CN113075061A - System and method for testing shear stiffness of shale bedding interface - Google Patents
System and method for testing shear stiffness of shale bedding interface Download PDFInfo
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Abstract
The invention provides a test system and a test method for shear stiffness of a shale bedding interface, wherein the test system comprises a pressurizing module, a measuring module and an interface stiffness calculating module, and the pressurizing module can apply load to a rock core test piece so that the test piece is subjected to shear failure along a prefabricated crack; the measuring module can shoot a digital image of the surface of the test piece in the load application process and calculate and output position coordinates of specified calculation points at different moments; the interface shear stiffness calculation module can calculate and output the interface shear stiffness according to an interface shear stiffness formula algorithm. The testing method comprises the steps of processing or preparing a core test piece containing a prefabricated crack; applying load to the test piece by using a test system, and recording the whole process of the test piece fracture; and calculating according to an interface shear stiffness formula algorithm to obtain the interface shear stiffness of the specified position at all loading moments. The method has the advantages of simple operation process and accurate and reliable result, and the test result can be used for reasonably predicting the crack trend.
Description
Technical Field
The invention relates to the technical field of oil exploration and development, in particular to a system and a method for testing shear stiffness of a shale bedding interface.
Background
In view of reservoir characteristics of shale gas resources such as low porosity, hypotonicity and heterogeneity, a hydraulic fracturing production increasing technology becomes one of key technologies for development of the hydraulic fracturing production increasing technology, in the hydraulic fracturing process, a shale layered structure influences the fracture propagation behavior of hydraulic fracturing, the hydraulic fracture interlayer propagation relationship relates to the effect and success or failure of hydraulic fracturing, and the shear stiffness of a shale interface is the basis for developing hydraulic fracture interlayer propagation research.
The determination of the shear stiffness of the interlaminar interface of the layered shale is beneficial to providing guidance for some practical engineering problems, for example, in the hydraulic fracturing of shale gas, coal bed gas and thin interbedded oil and gas resources, the shear stiffness of the interface of the shale interface has a close relation with the formation of hydraulic fractures, and the formation of the hydraulic fractures determines the quality of the fracturing effect, and directly influences the oil and gas yield.
However, the existing test method for shear stiffness of shale bedding interface has no mature method, and the accuracy of the shear stiffness test cannot be guaranteed.
Disclosure of Invention
The present invention aims to address at least one of the above-mentioned deficiencies of the prior art. For example, one of the objectives of the present invention is to provide a shale bedding interface shear stiffness test system and test method with simple operation and reliable results, so as to make reasonable prediction of fracture strike.
In order to achieve the above object, in one aspect, the present invention provides a shale bedding interface shear stiffness testing system, which includes a pressurizing module, a measuring module and an interface shear stiffness calculating module, wherein the pressurizing module includes a press machine and a press machine control unit, the press machine includes a shear fixture, the shear fixture has two load-carrying members with two support members and two support members arranged in a staggered manner, the two support members can contact with the lower surface of a natural core test piece to form two symmetrically distributed support points, the two load-carrying members can contact with the upper surface of the natural core test piece to form two symmetrically distributed load application points, so that the natural core test piece can generate shear failure along a prefabricated crack under the action of shear load, the press machine control unit is connected with the press machine and configured to adjust the load application speed, and outputting the press machine load at different moments; the measuring module comprises a measuring camera, a control box and an image processing unit, wherein the measuring camera is arranged right in front of the press and can shoot digital images of the surface of the natural rock core test piece with the speckle patterns in the load application process, the control box is connected with the measuring camera and can control the measuring camera, and the image processing unit is connected with the control box and is configured to process the acquired digital images and calculate and output position coordinates of designated calculating points at designated positions at different loading moments; the interface shear stiffness calculation module is configured to process data output by the press control unit and the image processing unit, and calculate and output interface shear stiffness according to an interface shear stiffness formula algorithm.
In an exemplary embodiment of the shale bedding interface shear stiffness testing system of the present invention, the span between the two supports may be adjusted according to the size of the natural core test piece.
In an exemplary embodiment of the shale bedding interface shear stiffness test system of the present invention, the interface shear stiffness formula algorithm may be as shown in formula (1):
in the formula, KτIs interfacial shear stiffness, N/m3D tau is the variation of the interface shear stress, Pa, dv is the variation of the interface shear displacement, m.
In an exemplary embodiment of the shale bedding interface shear stiffness test system of the present invention, the interface shear stress variation may be obtained by subtracting shear stresses at the same position at two different times, and a calculation formula of the shear stress is as shown in formula (2):
in the formula, tau is shear stress, Pa, T are press load, N, A are cross section area of the test piece, m2。
In an exemplary embodiment of the shale bedding interface shear stiffness test system of the present invention, the calculation formula of the interface shear displacement variation may be as shown in formula (3):
dv＝|YM1-yMj|+|YNj-YN1| (3)
in the formula, YM1Is the coordinate value of the point M at the initial time on the Y axis, M, YMiIs the coordinate value of the point M at the j time point on the Y axis, M, YN1Is the coordinate value of the point N at the initial time on the Y axis, m, YNiIs the coordinate value of the point N at the j time point on the y axis, M, MThe point and the N point are designated calculation points selected in the image processing unit.
In an exemplary embodiment of the shale bedding interface shear stiffness test system, the test system may further include a data transmission device and a speckle spraying module, the data transmission device may connect the press machine control unit, the image processing unit and the interface shear stiffness calculation module, and the speckle spraying module may form the speckle pattern on the surface of the natural core test piece.
In an exemplary embodiment of the shale bedding interface shear stiffness testing system of the present invention, the press may comprise a servo hydraulic press, the measurement camera may comprise a high speed industrial measurement camera, and the interface shear stiffness calculation module comprises a terminal device processor.
The invention provides a method for testing shear stiffness of a shale bedding interface, which comprises the following steps: s1, sampling the shale core to be tested, processing or preparing a natural core test piece containing a preformed crack, wherein the crack surface of the preformed crack is superposed with the rock bedding surface and is vertical to the middle point of the long edge of the test piece, and a speckle pattern is prepared on one side surface of the test piece, which is provided with the preformed crack; s2, placing the test piece on a shearing clamp of a press machine, placing a measuring camera right in front of the test piece so that a lens of the measuring camera is right opposite to the side face of the test piece with the speckle pattern, controlling the press machine to apply shearing load to the test piece, simultaneously controlling the measuring camera to shoot the whole process of shearing damage of the test piece along the prefabricated crack, and acquiring a digital image; s3, calculating initial time t0And a loading time tjAnd subtracting the interface shear stress at the lower appointed position to obtain the interface shear stress variation, wherein t0≤tj≤tmJ denotes the time node of the crack propagation process, t0Initial time, t, indicating the beginning of the crack propagation processmIndicating the terminal time of the crack propagation process; s4, calculating initial time t0And a loading time tjThe position coordinates of two appointed calculation points of the lower appointed position are calculated according to a preset formula to obtain the interface shear displacement variation; s5. Dividing the interface shear stress variation and the interface shear displacement variation of the specified position to obtain the loading time t of the specified positionjLower interfacial shear stiffness; and S6, repeating the steps S3 to S5, and calculating to obtain the interface shear stiffness of the specified position at all loading moments.
In an exemplary embodiment of the method for testing shear stiffness of shale bedding interface of the present invention, the calculating of the initial time t0And a loading time tjThe method comprises the following steps of:
s41, selecting a calculation area which always contains cracks on the speckle surface, and establishing a rectangular coordinate system by taking the prefabricated cracks as coordinate origin, wherein the crack extension direction is defined as y-axis forward direction, and the direction which is vertical to the y-axis and horizontally rightward is defined as x-axis forward direction;
s42, taking a line segment M symmetrical along the y axis on the x axis1N1One and M are selected every k pixels along the y-axis1N1Straight lines with equal length and parallel, and so on, sequentially selecting parallel straight lines to the line segment MiNiLine segment M1N1To MiNiAll the positions are the designated positions, and i represents a selected several parallel straight lines;
s43, selecting two designated calculation points which are symmetrical along the y axis and spaced by l pixels on each parallel straight line, and obtaining the initial time t0And a loading time tjThe position coordinates of the next two designated calculation points;
s44, substituting the coordinates of the two specified calculation points on each parallel straight line into a predetermined formula of the interface shearing displacement variation, and calculating the interface shearing displacement variation corresponding to each parallel straight line respectively, wherein the predetermined formula of the interface shearing displacement variation is shown as the following formula:
dv＝|YM1-yMj|+|YNj-YN1|
in the formula, YM1Is an initial time t0Coordinate value of lower designated calculation point M on y axis，m，YMjFor loading time tjCoordinate values of the calculation point M on the Y-axis, M, Y, are specifiedN1Is an initial time t0Coordinate values of the calculation point N on the Y-axis, m, Y, are specifiedNjFor loading time tjThe coordinate value, m, of the calculation point N on the y-axis is specified below.
In an exemplary embodiment of the shear stiffness test method of the shale bedding interface, k is more than or equal to 2 and less than or equal to 10, l is more than or equal to 1 and less than or equal to 4, and i is more than or equal to 5.
Compared with the prior art, the beneficial effects and advantages of the invention comprise at least one of the following:
(1) the shear stiffness of the shale bedding interface obtained by the method has important significance for simulating fracture propagation path judgment, and can reasonably predict the fracture trend;
(2) according to the method, for the laminar shale with the weak interface, a four-point bending test (FPB) and a digital image correlation method (DIC) are combined, shear damage is generated on the shale physical interface through the four-point bending test, the DIC is used for obtaining the shear deformation, and the shear rigidity is obtained through calculation according to a formula. The operation process is simple, the result is accurate and reliable, and technical reference can be provided for related work in the field of shale gas hydraulic fracturing.
Drawings
FIG. 1 shows a schematic diagram of a speckle pattern in an exemplary embodiment of the invention;
FIG. 2 illustrates a structural diagram of coordinate axes of a selected computing area in an exemplary embodiment of the invention;
FIG. 3 illustrates a diagram of specifying symmetric computation points in an exemplary embodiment of the invention;
FIG. 4 shows a schematic diagram of the specimen loading principle of a four-point bending test in an exemplary embodiment of the invention;
FIG. 5 is a schematic illustrating shale bedding and cores in different cut orientations in an exemplary embodiment of the invention;
FIG. 6 illustrates a schematic of the location of a prepared fracture and rock bedding plane in an exemplary embodiment of the invention;
FIG. 7 illustrates a schematic structural diagram of an interface shear stiffness testing system in an exemplary embodiment of the invention.
The reference numerals are explained below:
the method comprises the following steps of 1-a servo hydraulic press, 2-a press control unit, 3-a high-speed industrial measurement camera, 4-a control box, 5-an image processing unit, 6-an interface shear stiffness calculation module and 7-a data transmission device.
Detailed Description
Hereinafter, the test method of the shale bedding interface shear stiffness test system according to the present invention will be described in detail with reference to the exemplary embodiments and the accompanying drawings.
The invention provides a shear stiffness test system for a shale bedding interface.
In an exemplary embodiment of the shale bedding interface shear stiffness testing system of the present invention, the shale bedding interface shear stiffness testing system may include a pressurization module, a measurement module, and an interface shear stiffness calculation module.
In particular, the pressurizing module comprises a press and a press control unit. The press machine comprises a shearing clamp, wherein the shearing clamp is provided with two supporting pieces and two loading pieces arranged in a staggered mode on the two supporting pieces. The two supporting pieces can be in contact with the lower surface of the natural core test piece to form two symmetrically distributed supporting points, and the two load-bearing pieces can be in contact with the upper surface of the natural core test piece to form two symmetrically distributed load application points, so that the natural core test piece can be subjected to shear failure along the prefabricated crack under the action of shear load. The press control unit is connected with the press and is configured to adjust the speed of load application and output the press load at different times.
The press machine is used for performing a four-point bending loading test on a natural core test piece to be tested to form shear failure, so that interface shear stress is obtained. The natural rock core test piece to be tested is placed in a shearing clamp of a press machine, and two supporting pieces and two loading pieces on the natural rock core test piece form four-point contact with the natural rock core test piece, so that a four-point bending loading rule is met. The span between the two supporting pieces can be adjusted according to the size of the natural rock core test piece. For example, the press may be a servo hydraulic press. The press machine control unit is used for recording the application process of the load, and when the load of the load piece acting on the upper surface of the natural rock core test piece is applied to a certain value, the natural rock core test piece is sheared and damaged along the prefabricated crack.
The measuring module comprises a measuring camera, a control box and an image processing unit. The measuring camera is arranged right in front of the press machine and can shoot digital images of the surface of the natural rock core test piece with the speckle patterns in the load applying process. The control box is connected with the measuring camera and can control the measuring camera. The image processing unit is connected with the control box and is configured to process the acquired digital images and calculate and output position coordinates of specified calculation points at different times.
The speckle pattern is a pattern manufactured by using black and white spray paints on a natural rock core test piece to be measured so as to ensure that the shearing deformation displacement can be obtained by using a Digital Image Correlation (DIC) method. Specifically, one side of the test piece is sprayed with white paint to be white, then black paint is used for slightly spraying granular paint liquid drops at a position away from the surface of the test piece by a certain distance, the paint liquid drops float on the white paint to form a speckle pattern, and the sprayed speckle pattern is shown in fig. 1.
The image processing unit is used for calculating and analyzing the acquired digital image by using image processing software to obtain the coordinate position of a specified calculation point of a specified position on a rock stratum interface at any moment so as to obtain the shearing displacement through coordinate calculation in the following process. The calculation flow of the image processing software comprises the following contents:
(1) firstly, a calculation area is selected on a scattered spot surface, the calculation area always contains formed cracks in the test piece damage process, and a rectangular coordinate system is established. The established rectangular coordinate system can be as shown in fig. 2, with the tip of the pre-fabricated fracture as the origin of coordinate, defining the direction in which the fracture extends and expands upward along the rock interface (i.e. the direction in which the fracture tip points to the non-fractured interface) as the y-axis forward direction, and the direction perpendicular to the y-axis and horizontally rightward as the x-axis forward direction, where the y-axis is the location of the interface.
(2) Then, a line segment M with the y axis as the symmetry axis is taken on the x axis1N1(as shown in fig. 2), the length of the line segment depends on the size of the pixels of the camera and the size of the selected calculation area, and on the basis, one line segment is taken every k pixels and M is taken every k pixels along the y-axis direction1N1Straight lines M of equal and parallel length2N2Continuing to select parallel straight lines to M by analogyiNi. k can range from 2 to 10, for example, k can be 4, i.e., one for every 4 pixels and M1N1Straight lines of equal and parallel length.
(3) After the parallel straight lines are selected, each parallel straight line (M)1N1To MiNi) Two calculation points with the y axis as the symmetry axis are selected, and the two calculation points are separated by one pixel, so that the distances between all the symmetry points and the interface are the same. When the designated calculation point is selected and piled, the pixel point between the two points is reasonable, and the two points are ensured to be symmetrically distributed on two sides of the cementing surface and have a sufficiently close distance. The value range of l can be 1-4. For example, as line M in FIG. 31N1Taking the two symmetric points M and N as an example, l may take 2, i.e. the designated computation point M and the designated computation point N may be separated by 2 pixels.
After the parallel straight lines and the designated calculation points on the parallel straight lines are selected, the digital image can be calculated and analyzed by using image processing software. And calculating and analyzing by taking each parallel straight line as one calculation. In a straight line M1N1For example, line M1N1The calculated points are designated as M and N points, assuming a shear stress of τ1Increase to tau2In the process, after the designated calculating point M and the designated calculating point N at all times are analyzed by using image processing software, the position coordinates of the designated calculating point M and the designated calculating point N at any time can be automatically calculated and output. The obtained coordinate positions of the calculation point M and the calculation point N can be used for subsequently calculating the size of the shearing displacement.
The interface shear stiffness calculation module can process data output by the press control unit and the image processing unit, and can calculate and output interface shear stiffness according to an interface shear stiffness formula algorithm. For example, the interface shear stiffness calculation module may be a terminal device processor configured with an interface shear stiffness formula algorithm, and capable of calculating the output interface shear stiffness according to the recorded data of the press load input by the press control unit and the position coordinates of the specified calculation point input by the image processing unit.
Here, the interfacial shear stiffness formula algorithm may be represented by formula (1):
in the formula, KτIs interfacial shear stiffness, N/m3D tau is the variation of the interface shear stress, Pa, dv is the variation of the interface shear displacement, m.
The interface shear stress variation can be obtained by subtracting the shear stresses at the same position at two different times, and the calculation formula of the shear stress is as shown in formula (2):
in the formula, tau is shear stress, Pa, T are press load, N, A are cross section area of the test piece, m2. It should be noted that T is the recorded data of the press load, and is also a function of the loading time. Therefore, the interfacial shear stress can be calculated under a certain load at a certain moment, when the shear stress is t1Calculating the time to obtain the interface shear stress at the time, and calculating the shear stress at t2The shear stress of the interface is calculated again at any moment, and the shear stress variation d of the corresponding position can be obtained by subtracting the two distributed stressesτ。
The interface shearing displacement variation can be obtained by calculating the coordinates of the specified calculation points at two different times, and the calculation formula of the interface shearing displacement variation can be shown as formula (3):
dv＝|YM1-yMj|+|YNj-YN1| (3)
in the formula, YM1Is the coordinate value of the point M at the initial time on the Y axis, M, YMjIs the coordinate value of the point M at the j time point on the Y axis, M, YN1Is the coordinate value of the point N at the initial time on the Y axis, m, YNjAnd the coordinate value of the point N at the j time on the y axis, and the points M, M and N are designated calculation points selected in the image processing unit.
At this time, the tensile stress variation and the interface corresponding displacement variation on all the parallel straight lines are measured by experiments, and the interface shear stiffness can be calculated by substituting the tensile stress variation and the interface corresponding displacement variation into a definition formula (namely formula (1)) of the interface shear stiffness.
In another exemplary embodiment of the present invention, the test system may further include a data transmission device and a speckle spray module. The data transmission device can connect the press machine control unit and the image processing unit with the interface shear stiffness calculation module. The speckle pattern can be formed on the surface of the natural rock core test piece by the speckle spraying module.
The invention further provides a method for testing shear stiffness of the shale bedding interface.
In an exemplary embodiment of the shale bedding interface shear stiffness test method of the present invention, the test method may comprise the steps of:
s1, sampling the shale core to be tested, processing or preparing a natural core test piece containing a preformed crack, wherein the crack surface of the preformed crack is coincident with the rock bedding surface and is vertical to the middle point of the long edge of the test piece, and a speckle pattern is made on one side surface of the test piece, which is provided with the preformed crack.
S2, placing the test piece on a shearing clamp of a press machine, placing a measuring camera right in front of the test piece so that a lens of the measuring camera is right opposite to the side face of the test piece with the speckle pattern, controlling the press machine to apply shearing load to the test piece, simultaneously controlling the measuring camera to shoot the whole process that the test piece is sheared and damaged along the prefabricated crack, and collecting digital images.
S3, calculating initial time t0And a loading time tjAnd subtracting the interface shear stress at the lower appointed position to obtain the interface shear stress variation, wherein t0≤tj≤tmJ denotes the time node of the crack propagation process, t0Initial time, t, indicating the beginning of the crack propagation processmIndicating the end time at which the crack propagation process is ended.
S4, calculating initial time t0And a loading time tjAnd the position coordinates of two appointed calculation points at the lower appointed position are calculated according to a preset formula to obtain the interface shear displacement variation. Specifically, the specific implementation steps for obtaining the interface shear displacement variation may include the following steps:
s41, selecting a calculation area which always contains the crack on the speckle surface, and establishing a rectangular coordinate system by taking the prefabricated crack as a coordinate origin, wherein the extension direction of the crack (namely the direction in which the crack tip points to the crack interface which does not appear) is defined as the y-axis forward direction, and the direction which is vertical to the y-axis and horizontally rightwards is defined as the x-axis forward direction.
S42, taking a line segment M symmetrical along the y axis on the x axis1N1One and M are selected every k pixels along the y-axis1N1Straight lines with equal length and parallel, and so on, sequentially selecting parallel straight lines to the line segment MiNiLine segment M1N1To MiNiAll the positions are the designated positions, and i represents the selected several parallel straight lines. Here, k is 2. ltoreq. k.ltoreq.10, i.gtoreq.5.
S43, selecting two designated calculation points which are symmetrical along the y axis and spaced by l pixels on each parallel straight line, and obtaining the initial time t0And a loading time tjThe next two specify the position coordinates of the calculated points. Here, 1. ltoreq. l.ltoreq.4.
S44, substituting the coordinates of the two specified calculation points on each parallel straight line into a predetermined formula of interface shearing displacement variation, and calculating the interface shearing displacement variation corresponding to each parallel straight line respectively, wherein the predetermined formula of the interface shearing displacement variation is shown as the following formula:
dv＝|YM1-yMj|+|YNj-YN1|
in the formula, YM1Is an initial time t0Coordinate values of the calculation point M on the Y-axis, M, Y, are specifiedMjFor loading time tjCoordinate values of the calculation point M on the Y-axis, M, Y, are specifiedN1Is an initial time t0Coordinate values of the calculation point N on the Y-axis, m, Y, are specifiedNjFor loading time tjThe coordinate value, m, of the calculation point N on the y-axis is specified below.
S5, dividing the interface shear stress variation and the interface shear displacement variation of the designated position to obtain the loading time t of the designated positionjLower interfacial shear stiffness.
And S6, repeating the steps S3 to S5, and calculating to obtain the interface shear stiffness of the specified position at all loading moments.
For a better understanding of the above-described exemplary embodiments of the present invention, reference is made to the following description taken in conjunction with the accompanying drawings and specific examples.
Before a four-point bending test is carried out, a shale core to be tested needs to be obtained, the size and shape of the shale core are required to meet the size specified by the measurement specification of related industries, and factors such as the size, the shape and the prefabricated crack depth ratio of a test piece are required to be considered in the processing of the shale core.
For example, considering that the cylindrical curved surface is not beneficial to data acquisition and processing of the measuring camera, in order to facilitate the measuring camera to acquire the surface image of the shale core in the loading process, the natural core test piece can uniformly adopt a cuboid shape, and the depth ratio alpha/H of the prefabricated fracture can be 0.2. According to the analysis of the experimental results, the cutting size of the shale core can adopt that the length (S) multiplied by the width (B) multiplied by the height (H) is 240mm multiplied by 50mm, the span (L) is 200mm when loading, and the prefabricated crack a0And the thickness is 10mm, the plane of the prefabricated crack is the plane of the shale bedding plane, and four-point bending loading is adopted, as shown in figure 4.
In addition, for the shale with the interface, different sampling cutting directions are adopted, and the obtained natural core test pieces are different. By combining the four-point bending loading law, sampling can be carried out in the 90-degree direction or the 0-degree direction shown in fig. 5, and the bedding surface of the obtained cuboid shale core is ensured to be parallel to the left side surface and the right side surface of the cuboid and to be vertical to the front surface, the rear surface and the upper surface. On the basis, a preformed fracture is formed in the position, parallel to the two side faces, of the midpoint of the front face of the cuboid shale core, and the formed preformed fracture is overlapped with the bedding face of the shale, as shown in fig. 6. In addition, in order to better realize the shearing of the test piece and ensure the crack to initiate at the interface, a prefabricated crack can be processed on the upper surface and the lower surface of the natural rock core test piece.
After a to-be-tested piece is obtained, the speckle pattern is required to be manufactured on one surface of the to-be-tested piece of the rock core, and the test can be carried out after the paint is air-dried.
As shown in fig. 7, the shale bedding interface shear stiffness test system comprises a servo hydraulic machine 1, a press control unit 2, a high-speed industrial measurement camera 3, a control box 4, an image processing unit 5, an interface shear stiffness calculation module 6 and a data transmission device 7. The servo hydraulic machine 1 and the press machine control unit 2 control the servo hydraulic machine 1 to shear and destroy a piece to be tested through the press machine control unit 2. The high-speed industrial measurement camera 3 is arranged on a tripod and is placed right in front of the servo hydraulic machine 1, so that the side, with the speckle pattern, of the core to-be-tested piece faces the high-speed industrial measurement camera 3. The control box 4 is respectively connected with the high-speed industrial measurement camera 3 and the image processing unit 5, and is used for controlling the high-speed industrial measurement camera 3 to photograph the shearing and breaking process of the piece to be tested and inputting the acquired digital image to the image processing unit 5. The number of the data transmission devices 7 is two, and the data transmission devices are respectively connected between the press machine control unit 2 and the interface shear stiffness calculation module 6 and between the image processing unit 5 and the interface shear stiffness calculation module 6 so as to transmit the data output by the press machine control unit 2 and the image processing unit 5 to the interface shear stiffness calculation module 6.
After the shale bedding interface shear stiffness test system is built, the servo hydraulic machine 1 and the high-speed industrial measurement camera 3 are started simultaneously, and load application at a certain speed is carried out on the test piece until the test piece is damaged. In the process of applying the load, the press machine control unit 2 records the applying process of the load, and the high-speed industrial measurement camera 3 records the damage process of the test piece.
In this example, the shale bedding interface shear stiffness test method may include the steps of:
step 1: sampling a shale core to be tested, wherein the cutting size of the shale core is 240mm in length, 50mm in width and 50mm in height. And (3) obtaining a natural core test piece containing a prefabricated crack by adopting linear cutting processing or preparation, wherein the depth of the prefabricated crack is 10mm, and the depth ratio alpha/H of the prefabricated crack is 0.2. 2 prefabricated cracks can be formed and are respectively formed on the upper surface and the lower surface of the natural rock core test piece, and the crack surface of the prefabricated crack is coincided with the rock bedding surface and is perpendicular to the middle point of the long edge of the test piece. And manufacturing a speckle pattern on one side surface of the test piece with the prefabricated crack by using the speckle spraying module.
Step 2: and (5) building a shear stiffness test system of the shale bedding interface. And placing the test piece on a shearing clamp of a press machine, wherein the loading span of the test piece in the press machine is 200 mm. And placing a measuring camera right ahead the test piece so that a lens of the measuring camera is right opposite to the side face of the test piece with the speckle pattern, controlling a press machine to apply shearing load to the test piece, simultaneously controlling the measuring camera to shoot the whole process that the test piece is sheared and damaged along the prefabricated crack, and acquiring a digital image.
Step 3: calculating an initial time t0And a loading time tjAnd subtracting the interface shear stress at the lower appointed position to obtain the interface shear stress variation, wherein t0≤tj≤tmJ denotes the time node of the crack propagation process, t0Initial time, t, indicating the beginning of the crack propagation processmIndicating the end time at which the crack propagation process is ended. Wherein the shear stress is calculated as follows:
in the formula, tau is shear stress, Pa, T are press load, N, A are cross section area of the test piece, m2。
Step 4: calculating an initial time t0And a loading time tjLower fingerAnd (3) the position coordinates of two appointed calculation points of the fixed position are calculated according to a preset formula to obtain the interface shear displacement variation. Specifically, the calculation process of the interface shear displacement variation may include the following:
(1) and selecting a calculation area on the speckle surface, wherein the calculation area always contains formed cracks in the test piece damage process, and establishing a rectangular coordinate system. The established rectangular coordinate system takes the tip of the prefabricated crack as the origin of coordinates, the direction of the crack extending and expanding upwards along the rock interface (namely the direction of the crack tip pointing to the crack interface without crack) is defined as the positive direction of the y axis, and the direction which is vertical to the y axis and horizontally faces right is defined as the positive direction of the x axis.
(2) Taking a line segment M on the x axis with the y axis as the symmetry axis1N1(as shown in FIG. 2), the length of the line segment depends on the size of the camera pixels and the size of the selected calculation area, and on the basis of the length, one is taken for every 3 pixels (i.e. k can be 3) in the y-axis direction and M is taken for every 3 pixels1N1Parallel line segments M of equal length2N2Continuing to select parallel straight lines to the line segment M by analogyiNi. Here, i can take 8, i.e. a total of 8 parallel lines of equal length (line segment M)1N1To M8N8)。
(3) After the parallel straight lines are selected, each parallel straight line (line segment M)1N1To M8N8) Two calculation points with the y-axis as the symmetry axis are selected, the interval between the two calculation points is 2 (i.e. one can take 2) pixels, and the initial time t is obtained through calculation0And a loading time tjThe position coordinates of two specified calculation points on each next parallel straight line.
(4) Each parallel straight line (line segment M)1N1To M8N8) Substituting the coordinates of the two specified calculation points into a predetermined formula of the interface shearing displacement variation, and respectively calculating the interface shearing displacement variation corresponding to each parallel straight line, wherein the predetermined formula of the interface shearing displacement variation is shown as the following formula:
dv＝|YM1-yMj|+|YNj-YN1|
in the formula, YM1Is an initial time t0Coordinate values of the calculation point M on the Y-axis, M, Y, are specifiedMjFor loading time tjCoordinate values of the calculation point M on the Y-axis, M, Y, are specifiedN1Is an initial time t0Coordinate values of the calculation point N on the Y-axis, m, Y, are specifiedNjFor loading time tjThe coordinate value, m, of the calculation point N on the y-axis is specified below.
Step 5: substituting the interface shear stress variation and the interface shear displacement variation of the specified position into an interface shear stiffness formula, and calculating to obtain the loading time t of the specified positionjLower interfacial shear stiffness. The interfacial shear stiffness formula algorithm can be shown as follows:
in the formula, KτIs interfacial shear stiffness, N/m3D tau is the variation of the interface shear stress, Pa, dv is the variation of the interface shear displacement, m.
Step 6: and repeating the steps from Step3 to Step5, and calculating to obtain the interface shear stiffness of the specified position at all loading moments.
In summary, the beneficial effects and advantages of the invention include:
(1) the shear stiffness of the shale bedding interface obtained by the method has important significance for simulating fracture propagation path judgment, and can reasonably predict the fracture trend;
(2) according to the method, for the laminar shale with the weak interface, a four-point bending test (FPB) and a digital image correlation method (DIC) are combined, shear damage is generated on the shale physical interface through the four-point bending test, the DIC is used for obtaining the shear deformation, and the shear rigidity is obtained through calculation according to a formula. The operation process is simple, the result is accurate and reliable, and technical reference can be provided for related work in the field of shale gas hydraulic fracturing.
Although the present invention has been described above in connection with the exemplary embodiments and the accompanying drawings, it will be apparent to those of ordinary skill in the art that various modifications may be made to the above-described embodiments without departing from the spirit and scope of the claims.
Claims (10)
1. A shale bedding interface shear stiffness test system is characterized by comprising a pressurizing module, a measuring module and an interface shear stiffness calculating module, wherein,
the pressurizing module comprises a press machine and a press machine control unit, the press machine comprises a shearing clamp, the shearing clamp is provided with two supporting pieces and two load-carrying pieces which are arranged in a staggered mode with the two supporting pieces, the two supporting pieces can be in contact with the lower surface of a natural rock core test piece to form two supporting points which are symmetrically distributed, the two load-carrying pieces can be in contact with the upper surface of the natural rock core test piece to form two load applying points which are symmetrically distributed, so that the natural rock core test piece can be subjected to shearing failure along a prefabricated crack under the action of shearing load, and the press machine control unit is connected with the press machine and is configured to be capable of adjusting the load applying speed and outputting the load of the press machine at different moments;
the measuring module comprises a measuring camera, a control box and an image processing unit, wherein the measuring camera is arranged right in front of the press and can shoot digital images of the surface of the natural rock core test piece with the speckle patterns in the load application process, the control box is connected with the measuring camera and can control the measuring camera, and the image processing unit is connected with the control box and is configured to process the acquired digital images and calculate and output position coordinates of designated calculating points at designated positions at different loading moments;
the interface shear stiffness calculation module is configured to process data output by the press control unit and the image processing unit, and calculate and output interface shear stiffness according to an interface shear stiffness formula algorithm.
2. The shale bedding interface shear stiffness testing system of claim 1, wherein a span between the two support members is adjustable according to a size of a natural core test piece.
3. The shale bedding interface shear stiffness test system of claim 1, wherein the interface shear stiffness formula algorithm is as shown in equation (1):
in the formula, KτIs interfacial shear stiffness, N/m3D tau is the variation of the interface shear stress, Pa, dv is the variation of the interface shear displacement, m.
4. The shale bedding interface shear stiffness test system of claim 3, wherein the interface shear stress variation is obtained by subtracting shear stresses at the same position at two different times, and the shear stress calculation formula is as shown in formula (2):
in the formula, tau is shear stress, Pa, T are press load, N, A are cross section area of the test piece, m2。
5. The shale bedding interface shear stiffness test system of claim 3, wherein the calculation formula of the interface shear displacement variation is as shown in formula (3):
dv＝|YM1-YMj|+|YNj-YN1| (3)
in the formula, YM1Is the coordinate value of the point M at the initial time on the Y axis, M, YMjIs the coordinate value of the point M at the j time point on the Y axis, M, YN1Is the coordinate value of the point N at the initial time on the Y axis, m, YNjAnd the coordinate value of the point N at the j time on the y axis, and the points M, M and N are designated calculation points selected from the image processing unit.
6. The shale bedding interface shear stiffness test system of claim 1, further comprising a data transmission device and a speckle spraying module, wherein the data transmission device is capable of connecting the press machine control unit, the image processing unit and the interface shear stiffness calculation module, and the speckle spraying module is capable of forming the speckle pattern on the surface of the natural core test piece.
7. The shale bedding interface shear stiffness testing system of claim 1, wherein the press comprises a servo hydraulic press, the measurement camera comprises a high speed industrial measurement camera, and the interface shear stiffness calculation module comprises a terminal device processor.
8. A shear stiffness test method for a shale bedding interface is characterized by comprising the following steps:
s1, sampling the shale core to be tested, processing or preparing a natural core test piece containing a preformed crack, wherein the crack surface of the preformed crack is superposed with the rock bedding surface and is vertical to the middle point of the long edge of the test piece, and a speckle pattern is prepared on one side surface of the test piece, which is provided with the preformed crack;
s2, placing the test piece on a shearing clamp of a press machine, placing a measuring camera right in front of the test piece so that a lens of the measuring camera is right opposite to the side face of the test piece with the speckle pattern, controlling the press machine to apply shearing load to the test piece, simultaneously controlling the measuring camera to shoot the whole process of shearing damage of the test piece along the prefabricated crack, and acquiring a digital image;
s3, calculating initial time t0And a loading time tjAnd subtracting the interface shear stress at the lower appointed position to obtain the interface shear stress variation, wherein t0≤tj≤tmJ denotes the time node of the crack propagation process, t0Initial time, t, indicating the beginning of the crack propagation processmIndicating the terminal time of the crack propagation process;
s4, calculating initial time t0And a loading time tjThe position coordinates of two appointed calculation points of the lower appointed position are calculated according to a preset formula to obtain the interface shear displacement variation;
s5, dividing the interface shear stress variation and the interface shear displacement variation of the designated position to obtain the loading time t of the designated positionjLower interfacial shear stiffness;
and S6, repeating the steps S3 to S5, and calculating to obtain the interface shear stiffness of the specified position at all loading moments.
9. The method for testing shear stiffness of a shale bedding interface of claim 8, wherein the initial time t is calculated0And a loading time tjThe method comprises the following steps of:
s41, selecting a calculation area which always contains cracks on the speckle surface, and establishing a rectangular coordinate system by taking the prefabricated cracks as coordinate origin, wherein the crack extension direction is defined as y-axis forward direction, and the direction which is vertical to the y-axis and horizontally rightward is defined as x-axis forward direction;
s42, taking a line segment M symmetrical along the y axis on the x axis1N1One and M are selected every k pixels along the y-axis1N1Straight lines with equal length and parallel, and so on, sequentially selecting parallel straight lines to the line segment MiNiLine segment M1N1To MiNiAll the positions are the designated positions, and i represents a selected several parallel straight lines;
s43, selecting two designated calculation points which are symmetrical along the y axis and spaced by l pixels on each parallel straight line, and obtaining the initial time t0And a loading time tjThe position coordinates of the next two designated calculation points;
s44, substituting the coordinates of the two specified calculation points on each parallel straight line into a predetermined formula of the interface shearing displacement variation, and calculating the interface shearing displacement variation corresponding to each parallel straight line respectively, wherein the predetermined formula of the interface shearing displacement variation is shown as the following formula:
dv＝|YM1-YMj|+|YNj-YN1|
in the formula, YM1Is an initial time t0Coordinate values of the calculation point M on the Y-axis, M, Y, are specifiedMjFor loading time tjCoordinate values of the calculation point M on the Y-axis, M, Y, are specifiedN1Is an initial time t0Coordinate values of the calculation point N on the Y-axis, m, Y, are specifiedNjFor loading time tjThe coordinate value, m, of the calculation point N on the y-axis is specified below.
10. The method for testing shear stiffness of the shale bedding interface according to claim 9, wherein k is more than or equal to 2 and less than or equal to 10, l is more than or equal to 1 and less than or equal to 4, and i is more than or equal to 5.
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