A method for measuring horizontal ground stress and its application

Abstract

The present invention relates to a method and application for measuring horizontal geostress, comprising the following steps: performing casing fracturing, causing a packer to be filled with water and expanded at a target formation to generate an initial induced crack as a starting point for a fracturing test; using a straddle packer to place the initial induced crack between an upper packer and a lower packer to form a test interval; checking the sealing of the test interval; performing a leakage test, and if the fluid diffuses into the formation, changing the target formation; performing a hydraulic fracturing cycle to obtain test parameters, and using an interpretation method with pressure and time as variables to obtain an estimated geostress value; based on a local fault mechanism, using a stress polygon method to determine the horizontal component of the estimated geostress value. Compared with the prior art, the present invention combines casing fracturing with a hydraulic fracturing cycle, and can achieve testing of horizontal geostress under reverse fault geostress conditions, with simple operation and significant effect.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic fracturing ground stress measurement, and relates to a horizontal ground stress measurement method and application. Background Art

[0002] Geostress measurement has theoretical and practical significance for geological structure research, earthquake prediction, and solving related problems in mining, water conservancy, national defense and other projects. It is one of the important contents of geomechanics research. Through measurement, it is found that the direction of the maximum principal stress is almost horizontal. Measuring the magnitude and direction of horizontal geostress can determine the underground far-field stress distribution. For projects such as oil field exploitation, underground gas storage and gas injection, when injecting fluid under high pressure, understanding the in-situ stress, especially the horizontal geostress in the sealing layer overlying the reservoir or caprock, is crucial for selecting the maximum working pressure. Micro-hydraulic fracturing test is the most commonly used borehole stress measurement method. The pressure response obtained during the injection or closure of circulating fluid can explain the initiation, expansion and closure process of induced fractures caused by hydraulic fracturing. The pressure response is generated by injecting fluid at a constant rate into the sealed borehole area between two packers.

[0003] Retrieving Chinese publication number CN109469479A discloses a hydraulic fracturing ground stress measurement system and method, including a packer, a packer water injection pipeline and a fracturing water injection pipeline, the upper packer and the lower packer are sealed with the wall of the test hole to form two packer sections, and a fracturing test section is formed between the upper packer and the lower packer. The packer is provided with a packer core tube and a packer sealing water injection pipe, and also includes a regulating connection device, the regulating connection device is connected to the upper packer, and the packer sealing water injection pipe and the external environment are regulated by the regulating connection device.

[0004] During oilfield exploitation, the hydraulic fracturing geostress measurement method in the prior art can generate vertical cracks to test horizontal stress. However, under the geostress state of a reverse fault, the vertical geostress is less than the horizontal geostress. The cracks induced by hydraulic fracturing using the prior art must be horizontal cracks, so the horizontal stress cannot be tested. Summary of the invention

[0005] The purpose of the present invention is to provide a method for measuring geostress and its application, so as to overcome the problem that horizontal geostress cannot be tested due to horizontal cracks generated under the geostress state of reverse fault.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A method for measuring horizontal ground stress comprises the following steps:

[0008] Step S1, performing casing fracturing, filling a packer with water to expand it at the target formation to generate an initial induced fracture, which serves as the starting point of the fracturing test;

[0009] Step S2, using a straddle packer to place the initial induced fracture between the upper packer and the lower packer to form a test interval;

[0010] Step S3, checking the sealing of the test area;

[0011] Step S4, perform a leakage test. If the fluid diffuses into the formation, change the target formation and return to step S1;

[0012] Step S5, performing a hydraulic fracturing cycle, obtaining test parameters, and obtaining an estimated value of ground stress using an interpretation method with pressure and time as variables;

[0013] Step S6: Based on the local fault mechanism, use the stress polygon method to determine the horizontal component of the estimated ground stress value.

[0014] Furthermore, the initial induced cracks are discontinuous vertical cracks.

[0015] Furthermore, the checking of the sealing of the test area includes:

[0016] Step S301, filling the upper packer and the lower packer with water to expand until the pressure in the test interval begins to rise;

[0017] Step S302: observe the subsequent pressure drop to check the sealing of the test area.

[0018] Furthermore, the leakage test is to inject fluid into the test interval in the wellbore at a constant flow rate to pressurize the wellbore to a critical value of pressure that causes formation rupture.

[0019] Furthermore, the hydraulic fracturing cycle includes:

[0020] Step S501, injecting fluid into the test interval in the wellbore at a constant flow rate, extending the fracture from the starting point of the fracturing test until the pressure is stable;

[0021] Step S502, stop injection, let the pressure decay to a pressure level that ensures the closure of the crack, and return to step S501 until the set period is reached;

[0022] During the hydraulic fracturing cycle, the initiation, propagation, closure and re-initiation of induced fractures are monitored.

[0023] Furthermore, the setting period is 2 to 5.

[0024] Furthermore, the test parameters include fracture pressure, propagation pressure, instantaneous shut-in pressure, closing pressure, reopening pressure and rebound pressure.

[0025] Furthermore, the interpretation method is: an image is made with pressure and time as variables, and the inflection point of the curve in the image is taken as the minimum ground stress.

[0026] Further, the stress polygon method is described to estimate the range of possible stress states at any given depth and pore pressure using the Anderson stress and fault classification system.

[0027] A method for determining rock stability uses the horizontal geostress measurement method to measure the geostress in a target area, compares the geostress value with the rock quality grade, and thereby determines the stability of the rock.

[0028] Compared with the prior art, the present invention has the following characteristics:

[0029] 1. The present invention makes the packer expand by filling with water, and vertical cracks are induced in both the reverse fault in-situ stress state and the non-reverse fault in-situ stress state. Horizontal in-situ stress can be measured according to the vertical cracks. Compared with the prior art, the horizontal in-situ stress can be tested in the reverse fault in-situ stress state, and the operation is simple.

[0030] 3. The present invention refers to stress polygons and local fault mechanisms, analyzes according to the geological background conditions of the project site, and achieves relatively accurate determination of horizontal ground stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the process of the present invention;

[0032] Figure 2 It is a schematic diagram of the arrangement of the packer of the present invention;

[0033] Figure 3 A complete record of the packer water filling pressure time during the hydraulic fracturing cycle phase of the pilot project provided by the embodiment of the present invention;

[0034] Figure 4 The pressure history of a hydraulic fracturing cycle test at a certain depth recorded in a pilot project provided by an embodiment of the present invention;

[0035] Figure 5 A stress polygon diagram of a pilot project at a certain depth in a test section provided in an embodiment of the present invention.

[0036] Description of the markings in the figure:

[0037] 1—filling of upper and lower packers with water, 2—leakage test, 3—pressure drop, 4—first cycle, 5—first drawdown, 6—flowback, 7—second cycle, 8—second drawdown, 9—flowback, 10—degassing of packers, 11—vertical fractures in the wellbore, 12—upper packer, 13—test interval, 14—lower packer, 15—hydraulic expansion of fractures. DETAILED DESCRIPTION

[0038] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0039] Example 1

[0040] The present invention provides a method for measuring horizontal ground stress. Figure 1 , including the following steps:

[0041] Step S1: Casing fracturing is performed. A straddle packer is placed at the target formation to allow the upper packer to expand with water to generate initial induced fractures. The purpose is to generate a series of discontinuous fractures on the well wall as the starting point of the fracturing test. Figure 2 ;

[0042] The straddle packer allows the casing fracturing and hydraulic fracturing cycle to be carried out according to the test interval or depth, and the initial induced fractures are discontinuous vertical fractures.

[0043] Under normal conditions, horizontal geostress is less than vertical geostress, and the cracks generated during hydraulic fracturing will be perpendicular to the direction of the minimum principal stress, that is, the induced cracks are vertical cracks. At this time, the horizontal geostress can be measured by the hydraulic fracturing method. However, under the reverse fault geostress state, the vertical geostress is less than the horizontal geostress, and the cracks induced by hydraulic fracturing must be horizontal cracks. At this time, the horizontal geostress test cannot be performed at all using the conventional hydraulic fracturing method. The sleeve fracturing stage in this patent can ensure that the induced cracks generated are in a vertical direction, so as to perform horizontal geostress tests under the reverse fault geostress state. The primary goal of geostress testing using hydraulic fracturing technology is to create cracks to provide a closure stress estimate that is as close to the minimum in-situ stress as possible. In multiple test stages, when the crack is opened or closed, the estimate of the closure stress (or pressure) can be detected by checking the flow state associated with the sudden change in the fracture conductivity. At each stage, various estimates of the closure pressure are determined. By coordinating the estimates of all cycles, the closure stress and the minimum in-situ stress can be estimated.

[0044] At the beginning of the test, fluid should be pumped into a packer of the test device at a constant rate until the maximum allowable pressure that the packer can withstand is reached, resulting in the formation of stable cracks. At this stage, the initiation of the crack can be identified by rupture or pressure platform. The packer is then drained and repositioned so that the packer is in the same horizontal section as the generated crack. This process ensures that the crack extends from the test section level. In addition, casing fracturing is a method used to test horizontal stress under reverse fault in-situ stress state. Vertical tensile cracks are first induced in the well wall by expanding the casing, thereby greatly increasing the possibility of vertical cracks generated by hydraulic fracturing in the later stage.

[0045] Step S2: Pressure reduction is applied to the upper packer to place the initial induced fracture between the upper packer and the lower packer to form a test interval. Figure 2 ;

[0046] Step S3, checking the sealing of the test area;

[0047] Once the test equipment is correctly positioned, isolate the test zone by pressurizing the straddle packer assembly until the pressure in the test zone begins to rise. The subsequent pressure drop is then observed to check the quality of the packer seal. If the seal is not satisfactory, further pressurize the packer.

[0048] Step S4, perform a leakage test. If the fluid diffuses rapidly into the formation, change the target formation and return to step S1;

[0049] Fluid is injected into the well section at a constant flow rate to pressurize the wellbore to a pressure below the formation fracture pressure. The purpose is to check the possibility that the downhole pump can provide enough flow to overcome the diffusion of fluid into the formation through the mud.

[0050] Step S5, performing a hydraulic fracturing cycle, obtaining test parameters, and using a pressure-time data analysis variable interpretation method to obtain a minimum in-situ stress estimate;

[0051] The test parameters include fracture pressure, propagation pressure, instantaneous shut-in pressure, closing pressure, reopening pressure and rebound pressure.

[0052] Fluid is injected into the test section again at a constant flow rate, and then the fracture is extended. After the fracture is extended, fluid injection continues until the pressure stabilizes. The injection is then stopped to allow the pressure to decay to a certain pressure level to ensure that the fracture is closed. The stress value perpendicular to the fracture surface is determined by monitoring the initiation, expansion, closure and reopening of the induced fracture. Generally speaking, the fracture grows in a direction perpendicular to the minimum stress. The pressure test usually contains two to five such cycles. Once the operator is satisfied with the quality of the data obtained in the test well section, the packer is deflated and the test device moves to the next test well section.

[0053] A variety of interpretation methods have been developed to estimate the magnitude of the minimum principal stress from the pressure records obtained during hydraulic fracturing cycle tests. The following parameters may be used in these interpretation methods (if they can be measured during the test phase): fracture pressure, propagation pressure, instantaneous shut-in pressure, closing pressure, reopening pressure, and rebound pressure.

[0054] An estimate of the closure stress (or pressure) can be detected by examining the flow regime associated with abrupt changes in fracture conductivity as the fracture opens or closes during multiple test phases. At each phase, various estimates of the closure stress are determined. By coordinating the estimates from all cycles, the closure stress and the minimum in situ stress can be estimated.

[0055] After obtaining the data from the test phase, the method for determining the minimum ground stress using the pressure-time data analysis variables is as follows:

[0056] (1) Pressure vs. square root of shut-in time: Assume that after the injection of fluid into the formation stops, the pressure drop due to fluid leakage should be linear as long as the fracture remains open. This method is often used in conjunction with a logarithmic pressure-time plot. On a logarithmic plot, the linear flow regime of the fracture is determined by its characteristic half slope. This plot indicates whether the closure stress has been accurately identified on the pressure vs. square root of shut-in time plot. Due to shut-in, the relationship between pressure and square root of shut-in time is usually a straight line that decreases, but once the fracture is closed, it deviates from the straight line. This point is considered the closure stress or minimum in situ stress.

[0057] (2) Linear flow diagram: In a linear flow diagram, this type of curve superimposes the linear flow pressure during injection on the linear flow pressure during drawdown. The basic assumption is that before fracture closure, the flow is linear and shortly thereafter, the flow becomes nonlinear.

[0058] (3) Relationship between logging pressure and shut-in time: Usually the relationship between the logging curve of bottom hole pressure and shut-in time. This type of graph is based on the exponential leakage after closure. Therefore, the inflection point on the slope of the curve is considered to be equal to the instantaneous shut-in pressure or minimum ground stress.

[0059] (4) Log dP-Log dt curve: This curve is based on exponential leakage after closure. This plot is often used in reservoir engineering to estimate the duration of wellbore storage. After wellbore storage, linear or bilinear flow occurs in the fracture as long as the fracture remains open. When the fracture closes, the dP / dt curve deviates from the straight line. The dP value corresponding to the deviation from the straight line indicates fracture closure. Once this value dP is known, the instantaneous shut-in pressure or minimum ground stress can be obtained.

[0060] (5) Pressure decay rate: The pressure decay after the injection of fluid is stopped is caused by the expansion of the fracture and the leakage of the injected fluid from the fracture into the formation. As these processes continue, the pressure eventually reaches a value that is equal to the far-field stress perpendicular to the fracture, resulting in the closure of the fracture. This method is based on the assumption that the pressure decay consists of two different stages: the linear flow stage when the fracture is still open and the radial flow stage after the fracture is closed. Therefore, the pressure change per unit time (dP / dt) and the pressure (P) can be plotted to form a bilinear curve representing the pressure state of the open and closed fractures. The intersection of these two linear curves is considered to be the closure stress or minimum ground stress.

[0061] (6) Fracture reopening test: The minimum in situ stress can be estimated from the fracture reopening test. When fluid is subsequently injected at a constant rate after the initial fracture of the formation, the slope of the pressure vs. time curve should be constant until the fracture reopens. When the fracture reopens, there will be a deviation from the straight line. This deviation point is considered the reopening pressure, which is theoretically the upper limit of the minimum in situ stress.

[0062] (7) Horner diagram: It is a plot of the relationship between pressure and log(Tinj+dt) / dt. This diagram was originally used for pressure drop analysis in reservoir engineering. When the pressure drop data are plotted on the Horner diagram, the slope at the fracture closure may change. This method assumes that until the fracture is completely closed, the pressure transient effect is controlled by radial flow and a straight line is observed on the Horner curve; before the fracture is closed, the relationship is considered to deviate from the straight line.

[0063] Step S6: Based on the local fault mechanism, use the stress polygon method to determine the horizontal component of the ground stress.

[0064] After the minimum geostress is determined, an analysis is performed based on the geological background conditions of the project site with reference to the stress polygon and local fault mechanism to determine the horizontal component of the geostress relatively accurately.

[0065] The present invention combines sleeve fracturing with hydraulic fracturing cycles, and can test horizontal ground stress even in the reverse fault ground stress state, thereby obtaining a series of test parameters, with simple operation and significant effect.

[0066] The hydraulic fracturing cycle test problem is explained based on the above-mentioned device and method. The following section takes the results obtained from the hydraulic fracturing cycle test conducted at a certain depth of Well A as an example.

[0067] Figure 3A complete summary of the pressure history of the casing fracturing phase during the horizontal in-situ stress test is provided. The figure also shows the change in the slope of the pressure-time curve for two cycles to interpret the potential formation fracture pressure. As shown in the figure, the magnitude of the fracture initiation pressure is estimated to be between 13.5MPa and 14.2MPa.

[0068] One of the issues that may complicate the interpretation of stresses during this phase of testing is the process of creep and stress relaxation. The soft rock formations in this test section will undergo significant creep strains when subjected to sustained loading. In contrast, if the water-swollen packer is held constant for a period of time and is in a fixed state of deformation, stress relaxation may occur, where stress decreases with time. Stress relaxation can be conceptually viewed as the reverse process of creep, in that creep is pressure controlled, while relaxation is volume controlled. Figure 3 As shown, 11 points were identified during the two casing fracturing stages where creep and / or stress relaxation effects could affect the two boundary conditions of the test.

[0069] After the casing fracturing stage, the upper and lower packers of the mobile test device were tested for leakage and hydraulic fracturing cycles across a certain depth. A leakage test was conducted and the leakage rate was relatively slow, which indicated that the formation had low permeability.

[0070] Figure 4 A record of injection rate and pressure over time for a hydraulic fracturing cycle test is shown. The hydraulic fracturing cycle test sequence analyzed here includes two fracturing cycles. The figure summarizes the initial leak test of the test, the first injection and drawdown test, the second injection and drawdown test, and the first and second rebound phases of the test. The first and second injection cycles were analyzed using the above methods to determine reopening, fracture propagation, transient shut-in pressure, and fracture closure pressure.

[0071] The project site was selected for analysis with reference to the stress polygon and the local fault mechanism to determine the horizontal component of the geostress. First of all, it is necessary to consider the stress state in the area where the project is located, where the friction coefficient is in the relative range of 0.6 to 1, independent of depth. In addition, it is necessary to consider the large difference between the sample size used for laboratory experiments and the actual fault size and the variability of the sliding surface roughness, and the friction coefficient should be higher than the value measured in the laboratory. It is also assumed that the geostress cannot exceed the friction strength that will activate the fault. Therefore, this parameter limits the possible range of stress magnitudes at any given depth and pore pressure. The stress polygon uses the Anderson stress and fault classification system to estimate the range of possible stress states at any given depth and pore pressure. It illustrates the range of allowed values ​​of the horizontal component of the principal stress under normal fault, strike-slip fault and reverse fault conditions. For this example, a hydrostatic pore pressure state is assumed. Figure 5 This is the stress polygon diagram of the test section of the pilot project at a certain depth.

[0072] Example 2

[0073] A method for determining rock stability, by using a horizontal ground stress measurement method to measure the ground stress in a target area, and comparing the ground stress value with the rock quality grade to determine the stability of the rock.

[0074] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for measuring horizontal ground stress, characterized in that: The following steps are involved: Step S1, performing casing fracturing, filling a packer with water to expand it at the target formation to generate an initial induced fracture, which serves as the starting point of the fracturing test; Step S2, using a straddle packer to place the initial induced fracture between the upper packer and the lower packer to form a test interval; Step S3, checking the sealing of the test area; Step S4, perform a leakage test. If the fluid diffuses into the formation, change the target formation and return to step S1; Step S5, performing a hydraulic fracturing cycle, obtaining test parameters, and obtaining an estimated value of ground stress using an interpretation method with pressure and time as variables; Step S6, based on the local fault mechanism, using the stress polygon method, determining the horizontal component of the estimated ground stress value; The inspection and test interval sealing includes: Step S301, filling the upper packer and the lower packer with water to expand until the pressure in the test interval begins to rise; Step S302, observing the subsequent pressure drop to check the sealing of the test area; The hydraulic fracturing cycle includes: Step S501, injecting fluid into the test interval in the wellbore at a constant flow rate, extending the fracture from the starting point of the fracturing test until the pressure is stable; Step S502, stop injection, let the pressure decay to a pressure level that ensures the closure of the crack, and return to step S501 until the set period is reached; During the hydraulic fracturing cycle, monitor the initiation, expansion, closure and re-initiation of induced fractures; The test parameters include fracture pressure, propagation pressure, instantaneous shut-in pressure, closing pressure, reopening pressure and rebound pressure; The interpretation method is as follows: an image is drawn with pressure and time as variables, and the inflection point of the curve in the image is taken as the minimum ground stress.

2. A method for measuring horizontal ground stress according to claim 1, characterized in that: The initial induced cracks are discontinuous vertical cracks.

3. A method for measuring horizontal ground stress according to claim 1, characterized in that: The leakage test is to inject fluid into the test interval in the wellbore at a constant flow rate to pressurize the wellbore to a critical value of pressure that causes formation rupture.

4. A method for measuring horizontal ground stress according to claim 1, characterized in that: The setting period is 2 to 5.

5. A method for measuring horizontal ground stress according to claim 1, characterized in that: The stress polygon method described uses the Anderson stress and fault classification system to estimate the range of stress states at any given depth and pore pressure.

6. A method for determining rock stability, characterized in that: By using the horizontal geostress measurement method described in any one of claims 1 to 5, the geostress of the target area is measured, and the rock quality grade is compared according to the geostress value, thereby determining the stability of the rock.

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