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CN109959595B - Method and device for testing permeability in hydraulic sand fracturing process of tight reservoir - Google Patents

Method and device for testing permeability in hydraulic sand fracturing process of tight reservoir Download PDF

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Publication number
CN109959595B
CN109959595B CN201811143455.3A CN201811143455A CN109959595B CN 109959595 B CN109959595 B CN 109959595B CN 201811143455 A CN201811143455 A CN 201811143455A CN 109959595 B CN109959595 B CN 109959595B
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test piece
pressure
permeability
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CN109959595A (en
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张登文
钟炳成
车航
刘国华
朱杰
余曦
张德军
喻鹏
李建召
谷胜群
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Petrochina Co Ltd
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    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
    • G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
    • G01N15/082—Investigating permeability by forcing a fluid through a sample
    • G01N15/0826—Investigating permeability by forcing a fluid through a sample and measuring fluid flow rate, i.e. permeation rate or pressure change

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Abstract

The invention discloses a method and a device for testing permeability in a hydraulic sand fracturing process of a compact reservoir, wherein the method for testing the permeability in the hydraulic sand fracturing process of the compact reservoir comprises the steps of fixing a test piece to be tested of saturated fracturing fluid in a pressure container, and applying corresponding initial confining pressure, initial axial pressure, initial temperature and initial pore pressure to the test piece to be tested according to the pressure and the temperature of the compact reservoir under the stratum condition so as to simulate the stratum state of the test piece to be tested; obtaining a relation curve of the permeability of the test piece to be tested before fracture and the effective stress through fitting, calculating to obtain a change multiple of the permeability of the test piece to be tested after hydraulic sand fracturing, and determining a change rule of the permeability in the hydraulic sand fracturing process of the compact reservoir by using the relation curve and the change multiple, so as to provide a theoretical basis for reasonable and efficient development of the compact reservoir.

Description

Method and device for testing permeability in hydraulic sand fracturing process of tight reservoir
Technical Field
The invention relates to the technical field of reservoir evaluation, in particular to a method and a device for testing permeability in a hydraulic sand fracturing process of a compact reservoir.
Background
The compact reservoir has the characteristics of low porosity and low permeability, so that the exploitation difficulty is high, and the oil gas yield is low. For the development of compact reservoirs, the reservoir can be modified by a hydraulic sand fracturing mode to increase the permeability of the reservoir and improve the yield of oil and gas wells. And the permeability is a key parameter required for evaluating the oil and gas reservoir, calculating the capacity and formulating a reasonable development scheme. Therefore, the change of the permeability in the hydraulic sand fracturing process of the compact reservoir is determined through testing, and a theoretical basis can be provided for reasonable and efficient development of the compact reservoir.
At present, a test method for determining permeability by hydraulic fracturing of a compact reservoir is generally based on a reservoir core permeability test experiment before and after hydraulic fracturing under a triaxial condition, so as to determine the permeability before and after hydraulic fracturing of a reservoir permeable medium under a triaxial stress condition.
In the process of implementing the invention, the inventor finds that at least the following problems exist in the prior art:
because the existing testing method can only realize the testing of the permeability of the rock core of the reservoir before and after hydraulic fracturing under a triaxial condition, the method is not only not a research on hydraulic sand fracturing of a compact reservoir, but also does not simulate the hydraulic sand fracturing process under a stratum condition, and cannot obtain the change of the permeability in the hydraulic sand fracturing process.
Disclosure of Invention
In view of the above, the invention provides a method and a device for testing permeability in a hydraulic sand fracturing process of a compact reservoir, so as to determine a change rule of the permeability in the hydraulic sand fracturing process of the compact reservoir and provide a theoretical basis for reasonable and efficient development of the compact reservoir.
Specifically, the method comprises the following technical scheme:
in one aspect, the invention provides a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir, which is based on the device for testing permeability in the hydraulic sand fracturing process of the tight reservoir, and the method comprises the following steps:
fixing a test piece to be tested of the saturated fracturing fluid in a pressure container;
according to the pressure and the temperature of a compact reservoir under the stratum condition, applying initial confining pressure to a test piece to be tested through a confining pressure booster, applying initial axial pressure to the test piece to be tested through an axial actuator, applying initial temperature to the test piece to be tested through a temperature control assembly, and applying initial pore pressure to the test piece to be tested through a pore pressure booster;
applying a plurality of variable pore pressures to the upper end of the test piece to be tested through the pore pressure booster, wherein the difference values between two adjacent variable pore pressures are the same, and obtaining an initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time length by using a pulse tester;
obtaining an initial permeability and a plurality of first permeabilities of the test piece to be tested before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied, an application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time length;
fitting to obtain a relation curve of the permeability of the test piece to be tested before fracture and the effective stress according to the first permeabilities of the test piece to be tested before fracture, the initial confining pressure and the variable pore pressures;
when the test piece to be tested is broken, closing the pulse tester, applying a first preset pressure to the upper end of the test piece to be tested through the pore pressure booster, adding sand to the upper end of the test piece to be tested through the sand adding component, injecting fracturing fluid into the upper end of the test piece to be tested through the fracturing fluid injection component, applying a second preset pressure to the lower end of the test piece to be tested through the flow measuring component, and obtaining the flow with stable seepage after the second preset pressure is applied;
obtaining a second permeability of the test piece to be tested after the test piece to be tested is broken according to the flow, the first preset pressure and the second preset pressure;
and obtaining the change multiple of the permeability of the test piece to be tested after hydraulic sand fracturing according to the initial permeability of the test piece to be tested and the second permeability of the test piece to be tested after cracking.
Optionally, before the test piece to be tested saturated with water is fixed in the pressure container, the method further comprises: and acquiring the length and the diameter of the cross section of the test piece to be tested.
Optionally, after obtaining the length and the diameter of the cross section of the test piece to be tested, the method further comprises: and photographing or carrying out nuclear magnetic resonance imaging on the test piece to be tested to obtain the natural microcrack condition of the test piece to be tested.
Optionally, after the test piece to be tested is dried and then photographed or subjected to nuclear magnetic resonance imaging to obtain the natural microcrack condition of the test piece to be tested, the method further includes: and saturating the fracturing fluid after vacuumizing the test piece to be tested, and continuously vacuumizing.
Optionally, the initial permeability and the plurality of first permeabilities before fracture of the test piece to be tested are obtained according to the following calculation formula:
Figure BDA0001816304390000031
in the formula: k is the initial or first permeability in x 10-3μm2(ii) a Mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa-sec; beta is the compression coefficient of the fracturing fluid and has the unit of Pa-1(ii) a V is the volume of the pressure vessel in cm3；ΔpiThe initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied is expressed in kPa; Δ t is an application time period of each of the variable orifice pressures in sec; Δ pfThe unit of the ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time is kPa; a. thesIs the cross-sectional area, cm, of the test piece to be tested2；LsIs the length, cm, of the test piece to be tested.
Optionally, the effective stress is a difference between the initial confining pressure and the variable orifice pressure.
Alternatively, the second permeability is obtained according to the following calculation:
Figure BDA0001816304390000032
wherein k is the second permeability in units of D; mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa-sec; q is the flow rate in m3S; a is the cross-sectional area of the test piece to be tested, m2(ii) a Δ p is the difference between the first preset pressure and the second preset pressure in Pa; and L is the length of the test piece to be tested and has the unit of m.
Optionally, a value of a difference between the first preset pressure and the second preset pressure ranges from 0 MPa to 0.7 MPa.
In another aspect, the present invention further provides a permeability testing apparatus for a tight reservoir in a hydraulic sand fracturing process, the apparatus comprising: the device comprises a pressure container, a confining pressure booster, an axial actuator, a temperature control assembly, a pore pressure booster, a pulse tester, a sand adding assembly, a fracturing fluid injection assembly and a flow measuring assembly, wherein,
the axial actuator, the temperature control assembly and a test piece to be tested are arranged in the pressure container, the axial actuator is arranged at the upper end of the test piece to be tested, and the temperature control assembly is arranged on the outer wall of the test piece to be tested;
the confining pressure pressurizer is communicated with a cavity between the inner wall of the pressure container and the test piece to be tested, the pore pressure pressurizer is connected with the pulse tester, and the pulse tester is respectively connected with the upper end and the lower end of the test piece to be tested;
the sand adding assembly is connected with the fracturing fluid injection assembly, and the fracturing fluid injection assembly is connected with the upper end of the test piece to be tested;
the flow measuring component is connected with the lower end of the test piece to be tested.
Optionally, the apparatus further comprises: strain gauges and data collectors;
the strain gauge is arranged on the test piece to be tested;
and the data acquisition unit is connected with the strain gauge and the pulse tester through signals.
The technical scheme provided by the embodiment of the invention has the beneficial effects that at least:
1. fixing a test piece to be tested of saturated fracturing fluid in a pressure container, and applying corresponding initial confining pressure, initial axial pressure, initial temperature and initial pore pressure to the test piece to be tested according to the pressure and temperature of a compact reservoir under the stratum condition, so that the test piece to be tested realizes the simulation of the stratum state;
2. before the test piece to be tested is not broken, applying a plurality of variable pore pressures on the upper end of the test piece to be tested, and obtaining an initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied, an application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time length by using a pulse tester, so as to fit and obtain a relation curve of the permeability before the test piece to be tested is not broken and the effective stress;
3. after a test piece to be tested is broken, applying first preset pressure to the upper end of the test piece to be tested, changing second preset pressure at the lower end of the test piece to be tested through a flow measuring assembly, simulating a sand fracturing process by utilizing a sand adding assembly and a fracturing fluid injection assembly, obtaining a flow with stable seepage after the second preset pressure is applied through the flow measuring assembly, obtaining a second permeability after the test piece to be tested is broken, and further obtaining a change multiple of the permeability after the hydraulic sand fracturing of the test piece to be tested;
4. determining the change rule of the permeability in the hydraulic sand fracturing process of the compact reservoir according to the relation curve of the permeability before the test piece to be tested breaks and the effective stress and the change multiple of the permeability after the hydraulic sand fracturing, and providing a theoretical basis for reasonable and efficient development of the compact reservoir.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a schematic structural diagram of a permeability testing device in a hydraulic sand fracturing process of a tight reservoir provided by the invention;
FIG. 2 is a flow chart of a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir provided by the invention;
fig. 3 is a relation curve of permeability before fracture of a test piece to be tested and effective stress, which is obtained by using a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir according to an embodiment of the present invention;
fig. 4 is a relation curve of permeability before fracture of a test piece to be tested and effective stress, which is obtained by using the method for testing permeability in the hydraulic sand fracturing process of a tight reservoir provided by the second embodiment of the invention;
fig. 5 is a relation curve of permeability before fracture of a test piece to be tested and effective stress, which is obtained by using the method for testing permeability in the hydraulic sand fracturing process of a tight reservoir provided by the third embodiment of the invention;
fig. 6 is a relation curve of the permeability before fracture of the test piece to be tested and the effective stress, which is obtained by using the method for testing the permeability in the hydraulic sand fracturing process of the tight reservoir in the fourth embodiment of the invention.
The reference numerals in the figures are denoted respectively by:
1-pressure vessel, 101-first ram, 102-first pressure pad, 103-second pressure pad, 104-second ram, 105-base,
2-a confining pressure booster, and the pressure booster,
3-an axial actuator, wherein the axial actuator is provided with a plurality of axial actuators,
4-a temperature control component, wherein the temperature control component is arranged on the base,
5-a pore pressure booster is arranged in the pore space,
6-a pulse tester, namely a pulse tester,
7-a sand adding component, 701-a sand conveying tank, 702-a second valve,
8-fracturing fluid injection component, 801-liquid storage tank, 802-first valve, 803-pump body,
9-flow measuring component, 901-back pressure valve, 902-pressure gauge, 903-fourth valve, 904-liquid container, 905-electronic balance,
10-the test piece to be tested,
11-a strain gauge, wherein the strain gauge is arranged on the shell,
12-a data acquisition unit for acquiring data,
13-a third valve, which is arranged in the first valve,
14-a fifth valve, which is provided with a valve,
15-a sixth valve, wherein the first valve is connected with the second valve,
16-a seventh valve, which is provided with a valve,
17-an eighth valve, wherein the first valve is connected with the second valve,
18-a ninth valve, which is arranged in the first valve,
19-a data processor.
Detailed Description
Before the embodiments of the present invention are described in further detail, the terms of orientation, such as "upper end" and "lower end", in the embodiments of the present invention, are based on the orientation shown in fig. 1, and are only used to clearly describe the permeability testing method and apparatus in the hydraulic sand fracturing process of the tight reservoir according to the embodiments of the present invention, and have no meaning of limiting the protection scope of the present invention.
In order to make the technical solutions and advantages of the present invention clearer, the following will describe embodiments of the present invention in further detail with reference to the accompanying drawings.
In one aspect of the present invention, as shown in fig. 1, the present invention provides a permeability testing apparatus for a tight reservoir hydraulic sand fracturing process, the apparatus comprising: the device comprises a pressure container 1, a confining pressure booster 2, an axial actuator 3, a temperature control assembly 4, a pore pressure booster 5, a pulse tester 6, a sand adding assembly 7, a fracturing fluid injection assembly 8 and a flow measuring assembly 9.
The axial actuator 3, the temperature control assembly 4 and the test piece 10 to be tested are arranged in the pressure container 1, the axial actuator 3 is arranged at the upper end of the test piece 10 to be tested, and the temperature control assembly 4 is arranged on the outer wall of the test piece 10 to be tested;
the confining pressure booster 2 is communicated with a cavity between the inner wall of the pressure container 1 and the test piece 10 to be tested, the pore pressure booster 5 is connected with the pulse tester 6, and the pulse tester 6 is respectively connected with the upper end and the lower end of the test piece 10 to be tested;
the sand adding component 7 is connected with the fracturing fluid injection component 8, and the fracturing fluid injection component 8 is connected with the upper end of a test piece 10 to be tested;
the flow measuring component 9 is connected with the lower end of a test piece 10 to be tested.
The following description is given of the use method of the permeability testing device in the hydraulic sand fracturing process of the tight reservoir provided by the embodiment of the invention:
fixing a test piece 10 to be tested of saturated fracturing fluid in a pressure container 1;
according to the pressure and the temperature of the compact reservoir under the stratum condition, initial confining pressure is applied to a test piece 10 to be tested through a confining pressure booster 2, initial axial pressure is applied to the test piece 10 to be tested through an axial actuator 3, initial temperature is applied to the test piece 10 to be tested through a temperature control assembly 4, and initial pore pressure is applied to the test piece 10 to be tested through a pore pressure booster 5, so that the test piece 10 to be tested realizes the simulation of the stratum state;
when the test piece 10 to be tested is not cracked, applying a plurality of variable pore pressures to the upper end of the test piece 10 to be tested through the pore pressure booster 5, wherein the difference values between two adjacent variable pore pressures are the same, obtaining an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length by using the pulse tester 6, realizing the pulse attenuation permeability test, and obtaining a relation curve of the permeability of the test piece 10 to be tested before cracking and the effective stress;
when a test piece 10 to be tested is broken, the pulse tester 6 is closed, a first preset pressure is applied to the upper end of the test piece 10 to be tested through the pore pressure booster 5, sand is added to the upper end of the test piece 10 to be tested through the sand adding component 7, fracturing fluid is injected into the upper end of the test piece 10 to be tested through the fracturing fluid injection component 8, a second preset pressure is applied to the lower end of the test piece 10 to be tested through the flow measuring component 9, the flow with stable seepage after the second preset pressure is applied is obtained, the steady-state Kerschner permeability test is realized, and the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand adding fracturing is obtained.
Therefore, the permeability testing device for the compact reservoir in the hydraulic sand fracturing process of the embodiment of the invention utilizes the pressure container 1, the confining pressure booster 2, the axial actuator 3, the temperature control component 4, the pore pressure booster 5, the pulse tester 6, the sand adding component 7, the fracturing fluid injection component 8 and the flow measuring component 9 to realize the permeability testing of the test piece 10 to be tested in the hydraulic sand fracturing process under the simulated formation state, and utilizes the relation curve of the permeability before fracture of the test piece 10 to be tested and the effective stress and the change multiple of the permeability after hydraulic sand fracturing of the test piece 10 to be tested to determine the change rule of the permeability in the hydraulic sand fracturing process of the compact reservoir, thereby providing theoretical basis for reasonable and efficient development of the compact reservoir.
In view of the above, in order to facilitate the fixing of the test piece 10 to be tested, a first pressure head 101, a first pressure pad 102, a second pressure pad 103, a second pressure head 104 and a base 105 are further disposed in the pressure vessel 1, as shown in fig. 1.
Specifically, a first pressure head 101 is arranged at the upper end of a test piece 10 to be tested, a second pressure head 104 is arranged at the lower end of the test piece 10 to be tested, a first pressure pad 102 is arranged between the first pressure head 101 and the upper end of the test piece 10 to be tested, a second pressure pad 103 is arranged between the second pressure head 104 and the lower end of the test piece 10 to be tested, and a base 105 is arranged at the lower end of the second pressure head 104, so that the test piece 10 to be tested can be firmly arranged in the pressure container 1.
The pressure container 1 is filled with oil, that is, the cavity between the inner wall of the pressure container 1 and the test piece 10 to be tested is filled with oil, and the confining pressure booster 2 can set the confining pressure value by controlling the amount of oil injected into the cavity.
In addition, the confining pressure booster 2 is communicated with the cavity through a pipeline, in order to ensure the circulation of liquid in the cavity, a loop is formed between the confining pressure booster 2 and the cavity, the liquid is conveyed into the cavity through a liquid inlet pipeline, the liquid flowing out of the cavity is conveyed through a liquid outlet pipeline, an eighth valve 17 is arranged on the liquid inlet pipeline to control the liquid inlet pipeline to be communicated with the cavity, and a ninth valve 18 is arranged to control the liquid outlet pipeline to be communicated with the cavity.
Similarly, the pulse tester 6 is connected to the upper end and the lower end of the test piece 10 to be tested through pipes, the pulse tester 6 can be controlled to communicate with the upper end of the test piece 10 to be tested by setting the sixth valve 15, and the pulse tester 6 can be controlled to communicate with the lower end of the test piece 10 to be tested by setting the seventh valve 16.
For the sand adding assembly 7, the sand adding assembly 7 comprises a sand conveying tank 701 and a second valve 701, as shown in fig. 1, sand is filled in the sand conveying tank 701, the sand amount can be determined according to actual needs, and the type of the filled sand needs to be determined according to the reservoir type; a second valve 701 may control the sand feed tank 701 to communicate with the fracturing fluid injection assembly 8.
For the fracturing fluid injection assembly 8, the fracturing fluid injection assembly 8 comprises a liquid storage tank 801, a first valve 802 and a pump body 803, the prepared fracturing fluid is filled in the liquid storage tank 801, the first valve 802 can control the liquid storage tank 801 to be communicated with the sand adding assembly 7, and the pump body 803 is a constant-pressure constant-speed pump and can control the outflow pressure and outflow speed of the fracturing fluid flowing out of the liquid storage tank 801.
As can be understood by those skilled in the art, the sand adding assembly 7 and the fracturing fluid injection assembly 8 are connected with the upper end of a test piece 10 to be tested in the pressure vessel 1 through pipelines.
Based on this, in order to ensure that the test piece 10 to be tested is subjected to sand fracturing after being fractured, the third valve 13 may be arranged to control the sand adding assembly 7 and the fracturing fluid injection assembly 8 to be communicated with the upper end of the test piece 10 to be tested.
For the flow rate measuring component 9, the flow rate measuring component 9 includes a back pressure valve 901, a pressure gauge 902, a fourth valve 903, a liquid container 904 and an electronic balance 905, as shown in fig. 1, the electronic balance 905 is stably fixed on the ground or an operation platform, the liquid container 904 is disposed on the electronic balance 905, the back pressure valve 901, the pressure gauge 902 and the fourth valve 903 are communicated through a pipeline, one end of the pipeline is aligned to the liquid container 904, so that liquid flowing out from the lower end of the test piece 10 to be tested can enter the liquid container 904 through the pipeline, and the back pressure valve 901 can control and set a second preset pressure applied to the lower end of the test piece 10 to be tested.
By recording the volume of liquid flowing into the liquid container 904 and the time of the inflow, the flow rate can be calculated.
In order to ensure that the sample 10 to be tested can obtain a stable seepage flow after the sample is broken and the second preset pressure is applied, the fifth valve 14 is arranged to control the flow measuring assembly 9 to be communicated with the lower end of the sample 10 to be tested.
On the basis of the structure, the device for testing the permeability of the compact reservoir in the hydraulic sand fracturing process further comprises: strain gauges 11 and data collectors 12, as shown in fig. 1;
the strain gauge 11 is arranged on the test piece 10 to be tested;
the data acquisition unit 12 is in signal connection with the strain gauge 11 and the pulse tester 6.
Whether the test piece 10 to be tested is broken or not can be measured and determined through the strain gauge 11, and data in the strain gauge 11 and the pulse tester 6 can be obtained through the data acquisition unit 12, so that subsequent data statistics is facilitated.
Further, the device for testing the permeability of the tight reservoir in the hydraulic sand fracturing process of the tight reservoir in the embodiment of the invention further comprises: a data processor 19, as shown in FIG. 1;
data processor 19 is in signal communication with data collector 12.
Data acquired by data acquisition unit 12 can be processed by data processor 19 and the processed result can be displayed on a display screen.
In another aspect of the present invention, by using the apparatus for testing permeability during hydraulic sand fracturing of tight reservoir, the present invention provides a method for testing permeability during hydraulic sand fracturing of tight reservoir, whose flow chart is shown in fig. 2, the method includes:
step 201: a test piece 10 to be tested of saturated fracturing fluid is fixed in the pressure vessel 1.
Before this step, the preparation of a test piece 10 to be tested is first carried out. Selecting a rock mass of shale, raw coal or compact sandstone, conglomerate or other low-permeability-ultra-low-permeability reservoir stratum from the site, drilling a cylinder with the height-diameter ratio of 2-2.5: 1 from the rock mass to serve as a test piece 10 to be tested, cutting two ends of the test piece 10 to be tested flat and neat, and checking whether the surface of the obtained test piece 10 to be tested has obvious defects, unfilled corners and cracks and cannot develop or not if the surface has obvious defects, unfilled corners and cracks, discarding the test piece and then manufacturing the next test piece 10 to be tested; if not, the subsequent steps are continued.
After the test piece 10 to be tested is prepared, the test piece 10 to be tested is subjected to conventional physical property parameter testing and description. Preparing data for subsequent calculation of permeability by acquiring the length of the test piece 10 to be tested and the diameter of the cross section; after measuring the length and the diameter of the cross section of a test piece 10 to be tested, placing the test piece 10 to be tested in a thermostat, drying to constant weight, cooling, and measuring conventional physical property parameters such as dry weight, gas porosity and the like; then, photographing or nuclear magnetic resonance imaging is carried out on the test piece 10 to be tested to obtain the natural micro-crack condition of the test piece 10 to be tested, and the test piece 10 to be tested is described, wherein the photographing is carried out on the outer part of the test piece 10 to be tested, and the nuclear magnetic resonance imaging is carried out on the inner part of the test piece 10 to be tested; the test piece 10 to be tested is then placed in a drying bottle for use.
After the test piece 10 to be tested is subjected to conventional physical property parameter testing and description, the test piece 10 to be tested is assembled. After a high-strength binder is smeared on the upper section and the end face on the same side of the special nozzle for hydraulic fracturing, the special nozzle is placed into a fracturing round hole of a test piece 10 to be tested, is pressed to enable the special nozzle to be aligned with the end face of the test piece 10 to be tested to be flat, and is placed to be naturally dried.
After the test piece 10 to be tested is assembled, the test piece 10 to be tested is saturated with the fracturing fluid. The test piece 10 to be tested can be saturated with the fracturing fluid after being vacuumized, and the vacuum pumping is continued to ensure that no gas exists in the test piece 10 to be tested, and the fracturing fluid is completely saturated.
For example, the test piece 10 to be tested after natural drying can be put into a vacuum device, vacuumized for 4 hours, saturated with the prepared fracturing fluid, and continuously vacuumized, and the test piece 10 to be tested is observed to stop vacuuming when no bubble overflows in the process of saturating the fracturing fluid.
In this step, in the process of completing the saturated fracturing fluid of the test piece 10 to be tested, the test piece 10 to be tested of the saturated fracturing fluid may be installed and fixed in the pressure vessel 1. The method comprises the steps that a test piece 10 to be tested is placed between a first pressure head 101 and a second pressure head 104, so that hydraulic through holes on each surface of a first pressure pad 102 and a second pressure pad 103 at two ends of the test piece 10 to be tested are correspondingly aligned with hydraulic through holes of the first pressure head 101 and the second pressure head 104, the outer seal of the test piece 10 to be tested is sleeved outside a structure formed by the test piece 10 to be tested and the first pressure head 101 and the second pressure head 104, a temperature control assembly 4 is uniformly blown tightly from the middle part to two ends of the test piece 10 to be tested by hot air blowing, and the temperature control assembly is tightly contacted and sealed with the test piece 10 to be tested, the first pressure head 101 and the second pressure head 104; fixing the axial actuator 3, and then installing the strain gauge 11 at the middle position of the test piece 10 to be tested; connecting pipelines and transmission cables among all devices in the device.
Step 202: according to the pressure and the temperature of the compact reservoir under the stratum condition, initial confining pressure is applied to a test piece 10 to be tested through a confining pressure booster 2, initial axial pressure is applied to the test piece 10 to be tested through an axial actuator 3, initial temperature is applied to the test piece 10 to be tested through a temperature control assembly 4, and initial pore pressure is applied to the test piece 10 to be tested through a pore pressure booster 5.
Before this step, the test piece 10 to be tested needs to be debugged after installation. By placing the base 105 in place, the combination of the test piece 10 to be tested and the first pressure pad 102 and the second pressure pad 103 is loaded into the triaxial apparatus cavity, the axial load cable is connected, the debugging axial actuator 3 pushes the piston to contact the first pressure head 101, the strain gauge 11 is debugged in place, and the thick-walled cylinder of the pressure vessel 1 is slowly lowered to seal the triaxial apparatus cavity.
In the step, the confining pressure booster 2 can be started by controlling the high-precision servo hydraulic station according to the stress condition of the original rock in the tight reservoir so as to apply initial confining pressure and keep the initial confining pressure constant; starting the axial actuator 3, increasing the axial pressure to the initial axial pressure step by step and keeping the axial pressure constant; starting the temperature control component 4, setting the initial temperature and keeping the initial temperature constant; the pore pressure booster 5 is activated to apply an initial pore pressure and to keep it constant.
Step 203: a plurality of variable pore pressures are applied to the upper end of a test piece 10 to be tested through a pore pressure booster 5, the difference value between two adjacent variable pore pressures is the same, and an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length are obtained by using a pulse tester 6.
Specifically, the pore pressure of the test piece 10 to be tested is changed by the pore pressure booster 5, so that the test piece 10 to be tested generates a negative sudden change. Since the difference values of the applied variable pore pressures are the same, that is, the variable pore pressures are applied to the upper end of the test piece 10 to be tested step by step until the volume strain of the test piece 10 to be tested is accelerated to enter a negative sudden change stage, that is, the test piece 10 to be tested is broken.
In this step, it can be determined whether the test piece 10 to be tested is fractured or deformed by the strain gauge 11.
Because the pore pressure booster 5 is connected with the pulse tester 6, the pulse tester 6 can measure and obtain the initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the applying time length of each variable pore pressure and the finishing pressure difference between the upper end and the lower end of the test piece 10 to be tested after each applying time length.
Step 204: and obtaining the initial permeability and a plurality of first permeabilities of the test piece 10 to be tested before rupture according to the initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and the finishing pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length.
Specifically, the initial permeability and the plurality of first permeabilities before rupture of the test piece 10 to be tested are obtained according to the following calculation formulas:
Figure BDA0001816304390000121
in the formula: k is the initial or first permeability inIs x 10-3μm2(ii) a Mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa.sec; beta is the compression coefficient of the fracturing fluid and has the unit of Pa-1(ii) a V is the volume of the pressure vessel 1 in cm3；ΔpiThe initial pressure difference in kPa between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied; Δ t is the application time period of each variable orifice pressure in sec; Δ pfThe unit of the ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time is kPa; a. thesIs the cross-sectional area, cm, of the test piece 10 to be tested2；LsIs the length, cm, of the test piece 10 to be tested.
The viscosity coefficient of the fracturing fluid and the compression coefficient of the fracturing fluid can be obtained by a viscosity measuring instrument and a compression coefficient measuring instrument.
Step 205: and fitting to obtain a relation curve of the permeability of the test piece 10 to be tested before fracture and the effective stress according to the plurality of first permeability, the initial confining pressure and the plurality of variable pore pressures of the test piece 10 to be tested before fracture.
Wherein, the effective stress is the difference value between the initial confining pressure and the variable pore pressure.
It is understood that since the number of variable pore pressures is plural, the number of effective stresses is correspondingly plural. Through the effective stresses and the corresponding permeability before fracture, a relation curve between the permeability before fracture and the effective stresses of the test piece 10 to be tested can be obtained through fitting.
Step 206: after a test piece 10 to be tested is broken, the pulse tester 6 is closed, a first preset pressure is applied to the upper end of the test piece 10 to be tested through the pore pressure booster 5, sand is added to the upper end of the test piece 10 to be tested through the sand adding component 7, fracturing fluid is injected into the upper end of the test piece 10 to be tested through the fracturing fluid injection component 8, a second preset pressure is applied to the lower end of the test piece 10 to be tested through the flow measuring component 9, and the flow with stable seepage after the second preset pressure is applied is obtained.
In this step, after the test piece 10 to be tested is broken, the pulse tester 6 is closed, the pulse attenuation permeability test is ended, and the steady-state Facker permeability test is performed.
Step 207: and obtaining a second permeability of the test piece 10 to be tested after the test piece is broken according to the flow, the first preset pressure and the second preset pressure.
Specifically, the second permeability is obtained according to the following calculation formula:
Figure BDA0001816304390000131
wherein k is the second permeability in units of D; mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa.sec; q is the flow rate in m3S; a is the cross-sectional area of the test piece 10 to be tested, m2(ii) a Δ p is the difference between the first preset pressure and the second preset pressure in Pa; l is the length of the test piece 10 to be tested in m.
It should be noted that the back-pressure valve can be controlled by the high-precision multistage plunger displacement pump, so that the value range of the difference between the first preset pressure and the second preset pressure is 0-0.7MPa, and the problem that seepage stability is difficult to realize due to overlarge difference between the first preset pressure and the second preset pressure is avoided.
Step 208: and obtaining the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand fracturing according to the initial permeability of the test piece 10 to be tested and the second permeability of the test piece 10 to be tested after cracking.
Specifically, the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand fracturing is the ratio of the second permeability after fracturing to the initial permeability.
After the test of the test piece 10 to be tested is completed by the steps, the test piece 10 to be tested is disassembled, the expansion condition of the crack of the test piece 10 to be tested after fracturing is described by photographing or nuclear magnetic resonance imaging again, and the steps can be repeated subsequently according to a set scheme so as to perform the next group of tests.
Example 1
The embodiment provides a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir.
Preparation of test piece 10 to be tested: selecting a No. 3 coal seam of a Jincheng Temple river coal mine, burying the coal seam for 550m at the temperature of 28.5 ℃, and preparing a standard cylindrical original rock test piece;
early preparation: and (3) putting the original rock sample into an oven for drying, and drilling holes with the diameter phi of 8.0mm and the depth H6.0mm in the center of two end surfaces of the test piece by using a bench drill. Measuring the dimensions of the test piece 10 to be tested: height 101.4mm, test piece diameter 50 mm. After a test piece 10 to be tested is naturally dried, the test piece is placed into a fracturing fluid to be soaked so that the test piece is in a water saturation state, wherein the formula of the fracturing fluid is 0.5% of KCL + 0.2% of JH-20 cleanup additive 0.6% + 98.1% of clear water.
The method specifically comprises the following steps:
assembling a test piece 10 to be tested of saturated fracturing fluid and a corresponding pressure pad into a cavity of a triaxial apparatus, pushing a piston to be contacted with a first pressure head 101 by a debugging axial actuator 3, debugging a strain gauge 11 in place, putting down a thick-wall cylinder of a pressure container 1 to realize sealing, and fixing the test piece 10 to be tested of the saturated fracturing fluid in the pressure container 1;
according to the original rock stress condition, an initial confining pressure with the value of 11.8MPa is applied to a test piece 10 to be tested through a confining pressure booster 2 and is kept constant, an initial axial pressure with the value of 14.8MPa is applied to the test piece 10 to be tested through an axial actuator 3 and is kept constant, an initial pore pressure with the value of 3.8MPa is applied to the test piece 10 to be tested through a pore pressure booster 5, and the initial temperature of the test piece 10 to be tested is set to be 28.5 ℃ through a temperature control assembly 4 and is kept constant;
according to the stress-strain condition, applying a plurality of variable pore pressures to the upper end of a test piece 10 to be tested through a pore pressure booster 5, wherein the difference value between two adjacent variable pore pressures is 2MPa, the variable pore pressures are increased to 13.75MPa step by step, and an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length are obtained by recording through a pulse tester 6;
calculating according to a formula (1) to obtain an initial permeability and a plurality of first permeabilities of the test piece to be tested 10 before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested 10 when each variable pore pressure is applied, an application time of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested 10 after each application time;
when the volume strain of the test piece 10 to be tested is accelerated to enter a negative sudden change stage, the strain gauge 11 records whether the test piece 10 to be tested under the corresponding pore pressure condition is deformed, namely, the test piece 10 to be tested is judged to be damaged under pressure. After a test piece 10 to be tested is broken, applying a first preset pressure to the upper end of the test piece 10 to be tested through a pore pressure booster 5, opening a sand adding assembly 7, paving 250g of 700-mesh carborundum and 50g of 800-mesh carborundum propping agent in a pressed dynamic crack, injecting fracturing fluid into the upper end of the test piece 10 to be tested through a fracturing fluid injection assembly 8, applying a second preset pressure to the lower end of the test piece 10 to be tested through a flow measuring assembly 9, and obtaining a flow with stable permeation after the second preset pressure is applied;
and calculating according to the flow, the first preset pressure and the second preset pressure and the formula (2) to obtain a second permeability of the test piece 10 to be tested after the test piece is broken.
The fitting result of experimental data shows that: the curve of the permeability before fracture of the test piece 10 to be tested as a function of the effective stress is K0.1733 e-Δδ，R20.8667, as shown in fig. 3;
the permeability of the test piece 10 to be tested after hydraulic sand fracturing changes 2480 times.
Example 2
The embodiment provides a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir.
Preparation of test piece 10 to be tested: selecting a No. 3 coal seam of a Jincheng Temple river coal mine, burying the coal seam for 550m at the temperature of 28.5 ℃, and preparing a standard cylindrical original rock test piece;
early preparation: and (3) putting the original rock sample into an oven for drying, and drilling holes with the diameter phi of 8.0mm and the depth H6.3mm in the center of the two end surfaces of the test piece by using a bench drill. Measuring the dimensions of the test piece 10 to be tested: height 102.0mm, specimen diameter 50 mm. After a test piece 10 to be tested is naturally dried, the test piece is placed into a fracturing fluid to be soaked so that the test piece is in a water saturation state, wherein the formula of the fracturing fluid is 0.5% of KCL + 0.2% of JH-20 cleanup additive 0.6% + 98.1% of clear water.
The method specifically comprises the following steps:
assembling a test piece 10 to be tested of saturated fracturing fluid and a corresponding pressure pad into a cavity of a triaxial apparatus, pushing a piston to be contacted with a first pressure head 101 by a debugging axial actuator 3, debugging a strain gauge 11 in place, putting down a thick-wall cylinder of a pressure container 1 to realize sealing, and fixing the test piece 10 to be tested of the saturated fracturing fluid in the pressure container 1;
according to the original rock stress condition, an initial confining pressure with the value of 11.8MPa is applied to a test piece 10 to be tested through a confining pressure booster 2 and is kept constant, an initial axial pressure with the value of 14.8MPa is applied to the test piece 10 to be tested through an axial actuator 3 and is kept constant, an initial pore pressure with the value of 3.8MPa is applied to the test piece 10 to be tested through a pore pressure booster 5, and the initial temperature of the test piece 10 to be tested is set to be 28.5 ℃ through a temperature control assembly 4 and is kept constant;
according to the stress-strain condition, applying a plurality of variable pore pressures to the upper end of a test piece 10 to be tested through a pore pressure booster 5, wherein the difference value between two adjacent variable pore pressures is 2MPa, the variable pore pressures are increased to 13.62MPa step by step, and an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length are obtained by recording through a pulse tester 6;
calculating according to a formula (1) to obtain an initial permeability and a plurality of first permeabilities of the test piece to be tested 10 before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested 10 when each variable pore pressure is applied, an application time of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested 10 after each application time;
when the volume strain of the test piece 10 to be tested is accelerated to enter a negative sudden change stage, the strain gauge 11 records whether the test piece 10 to be tested under the corresponding pore pressure condition is deformed, namely, the test piece 10 to be tested is judged to be damaged under pressure. After a test piece 10 to be tested is broken, applying a first preset pressure to the upper end of the test piece 10 to be tested through a pore pressure booster 5, opening a sand adding assembly 7, paving 250g of 700-mesh carborundum and 50g of 600-mesh carborundum propping agent in a pressed dynamic crack, injecting fracturing fluid into the upper end of the test piece 10 to be tested through a fracturing fluid injection assembly 8, applying a second preset pressure to the lower end of the test piece 10 to be tested through a flow measuring assembly 9, and obtaining a flow with stable permeation after the second preset pressure is applied;
and calculating according to the flow, the first preset pressure and the second preset pressure and the formula (2) to obtain a second permeability of the test piece 10 to be tested after the test piece is broken.
The fitting result of experimental data shows that: the relation curve of the permeability before the test piece 10 to be tested is cracked and the effective stress is that K is 0.002e-0.274Δδ，R20.8314, as shown in fig. 4;
the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand fracturing is 2950 times.
Example 3
The embodiment provides a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir.
Preparation of test piece 10 to be tested: selecting a No. 3 coal seam of a Jincheng Temple river coal mine, burying the coal seam for 550m at the temperature of 28.5 ℃, and preparing a standard cylindrical original rock test piece;
early preparation: and (3) putting the original rock sample into an oven for drying, and drilling holes with the diameter phi of 8.0mm and the depth H6.5mm in the center of the two end surfaces of the test piece by using a bench drill. Measuring the dimensions of the test piece 10 to be tested: height 102.76mm, test piece diameter 50 mm. After a test piece 10 to be tested is naturally dried, the test piece is placed into a fracturing fluid to be soaked so that the test piece is in a water saturation state, wherein the formula of the fracturing fluid is 0.5% of KCL + 0.2% of JH-20 cleanup additive 0.6% + 98.1% of clear water.
The method specifically comprises the following steps:
assembling a test piece 10 to be tested of saturated fracturing fluid and a corresponding pressure pad into a cavity of a triaxial apparatus, pushing a piston to be contacted with a first pressure head 101 by a debugging axial actuator 3, debugging a strain gauge 11 in place, putting down a thick-wall cylinder of a pressure container 1 to realize sealing, and fixing the test piece 10 to be tested of the saturated fracturing fluid in the pressure container 1;
according to the original rock stress condition, an initial confining pressure with the value of 11.8MPa is applied to a test piece 10 to be tested through a confining pressure booster 2 and is kept constant, an initial axial pressure with the value of 14.8MPa is applied to the test piece 10 to be tested through an axial actuator 3 and is kept constant, an initial pore pressure with the value of 3.8MPa is applied to the test piece 10 to be tested through a pore pressure booster 5, and the initial temperature of the test piece 10 to be tested is set to be 28.5 ℃ through a temperature control assembly 4 and is kept constant;
according to the stress-strain condition, applying a plurality of variable pore pressures to the upper end of a test piece 10 to be tested through a pore pressure booster 5, wherein the difference value between two adjacent variable pore pressures is 2MPa, the variable pore pressures are increased to 13.75MPa step by step, and an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length are obtained by recording through a pulse tester 6;
calculating according to a formula (1) to obtain an initial permeability and a plurality of first permeabilities of the test piece to be tested 10 before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested 10 when each variable pore pressure is applied, an application time of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested 10 after each application time;
when the volume strain of the test piece 10 to be tested is accelerated to enter a negative sudden change stage, the strain gauge 11 records whether the test piece 10 to be tested under the corresponding pore pressure condition is deformed, namely, the test piece 10 to be tested is judged to be damaged under pressure. After a test piece 10 to be tested is broken, applying a first preset pressure to the upper end of the test piece 10 to be tested through a pore pressure booster 5, opening a sand adding component 7, paving 250g of 600-mesh carborundum +50g of 500-mesh carborundum propping agent in a pressed dynamic crack, injecting fracturing fluid into the upper end of the test piece 10 to be tested through a fracturing fluid injection component 8, applying a second preset pressure to the lower end of the test piece 10 to be tested through a flow measuring component 9, and obtaining a flow with stable permeation after the second preset pressure is applied;
and calculating according to the flow, the first preset pressure and the second preset pressure and the formula (2) to obtain a second permeability of the test piece 10 to be tested after the test piece is broken.
The fitting result of experimental data shows that: the curve of the permeability before fracture of the test piece 10 to be tested as a function of the effective stress is K0.0208 e-0.775Δδ，R20.9084, as shown in the figure5 is shown in the specification;
the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand fracturing is 3215 times.
Example 4
The embodiment provides a method for testing permeability in a hydraulic sand fracturing process of a tight reservoir.
Preparation of test piece 10 to be tested: selecting a No. 3 coal seam of a Jincheng Temple river coal mine, burying the coal seam for 550m at the temperature of 28.5 ℃, and preparing a standard cylindrical original rock test piece;
early preparation: and (3) putting the original rock sample into an oven for drying, and drilling holes with the diameter phi of 8.0mm and the depth H6.4mm in the center of the two end surfaces of the test piece by using a bench drill. Measuring the dimensions of the test piece 10 to be tested: the height is 103.7mm, and the diameter of the test piece is 50 mm. After a test piece 10 to be tested is naturally dried, the test piece is placed into a fracturing fluid to be soaked so that the test piece is in a water saturation state, wherein the formula of the fracturing fluid is 0.5% of KCL + 0.2% of JH-20 cleanup additive 0.6% + 98.1% of clear water.
The method specifically comprises the following steps:
assembling a test piece 10 to be tested of saturated fracturing fluid and a corresponding pressure pad into a cavity of a triaxial apparatus, pushing a piston to be contacted with a first pressure head 101 by a debugging axial actuator 3, debugging a strain gauge 11 in place, putting down a thick-wall cylinder of a pressure container 1 to realize sealing, and fixing the test piece 10 to be tested of the saturated fracturing fluid in the pressure container 1;
according to the original rock stress condition, an initial confining pressure with the value of 11.8MPa is applied to a test piece 10 to be tested through a confining pressure booster 2 and is kept constant, an initial axial pressure with the value of 14.8MPa is applied to the test piece 10 to be tested through an axial actuator 3 and is kept constant, an initial pore pressure with the value of 3.8MPa is applied to the test piece 10 to be tested through a pore pressure booster 5, and the initial temperature of the test piece 10 to be tested is set to be 28.5 ℃ through a temperature control assembly 4 and is kept constant;
according to the stress-strain condition, applying a plurality of variable pore pressures to the upper end of a test piece 10 to be tested through a pore pressure booster 5, wherein the difference value between two adjacent variable pore pressures is 2MPa, the variable pore pressures are increased to 12.36MPa step by step, and an initial pressure difference between the upper end and the lower end of the test piece 10 to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece 10 to be tested after each application time length are obtained by recording through a pulse tester 6;
calculating according to a formula (1) to obtain an initial permeability and a plurality of first permeabilities of the test piece to be tested 10 before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested 10 when each variable pore pressure is applied, an application time of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested 10 after each application time;
when the volume strain of the test piece 10 to be tested is accelerated to enter a negative sudden change stage, the strain gauge 11 records whether the test piece 10 to be tested under the corresponding pore pressure condition is deformed, namely, the test piece 10 to be tested is judged to be damaged under pressure. After a test piece 10 to be tested is broken, applying a first preset pressure to the upper end of the test piece 10 to be tested through a pore pressure booster 5, opening a sand adding component 7, paving 250g of 500-mesh carborundum and 50g of 400-mesh carborundum propping agent in a pressed dynamic crack, injecting fracturing fluid into the upper end of the test piece 10 to be tested through a fracturing fluid injection component 8, applying a second preset pressure to the lower end of the test piece 10 to be tested through a flow measuring component 9, and obtaining a flow with stable permeation after the second preset pressure is applied;
and calculating according to the flow, the first preset pressure and the second preset pressure and the formula (2) to obtain a second permeability of the test piece 10 to be tested after the test piece is broken.
The fitting result of experimental data shows that: the relation curve of the permeability before the test piece 10 to be tested is cracked and the effective stress is that K is 0.0036e-0.354Δδ，R20.8260, as shown in fig. 6;
the permeability of the test piece 10 to be tested after hydraulic sand fracturing changes by 3689 times.
In conclusion, the method for testing the permeability of the compact reservoir in the hydraulic sand fracturing process can realize the test of the permeability of the test piece 10 to be tested in the hydraulic sand fracturing process under the simulated formation state, and the change rule of the permeability of the compact reservoir in the hydraulic sand fracturing process is determined by utilizing the relation curve of the permeability of the test piece 10 to be tested before fracture and the effective stress and the change multiple of the permeability of the test piece 10 to be tested after hydraulic sand fracturing, so that the theoretical basis is provided for reasonable and efficient development of the compact reservoir.
The above description is only for facilitating the understanding of the technical solutions of the present invention by those skilled in the art, and is not intended to limit the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. A method for testing permeability of a tight reservoir in a hydraulic sand fracturing process is characterized by comprising the following steps:
fixing a test piece to be tested of the saturated fracturing fluid in a pressure container;
according to the pressure and the temperature of a compact reservoir under the stratum condition, applying initial confining pressure to a test piece to be tested through a confining pressure booster, applying initial axial pressure to the test piece to be tested through an axial actuator, applying initial temperature to the test piece to be tested through a temperature control assembly, and applying initial pore pressure to the test piece to be tested through a pore pressure booster;
applying a plurality of variable pore pressures to the upper end of the test piece to be tested through the pore pressure booster, wherein the difference values between two adjacent variable pore pressures are the same, and obtaining an initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied, the application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time length by using a pulse tester;
obtaining an initial permeability and a plurality of first permeabilities of the test piece to be tested before rupture according to an initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied, an application time length of each variable pore pressure and an ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time length;
fitting to obtain a relation curve of the permeability of the test piece to be tested before fracture and the effective stress according to the first permeabilities of the test piece to be tested before fracture, the initial confining pressure and the variable pore pressures;
when the test piece to be tested is broken, closing the pulse tester, applying a first preset pressure to the upper end of the test piece to be tested through the pore pressure booster, adding sand to the upper end of the test piece to be tested through the sand adding component, injecting fracturing fluid into the upper end of the test piece to be tested through the fracturing fluid injection component, applying a second preset pressure to the lower end of the test piece to be tested through the flow measuring component, and obtaining the flow with stable seepage after the second preset pressure is applied;
obtaining a second permeability of the test piece to be tested after the test piece to be tested is broken according to the flow, the first preset pressure and the second preset pressure;
and obtaining the change multiple of the permeability of the test piece to be tested after hydraulic sand fracturing according to the initial permeability of the test piece to be tested and the second permeability of the test piece to be tested after cracking.
2. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process of the tight reservoir according to claim 1, wherein before the test piece to be tested of the saturated fracturing fluid is fixed in the pressure container, the method further comprises the following steps: and acquiring the length and the diameter of the cross section of the test piece to be tested.
3. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process of the tight reservoir according to claim 2, wherein after the length and the diameter of the cross section of the test piece to be tested are obtained, the method further comprises the following steps: and photographing or carrying out nuclear magnetic resonance imaging on the test piece to be tested to obtain the natural microcrack condition of the test piece to be tested.
4. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process according to claim 3, wherein after the test piece to be tested is dried and then photographed or subjected to nuclear magnetic resonance imaging to obtain the natural microcrack condition of the test piece to be tested, the method further comprises the following steps: and saturating the fracturing fluid after vacuumizing the test piece to be tested, and continuously vacuumizing.
5. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process according to claim 1, wherein the initial permeability and the plurality of first permeabilities before fracture of the test piece to be tested are obtained according to the following calculation formula:
Figure FDA0003316246110000021
in the formula: k is the initial or first permeability in x 10-3μm2(ii) a Mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa-sec; beta is the compression coefficient of the fracturing fluid and has the unit of Pa-1(ii) a V is the volume of the pressure vessel in cm3；ΔpiThe initial pressure difference between the upper end and the lower end of the test piece to be tested when each variable pore pressure is applied is expressed in kPa; Δ t is an application time period of each of the variable orifice pressures in sec; Δ pfThe unit of the ending pressure difference between the upper end and the lower end of the test piece to be tested after each application time is kPa; a. thesIs the cross-sectional area, cm, of the test piece to be tested2；LsIs the length, cm, of the test piece to be tested.
6. The method for testing permeability during tight reservoir hydraulic sanding fracturing of claim 1, wherein the effective stress is the difference between the initial confining pressure and the variable pore pressure.
7. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process according to claim 1, wherein the second permeability is obtained according to the following calculation formula:
Figure FDA0003316246110000031
in the formula, k isThe second permeability in units of D; mu is the viscosity coefficient of the fracturing fluid, and the unit is Pa-sec; q is the flow rate in m3S; a is the cross-sectional area of the test piece to be tested, m2(ii) a Δ p is the difference between the first preset pressure and the second preset pressure in Pa; and L is the length of the test piece to be tested and has the unit of m.
8. The method for testing the permeability of the tight reservoir in the hydraulic sand fracturing process of the tight reservoir according to claim 7, wherein the difference value between the first preset pressure and the second preset pressure ranges from 0 MPa to 0.7 MPa.
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