An in-situ stress inversion method based on borehole wall strain monitoring

Abstract

The present application discloses a kind of present-day ground stress inversion method based on well wall strain monitoring, including in the local well wall wall surface of the far away from the influence area of orifice and hole bottom in ground stress measurement borehole three groups of right angle strain flowers are placed in the same horizontal position;On the well wall mud column pressure is applied, while recording the strain value on the strain flower, then respectively determine the circumferential strain and axial strain at 3 positions on the well wall;According to the relationship between the elastic theory and the global stress field of the measured point hole wall strain at different positions, the maximum and minimum horizontal stresses of the in-situ stress field at the three positions on the well wall are determined;According to the maximum and minimum horizontal stresses of the in-situ stress field at the three positions, the average value is obtained to determine the maximum and minimum horizontal stresses of the actual in-situ stress field.The present application can make up for the deficiency of traditional stress test, compared with traditional stress test, it is not necessary to drill core on the well wall, avoiding the complex construction process, saving the manpower and material resources required for drilling core.

Description

Technical Field

[0001] This invention relates to a current geostress inversion method based on wellbore strain monitoring, belonging to the field of petroleum exploration and development technology. Background Technology

[0002] In oil exploration and development, the assessment of initial geostress state is indispensable. Geostress refers to the natural stress existing in the Earth's crust without engineering disturbance. It is not only a major controlling factor affecting the mechanical behavior of rock masses, but also one of the main force sources leading to rock mass deformation and failure. Accurate measurement of the geostress field can significantly improve the safety of underground engineering: during drilling, understanding the geostress field helps construction personnel design reasonable drilling plans, such as the drilling fluid density required for wellbore stability and the casing design required for wellbore support, ensuring the safety and stability of the wellbore and reducing downhole accidents and drilling costs. During production and development, geostress measurement data can be used to understand reservoir deformation and fracture development, assess reservoir productivity and permeability, and optimize oil and gas extraction plans. Therefore, accurate measurement of the geostress field plays a crucial role in oil exploration and development.

[0003] There are numerous methods for measuring in-situ stress, among which the stress relief method is widely used. This method involves drilling a core sample from the surrounding rock and using a measuring sensor to detect changes in strain or displacement before and after stress relief, thereby calculating the magnitude and direction of the stress value. Generally, the stress relief method can be divided into borehole diameter deformation measurement, borehole wall stress relief, and borehole bottom strain measurement methods, depending on the measurement location and parameters. The borehole diameter deformation measurement method places a sensor inside the borehole and uses the change in borehole diameter before and after stress relief to determine the in-situ stress state; however, this method requires the core sample to be longer than 300mm and intact. The borehole wall stress relief method installs a sensor on the borehole wall and uses the strain difference on the borehole wall before and after stress relief to obtain the in-situ stress state; however, this method also requires the core sample to be longer than 300mm and intact. The borehole bottom strain measurement method places the measuring device at the bottom of a treated borehole and uses the strain difference at the borehole end before and after stress relief to determine the in-situ stress state; however, the accuracy of this method is affected by stress concentration at the bottom of the borehole.

[0004] Therefore, it is not difficult to see that in practical applications, the stress relief method still has room for improvement in obtaining reliable geostress states due to the difficulty of drilling complete rock cores and the limitations of measuring devices. To this end, a current geostress inversion method based on wellbore strain monitoring is proposed. The advantage of this method is that it does not require drilling rock cores, but uses the stress information of local wall measurement points under mud column pressure to deduce the magnitude of the principal stress of the stress field in the area. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention aims to provide a current geostress inversion method based on wellbore strain monitoring. This method assumes that the rock mass in the borehole wall area being measured is a homogeneous, continuous, and isotropic linear elastic body. The magnitude of the principal stress of the stress field in the local area is derived from the stress information of the measurement points on the local wall surface. Compared with existing drilling local wall stress complete relief technology, the advantage of this method is that it does not require drilling rock cores on the wellbore.

[0006] The technical solution provided by this invention to solve the above-mentioned technical problems is: a current geostress inversion method based on wellbore strain monitoring, comprising:

[0007] Step S10: Place three sets of right-angle strain flowers at the same horizontal position on the local well wall surface outside the influence zone of the borehole opening and bottom in the geostress measurement borehole;

[0008] Step S20: Apply mud column pressure to the well wall and record the strain value on the strain rosette. Based on the strain value of the strain rosette at the three locations, determine the circumferential strain and axial strain at the three locations on the well wall.

[0009] Step S30: Based on elasticity theory, using the relationship between the borehole wall strain at different measurement points and the global geostress field, determine the maximum and minimum horizontal stresses of the in-situ stress field at three locations on the well wall.

[0010] Step S40: Calculate the average value of the maximum and minimum horizontal stresses of the in-situ stress field at the three locations to determine the maximum and minimum horizontal stresses of the actual in-situ stress field.

[0011] A further technical solution is that the interval between two adjacent right-angle strain gauges in step 10 is 120°.

[0012] A further technical solution is that the calculation formula in step S20 is:

[0013]

[0014] In the formula: , , It is the angle between the attached strain gauge and the circumferential direction; , , The circumferential angle is , , The strain; , These are the circumferential and axial strains on the well wall.

[0015] A further technical solution is that the specific steps of step S30 are as follows:

[0016] Step S31: Determine the principal stress in the vertical direction based on the relationship between vertical ground stress and burial depth. σ z ;

[0017] Step S32, then based on the principal stress in the vertical direction σ z The circumferential strain and axial strain on the well wall determine the maximum and minimum horizontal stress of the in-situ stress field.

[0018] A further technical solution is that the calculation formula in step S31 is:

[0019]

[0020] In the formula: For density; The depth of burial; The principal stress is in the vertical direction.

[0021] A further technical solution is that the calculation formula in step S32 is:

[0022]

[0023] In the formula: and The horizontal stress at the measurement point; θ The location of the strain gauge on the well wall; Poisson's ratio; It is the elastic modulus; and The circumferential and axial strains occurring on the well wall at the measurement point are given.

[0024] The present invention has the following beneficial effects: The present invention is a novel method for testing geostress, which can make up for the shortcomings of traditional stress testing. Compared with traditional stress testing, it does not require drilling rock cores on the well wall, avoiding the complex construction process and saving the manpower and material resources required for drilling rock cores; instead, it uses the relationship between the stress state of the measurement point on the local wall surface and the global geostress field to deduce the principal stress of the in-situ stress field, providing a new solution for identifying geostress in underground geotechnical engineering. Attached Figure Description

[0025] Figure 1 This is a flowchart of the present invention;

[0026] Figure 2 It is a strain grub arrangement method for the complete relief of local wall stress in boreholes;

[0027] Figure 3 It represents the strain at any point on the borehole wall. Detailed Implementation

[0028] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 As shown, the present invention provides a current geostress inversion method based on wellbore strain monitoring, comprising the following steps:

[0030] Step S10: Place three sets of right-angle strain rosettes at the same horizontal position on the local well wall surface outside the influence zone of the borehole opening and bottom in the geostress measurement borehole; the interval between two adjacent right-angle strain rosettes is 120°.

[0031] Step S20: Apply mud column pressure to the well wall and record the strain value on the strain rosette. Based on the strain value of the strain rosette at the three locations, determine the circumferential strain and axial strain at the three locations on the well wall.

[0032] In a vertical well, the relationship between strain on a strain rosette and axial and circumferential strain of the wellbore:

[0033]

[0034] Therefore, the circumferential and axial strains on the well wall can be solved using the least squares method:

[0035]

[0036] In the formula: , , It is the angle between the attached strain gauge and the circumferential direction; , , The circumferential angle is , , The strain; , These are the circumferential and axial strains on the well wall;

[0037] Step S30: Based on elasticity theory, using the relationship between the borehole wall strain at different measurement points and the global geostress field, determine the maximum and minimum horizontal stresses of the in-situ stress field at three locations on the well wall.

[0038] Step S31: In the vertical well, = Therefore, the principal stress in the vertical direction can be determined based on the relationship between vertical ground stress and burial depth. ;

[0039]

[0040] In the formula: For density; The depth of burial; The principal stress is in the vertical direction;

[0041] Step S32, then based on the principal stress in the vertical direction The circumferential strain and axial strain on the well wall determine the maximum and minimum horizontal stress in the in-situ stress field;

[0042]

[0043] In the formula: and The horizontal stress is to be determined at the three measurement points; θ The location of the strain gauge on the well wall; Poisson's ratio; It is the elastic modulus; and The values ​​represent the circumferential and axial strains occurring on the well wall at three measurement points.

[0044] Step S40: Calculate the average value of the maximum and minimum horizontal stresses of the in-situ stress field at the three locations to determine the maximum and minimum horizontal stresses of the actual in-situ stress field.

[0045] Example

[0046] The elastic modulus and Poisson's ratio in the rock mass mechanical properties are 14000 MPa and 0.324, respectively.

[0047] I. Treating the borehole for geostress measurement as a vertical circular hole in an infinitely large rock mass, and taking a local wall surface far from the influence zone of the borehole opening and bottom as the research object, strain gauges are installed at the same horizontal position on the surface of the object being measured using adhesive. A set of three right-angle strain gauges is placed at 120° intervals, with the included angle between the strain gauges being 45°. Figure 2 As shown. According to the formula for calculating the principal stress in the vertical direction, the principal stress in the vertical direction can be determined. =20MPa.

[0048] 2. Apply mud column pressure to the wellbore. =40MPa, the strain values ​​on the strain rosette were recorded. The strain values ​​at the 0° position were -0.0033, -0.0015, and 0.0003468, respectively. The strain values ​​at the 120° position were -0.0012, -0.0003627, and 0.00034667, respectively. The strain values ​​at the 240° position were -0.00119, -0.0003619, and 0.0003501.

[0049] 3. Based on the relationship between the strain at the measurement point and the strain of the well wall, the axial and circumferential strain values ​​of the well wall at the measurement point are obtained. At the 0° position, the circumferential strain and axial strain of the well wall are -0.0033 and 0.000339, respectively. At the 120° position, the circumferential strain and axial strain of the well wall are -0.00119 and 0.000339, respectively. At the 240° position, the circumferential strain and axial strain of the well wall are -0.0012 and 0.00036945, respectively.

[0050] IV. Based on the wellbore strain at different measurement points, the horizontal principal stresses of the geostress field were calculated. The magnitudes of the horizontal principal stresses calculated at the 0° position were 25.4606 MPa and 11.8269 MPa, respectively. The magnitudes of the horizontal principal stresses calculated at the 120° position were 24.1273 MPa and 12.1602 MPa, respectively. The magnitudes of the horizontal principal stresses calculated at the 240° position were 24.2006 MPa and 11.9451 MPa, respectively.

[0051] V. Determine the horizontal principal stresses of the in-situ stress field based on the average horizontal principal stresses at the three measurement points. , The values ​​are 24.5961 MPa and 11.9774 MPa, respectively.

[0052] It is easy to see that the overall error of the horizontal stress calculated at different measurement points is less than 5%, indicating that the current geostress inversion method based on wellbore strain monitoring is reliable.

[0053] The above description is not intended to limit the present invention in any way. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention shall still fall within the scope of the present invention.

Claims

1. A current-state stress inversion method based on wellbore strain monitoring, characterized in that, include: Step S10: Place three sets of right-angle strain flowers at the same horizontal position on the local well wall surface outside the influence zone of the borehole opening and bottom in the geostress measurement borehole; Step S20: Apply mud column pressure to the well wall and record the strain value on the strain rosette. Based on the strain value of the strain rosette at the three locations, determine the circumferential strain and axial strain at the three locations on the well wall. Step S30: Based on elasticity theory, using the relationship between the borehole wall strain at different measurement points and the global geostress field, determine the maximum and minimum horizontal stresses of the in-situ stress field at three locations on the well wall. Step S31: Determine the principal stress in the vertical direction based on the relationship between vertical ground stress and burial depth. ; In the formula: For density; The depth of burial; The principal stress is in the vertical direction; Step S32, then based on the principal stress in the vertical direction σ z The circumferential strain and axial strain on the well wall determine the maximum and minimum horizontal stress in the in-situ stress field; In the formula: and The horizontal stress at the measurement point; θ The location of the strain gauge on the well wall; Poisson's ratio; It is the elastic modulus; and The circumferential and axial strains occurring on the well wall at the measurement point; Step S40: Calculate the average value of the maximum and minimum horizontal stresses of the in-situ stress field at the three locations to determine the maximum and minimum horizontal stresses of the actual in-situ stress field.

2. The current geostress inversion method based on wellbore strain monitoring according to claim 1, characterized in that, In step S10, the interval between two adjacent right-angle strain patterns is 120°.

3. The current geostress inversion method based on wellbore strain monitoring according to claim 1, characterized in that, The calculation formula in step S20 is: In the formula: , , It is the angle between the attached strain gauge and the circumferential direction; , , The circumferential angle is , , The strain; , These are the circumferential and axial strains on the well wall.

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