Multi-scale fracture core sample based on tight reservoir, experimental device and method

Abstract

The invention relates to a multi-scale fracture core sample based on a tight reservoir, an experimental device and a method, the core sample comprises four core units, the four core units form a cube, longitudinal fractures or transverse fractures are respectively formed between the adjacent core units, and the longitudinal fractures or the transverse fractures are formed between the adjacent core units. The longitudinal crack and the transverse crack respectively penetrate through the rock core sample; the rock core unit is selected from any one or a combination of two or more of a matrix rock core, a micro-fracture rock core and a fractured rock core. The device can effectively simulate the real tight reservoir in the presence of nano-micron cracks, micro-cracks and cracks, and solves the problem that the gas permeability determination experiment of the core sample and the experiment for judging whether the core sample is sensitive to the flow velocity cannot be carried out due to the lack of the core sample based on the multi-scale cracks of the tight reservoir at present.

Description

Technical Field

[0001] The invention relates to the field of oil and gas field development, and in particular to a core sample, an experimental device and a method for multi-scale fractures based on a dense reservoir. Background Art

[0002] Some researchers in China have proposed that the production process of tight reservoir gas is the result of multi-field coupling in the pore-permeability space such as microscopic pore throats, microcracks, macroscopic cracks and hydraulic fractures. Therefore, the flow law of gas in the pore throats of low permeability and tight gas reservoirs is more complex, which is mainly manifested in: (1) The flow of gas in tight reservoirs not only involves seepage, but also nonlinear flows such as slip, diffusion, and desorption; (2) Fracturing is the main technology for achieving effective development of tight reservoirs. Microcracks and fracturing cracks in tight reservoirs are the main channels for fluid circulation; (3) Due to the presence of fractures, the conductivity of the fractures near the bottom of the well is very strong, resulting in a high-speed non-Darcy flow effect. At present, the basic theoretical and methodological research on gas flow in tight reservoirs in China is still insufficient, especially when pores and fractures of various scales coexist, there is a lack of accurate description of the seepage law of fluids in the reservoir. Therefore, designing a physical simulation device and method that can simulate the flow of fluids in the matrix of tight reservoirs in the presence of microcracks of different scales is of great significance for studying the flow law in tight gas reservoirs.

[0003] At present, in the process of studying the seepage laws of fluids in reservoirs where pores and fractures of various scales coexist, there is a lack of core samples based on multi-scale fractures in tight reservoirs, and it is impossible to carry out experiments to measure the gas permeability of core samples and to determine whether core samples are sensitive to flow velocity. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a core sample, experimental device and method based on multi-scale fractures of tight reservoirs, which are used to solve the problem that in the current study on the seepage laws of fluids in reservoirs where pores and fractures of various scales coexist, there is a lack of core samples based on multi-scale fractures of tight reservoirs, and it is impossible to carry out experiments to measure the gas permeability of core samples and to determine whether the core samples are sensitive to flow velocity.

[0005] To achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention discloses a core sample with multi-scale fractures based on a tight reservoir, comprising four core units, wherein the four core units form a cube, longitudinal fractures or transverse fractures are formed between adjacent core units, and the longitudinal fractures and the transverse fractures respectively penetrate the core sample; The core unit is selected from any one of matrix core, micro-fracture core and fracture core or a combination of two or more thereof; The matrix core is tested by nuclear magnetic resonance testing technology to select the core with suitable nano-micron pore size; The micro-crack core is heated by microwaves to generate micro-cracks inside the core, and then the core is tested by nuclear magnetic resonance testing technology to select the core with appropriate crack size; The fractured core is a core with obvious fractures visible to the naked eye obtained by fracturing the matrix core using the Brazilian splitting method.

[0006] Preferably, the width, height and length of the core unit are 25, 25 and 100 mm respectively; the width, height and length of the core sample are 50, 50 and 100 mm respectively.

[0007] Further, the core samples include 12 combinations, which are respectively a combination of four matrix cores, or a combination of three matrix cores and one microfracture core, or a combination of three matrix cores and one fracture core, or a combination of two matrix cores and two microfracture cores, or a combination of two matrix cores and two fracture cores, or a combination of two matrix cores, one microfracture core and one fracture core, or a combination of one matrix core and three microfracture cores, or a combination of one matrix core, two microfracture cores and one fracture core, or a combination of one matrix core, one microfracture core and two fracture cores, or a combination of one matrix core and three fracture cores, or a combination of four microfracture cores, or a combination of four fracture cores; each combination constitutes a core sample, and a total of 12 core samples are obtained.

[0008] In a second aspect, the present invention further discloses an experimental device for multi-scale fractures based on a tight reservoir, comprising a core holder, an injection pump, a confining pressure pump, a back pressure pump, a gas intermediate container, a liquid intermediate container and an exhaust valve, wherein the core holder comprises an inlet, an outlet and a confining pressure port, and is pre-installed with the above-mentioned core sample, the gas intermediate container is pre-stored with gas, and the liquid intermediate container is pre-stored with liquid; the outlet of the injection pump is respectively connected to the inlet of the liquid intermediate container and the gas intermediate container through a pipeline, and the outlet of the liquid intermediate container and the gas intermediate container is respectively connected to the inlet of the core holder through a pipeline; The outlet of the core holder is connected to the inlet of the back pressure pump through a pipeline; the confining pressure port of the core holder is connected to the outlet of the confining pressure pump through a pipeline; Wherein, an exhaust valve is arranged on the main pipeline connecting the outlet of the liquid intermediate container and the gas intermediate container with the inlet of the core holder; A back-pressure valve is provided in a pipeline connecting the outlet of the core holder with the outlet of the back-pressure pump.

[0009] Preferably, the injection pump, the confining pressure pump and the back-pressure pump are all constant speed and constant pressure pumps.

[0010] Furthermore, it also includes an upstream pressure sensor, a downstream pressure sensor and a differential pressure sensor, wherein the upstream pressure sensor and the downstream pressure sensor are respectively arranged at the inlet and the outlet of the core clamp; the inlet and the outlet of the core clamp are connected by a pipeline, and a differential pressure sensor is arranged on the pipeline connecting the inlet and the outlet of the core clamp.

[0011] Furthermore, a liquid outlet is provided on the back-pressure valve, and a fluid collector is provided directly below the liquid outlet of the back-pressure valve, and the fluid collector is a measuring cylinder or a beaker.

[0012] Furthermore, a flow meter is provided on the pipeline connecting the outlet of the core holder and the back pressure valve.

[0013] In a third aspect, the present invention also discloses an experimental method for determining the gas permeability of a core sample, using the above-mentioned experimental device, characterized in that it includes: Step A: Place the core sample mentioned above into a square core holder; Step B: introducing gas into the core holder to measure the gas permeability of the core sample, including the following steps: Step B1: start the confining pressure pump to inject liquid into the core holder; Step B2: Start the injection pump and inject the gas stored in the gas intermediate container into the inlet of the core holder to perform a gas permeability test until the gas flow at the outlet of the core holder is stable. The gas flow at the outlet of the core holder is collected by a flow meter. The pressure values ​​of the inlet of the core holder are collected by the upstream pressure sensor and the downstream pressure sensor respectively. and the pressure at the core holder outlet ; Step B3: Calculate the gas permeability of the core sample, and the expression is:

[0014] In the formula, represents the gas permeability of the core sample; Indicates the gas flow rate at the outlet of the core holder; represents the viscosity of the gas, represents the length of the core sample, Represents the cross-sectional area of ​​the core sample.

[0015] Indicates the pressure value at the inlet of the core holder; Indicates the pressure value at the outlet of the core holder.

[0016] Step B4: exhausting the gas in the pipeline connecting the outlet of the gas intermediate container to the inlet of the core holder.

[0017] In a fourth aspect, the present invention also discloses an experimental method for determining whether a core sample is sensitive to flow velocity, using the above-mentioned experimental device, characterized in that it includes: Step S1: placing the core sample mentioned above into a square core holder; Step S2: introducing liquid into the core holder to measure the liquid permeability of the core sample at different flow rates and the pressure gradient between the inlet and outlet ends of the core sample, including the following steps: Step S21: adjusting the flow rate of the injection pump to a minimum flow rate, wherein the minimum flow rate is 0.1 ml / min; Step S22: Calculating the liquid permeability of the core sample when the flow rate of the injection pump is adjusted to the minimum flow rate, including the following steps: Step S221: starting the confining pressure pump to inject liquid into the core holder; Step S222: Start the injection pump to inject the liquid stored in the liquid intermediate container into the inlet of the core holder, start the back pressure pump to form a barrier to the liquid at the outlet of the core holder to simulate the pressure environment of the real formation, and conduct a liquid displacement experiment until the liquid flow at the outlet of the core holder is stable. The flow meter collects the liquid flow at the outlet of the core holder when the flow of the injection pump is adjusted to the minimum flow. The pressure value of the inlet of the core holder when the flow rate of the injection pump is adjusted to the minimum flow rate is collected by the upstream pressure sensor and the downstream pressure sensor. and the pressure at the core holder outlet ; Step S223: Calculate the liquid permeability of the core sample when the flow rate of the injection pump is adjusted to the minimum flow rate, and the expression is:

[0018] In the formula, represents the liquid permeability of the core sample 200 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate; It indicates the liquid flow rate at the outlet of the core holder when the injection pump is adjusted to the minimum flow rate; The viscosity of the liquid. represents the length of the core sample, Represents the cross-sectional area of ​​the core sample.

[0019] It indicates the pressure value at the inlet of the core holder when the flow rate of the injection pump is adjusted to the minimum flow rate; It indicates the pressure value at the outlet of the core holder when the flow rate of the injection pump is adjusted to the minimum flow rate; Step S224: The pressure value of the inlet of the core holder when the flow rate of the injection pump is adjusted to the minimum flow rate and the pressure at the core holder outlet and the length of the core sample, calculate the pressure gradient between the inlet and outlet of the core sample when the injection pump flow rate is adjusted to the minimum flow rate; Step S23: the flow rate of the injection pump is increased by 0.1 ml / min, and step S22 is repeated to obtain the liquid permeability of the core sample after the flow rate of the injection pump is increased by 0.1 ml / min and the pressure gradient at the inlet and outlet ends of the core sample; Step S24: increasing the flow rate of the injection pump by 0.1 ml / min each time, and repeating step S23 until the liquid permeability of the core sample and the pressure gradient between the inlet and outlet ends of the core sample when the flow rate of the injection pump is 0.5 ml / min are obtained; Step S3: Compare the liquid permeability of the core samples at all flow states of the injection pump and the pressure gradients at the inlet and outlet ends of the core samples, and determine whether the core samples are sensitive to the flow rate.

[0020] The specific method is: if the difference in liquid permeability of the core samples at all flow states of the injection pump is less than 0.1×10 -15 m 2 , and the pressure gradient between the inlet and outlet of the core sample is not less than 3MPa / cm, the core sample is judged to be insensitive to flow velocity; otherwise, the core sample is judged to be sensitive to flow velocity.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (I) The present invention discloses a core sample, experimental device and method based on multi-scale fractures of a tight reservoir. First, four core units of any one of matrix core, micro-fracture core and fracture core or a combination of two or more thereof are used to form a cube to form a core sample; second, the core sample is placed in the core holder of the experimental device; finally, experiments are carried out to determine the gas permeability of the core sample and to determine whether the core sample is sensitive to flow rate. The present invention can effectively simulate real tight reservoirs with nano-micron fractures, micro-fractures and fractures, solving the problem that the gas permeability determination experiment of the core sample and the determination experiment of whether the core sample is sensitive to flow rate cannot be carried out due to the lack of core samples with multi-scale fractures based on tight reservoirs.

[0022] (ii) The present invention discloses a core sample, experimental device and method based on multi-scale fractures of a tight reservoir. By means of physical experiments, the process of fluid flow in a tight reservoir matrix in the presence of micro-cracks of different scales is simulated, and the foundation is laid for studying the seepage law of fluids when pores and fractures of multiple scales coexist. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of a core sample composed of four core units provided in Example 1 of the present invention; Figure 2 It is a schematic structural diagram of a multi-scale flow experimental device for dense reservoir matrix microfractures provided in Example 2 of the present invention.

[0024] Explanation of reference numerals: 100 - core unit, 200 - core sample; 1-Core holder; 21-injection pump, 22-confining pressure pump, 23-back pressure pump, 24-back pressure valve; 31-gas intermediate container, 32-liquid intermediate container, 33-exhaust valve; 41-upstream pressure sensor, 42-downstream pressure sensor, 43-differential pressure sensor, 5- Fluid collector; 6- Flow meter. DETAILED DESCRIPTION

[0025] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0026] Example 1: A core sample based on multi-scale fractures in a tight reservoir Embodiment 1 of the present invention provides a core sample of multi-scale fractures based on a dense reservoir, and its structure is described in detail below with reference to the accompanying drawings.

[0027] refer to Figure 1 The core sample based on multi-scale fractures of a tight reservoir includes four core units 100. The four core units 100 form a cube, and longitudinal cracks or transverse cracks are formed between adjacent core units, and the longitudinal cracks and the transverse cracks respectively penetrate the core sample 200; The core unit 100 is selected from any one of matrix core, micro-fracture core and fracture core or a combination of two or more thereof; The matrix core is tested by nuclear magnetic resonance testing technology to select the core with suitable nano-micron pore size; The micro-crack core is heated by microwaves to generate micro-cracks inside the core, and then the core is tested by nuclear magnetic resonance testing technology to select the core with appropriate crack size; The fractured core is a core with obvious fractures visible to the naked eye obtained by fracturing the matrix core using the Brazilian splitting method.

[0028] Since the core units 100 constituting the core sample 200 are selected from any one of a matrix core, a microfracture core and a fracture core, and longitudinal fractures or transverse fractures are formed between adjacent core units, and the longitudinal fractures and the transverse fractures respectively penetrate the core sample 200, the core sample 200 composed of four core units is a core sample containing microfractures.

[0029] Specifically, the width, height and length of the core unit 100 are 25, 25 and 100 mm respectively, that is, the size of the core unit 100 satisfies 25×25×100 mm.

[0030] The width, height and length of the core sample 200 are 50, 50 and 100 mm respectively, that is, the size of the core sample 200 satisfies 50×50×100 mm.

[0031] Specifically, the core sample 200 includes 12 combinations, namely: A combination of four matrix cores or A combination of three matrix cores and one microfracture core or A combination of three matrix cores and one fracture core or A combination of two matrix cores and two microfracture cores or A combination of two matrix cores and two fracture cores or A combination of two matrix cores, one microfracture core and one fracture core or A combination of one matrix core and three microfracture cores or A combination of one matrix core, two microfracture cores and one fracture core or A combination of one matrix core, one microfracture core and two fracture cores or A combination of one matrix core and three fracture cores or A combination of four microfracture cores or Combination of four fracture cores.

[0032] Each combination constitutes one kind of core sample 200, and a total of 12 kinds of core samples 200 are obtained.

[0033] The preparation method of the core sample 200 is as follows: Step 1: Take several matrix cores and heat them with microwaves to generate microcracks inside them, thereby obtaining microcrack cores; Step 2: Take several more matrix cores and perform fracturing treatment on the matrix cores using the Brazilian splitting method to form obvious cracks visible to the naked eye, thereby obtaining fractured cores; Step 3: Use nuclear magnetic resonance testing technology to test matrix cores, micro-crack cores, and fracture cores containing nano- and micro-pores, select cores with fracture scales that meet the experimental requirements, and cut them into cores of 25×25×100 mm in size; Step 4: Select four cores of the three prepared fracture scales for combination, forming a total of 12 combinations; Step 5: Apply external loads to each combination respectively, press the four core units 100 into a whole, and obtain a core sample 200 with a size of 50×50×100 mm.

[0034] Each combination constitutes a core sample 200, and a total of 12 core samples 200 are obtained. These 12 core samples 200 will be subsequently placed in the core holder 1 of the experimental device of Example 2 to carry out the experiment of measuring the gas permeability of the core samples in Example 3 and the experiment of determining whether the core samples are sensitive to flow velocity in Example 4.

[0035] Example 2: An experimental device based on multi-scale fractures in a dense reservoir Embodiment 2 of the present invention provides an experimental device for multi-scale fractures based on a dense reservoir, and its structure is described in detail below in conjunction with the accompanying drawings.

[0036] refer to Figure 2The experimental device includes a core holder 1, an injection pump 21, a confining pressure pump 22, a back pressure pump 23, a gas intermediate container 31, a liquid intermediate container 32 and an exhaust valve 33. The core holder 1 comprises an inlet, an outlet and a confining pressure port, and is pre-installed with the core sample 200 of Example 1; the gas intermediate container 31 is pre-stored with gas; and the liquid intermediate container 32 is pre-stored with liquid, including oil or water; The outlet of the injection pump 21 is connected to the inlet of the liquid intermediate container 32 and the gas intermediate container 31 through pipelines, and the outlet of the liquid intermediate container 32 and the gas intermediate container 31 is connected to the inlet of the core holder 1 through pipelines; The outlet of the core holder 1 is connected to the inlet of the back pressure pump 23 through a pipeline; The confining pressure port of the core holder 1 is connected to the outlet of the confining pressure pump 22 through a pipeline; The main pipeline connecting the outlets of the liquid intermediate container 32 and the gas intermediate container 31 with the inlet of the core holder 1 is provided with an exhaust valve 33; A back-pressure valve 24 is provided in the pipeline connecting the outlet of the core holder 1 with the outlet of the back-pressure pump 23 .

[0037] In order to facilitate the exhaust of gas, an exhaust hole is provided on the exhaust valve 33 .

[0038] It should be noted that the core sample 200 pre-installed in the core holder 1 is used to simulate the formation. Since the core sample 200 is a cube, in order to facilitate installation, the cross-sectional shape of the core holder 1 is a square. The core holder 1 is a prior art, including a holder tube, a rubber tube, a holder sealing cover, etc.

[0039] The confining pressure pump 22 applies confining pressure to the core holder 1 to simulate the pressure exerted by the surrounding rock mass on the core; The back pressure pump 23 is used to control the pressure at the outlet of the core holder 1 .

[0040] The liquid intermediate container 32 and the gas intermediate container 31 respectively include storage tanks for storing liquid and gas for experiments. The injection pump 21 injects fluid into the core holder 1 at different flow rates to simulate the flow of different fluids at different flow rates in the formation.

[0041] Specifically, the injection pump 21 , the confining pressure pump 22 and the back pressure pump 23 are all constant speed and constant pressure pumps.

[0042] To collect the pressure values ​​at the inlet and outlet of the core holder 1, continue to refer to Figure 2The experimental device also includes an upstream pressure sensor 41, a downstream pressure sensor 42 and a differential pressure sensor 43. The upstream pressure sensor 41 and the downstream pressure sensor 42 are respectively arranged at the inlet of the core holder 1 and the outlet of the core holder 1; The inlet and outlet of the core holder 1 are connected via a pipeline, and a differential pressure sensor 43 is provided on the pipeline connecting the inlet and outlet of the core holder 1 .

[0043] The upstream pressure sensor 41 and the downstream pressure sensor 42 are used to collect the pressure values ​​at the inlet and outlet of the core holder 1 respectively; the differential pressure sensor 43 is a high linear differential pressure sensor, used to collect the pressure difference between the inlet and outlet of the core holder 1 .

[0044] In order to collect waste liquid, a liquid outlet is provided on the back pressure valve 24 , and a fluid collector 5 is provided directly below the liquid outlet of the back pressure valve 24 .

[0045] Specifically, the fluid collector 5 is a measuring cylinder or a beaker.

[0046] The fluid collector 5 is used to collect the fluid flowing out of the outlet of the core holder 1 and record the fluid flow volume.

[0047] In order to collect the flow rate flowing out from the outlet of the core holder 1 , a flow meter 6 is provided on the pipeline connecting the outlet of the core holder 1 and the back pressure valve 24 .

[0048] In order to facilitate flow control, valves are respectively provided before and after the liquid intermediate container 32, and valves are respectively provided before and after the gas intermediate container 31. Valves are respectively provided on the pipeline connecting the upstream pressure sensor 41 and the pressure difference sensor 43, and on the pipeline connecting the downstream pressure sensor 42 and the pressure difference sensor 43. A valve is provided at the liquid outlet of the back pressure valve 24. A valve is provided at the pipeline connecting the outlet of the back pressure pump 23 and the back pressure valve 24.

[0049] Example 3: An experimental method for determining the gas permeability of a core sample Embodiment 3 of the present invention provides an experimental method for measuring the gas permeability of a core sample, using the core sample of embodiment 1 and the experimental device of embodiment 2, and the method comprises the following steps: Step A: placing the core samples 200 of Example 1 into the square core holders 1 respectively; Step B: introducing gas into the core holder 1 to measure the gas permeability of the core sample 200, including the following steps: Step B1: start the confining pressure pump 22 to inject liquid into the core holder 1; Step B2: Start the injection pump 21, inject the gas stored in the gas intermediate container 31 into the inlet end of the core holder 1, and perform a gas permeability test until the gas flow at the outlet end of the core holder 1 is stable. The gas flow at the outlet of the core holder 1 is collected by the flow meter 6. The pressure values ​​at the inlet of the core holder 1 are collected by the upstream pressure sensor 41 and the downstream pressure sensor 42. and the pressure value at the outlet of core holder 1 ; Among them, in order to ensure the confining pressure effect, the confining pressure must be 1 to 2 MPa higher than the internal pressure.

[0050] Step B3: Calculate the gas permeability of the core sample, and the expression is:

[0051] In the formula, represents the gas permeability of the core sample 200; represents the gas flow rate at the outlet of the core holder 1; Indicates the viscosity of the gas; represents the length of the core sample 200; represents the cross-sectional area of ​​the core sample 200; Indicates the pressure value at the inlet of the core holder 1; Indicates the pressure value at the outlet of the core holder 1.

[0052] Step B4: exhausting the gas in the pipeline connecting the outlet of the gas intermediate container 31 to the inlet of the core holder 1 .

[0053] The specific method is: keep the injection pump 21 in the starting state until liquid flows out of the exhaust hole of the exhaust valve.

[0054] Example 4: An experimental method for determining whether a core sample is sensitive to flow velocity Embodiment 4 of the present invention provides an experimental method for determining whether a core sample is sensitive to flow velocity, using the core sample of embodiment 1 and the experimental device of embodiment 2, the method comprising the following steps: Step S1: placing the core samples 200 of Example 1 into the square core holders 1 respectively; Step S2: introducing liquid into the core holder 1 to measure the liquid permeability of the core sample 200, including the following steps: Step S21: adjusting the flow rate of the injection pump 21 to a minimum flow rate, wherein the minimum flow rate is 0.1 ml / min; Step S22: Calculating the liquid permeability of the core sample 200 and the pressure gradient between the inlet and outlet of the core sample when the flow rate of the injection pump 21 is adjusted to the minimum flow rate, including the following steps: Step S221: starting the confining pressure pump 22 to inject liquid into the core holder 1; Step S222: Start the injection pump 21 to inject the liquid stored in the liquid intermediate container 32 into the inlet end of the core holder 1, start the back pressure pump 23 to form a barrier to the liquid at the outlet end of the core holder 1, so as to simulate the pressure environment of the real formation, and conduct a liquid displacement experiment until the liquid flow at the outlet end of the core holder 1 is stable, and collect the liquid flow at the outlet of the core holder 1 when the flow of the injection pump 21 is adjusted to the minimum flow through the flow meter 6 The pressure value at the inlet of the core holder 1 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate is collected by the upstream pressure sensor 41 and the downstream pressure sensor 42. and the pressure value at the outlet of core holder 1 ; Step S223: Calculate the liquid permeability of the core sample when the flow rate of the injection pump 21 is adjusted to the minimum flow rate, and the expression is:

[0055] In the formula, represents the liquid permeability of the core sample 200 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate; represents the liquid flow rate at the outlet of the core holder 1 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate; Indicates the viscosity of the liquid; represents the length of the core sample 200; represents the cross-sectional area of ​​the core sample 200; represents the pressure value at the inlet of the core holder 1 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate; It represents the pressure value at the outlet of the core holder 1 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate.

[0056] Step S224: The pressure value at the inlet of the core holder 1 when the flow rate of the injection pump 21 is adjusted to the minimum flow rate and the pressure value at the outlet of core holder 1 and the length of the core sample 200, calculate the pressure gradient between the inlet and outlet of the core sample when the flow rate of the injection pump 21 is adjusted to the minimum flow rate.

[0057] Step S23: the flow rate of the injection pump 21 is increased by 0.1 ml / min, and step S22 is repeated to obtain the liquid permeability of the core sample after the flow rate of the injection pump 21 is increased by 0.1 ml / min and the pressure gradient at the inlet and outlet ends of the core sample; Step S24: increasing the flow rate of the injection pump 21 by 0.1 ml / min each time, and repeating step S23 until the liquid permeability of the core sample and the pressure gradient between the inlet and outlet ends of the core sample when the flow rate of the injection pump 21 is 0.5 ml / min are obtained; Step S3: Compare the liquid permeability of the core samples at all flow states of the injection pump 21 and the pressure gradients at the inlet and outlet ends of the core samples, and determine whether the core samples are sensitive to the flow rate.

[0058] The specific method is: if the difference in liquid permeability of the core samples at all flow states of the injection pump 21 is less than 0.1×10 -15 m 2 , and the pressure gradient between the inlet and outlet of the core sample is not less than 3MPa / cm, the core sample is judged to be insensitive to flow velocity; otherwise, the core sample is judged to be sensitive to flow velocity.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A core sample of multi-scale fractures based on a tight reservoir, characterized in that: It consists of four core units (100), Four core units (100) form a cube, longitudinal cracks or transverse cracks are formed between adjacent core units, and the longitudinal cracks and transverse cracks respectively penetrate the core sample (200); The core unit (100) is selected from any one of matrix core, micro-fracture core and fracture core, or a combination of two or more thereof; The matrix core is tested by nuclear magnetic resonance testing technology to select the core with suitable nano-micron pore size; The micro-crack core is heated by microwaves to generate micro-cracks inside the core, and then the core is tested by nuclear magnetic resonance testing technology to select the core with appropriate crack size; The fractured core is a core with obvious fractures visible to the naked eye obtained by fracturing the matrix core using the Brazilian splitting method.

2. The core sample according to claim 1, characterized in that: The width, height and length of the core unit (100) are 25, 25 and 100 mm respectively; The width, height and length of the core sample (200) are 50, 50 and 100 mm respectively.

3. The core sample according to claim 1, characterized in that: The core samples (200) include 12 combinations, which are respectively a combination of four matrix cores, a combination of three matrix cores and one microfracture core, a combination of three matrix cores and one fracture core, a combination of two matrix cores and two microfracture cores, a combination of two matrix cores and two fracture cores, a combination of two matrix cores, one microfracture core and one fracture core, a combination of one matrix core and three microfracture cores, a combination of one matrix core, two microfracture cores and one fracture core, a combination of one matrix core, one microfracture core and two fracture cores, a combination of one matrix core and three fracture cores, a combination of four microfracture cores, or a combination of four fracture cores; Each combination constitutes one core sample (200), and a total of 12 core samples (200) are obtained.

4. An experimental device for multi-scale fractures based on tight reservoirs, characterized in that: The invention comprises a core holder (1), an injection pump (21), a confining pressure pump (22), a back pressure pump (23), a gas intermediate container (31), a liquid intermediate container (32) and an exhaust valve (33). The core holder (1) comprises an inlet, an outlet and a confining pressure port, and is pre-installed with a core sample (200) according to any one of claims 1 to 3; the gas intermediate container (31) is pre-stored with gas, and the liquid intermediate container (32) is pre-stored with liquid; The outlet of the injection pump (21) is respectively connected to the inlet of the liquid intermediate container (32) and the gas intermediate container (31) through pipelines, and the outlet of the liquid intermediate container (32) and the gas intermediate container (31) is respectively connected to the inlet of the core holder (1) through pipelines; The outlet of the core holder (1) is connected to the inlet of the back pressure pump (23) through a pipeline; The confining pressure port of the core holder (1) is connected to the outlet of the confining pressure pump (22) via a pipeline; Wherein, an exhaust valve (33) is provided on the main pipeline connecting the outlets of the liquid intermediate container (32) and the gas intermediate container (31) with the inlet of the core holder (1); A back-pressure valve (24) is provided in a pipeline connecting the outlet of the core holder (1) and the outlet of the back-pressure pump (23).

5. The experimental device according to claim 4, characterized in that: The injection pump (21), the confining pressure pump (22) and the back-pressure pump (23) are all constant-speed and constant-pressure pumps.

6. The experimental device according to claim 4, characterized in that: It also includes an upstream pressure sensor (41), a downstream pressure sensor (42) and a differential pressure sensor (43), The upstream pressure sensor (41) and the downstream pressure sensor (42) are respectively arranged at the inlet and the outlet of the core holder (1); The inlet and outlet of the core holder (1) are connected via a pipeline, and a differential pressure sensor (43) is provided on the pipeline connecting the inlet and outlet of the core holder (1).

7. The experimental device according to claim 4, characterized in that: The back-pressure valve (24) is provided with a liquid outlet, and a fluid collector (5) is provided directly below the liquid outlet of the back-pressure valve (24), wherein the fluid collector (5) is a measuring cylinder or a beaker.

8. The experimental device according to claim 7, characterized in that: A flow meter (6) is provided on the pipeline connecting the outlet of the core holder (1) and the back pressure valve (24).

9. An experimental method for determining the gas permeability of a core sample, using the experimental device according to any one of claims 4 to 8, characterized in that: include Step A: placing the core sample (200) according to any one of claims 1 to 3 into a square core holder (1); Step B: introducing gas into the core holder (1) to measure the gas permeability of the core sample (200).

10. An experimental method for determining whether a core sample is sensitive to flow velocity, using the experimental device according to any one of claims 4 to 8, characterized in that: include Step S1: placing the core sample (200) according to any one of claims 1 to 3 into a square core holder (1); Step S2: introducing liquid into the core holder (1) to measure the liquid permeability of the core sample (200) at different flow rates and the pressure gradient between the inlet and outlet ends of the core sample; Step S3: comparing the liquid permeability of the core samples at all flow states of the injection pump (21) and the pressure gradients at the inlet and outlet ends of the core samples, and determining whether the core samples are sensitive to the flow rate.

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