to the actual formation in-situ stress, well inclination and wellbore azimuth, transform the in-situ stress into the wellbore coordinate system:

[0040] [σ B ]=[R2][R1 T ][σ S ][R1][R2 T ]

[0041] in,

[0042]

[0043] Where: [σ s ] is the ground stress component in the ground stress coordinate system; [σ B ] is the ground stress component in the wellbore coordinate system; α1 and β1 are the azimuths of the maximum horizontal ground stress and the minimum horizontal ground stress, respectively, °; α2 and β2 are the wellbore azimuth and well inclination, respectively, °; σ H , σ h and σ V They are the maximum horizontal geostress, minimum horizontal geostress and overlying rock pressure in the geostress coordinate system, MPa.

[0044] Step 3: Assume that the direction of the maximum principal stress around the well is 0° and the direction of the minimum principal stress is 90°. According to the generalized plane strain solution method, the ground stress components, pore pressure, and fluid pressure in the wellbore coordinate system are superimposed to obtain the rock stress around the well:

[0045]

[0046] Where: ij (i,j∈x,y,z b ) is [σ B ], MPa; σ r and σ θ is the radial stress and circumferential stress around the well caused by load imbalance, MPa; A=(σ xx +σ yy ) / 2; B=(σ xx -σ yy ) / 2; C = r i 2 / r 2 ; D = r i 4 / r 4 ; θ is the angle between the radial direction of a point around the wellbore and the direction of the maximum horizontal principal stress, °; p i is the fluid pressure in the wellbore, MPa; p p is the formation pore pressure, MPa; r i is the wellbore radius, m; r is the distance from the wellbore center, m; α is the Biot coefficient, which is generally taken as 1.

[0047] Step 4: Assuming the horizontal bedding plane dip is 0°, calculate the normal stress on the bedding plane at different bedding plane dips based on the rock stress around the well:

[0048]

[0049] According to the rock stress around the well, the shear stress on the bedding plane at different bedding plane inclination angles is calculated:

[0050]

[0051] Where: β is the bedding plane dip, °; σ r , σ θ is the effective principal stress in radial and circumferential directions, MPa; σ w is the normal stress on the bedding plane, MPa; τ w is the shear stress on the bedding plane, MPa.

[0052] Example:

[0053] Step 1: Collect field data and obtain actual field formation data: overburden stratum pressure stress, maximum horizontal stress, minimum horizontal stress, maximum horizontal stress azimuth, pore pressure, and formation bedding plane inclination; actual field engineering data: well inclination angle, wellbore azimuth angle, wellbore fluid pressure, and wellbore radius. See Table 1 for details:

[0054] Table 1 Calculation parameters of the embodiment

[0055]

[0056]

[0057] Step 2: According to the reference values ​​of formation in-situ stress, well inclination angle and wellbore azimuth angle in the embodiment, the in-situ stress is converted to the wellbore coordinate system:

[0058]

[0059] Step 3: Superimpose the ground stress components, pore pressure, and fluid pressure in the wellbore coordinate system to obtain the rock stress around the well. The rock stress diagram around the well is shown in Figure 2 , (a) is radial stress, (b) is circumferential stress;

[0060] Step 4: Calculate the normal stress and shear stress on the bedding plane at different bedding plane inclination angles according to the surrounding rock stress of the well wall. See the stress diagram of the bedding plane around the well for details. Figure 3 , (a) is the normal stress of the bedding surface around the well, (b) is the shear stress of the bedding surface around the well;

[0061] Step 5: Repeat steps 2 to 4 to obtain the stress distribution diagram of the bedding surface around the well under different well inclination angles. Figure 4 In the figure, (a) is the normal stress of the bedding surface around the well, and (b) is the shear stress of the bedding surface around the well. The stress distribution of the bedding surface around the well under different wellbore azimuth conditions is shown in Figure 5 In the figure, (a) is the normal stress of the bedding surface around the well, and (b) is the shear stress of the bedding surface around the well. The stress distribution of the bedding surface around the well under different bedding surface inclination conditions is shown in Figure 6In the figure, (a) is the normal stress of the bedding surface around the well, and (b) is the shear stress of the bedding surface around the well.

[0062] The present invention collects on-site geological data and engineering data; calculates the geostress components around the wellbore according to the geostress, geostress azimuth, well inclination and wellbore azimuth; calculates the rock stress around the well according to the generalized plane strain solution method; and calculates the normal stress and shear stress on the bedding surface at different bedding surface inclinations. The present invention comprehensively considers the influence of the geostress components around the wellbore, the fluid pressure in the wellbore and the formation pore pressure on the stress of the bedding surface around the wellbore, and more truly and accurately reflects the stress distribution of the bedding surface around the wellbore under the conditions of different well inclinations, wellbore azimuths and bedding surface inclinations, and solves the problem that the current rock mechanics theory based on homogeneous and isotropic rock does not take into account the influence of multiple factors such as wellbore azimuth, well inclination and bedding surface inclination during evaluation, which makes the calculation of the stress state of the bedding surface around the wellbore lack of accuracy and reliability.

Claims

1. A method for calculating the stress of the bedding plane around a well in a bedding formation, characterized in that The steps include: Step 1: Collect field data to obtain actual field formation data and actual field engineering data; Step 2: According to the actual formation in-situ stress, well inclination angle and wellbore azimuth, the in-situ stress is converted into the wellbore coordinate system; Step 3: Assuming that the direction of the maximum principal stress around the well is 0° and the direction of the minimum principal stress is 90°, the ground stress component, pore pressure, and fluid pressure in the wellbore coordinate system are superimposed to obtain the rock stress around the well: Where: r is the radial stress around the well caused by load imbalance; σ θ Circumferential stress caused by unbalanced load, MPa; A=(σ xx +σ yy ) / 2; B=(σ xx -σ yy ) / 2; θ is the angle between the radial direction of a point around the wellbore and the direction of the maximum horizontal principal stress, °; p i is the fluid pressure in the wellbore, MPa; p p is the formation pore pressure, MPa; r i is the borehole radius, m; r is the distance from the borehole center, m; α is the Biot coefficient, which is taken as 1; Step 4: Assuming the horizontal bedding plane dip is 0°, calculate the normal stress on the bedding plane at different bedding plane dips based on the rock stress around the well: According to the rock stress around the well, the shear stress on the bedding plane at different bedding plane inclination angles is calculated: Where: β is the bedding plane dip; σ w is the normal stress on the bedding plane, MPa; τ w is the shear stress on the bedding plane, MPa.

2. The method for calculating the stress of the bedding plane around the well in the bedding formation according to claim 1, characterized in that: The actual on-site formation data in step 1 include the pressure stress of the overlying rock formation, the maximum horizontal stress, the minimum horizontal stress, the maximum horizontal stress azimuth, the pore pressure, and the inclination of the formation bedding surface; the actual on-site engineering data include the well inclination angle, the wellbore azimuth angle, the fluid pressure in the wellbore, and the wellbore radius.

3. The method for calculating the stress of the bedding plane around the well in the bedding formation according to claim 2, characterized in that: The second step is specifically: according to the actual formation in-situ stress, well inclination angle and wellbore azimuth, the in-situ stress is converted into the wellbore coordinate system: [σ B ]=[R2][R1 T ][σ S ][R1][R2 T ] in, Where: [σ s ] is the ground stress component in the ground stress coordinate system; [σ B ] is the ground stress component in the wellbore coordinate system; α1 is the azimuth of the maximum horizontal geostress; β1 is the azimuth of the minimum horizontal geostress; α2 is the wellbore azimuth and β2 is the well inclination; σ H is the maximum horizontal geostress in the geostress coordinate system; σ h is the minimum horizontal geostress in the geostress coordinate system; σ V is the overlying rock pressure in the geostress coordinate system.

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