CN107589020B - A kind of hydraulic fracturing test method based on stress path - Google Patents

Abstract

The hydraulic fracturing test method based on stress path of the present invention can simulate the hydraulic fracturing process under shale different structure face inclination angle, different three-dimensional stress states and different disturbance loads, and pressure break test specimen and bulk testing process have typical similitude with fractured in-situ shale reservoir.According to the experimental design without stress path, unidirectional supercharging decompression, bidirectional booster decompression disturbance crushing test are done under the load stress state according to the different stage of geology depth setting.Shale fracturing process under true triaxial state disturbance load action of the present invention and fracturing features are laid a good foundation to understand and grasp shale volume fracturing effect research under the influence of additional profiled bar, while can provide theoretical and technical support for live multistage more well pressure breaks.

Description

Stress path-based hydraulic fracturing test method

Technical Field

The invention relates to the technical field of hydraulic fracturing yield increase in shale reservoir transformation, in particular to a hydraulic fracturing test method based on a stress path.

Background

Along with the gradual increase of the exploration and exploitation strength of shale gas in China, the large-scale volume fracturing of shale reservoirs is increasingly regarded as a core scientific technology influencing the productivity. The permeability of the sea-land facies sedimentary shale and shale reservoir matrix in China is extremely low, the physical property difference of the shale is large, and the brittleness of the shale is lower than that of the North American shale, so that the formation of a three-dimensional seam network of the shale reservoir in fracturing operation is restricted, a shale airflow channel cannot be effectively formed, and the purpose of industrial exploitation cannot be achieved.

Aiming at the problems, a new process and a fracturing means are imperative. The horizontal well is segmented and clustered, and the volume fracturing of multiple wells is synchronous or pulse method, so that the shale reservoir is effectively improved. The novel volume fracturing methods in the starting stage can generate pressure disturbance on nearby shale bodies in the fracturing process of a reservoir stratum, change the crustal stress environment of the reservoir stratum, and aim to realize and control the formation and the steering of fracturing fractures and achieve the formation of an effective three-dimensional fracture network. On the basis of a new fracturing process, a large number of scholars point out the importance of the disturbance load effect on the improvement of the yield of the effective volume of the reservoir through researches on the superposition effect of a fracturing reservoir stress field, the density and the surface area of a hydraulic fracturing fracture network, the mechanical conditions of the diversion of the fracture to form the reticular fracture and the like.

However, the real shale reservoir is subjected to the superposition of stress fields such as structural stress, fluid pressure, gravity, thermal stress and the like, the stress condition is complex and changeable, stress shadow is possibly generated among all sections to cause stress offset in multi-well or multi-stage fracturing transformation, the extension of cracks is prevented to a certain extent, and the volume of the reservoir is reduced by pressing open. The condition is avoided as much as possible in the process of reservoir fracturing modification, the ground stress change condition of a fractured reservoir is monitored in real time, the injection pressure of the fluid is adjusted timely, and the multi-stage and multi-well fracturing implementation scheme is reasonably designed. The influence of the complex stress field on the fracturing modification effect of the shale reservoir is researched by utilizing an indoor simulation test, and the indoor simulation test for the fracturing modification of the shale by utilizing the complex stress field can be used for guiding field fracturing, but at present, no indoor simulation test for the fracturing modification of the shale by utilizing the complex stress field exists.

Disclosure of Invention

The invention aims to solve the technical problem that no indoor simulation test aiming at the complicated stress field for shale fracturing reformation exists at present.

The invention aims to provide a stress path-based hydraulic fracturing test method, which combines the original crustal stress state of a shale reservoir and an indoor simulated crustal stress environment, provides theoretical and technical support for on-site multistage multi-well fracturing, and improves the fracturing modification effect of the reservoir.

A hydraulic fracturing test method based on stress paths comprises the following steps:

cutting natural rock blocks into cubic rock blocks with preset sizes, wrapping concrete with a material similar to that of the natural rock outside the cubic rock blocks, and manufacturing a fracturing test piece;

drilling a circular hole with a preset diameter and depth at the center of the fracturing test piece to serve as a shaft, installing a fracturing fluid injection pipe in the shaft, sealing and fixing the injection pipe, and keeping a preset distance between the bottom of the injection pipe and the bottom of the circular hole;

and setting a true triaxial state three-way load according to the original ground stress state of the rock mass, and performing a hydraulic fracturing test on the fracturing test piece under the action of the true triaxial state three-way load.

The hydraulic fracturing test under true triaxial state three-dimensional loading effect includes that the hydraulic fracturing test under true triaxial state three-dimensional constant loading effect, the hydraulic fracturing test under true triaxial state three-dimensional constant loading effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the first horizontal load, the second horizontal load, and the vertical load;

applying the maximum value of the true triaxial state three-way load to the fracturing test piece and keeping the maximum value unchanged, and simultaneously injecting fracturing fluid into the shaft at a preset speed by using a fracturing fluid injection pipe until the test piece is fractured;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

The hydraulic fracturing test under true triaxial state three-way load effect includes that the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect, the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore; applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece and keeping the maximum values unchanged, injecting fracturing fluid into the shaft at the initial value of injection pressure by using an injection pipe, applying a second horizontal load to the fracturing test piece, gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each stage are the same or different, the load of each stage is kept for a preset time period, testing whether the test piece breaks, and if the test piece breaks, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore from an initial value to a first preset pressure and keeping the same, and simultaneously carrying out the following first unloading and loading operations: directly unloading the second horizontal load from the maximum value to a preset value, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the shaft from a first preset pressure to a second preset pressure, simultaneously carrying out the first unloading and loading operation, testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft and keeping the injection pressure for a preset time period, and simultaneously carrying out the first unloading and loading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

The hydraulic fracturing test under true triaxial state three-dimensional load effect includes that the hydraulic fracturing test under true triaxial state one-way step-down disturbance load effect, the hydraulic fracturing test under true triaxial state one-way step-down disturbance load effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: a vertical load, a maximum of the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection tube into the wellbore;

applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece and keeping the maximum values unchanged, injecting fracturing fluid into the shaft at the initial value of injection pressure by using an injection pipe, applying the maximum value of a second horizontal load to the fracturing test piece, reducing the second horizontal load from the maximum value to a preset value step by step according to preset step values, wherein the preset step values of each step are the same or different, the load of each step is kept for a preset time period, testing whether the test piece breaks or not, and if the test piece breaks, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and simultaneously carrying out the following first loading and unloading operations: the second horizontal load is loaded to the maximum value of the second horizontal load again, and then the second horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a second preset pressure and keeping the second preset pressure unchanged, simultaneously carrying out the first loading and unloading operation, testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft and keeping the injection pressure for a preset time period, and simultaneously carrying out the first loading and unloading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

The hydraulic fracturing test under true triaxial state three-way load effect includes that the hydraulic fracturing test under true triaxial state two-way pressure boost disturbance load effect, the hydraulic fracturing test under true triaxial state two-way pressure boost disturbance load effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore;

applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged; injecting fracturing fluid into the shaft at an initial value of injection pressure by using an injection pipe; applying a second horizontal load to the fracturing test piece, and gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; simultaneously applying a first horizontal load to the fracturing test piece, and gradually increasing the first horizontal load from 0 to the maximum value of the first horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and carrying out the following first unloading and loading operation: directly unloading the second horizontal load from the maximum value to a preset value, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

simultaneously performing the following second unloading and loading operation on the first horizontal load: directly unloading the first horizontal load from the maximum value to a preset value, and then gradually increasing the first horizontal load to the maximum value of the first horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a second preset pressure and keeping the injection pressure unchanged, simultaneously performing the first unloading and loading operation and the second unloading and loading operation, testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the well bore and keeping the injection pressure for a preset time period, and simultaneously carrying out the first unloading and loading operation and the second unloading and loading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

The hydraulic fracturing test under the action of true triaxial state three-dimensional load includes: the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of the true triaxial state, the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of the true triaxial state comprises:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore;

applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged; injecting fracturing fluid into the shaft at an initial value of injection pressure by using an injection pipe; applying the maximum value of a second horizontal load to the fracturing test piece, and gradually reducing the second horizontal load from the maximum value to a preset value according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; simultaneously, applying the maximum value of the first horizontal load to the fracturing test piece, and gradually reducing the first horizontal load from the maximum value to a preset value according to preset step values, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and carrying out the following first loading and unloading operations: the second horizontal load is loaded to the maximum value of the second horizontal load again, and then the second horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

simultaneously carrying out the following second loading and unloading operations on the first horizontal load: the first horizontal load is loaded to the maximum value of the first horizontal load again, then the first horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the well bore to a second preset pressure and keeping the same, and carrying out the first loading and unloading operation on the second horizontal load and simultaneously carrying out the second loading and unloading operation on the first horizontal load; testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft for a preset time period, carrying out the first loading and unloading operation on the second horizontal load, and simultaneously carrying out the second loading and unloading operation on the first horizontal load until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

The tool for drilling a circular hole with a preset diameter and depth in the center of the fracturing test piece is an electric drill; the tool for applying the true triaxial state three-way load to the fracturing test piece is a hydraulic jack;

the method for testing whether the test piece is cracked comprises the following steps: arranging a pressure sensor on the injection pipe, and measuring the pressure of the injection pipe for injecting fracturing fluid into the shaft; in the process of injecting fracturing fluid into a shaft by an injection pipe, if the pressure detected by a pressure sensor suddenly and rapidly drops, judging that the fracturing test piece is broken; or, a displacement sensor or a pressure sensor is arranged on the force application surface of a hydraulic jack on the fracturing test piece, and in the process that the hydraulic jack applies load to the fracturing test piece, if the pressure detected by the pressure sensor suddenly and rapidly drops, or the displacement detected by the displacement sensor suddenly and rapidly rises or rapidly drops, the fracturing test piece is judged to be broken.

The change range of the true triaxial three-way load is set according to a linear equation and a similar criterion that the vertical ground stress, the maximum horizontal ground stress and the minimum horizontal ground stress change along with the depth, by considering the original ground stress state of the rock body;

the rock mass is in an original ground stress state comprising an original ground stress state of the rock mass in a vertical well and an original ground stress state of the rock mass in a horizontal well, and a hydraulic fracturing test under the action of a true triaxial state three-way load is respectively carried out according to two different original ground stress states;

setting a first horizontal load and a horizontal maximum ground stress sigma according to the original ground stress state of a rock body in a vertical well_HThe same, the second horizontal load and the horizontal minimum stress σ_hSame, vertical load and vertical ground stress σ_vThe same;

setting a first horizontal load and a horizontal maximum ground stress sigma according to the original ground stress state of a rock mass in a horizontal well_HSame, second horizontal load and vertical ground stress σ_vSame, vertical load and horizontal minimum ground stress σ_hThe same;

horizontal maximum stress sigma_HThe variation range of (a) is 0-14 MPa, and the horizontal minimum ground stress sigma_hThe variation range of (A) is 0-12 MPa, and the vertical ground stress sigma is_vThe variation range of (2) is 0-2 MPa.

The method further comprises the following steps: cutting more than two cubic rock blocks with different structural plane dip angles, wherein each cubic rock block with the structural plane dip angle comprises more than two cubic rock blocks; and respectively carrying out a hydraulic fracturing test under the action of true triaxial state three-dimensional load on a fracturing test piece made of cubic rock blocks with each structural surface inclination angle.

The concrete similar with natural rock of material is wrapped up to outside cubic rock piece, makes into the fracturing test piece and includes:

deducing a similar criterion and a similar index of the fracturing test by using a fracturing control equation, determining a similar material mechanical parameter according to the similar criterion and the similar index, and manufacturing a concrete similar material according to the similar material mechanical parameter;

manufacturing a square mould according to the basic size of a fracturing test piece required by the test, wherein the mould comprises a cover plate, side plates and a bottom plate; the size of the mould is larger than that of the cubic rock block;

and placing the cubic rock block into the center of the mold, pouring the concrete similar material into the mold, pouring a fracturing test piece with the size required by the test, and wrapping concrete similar to natural rock on the cubic rock block to manufacture the fracturing test piece.

Compared with the prior art, the method has the technical effects that the method can simulate the hydraulic fracturing process of the shale under different structural surfaces, different three-dimensional stress states and different disturbance loads, and the fracturing test piece and the whole test process have typical similarity with the in-situ fracturing shale reservoir. According to the test design of the stress-free path, unidirectional pressure-boosting and pressure-reducing and bidirectional pressure-boosting and pressure-reducing disturbance fracturing tests are performed under different levels of load stress states set according to the geological depth. The shale fracturing process and the fracture characteristics under the action of the disturbance load in the true triaxial state lay a foundation for understanding and mastering the research on the shale volume fracturing effect under the influence of the additional stress field, and can provide theoretical and technical support for on-site multistage multi-well fracturing.

Drawings

FIG. 1 is a schematic view of a process for manufacturing a fracturing test piece according to the present invention;

FIG. 2 is a schematic structural diagram of a fracturing test piece of the present invention;

FIG. 3 is a diagram showing the original ground stress state of the rock mass in the vertical well according to the present invention;

FIG. 4 is a schematic diagram of the rock mass of the present invention in the original ground stress state in a horizontal well;

FIG. 5 is a schematic diagram of a hydraulic jack applying true triaxial loading to a fracturing test piece according to the present invention;

FIG. 6 is a schematic diagram showing changes of loads in a hydraulic fracturing test under a true triaxial state under a three-dimensional constant load effect according to the present invention;

FIG. 7 is a schematic diagram of changes in various loads in a hydraulic fracturing test under the action of a true triaxial state unidirectional pressurization disturbance load;

FIG. 8 is a schematic diagram of changes in various loads in a hydraulic fracturing test under the action of a true triaxial state unidirectional pressure reduction disturbance load;

FIG. 9 is a schematic diagram of changes in various loads in a hydraulic fracturing test under the action of a true triaxial state bidirectional supercharging disturbance load according to the present invention;

FIG. 10 is a schematic diagram of changes of all loads in a hydraulic fracturing test under the action of a true triaxial state bidirectional pressure reduction disturbance load.

Detailed Description

The invention provides a stress path-based hydraulic fracturing test method, which can be used for researching the influence of an additional disturbance stress field on the fracturing transformation effect of a shale reservoir indoors and guiding field fracturing.

The hydraulic fracturing test method based on the stress path comprises the following steps: as shown in fig. 1, natural rock pieces were cut into cubic rock pieces of a predetermined size, and this example was tested using natural shale. More than two cubic rock blocks of different structural face inclination angles are cut, each cubic rock block of structural face inclination angles comprising more than two, for example 3-6. The inclination angle of the structural surface can be from 0 degree to 90 degrees, and rock blocks with any inclination angle of the structural surface can be cut according to experimental requirements. After the cubic rock block is cut, the grinding treatment is carried out and the non-parallelism of the end faces of the rock block is ensured to be less than 0.01 mm. The dimensions of the rock mass may be, for example, 200mm x 200 mm.

And deducing a similar criterion and a similar index of a fracturing test by using a fracturing control equation, determining similar material mechanical parameters according to the similar criterion and the similar index, wherein the similar material mechanical parameters comprise elastic modulus, Poisson's ratio, compressive strength, permeability, porosity and the like, and then manufacturing the concrete similar material according to the similar material mechanical parameters. This is the prior art and will not be described further herein. The similar concrete material of this example adopts PO42.5 cement, and the cement and quartz sand ratio is 1: and 1, the volume ratio of the mixture to the water is 2:5, and finally, measuring the mechanical parameters of the concrete to test whether the mechanical parameters are the same as the mechanical parameters required by the test.

A square mold is manufactured according to the basic size of the fracturing test piece required by the test, and the basic size of the fracturing test piece required by the test of the embodiment is 300mm × 300mm × 300 mm. The mould can be made of PVC plastics, and comprises a cover plate, a side plate and a bottom plate, wherein a round hole is drilled in the center of the cover plate and used as a reserved channel of a well hole, and the diameter of the round hole can be 10 mm. The dimensions of the mould cavity are larger than the dimensions of the cubic rock mass. In order to ensure that the test piece does not deform in the pouring and curing process, the mould must have enough rigidity, namely, the PVC plate has enough thickness, the thickness of the bottom plate is designed to be 24mm, and the side plates and the cover plate are both 20 mm.

And placing the cubic rock block into the center of the mold, pouring the concrete similar material into the mold, and pouring the concrete similar material into the fracturing test piece with the size required by the test. As shown in fig. 2, the fracturing test piece is a rock block coated with concrete, on one hand, the damage of the loading pressure head to the shale test piece is prevented, and on the other hand, the actual ground stress state can be better simulated.

A circular hole with preset diameter and depth is drilled in the center of the fracturing test piece by an electric drill to form a shaft, wherein the diameter of the circular hole is 10mm, and the length of the circular hole is 140 mm. After drilling, the ear washing ball is used to blow out a great amount of dust quickly, a small piece of wet cloth is wound on the iron wire, the iron wire is scrubbed once, and then the dry cloth is replaced to scrub repeatedly until the iron wire is basically clean. A syringe is then inserted into the wellbore and secured. For example, the injection tube can be adhered by using an inlet 3M (AB) glue, the injection tube is mixed and stirred uniformly, the mixture is slowly poured into the hole, the injection tube is inserted into the shaft, and after 9 hours, the glue is solidified to the injection tube. A preset distance is reserved between the bottom of the injection tube and the bottom of the round hole to serve as an open hole section, and the open hole section is 20mm deep, for example. A20 mm length of plastic foam is cut with scissors and inserted into the open hole section. The open hole section is an action area of the fracturing fluid after the fracturing fluid is injected into the shaft and is also a position where the fracturing test piece starts to fracture.

Setting a true triaxial state three-way load according to the original ground stress state of the rock mass, wherein the true triaxial state three-way load comprises a first horizontal load, a second horizontal load and a vertical load; and performing a hydraulic fracturing test on the fracturing test piece under the action of true triaxial state three-dimensional load. And respectively carrying out a hydraulic fracturing test under the action of true triaxial state three-dimensional load on a fracturing test piece made of cubic rock blocks with each structural surface inclination angle.

The change range of the true triaxial three-way load is set according to a linear equation and a similar criterion that the vertical ground stress, the maximum horizontal ground stress and the minimum horizontal ground stress change along with the depth, by considering the original ground stress state of the rock body; the rock mass is in an original ground stress state comprising an original ground stress state of the rock mass in a vertical well and an original ground stress state of the rock mass in a horizontal well, and a hydraulic fracturing test is respectively carried out under the action of true triaxial state three-way load according to two different original ground stress states.

As shown in figure 3, a first horizontal load and a horizontal maximum ground stress sigma are set according to the original ground stress state of the rock body in the vertical well_HThe same, the second horizontal load and the horizontal minimum stress σ_hSame, vertical load and vertical ground stress σ_vThe same is true. As shown in FIG. 4, a first horizontal load and a horizontal maximum ground stress sigma are set according to the original ground stress state of the rock mass in the horizontal well_HSame, second horizontal load and vertical ground stress σ_vSame, vertical load and horizontal minimum ground stress σ_hThe same is true. Horizontal maximum stress sigma_HThe variation range of (a) is 0-14 MPa, and the horizontal minimum ground stress sigma_hThe variation range of (A) is 0-12 MPa, and the vertical ground stress sigma is_vThe variation range of (2) is 0-2 MPa.

The hydraulic fracturing test under the action of true triaxial state three-way load performed in the original ground stress state of the simulated rock mass in the vertical well is specifically described below as an example, and the fracturing test performed in the original ground stress state of the simulated rock mass in the vertical well is similar to the above, and is not repeated in this embodiment.

(1) The hydraulic fracturing test under the action of true triaxial state three-dimensional load includes: the hydraulic fracturing test under the effect of the invariable load of true triaxial state three-dimensional, the hydraulic fracturing test under the effect of the invariable load of true triaxial state three-dimensional includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: the maximum values of the first horizontal load, the second horizontal load, and the vertical load, and the initial value of the injection pressure of the injection tube into the wellbore.

As shown in fig. 5, a true triaxial state three-way load is applied to the fracturing test piece using a jack. As shown in FIG. 6, the three-way load is kept constant in the true triaxial state, for example, the first horizontal load is 14MPa, the second horizontal load is 12MPa, and the vertical load is 2MPa, after stabilizing the pressure, the fracturing fluid is injected into the well bore by using the injection pipe at a preset rate, for example, 0.1mm/s, until the test piece is broken.

Recording test procedure data including injection pressure in the wellbore at test piece fracture initiation, i.e. fracture initiation pressure P_iInjection pressure when the test piece is fractured, i.e. fracture pressure P_fAnd the time required from the start of the test to the initiation of the test piece, i.e., the initiation time T_iAnd the time required from the start of the test to the rupture of the test piece, i.e., the rupture time T_f。

(2) The hydraulic fracturing test under true triaxial state three-dimensional load effect includes that the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect, and the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect includes:

and setting the maximum values of the true triaxial state three-way load vertical first horizontal load, the second horizontal load and the vertical load according to the original ground stress state of the rock mass, and setting the initial value of the injection pressure of the injection pipe to the shaft.

Applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece, wherein the maximum values are 14MPa and 2MPa respectively in the graph 7 and are kept unchanged, injecting fracturing fluid into the shaft at an initial value of injection pressure of 5MPa by using an injection pipe, applying a second horizontal load to the fracturing test piece, gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period. As shown in fig. 7, the second horizontal load σ_hGradually increasing from 0 to 3MPa, 6MPa, 9MPa and 12MPa according to a preset step value of 3MPa, and keeping each stage of load for a preset time period, wherein the time period is 3 minutes in the figure 7. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore from the initial value to a first preset pressure and keeping the first preset pressure unchanged, wherein the first preset pressure is 10MPa in the figure 7, and simultaneously carrying out the following first unloading and loading operations: and directly unloading the second horizontal load from the maximum value to a preset value, namely 3MPa in the figure 7, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period. In FIG. 7, the second horizontal load was increased stepwise from 3MPa to 12MPa, i.e., 6MPa, 9MPa, 12MPa, and each load was maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, the injection pressure of the injection pipe to the well bore is continuously increased to a second preset pressure, wherein the second preset pressure is 15MPa in the figure 7, and the first unloading and loading operation is carried out simultaneously. As shown in fig. 7, the second horizontal load is unloaded to 3MPa, and then is increased to 12MPa, i.e., 6MPa, 9MPa, 12MPa, step by step according to the preset step value of 3MPa, and each step of load is maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not fractured, the test procedure is repeated, i.e. the injection pressure P of the injection tube to the borehole_Hole(s)Continuously increasing and keeping the preset time periodE.g., 20MPa, 25MPa, the first unloading loading operation described above is carried out with each increase in injection pressure by one step until the specimen fractures.

Recording test procedure data including injection pressure in the wellbore at test piece fracture initiation, i.e. fracture initiation pressure P_iInjection pressure when the test piece is fractured, i.e. fracture pressure P_fAnd the time required from the start of the test to the initiation of the test piece, i.e., the initiation time T_iAnd the time required from the start of the test to the rupture of the test piece, i.e., the rupture time T_f。

(3) The hydraulic fracturing test under the three-dimensional load effect of true triaxial state still includes: the hydraulic fracturing test under the effect of true triaxial state one-way step-down disturbance load, the hydraulic fracturing test under the effect of one-way step-down disturbance load includes:

and setting the maximum values of the true triaxial state three-way load vertical first horizontal load, the second horizontal load and the vertical load according to the original ground stress state of the rock mass, and setting the initial value of the injection pressure of the injection pipe to the shaft. Applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece, wherein the maximum values are 14MPa and 2MPa respectively in the graph 8 and are kept unchanged, injecting fracturing fluid into the shaft by using an injection pipe at the initial value of injection pressure, wherein the maximum value is 5MPa in the graph 8, applying the maximum value of a second horizontal load to the fracturing test piece, wherein the maximum value is 12MPa in the graph 8, gradually reducing the second horizontal load from the maximum value to a preset value according to preset step values, the preset step values of each stage are the same or different, the load of each stage is kept for a preset time period, and the second horizontal load sigma in the graph 8 is_hThe step-by-step reduction is carried out from 12MPa to 9MPa, 6MPa and 3MPa according to the preset step value of 3MPa, and the load of each step is kept for 3 minutes. Testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the first preset pressure unchanged, wherein the first preset pressure is 10MPa in the figure 8, and simultaneously carrying out the following first loading and unloading operations: the second horizontal load is loaded to the maximum value of 12MPa of the second horizontal load again, then the second horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; in fig. 8, the second horizontal load is gradually reduced from 12MPa to 9MPa, 6MPa, and 3MPa according to a preset step value of 3MPa, and each load is maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, the injection pressure of the injection pipe to the well bore is increased to a second preset pressure which is 15MPa in the figure 8 and is kept unchanged, and meanwhile, the first loading and unloading operation is carried out. As shown in fig. 8, the second horizontal load was loaded to 12MPa, and then increased stepwise from 12MPa to 9MPa, 6MPa, 3MPa according to a preset step value of 3MPa, with each step of load being maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, the test process is repeated, namely the injection pressure of the injection pipe to the well bore is continuously increased and kept for a preset time period, such as 20MPa and 25MPa, and the first loading and unloading operation is carried out while increasing every step until the test piece is broken.

Recording test procedure data including injection pressure in the wellbore at test piece fracture initiation, i.e. fracture initiation pressure P_iInjection pressure when the test piece is fractured, i.e. fracture pressure P_fAnd the time required from the start of the test to the initiation of the test piece, i.e., the initiation time T_iAnd the time required from the start of the test to the rupture of the test piece, i.e., the rupture time T_f。

(4) The hydraulic fracturing test under the three-dimensional load effect of true triaxial state still includes: the hydraulic fracturing test under the effect of the two-way pressure boost disturbance load of true triaxial state, the hydraulic fracturing test under the effect of the two-way pressure boost disturbance load of true triaxial state includes:

and setting the maximum values of the true triaxial state three-way load vertical first horizontal load, the second horizontal load and the vertical load according to the original ground stress state of the rock mass, and setting the initial value of the injection pressure of the injection pipe to the shaft.

Applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged, wherein the maximum value in the vertical load is 2MPa in the figure 9; injecting fracturing fluid into the shaft by using an injection pipe at an initial value of injection pressure of 5 MPa; applying a second horizontal load to the fracturing test piece, and gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; as shown in fig. 9, the second horizontal load was increased from 0 to 3MPa, 6MPa, 9MPa, 12MPa stepwise at a preset step value of 3MPa, and each step of the load was maintained for 3 minutes.

And meanwhile, applying a first horizontal load to the fracturing test piece, gradually increasing the first horizontal load from 0 to the maximum value of the first horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period. As shown in fig. 9, the first horizontal load was increased from 0 to 5MPa, 8MPa, 11MPa, 14MPa, and each load was maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the first preset pressure unchanged, wherein the first preset pressure is 10MPa in the figure 9, and carrying out the following first unloading and loading operation: directly unloading the second horizontal load from the maximum value to a preset value, which is 3MPa in fig. 9, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; in fig. 9, the second horizontal load was increased stepwise from 3MPa to 6MPa, 9MPa, 12MPa, and each load was held for 3 minutes.

Simultaneously performing the following second unloading and loading operation on the first horizontal load: directly unloading the first horizontal load from the maximum value to a preset value, which is 5MPa in fig. 9, and then gradually increasing the first horizontal load to the maximum value of the first horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; as shown in FIG. 9, the load is increased to 8MPa, 11MPa and 14MPa in steps according to the preset step value of 3MPa, and each step is kept for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

And if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a second preset pressure and keeping the same, namely 15MPa in the figure 9, simultaneously performing the first unloading and loading operation and the second unloading and loading operation, testing whether the test piece is broken, and if the test piece is broken, finishing the test.

If the test piece is not broken, the test process is repeated, namely the injection pressure of the injection pipe to the well bore is continuously increased, for example, 20MPa and 25MPa, and is kept for a preset time period, and the first unloading loading operation and the second unloading loading operation are carried out at the same time when the injection pressure of the injection pipe to the well bore is increased by one step each time until the test piece is broken.

Recording test procedure data including injection pressure in the wellbore at test piece fracture initiation, i.e. fracture initiation pressure P_iInjection pressure when the test piece is fractured, i.e. fracture pressure P_fAnd the time required from the start of the test to the initiation of the test piece, i.e., the initiation time T_iAnd the time required from the start of the test to the rupture of the test piece, i.e., the rupture time T_f。

(5) The hydraulic fracturing test under the three-dimensional load effect of true triaxial state still includes: the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of true triaxial state, the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of true triaxial state includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: the maximum value of the vertical load, the first horizontal load, and the second horizontal load, and the initial value of the injection pressure of the injection tube into the wellbore. Applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged, wherein the maximum value in the graph 10 is 2 MPa; injecting fracturing fluid into the shaft at an initial value of injection pressure by using an injection pipe; applying a maximum value of a second horizontal load to the fracturing test piece, wherein the maximum value is 12MPa in the graph 10, reducing the second horizontal load from the maximum value to a preset value step by step according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; as shown in fig. 10, the second horizontal load was decreased stepwise from 12MPa to 9MPa, 6MPa, 3MPa at a preset step value of 3MPa, and each step of the load was maintained for 3 minutes.

Simultaneously, applying a maximum value of a first horizontal load to the fracturing test piece, wherein the maximum value is 14MPa in the graph 10, gradually reducing the first horizontal load from the maximum value to a preset value according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; as shown in fig. 10, the first horizontal load was decreased stepwise from 14MPa to 11MPa, 8MPa, 5MPa at a preset step value of 3MPa, and each step of the load was maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the first preset pressure unchanged, wherein the first preset pressure is 10MPa in the figure 10, and carrying out the following first loading and unloading operations: and loading the second horizontal load to the maximum value of 12MPa of the second horizontal load again, and then gradually reducing the second horizontal load to a preset value according to preset step values, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period. As shown in FIG. 10, the gradient was decreased from 12MPa to 9MPa, 6MPa, and 3MPa in accordance with a predetermined step value of 3 MPa.

Simultaneously carrying out the following second loading and unloading operations on the first horizontal load: the first horizontal load is loaded to the maximum value of the first horizontal load again, 14MPa in the graph 10, and then the first horizontal load is gradually reduced to a preset value according to a preset step value, the preset step values of all the steps are the same or different, and the load of each step is kept for a preset time period; as shown in FIG. 10, the load was gradually decreased from 14MPa to 11MPa, 8MPa, and 5MPa at a predetermined step value of 3MPa, and the load was maintained for 3 minutes. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, the injection pressure P of the injection tube to the well bore_Hole(s)Increasing to a second predetermined pressure, which is 15MPa in fig. 10, and performing the first loading and unloading operation and the second loading and unloading operation. And testing whether the test piece is broken or not, and if the test piece is broken, finishing the test.

If the test piece is not broken, repeating the test process, namely continuously increasing the injection pressure of the injection pipe to the shaft, such as 20MPa and 25MPa, and keeping for a preset time period, and simultaneously performing the first loading and unloading operation and the second loading and unloading operation when the injection pressure of the injection pipe to the shaft is increased by one step until the test piece is broken.

Recording test procedure data including injection pressure in the wellbore at test piece fracture initiation, i.e. fracture initiation pressure P_iInjection pressure when the test piece is fractured, i.e. fracture pressure P_fAnd the time required from the start of the test to the initiation of the test piece, i.e., the initiation time T_iAnd the time required from the start of the test to the rupture of the test piece, i.e., the rupture time T_f。

In the test process, the method for testing whether the test piece is broken comprises the following steps: arranging a pressure sensor on the injection pipe, and measuring the pressure of the injection pipe for injecting fracturing fluid into the shaft; in the process of injecting the fracturing fluid into the shaft by the injection pipe, if the pressure detected by the pressure sensor suddenly and rapidly drops, the fracture test piece is judged to be broken; or,

the method comprises the steps that a displacement sensor or a pressure sensor is arranged on a force application surface of a hydraulic jack on a fracturing test piece, and in the process that the hydraulic jack applies load to the fracturing test piece, if pressure detected by the pressure sensor suddenly and rapidly drops, or displacement detected by the displacement sensor suddenly and rapidly rises or rapidly drops, the fracturing test piece is judged to be broken.

It should be noted that the magnitude and variation of the loading stress in the test schemes of fig. 6 to 10 are merely illustrative, and the specific data may be changed according to the test requirements.

In conclusion, the method can simulate the hydraulic fracturing process of the shale under different structural plane inclination angles, different three-way stress states and different disturbance loads, and the fracturing test piece and the whole test process have typical similarity with the in-situ fracturing shale reservoir. According to the test design of the stress-free path, unidirectional pressure-boosting and pressure-reducing and bidirectional pressure-boosting and pressure-reducing disturbance fracturing tests are performed under different levels of load stress states set according to the geological depth. The shale fracturing process and the fracture characteristics under the action of the disturbance load in the true triaxial state lay a foundation for understanding and mastering the research on the shale volume fracturing effect under the influence of the additional stress field, and can provide theoretical and technical support for on-site multistage multi-well fracturing.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various changes and modifications can be made without departing from the overall concept of the invention, and these should also be considered as the protection scope of the present invention.

Claims (8)

1. A method of hydraulic fracture testing based on stress paths, the method comprising:

cutting natural rock blocks into cubic rock blocks with preset sizes, wrapping concrete with a material similar to that of the natural rock outside the cubic rock blocks, and manufacturing a fracturing test piece;

drilling a circular hole with a preset diameter and depth at the center of the fracturing test piece to serve as a shaft, installing a fracturing fluid injection pipe in the shaft, sealing and fixing the injection pipe, and keeping a preset distance between the bottom of the injection pipe and the bottom of the circular hole;

setting a true triaxial state three-way load according to the original ground stress state of a rock mass, and performing a hydraulic fracturing test on the fracturing test piece under the action of the true triaxial state three-way load;

the hydraulic fracturing test under true triaxial state three-dimensional loading effect includes that the hydraulic fracturing test under true triaxial state three-dimensional constant loading effect, the hydraulic fracturing test under true triaxial state three-dimensional constant loading effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the first horizontal load, the second horizontal load, and the vertical load;

applying the maximum value of the true triaxial state three-way load to the fracturing test piece and keeping the maximum value unchanged, and simultaneously injecting fracturing fluid into the shaft at a preset speed by using a fracturing fluid injection pipe until the test piece is fractured;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f；

The hydraulic fracturing test under true triaxial state three-way load effect includes that the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect, the hydraulic fracturing test under true triaxial state one-way pressure boost disturbance load effect includes:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore; applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece and keeping the maximum values unchanged, injecting fracturing fluid into the shaft at the initial value of injection pressure by using an injection pipe, applying a second horizontal load to the fracturing test piece, gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each stage are the same or different, the load of each stage is kept for a preset time period, testing whether the test piece breaks, and if the test piece breaks, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore from an initial value to a first preset pressure and keeping the same, and simultaneously carrying out the following first unloading and loading operations: directly unloading the second horizontal load from the maximum value to a preset value, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the shaft from a first preset pressure to a second preset pressure, simultaneously carrying out the first unloading and loading operation, testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft and keeping the injection pressure for a preset time period, and simultaneously carrying out the first unloading and loading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

2. The stress-path-based hydraulic fracturing test method of claim 1, wherein the true triaxial state three-way loading hydraulic fracturing test comprises a true triaxial state one-way depressurization disturbance loading hydraulic fracturing test, and the true triaxial state one-way depressurization disturbance loading hydraulic fracturing test comprises:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: a vertical load, a maximum of the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection tube into the wellbore;

applying the maximum values of a first horizontal load and a vertical load to the fracturing test piece and keeping the maximum values unchanged, injecting fracturing fluid into the shaft at the initial value of injection pressure by using an injection pipe, applying the maximum value of a second horizontal load to the fracturing test piece, reducing the second horizontal load from the maximum value to a preset value step by step according to preset step values, wherein the preset step values of each step are the same or different, the load of each step is kept for a preset time period, testing whether the test piece breaks or not, and if the test piece breaks, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and simultaneously carrying out the following first loading and unloading operations: the second horizontal load is loaded to the maximum value of the second horizontal load again, and then the second horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a second preset pressure and keeping the second preset pressure unchanged, simultaneously carrying out the first loading and unloading operation, testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft and keeping the injection pressure for a preset time period, and simultaneously carrying out the first loading and unloading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

3. The stress-path-based hydraulic fracturing test method of claim 1, wherein the true triaxial state three-way loading hydraulic fracturing test comprises a true triaxial state two-way pressurization disturbance loading hydraulic fracturing test, and the true triaxial state two-way pressurization disturbance loading hydraulic fracturing test comprises:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore;

applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged; injecting fracturing fluid into the shaft at an initial value of injection pressure by using an injection pipe; applying a second horizontal load to the fracturing test piece, and gradually increasing the second horizontal load from 0 to the maximum value of the second horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; simultaneously applying a first horizontal load to the fracturing test piece, and gradually increasing the first horizontal load from 0 to the maximum value of the first horizontal load according to a preset step value, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and carrying out the following first unloading and loading operation: directly unloading the second horizontal load from the maximum value to a preset value, and then gradually increasing the second horizontal load to the maximum value of the second horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

simultaneously performing the following second unloading and loading operation on the first horizontal load: directly unloading the first horizontal load from the maximum value to a preset value, and then gradually increasing the first horizontal load to the maximum value of the first horizontal load according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a second preset pressure and keeping the injection pressure unchanged, simultaneously performing the first unloading and loading operation and the second unloading and loading operation, testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the well bore and keeping the injection pressure for a preset time period, and simultaneously carrying out the first unloading and loading operation and the second unloading and loading operation until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

4. The stress-path-based hydraulic fracturing test method of claim 1, wherein the hydraulic fracturing test under true triaxial conditions three-way loading comprises: the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of the true triaxial state, the hydraulic fracturing test under the effect of the bidirectional decompression disturbance load of the true triaxial state comprises:

according to the original ground stress state of the rock mass, setting a true triaxial state three-way load: maximum values of the vertical load, the first horizontal load and the second horizontal load, and an initial value of injection pressure of the injection pipe into the wellbore;

applying the maximum value of the vertical load to the fracturing test piece and keeping the maximum value unchanged; injecting fracturing fluid into the shaft at an initial value of injection pressure by using an injection pipe; applying the maximum value of a second horizontal load to the fracturing test piece, and gradually reducing the second horizontal load from the maximum value to a preset value according to preset step values, wherein the preset step values of each step are the same or different, and the load of each step is kept for a preset time period; simultaneously, applying the maximum value of the first horizontal load to the fracturing test piece, and gradually reducing the first horizontal load from the maximum value to a preset value according to preset step values, wherein the preset step values of each stage are the same or different, and the load of each stage is kept for a preset time period; testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the wellbore to a first preset pressure and keeping the same, and carrying out the following first loading and unloading operations: the second horizontal load is loaded to the maximum value of the second horizontal load again, and then the second horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

simultaneously carrying out the following second loading and unloading operations on the first horizontal load: the first horizontal load is loaded to the maximum value of the first horizontal load again, then the first horizontal load is reduced to a preset value step by step according to preset step values, the preset step values of each step are the same or different, and the load of each step is kept for a preset time period;

testing whether the test piece is broken or not, and if the test piece is broken, finishing the test;

if the test piece is not broken, increasing the injection pressure of the injection pipe to the well bore to a second preset pressure and keeping the same, and carrying out the first loading and unloading operation on the second horizontal load and simultaneously carrying out the second loading and unloading operation on the first horizontal load; testing whether the test piece is broken, and if the test piece is broken, finishing the test;

if the test piece is not broken, continuously increasing the injection pressure of the injection pipe to the shaft for a preset time period, carrying out the first loading and unloading operation on the second horizontal load, and simultaneously carrying out the second loading and unloading operation on the first horizontal load until the test piece is broken;

recording test procedure data including injection pressure P in the wellbore at test piece fracture initiation_iAnd the injection pressure P when the test piece is fractured_fAnd the time T required from the start of the test to the initiation of the test piece_iAnd the time T required from the start of the test to the breakage of the test piece_f。

5. The stress path-based hydraulic fracture testing method according to any one of claims 2 to 4, wherein the tool for drilling a circular hole with a preset diameter and depth in the center of the fracture specimen is an electric drill; the tool for applying the true triaxial state three-way load to the fracturing test piece is a hydraulic jack;

the method for testing whether the test piece is cracked comprises the following steps: arranging a pressure sensor on the injection pipe, and measuring the pressure of the injection pipe for injecting fracturing fluid into the shaft; in the process of injecting fracturing fluid into a shaft by an injection pipe, if the pressure detected by a pressure sensor suddenly and rapidly drops, judging that the fracturing test piece is broken; or, a displacement sensor or a pressure sensor is arranged on the force application surface of a hydraulic jack on the fracturing test piece, and in the process that the hydraulic jack applies load to the fracturing test piece, if the pressure detected by the pressure sensor suddenly and rapidly drops, or the displacement detected by the displacement sensor suddenly and rapidly rises or rapidly drops, the fracturing test piece is judged to be broken.

6. The hydraulic fracture test method based on stress path according to any one of claims 2-4, wherein the variation range of the true triaxial three-way load is set by considering the original ground stress state of the rock body and according to a linear equation of the vertical ground stress, the maximum horizontal ground stress and the minimum horizontal ground stress with the change of the depth and a similar criterion;

the rock mass is in an original ground stress state comprising an original ground stress state of the rock mass in a vertical well and an original ground stress state of the rock mass in a horizontal well, and a hydraulic fracturing test under the action of a true triaxial state three-way load is respectively carried out according to two different original ground stress states;

setting a first horizontal load and a horizontal maximum ground stress sigma according to the original ground stress state of a rock body in a vertical well_HThe same, the second horizontal load and the horizontal minimum stress σ_hSame, vertical load and vertical ground stress σ_vThe same;

setting a first horizontal load and a horizontal maximum ground stress sigma according to the original ground stress state of a rock mass in a horizontal well_HSame, second horizontal load and vertical ground stress σ_vSame, vertical load and horizontal minimum ground stress σ_hThe same;

horizontal maximum stress sigma_HThe variation range of (a) is 0-14 MPa, and the horizontal minimum ground stress sigma_hThe variation range of (A) is 0-12 MPa, and the vertical ground stress sigma is_vThe variation range of (2) is 0-2 MPa.

7. The stress path-based hydraulic fracture testing method of claim 1, further comprising: cutting more than two cubic rock blocks with different structural plane dip angles, wherein each cubic rock block with the structural plane dip angle comprises more than two cubic rock blocks; and respectively carrying out a hydraulic fracturing test under the action of true triaxial state three-dimensional load on a fracturing test piece made of cubic rock blocks with each structural surface inclination angle.

8. The hydraulic fracture test method based on stress path as claimed in claim 1, wherein the step of wrapping concrete similar to natural rock on the cubic rock mass to make a fracture test piece comprises:

deducing a similar criterion and a similar index of the fracturing test by using a fracturing control equation, determining a similar material mechanical parameter according to the similar criterion and the similar index, and manufacturing a concrete similar material according to the similar material mechanical parameter;

manufacturing a square mould according to the basic size of a fracturing test piece required by the test, wherein the mould comprises a cover plate, side plates and a bottom plate; the size of the mould is larger than that of the cubic rock block;

and placing the cubic rock block into the center of the mold, pouring the concrete similar material into the mold, pouring a fracturing test piece with the size required by the test, and wrapping concrete similar to natural rock on the cubic rock block to manufacture the fracturing test piece.