Stress relief effect evaluation method based on hydraulic fracturing ground stress test

Abstract

The embodiment of the invention discloses a stress relief effect evaluation method based on a hydraulic fracturing ground stress test. The stress relief effect evaluation method comprises the following steps: performing ground stress inversion on a to-be-relieved stress area according to hydraulic fracturing ground stress test data to determine an initial stress field; determining a stress concentration area in the excavation process of the stress to-be-relieved area according to the initial stress field; drilling a hole in the stress concentration area, carrying out a first hydraulic fracturing ground stress test in the drilled hole, recording a first pressure time history curve, and calculating the fracture surface stress before relieving; keeping the position of the packer in the first crustal stress test unchanged, continuously performing water injection fracturing, performing the hydraulic fracturing crustal stress test at the original position again, recording a second pressure time-history curve, and calculating the fracture surface stress after the second pressure time-history curve is relieved; and comparing fracture surface stress characteristic changes before and after fracturing, and evaluating the stress relief effect of the stress to-be-relieved area according to the fracture surface stress before and after relieving. The pain point that the stress relieving effect is difficult to quantitatively evaluate in the prior art is effectively solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of deep earth engineering, and in particular relates to a stress relief effect evaluation method based on a hydraulic fracturing ground stress test. Background Art

[0002] High geostress is a significant challenge that cannot be ignored in deep engineering construction, especially in tunnels and mines. As human engineering activities continue to extend deep into the Earth, the high geostress environment, resulting in large deformations of surrounding rock and rockbursts, is becoming increasingly prominent. During deep tunnel construction, high geostress can cause soft rock to deform or hard rock to suddenly burst, threatening not only the safety of construction workers but also serious consequences such as failure of support structures and damage to equipment.

[0003] Faced with the numerous challenges posed by high geostress, it is crucial to implement effective and scientific prevention and control measures. Stress relief is a key technology in geotechnical engineering, mining engineering, underground engineering, and other fields. It is primarily used to release or adjust initial stresses in rock or soil to improve engineering stability. In practice, stress relief methods primarily include stress relief blasting and stress relief drilling. Stress relief blasting involves creating fissures or fractured zones in the rock or coal mass through small-scale blasting, releasing localized high stresses and preventing stress concentrations from triggering hazards such as rockbursts and rock bursts. This method is fast and effective, suitable for high-stress hard rock, but it can disrupt the stability of the surrounding rock. Stress relief drilling involves drilling densely in areas of stress concentration, creating weakened zones through the drilling, and encouraging stress transfer and release to the surrounding areas. This method is simple to construct, minimally disturbs the surrounding rock, and can precisely locate stress zones. However, the stress relief range is limited and requires dense drilling. Furthermore, both of these methods present difficulties in quantifying the relief effect, necessitating the development of a method that can quantify the stress relief effect. Summary of the Invention

[0004] The purpose of the present invention is to address the shortcomings of the existing technology and provide a stress relief effect evaluation method based on hydraulic fracturing ground stress testing. This method utilizes the destructive effect of water on the surrounding rock during the hydraulic fracturing ground stress testing process to achieve stress concentration relief, and can evaluate the relief effect by comparing the stress difference before and after, effectively solving the pain point of the existing technology that the stress relief effect is difficult to quantify and evaluate.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A method for evaluating stress relief effect based on hydraulic fracturing ground stress testing comprises the following steps:

[0007] Step 1: Based on the hydraulic fracturing stress test data, perform in-situ stress inversion on the area to be relieved of stress to determine the initial stress field;

[0008] Step 2: Determine the stress concentration area during the excavation of the area to be relieved based on the initial stress field;

[0009] Step 3: Drill a hole in the stress concentration area and conduct the first hydraulic fracturing stress test in the hole, record the first pressure-time curve, and calculate the stress of the section before release based on the characteristic value of the first pressure-time curve;

[0010] Step 4: Keep the packer position unchanged from the first in-situ stress test, continue water injection and fracturing, conduct another hydraulic fracturing in-situ stress test at the original location, record the second pressure-time history curve, and calculate the post-release cross-section stress based on the characteristic value of the second pressure-time history curve;

[0011] Step 5: Compare the changes in cross-section stress characteristics before and after fracturing, and evaluate the stress relief effect of the area to be relieved based on the cross-section stress before and after relief.

[0012] Furthermore, in step 1, the multivariate regression method is used to perform inverse analysis of geostress. As the dependent variable, the self-weight stress field and tectonic stress field calculated based on the test data are calculated to the stress value corresponding to the measured point. As the independent variable, the regression equation of the ground stress field is:

[0013]

[0014] Where: k is the serial number of the observation point; is the regression calculation value of the kth observation point; L i is the multiple regression coefficient corresponding to the independent variable; and is a single-column matrix of the calculated values ​​of the corresponding stress components, and n is the number of working conditions.

[0015] Furthermore, the self-weight stress field calculation method is: using the measured density of the rock mass to calculate the self-weight stress field generated by the rock mass under the action of its own weight, wherein, during the calculation, displacement constraints are imposed on the side and bottom surfaces of the rock mass model, and only their normal direction displacement is restricted.

[0016] Furthermore, the tectonic stress field calculation method is as follows: horizontal triangular loads are applied to the sides of the two rock models to simulate the horizontal tectonic force, displacement constraints are applied to the non-loaded side boundaries and bottom, and only their normal direction displacement is restricted; and the shear stress in the horizontal plane is simulated by applying boundary displacement.

[0017] Furthermore, in step 3, a hole is drilled through the area to be relieved of stress.

[0018] Furthermore, the depth at which the bottom of the borehole passes through the boundary of the stress-relieving area is not less than the length of the packer.

[0019] Furthermore, the characteristic values ​​include fracture and fracturing P b , reopening pressure P r and closing pressure P s The calculated cross-sectional stress includes the maximum cross-sectional stress σ A and the minimum stress of the cross section σ B .

[0020] Furthermore, the maximum stress σ A and the minimum stress of the cross section σ B The calculation methods are:

[0021] σ A =3P s -P r -P0σ B =P r ;

[0022] Where P0 is the pore water pressure.

[0023] Furthermore, the unloading rate η is used as an evaluation index of the stress relief effect.

[0024] Furthermore, the unloading rate η is calculated as:

[0025] η=(σ A前 -σ A后 ) / σ A前 ×100%;

[0026] where σ A前 and σ A后 They correspond to the maximum stress of the section before and after release, respectively.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] This invention uses blasting vibration monitoring equipment to analyze the vibration signals obtained from monitoring and thus evaluate the intensity of deep rock bursts. It is simple and practical, does not require huge human and material investment, does not interfere with on-site construction, is flexible and convenient, and has strong adaptability to different projects. Specific advantages are as follows:

[0029] (1) The present invention utilizes the destructive effect of water on surrounding rocks during the hydraulic fracturing stress test to relieve stress concentration, and can evaluate the relief effect by comparing the stress difference before and after, effectively solving the problem of the difficulty in quantifying and evaluating the relief effect in existing stress relief technologies;

[0030] (2) Compared with the stress relief hole method, the present invention effectively increases the release range of a single hole, improves the utilization rate of the drill hole, reduces the number of drill holes, and reduces the project cost. Compared with the stress relief blasting method, the present invention has a more controllable disturbance to the surrounding rock and can more effectively ensure the stability of the surrounding rock. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the overall process of the stress relief effect evaluation method according to an embodiment of the present invention;

[0032] Figure 2 The stress distribution state of the flat plate with holes in the ground stress test according to the embodiment of the present invention;

[0033] Figure 3 A typical fracturing process curve and its pressure characteristic values ​​of the hydraulic fracturing method according to an embodiment of the present invention;

[0034] Figure 4 Schematic diagram of load and boundary constraints applied in an embodiment of the present invention;

[0035] Figure 5 Schematic diagram of stress relief area and drilling position according to an embodiment of the present invention;

[0036] Figure 6 This is a schematic diagram of the arrangement of a packer in a borehole according to an embodiment of the present invention;

[0037] Figure 7 This is a schematic diagram of a first pressure time history curve according to an embodiment of the present invention;

[0038] Figure 8 Schematic diagram of the second pressure-time curve according to an embodiment of the present invention. DETAILED DESCRIPTION

[0039] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0040] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0041] The present invention will be further described below with reference to specific examples, but they are not intended to limit the present invention.

[0042] like Figure 1 As shown, the embodiment of the present invention discloses a stress relief effect evaluation method based on hydraulic fracturing ground stress testing, comprising the following steps:

[0043] Step 1: Based on the hydraulic fracturing stress test data, perform in-situ stress inversion on the area to be relieved of stress to determine the initial stress field;

[0044] In this step, the principle of hydraulic fracturing ground stress testing is:

[0045] Under the assumptions of linear elasticity, homogeneity, and isotropy, the mechanical principle of hydraulic fracturing testing can be simplified as follows: Figure 2 The elastic plane problem mentioned above. A and σ B (σ A >σ B ) of an infinite plate with a hole, the secondary stress distribution near the hole can be expressed by formula (1).

[0046]

[0047] Where: a is the drilling radius, r is the radial distance, θ is the angle between the polar diameter and the axis x, σ r ′ is radial stress, σ θ ′ is the tangential stress, τ rθ ′ is the shear stress.

[0048] The stress state of the hole wall (r = a) can be expressed as:

[0049]

[0050] During the hydraulic fracturing test, the hydraulic pressure P W The additional stress generated is:

[0051]

[0052] The additional stress on the hole wall (r = a) is:

[0053]

[0054] Therefore, the stress on the borehole rock wall is:

[0055] σ θ =σ' θ +σ” θ =(σ A +σ B )-2(σ A -σ B )cos2θ-P W (5)

[0056] The rock on the hole wall is damaged by tension, so the cracks first appear at the location with the maximum tensile stress. From formula (5), we can get that the tangential stress is minimum at the hole wall where θ = 0 or θ = π.

[0057] σ θ =3σ B -σ A -P W (6)

[0058] Due to the existence of pore water pressure P0, the ground stress of the rock mass is σ-P0, where σ is the effective stress. During the test, as the test pressure increases, the effective stress of the surrounding rock gradually decreases until it becomes tensile stress. When the effective stress value on the hole wall is greater than or equal to the tensile strength of the surrounding rock, the surrounding rock will produce tensile failure and cracks will appear. The critical pressure for rock fracture (P b )for:

[0059]

[0060] Where: K is the pore permeability elastic coefficient, which varies in the range of 1 ≤ K ≤ 2. When the surrounding rock has poor permeability or no permeability (K ≈ 1), Equation (7) can be simplified to Equation (8).

[0061] P b -P0=3σ B -σ A +σ t -2P0 (8)

[0062] When the test hole is vertical, σ A and σ B are the maximum and minimum horizontal principal stresses σ H and σ h , if the effective stress in the above formula is replaced by the ground stress, we get:

[0063] P b =3σ h -σ H +σ t -P0 (9)

[0064] According to the principle that fractures propagate along the path of least resistance, after the pressure pump is turned off, the instantaneous closing pressure P that maintains the fracture open is s , is equal to the compressive stress in the direction perpendicular to the fracture surface, that is, the minimum horizontal principal stress:

[0065] σ h =P S (10)

[0066] The maximum horizontal principal stress is determined according to formula (9):

[0067] σ H =3σ h -P b -P0+σ t (11)

[0068] The tensile strength σ in formula (11) t It can be obtained by multiple cycles of pressurization. First, pressurization causes the complete hole wall surrounding rock to break, and the fracture pressure P is obtained. b , after which the tensile strength of the surrounding rock is 0, the re-tensioning pressure P r for:

[0069] P r =3σ h -σ H -P0 (12)

[0070] The maximum horizontal principal stress can also be expressed in terms of the re-tensioning pressure:

[0071] σ H =3σ h -P r -P0 (13)

[0072] Combining equations (9) and (12), the tensile strength of the surrounding rock can be obtained:

[0073] σ t =P b –P r (14)

[0074] The fracture surface of hydraulic fracturing generally extends along the direction of the maximum principal stress on the borehole cross section. The above stress calculation parameters are obtained from the pressure-time curve of the fracturing process. Figure 3 This is a typical fracturing process curve and its pressure characteristic value selection method.

[0075] Based on the above-mentioned hydraulic fracturing method ground stress test data, ground stress inversion is performed on the area where stress is to be released, wherein the ground stress inversion method is a multiple regression method.

[0076] According to the viewpoint of geomechanics, the main components of the geostress field are the self-weight stress field and the geological structural stress field. Based on this viewpoint, a mathematical calculation model can be established and the multivariate regression analysis method can be used for fitting analysis.

[0077] According to the principle of multiple regression method, the calculated value of ground stress regression is As the dependent variable, the self-weight stress field and tectonic stress field obtained by numerical calculation correspond to the stress calculation value of the measured point. As the independent variable, the regression equation is in the form of:

[0078]

[0079] Where: k is the serial number of the observation point; is the regression calculation value of the kth observation point; L i is the multiple regression coefficient corresponding to the independent variable; and is a single-column matrix of the calculated values ​​of the corresponding stress components, and n is the number of working conditions.

[0080] Assuming there are m observation points, the residual sum of squares of the least squares method is:

[0081]

[0082] Where: is the observed value of stress component j at observation point k, is the finite element calculated value of the stress component j at the k observation point under the i working condition.

[0083] According to the principle of least squares method, the equation that minimizes S residual is:

[0084]

[0085] Solve this equation and get n undetermined regression coefficients L=(L1,L2,...,L n ) T , then the regression initial stress of any point P in the calculation domain can be obtained by superimposing the numerical calculation values ​​of each working condition at that point:

[0086]

[0087] Where j = 1, 2, ..., 6 corresponds to the six components of initial stress.

[0088] For the regression analysis of the geostress field in this water diversion project area, based on the measured results, the geostress field in the calculation domain is regarded as the linear superposition of the self-weight stress field and the boundary-imposed tectonic stress field. The self-weight stress field and the boundary load stress field are decomposed and simulated, and finally combined into the calculated geostress field.

[0089] Self-weight stress field: The measured density of the rock mass is used to calculate the self-weight stress field generated by the self-weight. Displacement constraints are added to the sides and bottom of the calculation model, and only the displacement in the normal direction is restricted.

[0090] Tectonic stress field: horizontal triangular loads are applied on both sides to simulate horizontal tectonic forces. The constraints on the non-loaded side and bottom boundaries are the same as those for the self-weight stress field simulation. The simulation of shear stress in the horizontal plane is achieved by applying boundary displacements, that is, applying 2 unit displacements on the long boundary and 5 unit displacements on the short boundary. Under the action of the above basic factors alone, the corresponding loads and boundary constraints are as follows: Figure 4 shown.

[0091] Step 2: Determine the stress concentration area during the excavation of the stress-relieving area based on the initial stress field calculated in step 1. The stress concentration area is as follows: Figure 2 As shown in the middle area Z;

[0092] Step 3: Drill holes in the stress concentration area, and drill through the stress concentration area, such as Figure 5 Borehole AB, where the intersections of the borehole and the area to be released are C and D. A hydraulic fracturing ground stress testing device is arranged in the borehole, and the packer arrangement of the ground stress testing device is as follows: Figure 6 As shown, the packers are located at positions AC and BD, respectively, with the stress-relief zone located between the two packers. Assuming a packer length of 1 meter, the lengths of AC and BD should be no less than 1 meter. Furthermore, the extent to which the bottom of the borehole passes through the boundary of the stress-relief zone is determined by the size of the packers, and the depth should be no less than the packer length. Furthermore, the distance from the boundary of the stress-relief zone to the borehole opening is determined by the size of the packers, and the depth should also be no less than the packer length.

[0093] Then, the first hydraulic fracturing stress test is carried out in the borehole, the first pressure-time curve is recorded, and the stress before release is calculated based on the characteristic value of the first pressure-time curve; wherein the characteristic value includes the fracture pressure P b , reopening pressure P r and closing pressure P s , the cross-sectional stress calculated based on the characteristic value includes the maximum cross-sectional stress σ A and the minimum stress of the cross section σ B . Maximum stress of the section σ A and the minimum stress of the cross section σ B The calculation methods are:

[0094] σ A =3P s -P r -P0σ B =P r ;

[0095] Where P0 is the pore water pressure;

[0096] The first pressure-time curve recorded in this embodiment is as follows Figure 7 As shown, the burst pressure P b is 31.3MPa, and the reopening pressure P r 27.1MPa, closing pressure P s is 18.2MPa, then the maximum principal stress of the cross section before release is 27.5MPa, and the minimum principal stress of the cross section before release is 18.2MPa;

[0097] Step 4: Keep the packer position unchanged from the first in-situ stress test, continue water injection and fracturing, conduct another hydraulic fracturing in-situ stress test at the original location, record the second pressure-time history curve, and calculate the post-release cross-section stress based on the characteristic value of the second pressure-time history curve;

[0098] In this embodiment, the continuous water injection pressure should be no less than 18.2 MPa; the second pressure time course curve recorded is as follows: Figure 8 As shown, the reopening pressure P is read r 9.8MPa, closing pressure P s is 8.2MPa, then the calculated maximum principal stress of the cross section after release is 14.8MPa, and the minimum principal stress of the cross section after release is 8.2MPa;

[0099] Step 5: Compare the stress characteristic changes before and after fracturing, and evaluate the stress relief effect of the area to be relieved based on the stress before and after relief;

[0100] This embodiment uses the unloading rate η as the evaluation index of the stress relief effect. The unloading rate η is calculated as follows:

[0101] η=(σ A前 -σ A后 ) / σ A前 ×100%;

[0102] where σ A前 and σ A后 are the maximum stress of the section before and after release, respectively.

[0103] Finally, the unloading rate η calculated in this embodiment is (27.5-14.8) / 27.5*100%=46.2%.

[0104] The above are only preferred embodiments of the present invention and do not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the present invention specification should be included in the protection scope of the present invention.

Claims

1. A stress relief effect evaluation method based on hydraulic fracturing ground stress testing, characterized in that: The following steps are involved: Step 1: Based on the hydraulic fracturing stress test data, perform in-situ stress inversion on the area to be relieved of stress to determine the initial stress field; Step 2: Determine the stress concentration area during the excavation of the area to be relieved based on the initial stress field; Step 3: Drill a hole in the stress concentration area and conduct the first hydraulic fracturing stress test in the hole, record the first pressure-time curve, and calculate the stress of the section before release based on the characteristic value of the first pressure-time curve; Step 4: Keep the packer position unchanged from the first in-situ stress test, continue water injection and fracturing, conduct another hydraulic fracturing in-situ stress test at the original location, record the second pressure-time history curve, and calculate the post-release cross-section stress based on the characteristic value of the second pressure-time history curve; Step 5: Compare the changes in cross-section stress characteristics before and after fracturing, and evaluate the stress relief effect of the area to be relieved based on the cross-section stress before and after relief.

2. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 1, characterized in that: In step 1, the multivariate regression method is used to invert the geostress. As the dependent variable, the self-weight stress field and tectonic stress field calculated based on the test data are calculated to the stress value corresponding to the measured point. As the independent variable, the regression equation of the ground stress field is: Where: k is the serial number of the observation point; is the regression calculation value of the kth observation point; L i is the multiple regression coefficient corresponding to the independent variable; and is a single-column matrix of the calculated values ​​of the corresponding stress components, and n is the number of working conditions.

3. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 2, characterized in that: The self-weight stress field calculation method is as follows: the measured density of the rock mass is used to calculate the self-weight stress field generated by the rock mass under the action of its own weight. During the calculation, displacement constraints are imposed on the side and bottom surfaces of the rock mass model, and only their normal direction displacement is restricted.

4. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 2, characterized in that: The tectonic stress field calculation method is as follows: horizontal triangular loads are applied to the sides of two rock mass models to simulate horizontal tectonic forces, displacement constraints are applied to the non-loaded side boundaries and bottom, and only their normal direction displacement is restricted; and the shear stress in the horizontal plane is simulated by applying boundary displacement.

5. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 1, characterized in that: In step 3, a hole is drilled through the area to be relieved of stress.

6. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 5, characterized in that: The depth of the bottom of the borehole passing through the boundary of the stress-relieving area shall not be less than the length of the packer.

7. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 1, characterized in that: Characteristic values ​​include fracture and fracturing P b , reopening pressure P r and closing pressure P s The calculated cross-sectional stress includes the maximum cross-sectional stress σ A and the minimum stress of the cross section σ B .

8. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 7, characterized in that: Maximum stress of section σ A and the minimum stress of the cross section σ B The calculation methods are: s A =3P s -P r -P0 p B =P r ; Where P0 is the pore water pressure.

9. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 7, characterized in that: The unloading rate η is used as the evaluation index of the stress relief effect.

10. The stress relief effect evaluation method based on hydraulic fracturing ground stress testing according to claim 9, characterized in that: The unloading rate η is calculated as: the=(s A前 -s A后 ) / s A前 ×100%; where σ A前 and σ A后 They correspond to the maximum stress of the section before and after release, respectively.

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