Variable-diameter rock core displacement simulation device and simulation method for simulating underground fluid migration in near wellbore area

Abstract

The invention relates to the technical field of oil and gas field development engineering, carbon storage engineering, underground hydrogen storage technology, underground fluid seepage mechanical experiment technology and rock core displacement simulation devices, and particularly discloses a variable-diameter rock core displacement simulation device and method for simulating underground fluid migration in a near wellbore area. In order to solve the problem that a traditional equal-diameter rock core displacement device cannot simulate sudden drop of flow velocity and pressure gradient when fluid in a near wellbore zone flows radially, the device adopts a variable-diameter rock core design, the diameter of the variable-diameter rock core is gradually increased in the flowing direction, and a back pressure control system, a confining pressure system and a multi-point pressure measuring structure are matched. Under the fixed injection flow, the cross sectional area of the rock core is increased, so that the Darcy flow velocity and the pressure gradient are suddenly reduced from the injection end to the output end, and the flowing environment of the immediate vicinity of a wellbore is directly reproduced. The device can directly and efficiently simulate and test the phase state behavior, rheological property and seepage law of complex fluid under the condition of sudden change of the flow velocity and pressure gradient in the immediate vicinity of wellbore, and the engineering representativeness of experimental data is remarkably improved.

Description

Technical Field

[0001] This invention relates to the fields of oil and gas field development engineering, carbon storage engineering, underground hydrogen storage technology, underground fluid seepage mechanics experimental technology and core displacement simulation device technology, specifically to a variable diameter core displacement simulation device and simulation method for simulating underground fluid migration in the near-wellbore zone. Background Technology

[0002] Fluid seepage processes in underground rock pores are prevalent in various engineering fields, including oil and gas field development, carbon dioxide utilization and storage, underground hydrogen storage, and groundwater remediation. Quantitative evaluation and in-depth understanding of fluid seepage mechanisms in porous media are key technologies for effectively controlling and optimizing these processes. Due to limitations in objective technology and economic conditions, directly observing or detecting the migration and distribution patterns of fluids in rocks thousands of meters underground presents numerous challenges. Core displacement experiments are an experimental method that simulates the migration of multiphase fluids in rock pores under formation pressure and temperature conditions. It is the most commonly used experimental method for revealing the migration patterns and seepage mechanisms of fluids in porous media.

[0003] In oil and gas extraction, carbon dioxide utilization and storage, underground hydrogen storage, and groundwater remediation, injected fluids are injected into the formation through cylindrical wellbores (vertical or horizontal) and flow radially into the deeper formation until they become linear due to formation constraints. During this radial flow, pressure gradients and flow velocities drop sharply near the wellbore (for example, when the distance between injection and production wells is less than 500 meters, approximately 70% of the pressure drop occurs within about 1 meter of the wellbore). The physical properties of non-Newtonian fluids and other complex fluids, such as polymers and foams, are severely affected by flow velocities and pressure gradients, especially their rheological properties or phases, which are significantly influenced by pressure or pressure gradients. For instance, in carbon dioxide flooding or storage, when carbon dioxide flows from the high-pressure near-wellbore zone to the low-pressure deeper formation, it undergoes expansion and heat absorption (i.e., the Joule-Thomson effect), causing a sharp drop in temperature in its vicinity. Under specific high-pressure, low-temperature environments, carbon dioxide can react with water and even form solid carbon dioxide hydrates, blocking the formation and severely affecting its injection capacity and storage potential.

[0004] However, traditional core displacement simulation devices use constant-diameter cores. At a given volumetric flow rate, the Darcy velocity remains constant throughout the core, and the pressure gradient varies slowly along the flow direction or remains constant when steady-state flow is reached. Therefore, conventional constant-diameter core displacement devices cannot simulate the flow environment in actual formations where fluids flow radially from the wellbore towards the near-wellbore zone and then into deeper formations, experiencing a sudden drop in pressure gradient and velocity. This limitation of traditional constant-diameter core displacement devices makes it impossible to effectively simulate and evaluate the phase behavior, rheology, and overall seepage characteristics of fluids in the approximately 1-meter near-wellbore zone. Summary of the Invention

[0005] The purpose of this invention is to provide a variable-diameter core displacement simulation device based on Darcy's law to simulate the migration of subsurface fluids in the near-wellbore zone. The cores designed for this device have an increasing diameter along the flow direction, unlike traditional constant-diameter core displacement. The cross-sectional area of ​​the variable-diameter core displacement increases with the core diameter, thus achieving the flow conditions where Darcy's velocity and pressure gradient drop sharply from the injection end to the production end. Therefore, this invention can successfully simulate the flow environment of a sharp drop in velocity and pressure gradient during radial flow of fluids in the near-wellbore zone. It can be used to directly and effectively evaluate the phase behavior, rheology, seepage mechanism, and injection capacity of complex fluids under conditions of a sharp drop in velocity and pressure gradient in the near-wellbore zone. Furthermore, under a fixed injection volumetric flow rate, variable-diameter core displacement can simultaneously provide pressure gradient data at multiple flow velocities in a single displacement, making data acquisition more efficient and more representative of fluid seepage behavior in the near-wellbore zone.

[0006] To achieve the above objectives, the present invention provides a variable diameter core displacement simulation device for simulating the movement of underground fluids in the near-wellbore zone, comprising an injection system, a variable diameter core clamping system, a confining pressure system, a back pressure control system, a data processing system, and a gas-liquid separation system arranged in sequence. The variable diameter core clamping system is equipped with a pressure detection component, which is connected to the data processing system. The variable diameter core clamping system is housed in a constant temperature chamber.

[0007] The variable-diameter core clamping system includes a clamping cylinder, a constant-diameter rubber sleeve inside the clamping cylinder, a confining pressure system outside the constant-diameter rubber sleeve, a variable-diameter core inside the constant-diameter rubber sleeve, and a filling layer outside the variable-diameter core. The injection end of the clamping cylinder is connected to the injection system through an inlet connecting component; the output end of the clamping cylinder is connected to the back pressure control system through an outlet connecting component, and the back pressure control system is connected to a gas-liquid separation system.

[0008] Preferably, the variable diameter rock core is a conical variable diameter rock core or a stepped variable diameter rock core;

[0009] The diameter of the cone-shaped variable-diameter core gradually increases from the injection end to the production end;

[0010] The stepped variable diameter core consists of at least three different diameter sections, with the diameter increasing sequentially from the injection end to the production end.

[0011] Preferably, the injection system is used for the injection of different fluids to achieve single-phase fluid injection, two-phase fluid injection, and three-phase fluid injection;

[0012] Single-phase fluid injection can be gas phase injection, oil phase injection, aqueous phase injection, or aqueous solution injection.

[0013] Two-phase fluid injection can be either simultaneous injection of gas and liquid phases or alternating injection of gas and liquid phases.

[0014] Three-phase fluid injection is the simultaneous injection of oil, gas and water.

[0015] Choose the type and method of injection fluid according to the experimental requirements.

[0016] Preferably, the injection system includes an air injection unit, a water injection unit, and an oil injection unit;

[0017] The gas injection unit includes a gas cylinder, a gas injection pipeline connected to the gas cylinder, and a gas flow controller. The gas cylinder is equipped with a pressure regulating valve to control the pressure of the gas output from the gas cylinder. The gas flow controller is controlled by an industrial control computer to control the flow rate of the injected gas. The end of the gas injection pipeline is set in a constant temperature chamber, and the end of the gas injection pipeline is equipped with a gas phase preheating coil for the initial heating of the injected gas.

[0018] The water injection unit includes a constant pressure and constant speed water injection pump and a water injection pipeline. An intermediate water phase container for storing the water phase is installed on the water injection pipeline.

[0019] The oil injection unit includes an oil injection constant pressure and constant speed pump and an oil injection pipeline. An intermediate oil phase container for storing the oil phase is installed on the oil injection pipeline.

[0020] The ends of the gas injection line, water injection line, and oil injection line are all connected to one end of the fluid injection line of the variable diameter core holder, and the other end of the fluid injection line is connected to the inlet connection component of the core holder.

[0021] The gas phase preheating coil, the water phase intermediate container, and the oil phase intermediate container are all placed in a constant temperature chamber. Under the action of the constant temperature chamber, the fluid is heated to the temperature required for the experiment before being injected into the variable diameter rock core.

[0022] Preferably, the inlet connection assembly at the injection end of the clamping cylinder includes a clamping inlet plug, a clamping inlet adjusting cap, and a clamping inlet pressure cap arranged from the inside to the outside.

[0023] Preferably, the pressure detection component is connected to the data processing system;

[0024] The pressure detection assembly includes an inlet pressure sensor, an outlet pressure sensor, and a variable diameter core pressure sensor for collecting pressure from variable diameter cores. Several pressure measuring holes are distributed along the axial direction of the constant diameter rubber sleeve and the filling layer. These pressure measuring holes correspond to various pressure measuring points on the variable diameter core. One end of the pressure measuring pipeline is placed inside the pressure measuring hole, and the other end of the pressure measuring pipeline is connected to the variable diameter core pressure sensor to measure the pressure at each pressure measuring point on the variable diameter core.

[0025] The confining pressure system includes a ring pressure pump used to maintain the pressure around the core sleeve.

[0026] Preferably, the back pressure control system includes a back pressure pump and a back pressure valve installed on the fluid outlet pipeline. The back pressure pump is connected to the back pressure valve to apply the control pressure of the back pressure valve, so as to control the pressure of the fluid flowing out of the back pressure valve in the variable diameter core. One end of the fluid outlet pipeline is connected to the outlet communication component, and the other end of the fluid outlet pipeline is connected to a gas-liquid separation system.

[0027] Preferably, the outlet connection assembly at the output end of the clamping cylinder includes a clamping outlet plug, a clamping outlet adjusting cap, and a clamping outlet pressure cap arranged from the inside to the outside.

[0028] The simulation method based on the above-mentioned variable-diameter core displacement simulation device for simulating near-wellbore subsurface fluid transport includes the following specific steps:

[0029] Step S1: Prepare variable diameter rock cores;

[0030] Step S2: Apply a filler layer to make the outer diameter of the coated variable diameter rock core the same. Fit an equal diameter rubber sleeve, set a pressure measuring point through the pressure measuring hole to the surface of the variable diameter rock core, connect the pressure measuring point and the pressure sensor of the variable diameter rock core with a pressure measuring pipeline, and place the installed variable diameter rock core into the holder cylinder.

[0031] Step S3: Set the temperature of the constant temperature chamber, preheat the injected fluid and the variable diameter core clamping system to the required temperature for the experiment, maintain the pressure outside the variable diameter core sleeve at the set experimental pressure through the confining pressure system, select the injected fluid and injection method required for the experiment through the injection system, and inject the fluid from the variable diameter core injection end through the inlet connecting component of the injection end of the clamping cylinder.

[0032] Step S4: The fluid injection rate is regulated by the injection system, the pressure at each pressure measuring point during the fluid injection process is measured by the pressure detection component, the pressure at each pressure measuring point is collected and recorded by the data processing system as the injection time changes, and the pressure of the fluid flowing out of the back pressure valve in the core is controlled by the back pressure control system.

[0033] Step S5: The fluid flowing out of the back pressure valve flows into the gas-liquid separation system through the outflow fluid pipeline for gas and liquid separation.

[0034] Preferably, step S1 is as follows:

[0035] The variable diameter core size is designed based on the ratio of the actual radial outflow radius of formation fluid to the inflow radius into the near-wellbore zone, as follows:

[0036] The square of the ratio of the diameter of the producing end to the injection end in a conical variable diameter core is the same as the ratio of the radial outflow radius of the actual formation fluid to the flow radius of the fluid flowing into the near-wellbore zone.

[0037] The specific formula is as follows:

[0038] Among them, D 出 D is the diameter of the producing end in a conical variable-diameter core. 入 r is the diameter of the injection end in the tapered variable-diameter rock core. 出 r is the actual radial outflow radius of the formation fluid. 入 The actual radial inflow radius of the formation fluid; the design of the tapered variable-diameter core size is used to reduce the Darcy velocity of the fluid from the injection end to the production end of the variable-diameter core. times;

[0039] The square of the ratio of the maximum to the minimum diameter in a stepped variable-diameter core is the same as the ratio of the radial outflow radius of the actual formation fluid to the flow radius into the near-wellbore zone.

[0040] The specific formula is as follows:

[0041] Where D3 is the diameter of the largest end in the stepped variable-diameter core, D1 is the diameter of the smallest end in the stepped variable-diameter core, and r 出 r is the actual radial outflow radius of the formation fluid. 入 The stepped variable-diameter core size is designed to reduce the Darcy velocity of the fluid from the injection end to the production end, representing the actual radial inflow radius of the formation fluid. times.

[0042] Therefore, this invention can successfully simulate the flow environment of a sudden drop in velocity and pressure gradient during radial flow of fluids in the near-wellbore zone. It can be used to directly and effectively evaluate the phase behavior, rheology, seepage mechanism and injection capacity of complex fluids under the condition of a sudden drop in velocity and pressure gradient in the near-wellbore zone. In addition, under a fixed injection volume flow rate, variable diameter core displacement can provide pressure gradient data at multiple flow rates in a single displacement, making its data acquisition more efficient and more representative of the seepage behavior of fluids in the near-wellbore zone of the formation.

[0043] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a variable-diameter core displacement simulation device for simulating the transport of underground fluids in the near-wellbore zone according to the present invention;

[0045] Figure 2 This is a schematic diagram of the variable diameter rock core clamping system of the present invention clamping a conical variable diameter rock core;

[0046] Figure 3 This is a schematic diagram of the variable diameter rock core clamping system of the present invention clamping a stepped variable diameter rock core;

[0047] Figure 4 This is a schematic diagram of the tapered variable diameter rock core design of the present invention;

[0048] Figure 5 This is a schematic diagram of the stepped variable diameter rock core design of the present invention.

[0049] Figure Labels

[0050] 1. Injection system; 11. Gas cylinder; 12. Gas pressure regulating valve; 13. Gas flow controller; 14. Gas phase preheating coil; 15. Water injection constant pressure and constant speed pump; 16. Oil injection constant pressure and constant speed pump; 17. Aqueous phase intermediate container; 18. Oil phase intermediate container; 2. Variable diameter core clamping system; 20. Clamping device inlet plug; 21. Clamping device outlet plug; 22. Clamping device inlet adjusting cap; 23. Clamping device outlet adjusting cap; 24. Clamping device inlet pressure cap; 25. Clamping device outlet pressure cap; 26. Clamping device cylinder; 27. Equal diameter rubber sleeve; 28. Pressure measuring hole; 29. ​​Pressure measuring pipeline; 3. Constant temperature chamber; 4. Confining pressure system; 5. Back pressure control system; 51. Back pressure pump; 52. Back pressure valve; 6. Data processing system; 7. Gas-liquid separation system. Detailed Implementation

[0051] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0052] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0053] Words like "connect" or "link" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms like "up," "down," "left," and "right" are only used to indicate relative positional relationships; when the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0054] Example 1

[0055] like Figure 1As shown, a variable-diameter core displacement simulation device for simulating near-wellbore subsurface fluid transport includes, in sequence, an injection system 1, a variable-diameter core clamping system 2, a confining pressure system 4, a back pressure control system 5, a data processing system 6, and a gas-liquid separation system 7. The confining pressure system 4 includes an annular pressure pump for maintaining the pressure around the rubber sleeve of the variable-diameter core. A pressure detection component is installed on the variable-diameter core clamping system 2, which is connected to the data processing system 6. The variable-diameter core clamping system 2 is housed within a constant temperature chamber 3.

[0056] Injection system 1 is used for injecting different fluids to achieve single-phase, two-phase, and three-phase fluid injection. Single-phase fluid injection includes gas-phase injection, oil-phase injection, water-phase injection, or aqueous solution injection. Two-phase fluid injection includes simultaneous gas-liquid injection or alternating gas-liquid injection. Three-phase fluid injection includes simultaneous oil-gas-water injection. The type and method of injection fluid are selected according to experimental requirements.

[0057] The injection system 1 includes a gas injection unit, a water injection unit, and an oil injection unit. The gas injection unit includes a gas cylinder 11, a gas injection pipeline connected to the gas cylinder 11, and a gas flow controller 13. A pressure regulating valve 12 is installed on the gas cylinder 11 to control the pressure of the gas output from the gas cylinder 11. The gas flow controller 13 is controlled by an industrial computer to control the flow rate of the injected gas. The end of the gas injection pipeline is located in a constant temperature chamber 3, and a gas phase preheating coil 14 is installed at the end of the gas injection pipeline for initial heating of the injected gas. The water injection unit includes a constant pressure and constant speed water injection pump 15 and a water injection pipeline. An intermediate water phase container 17 for storing the water phase is installed on the water injection pipeline. The oil injection unit includes a constant pressure and constant speed oil injection pump 16 and an oil injection pipeline. An intermediate oil phase container 18 for storing the oil phase is installed on the oil injection pipeline. The ends of the gas injection pipeline, water injection pipeline, and oil injection pipeline are all connected to one end of the fluid injection pipeline of the variable diameter core holder. The other end of the fluid injection pipeline is connected to the inlet connection component of the variable diameter core holder. The gas phase preheating coil 14, the water phase intermediate container 17 and the oil phase intermediate container 18 are all placed in the constant temperature chamber 3. Under the action of the constant temperature chamber 3, the fluid is heated to the temperature required for the experiment before being injected into the variable diameter rock core.

[0058] like Figure 2As shown, the variable-diameter core clamping system 2 includes a clamping cylinder 26, an equal-diameter rubber sleeve 27 inside the clamping cylinder 26, a confining pressure system 4 outside the equal-diameter rubber sleeve 27, and a variable-diameter core inside the equal-diameter rubber sleeve 27. In this embodiment, the variable-diameter core is a conical variable-diameter core, with the diameter gradually increasing from the injection end to the output end. A filling layer is provided on the outside of the variable-diameter core. The filling layer can be replaced with epoxy resin or other chemically stable thermosetting materials. The filling layer ensures that the variable-diameter core has the same outer surface diameter. The equal-diameter rubber sleeve 27 and the variable-diameter core inside it are then placed into the clamping cylinder 26. The equal-diameter rubber sleeve 27 isolates the variable-diameter core from the clamping cylinder 26, facilitating the application of confining pressure and preventing fluid lateral flow within the core. The injection end of the clamp cylinder 26 is connected to the injection system 1 through an inlet communication assembly; the inlet communication assembly of the injection end includes a clamp inlet plug 20, a clamp inlet adjusting cap 22, and a clamp inlet pressure cap 24 arranged from the inside to the outside. The clamp inlet adjusting cap 22 is used to adjust the screw-in depth of the clamp inlet plug 20.

[0059] The pressure detection assembly is connected to the data processing system 6 via pressure measurement pipeline 29. The pressure detection assembly includes an inlet pressure sensor, an outlet pressure sensor, and a variable-diameter core pressure sensor. Figure 1 In the diagram, P1 and P7 are the inlet and outlet pressure sensors, respectively, while P2-P6 are variable-diameter core pressure sensors. Several pressure measuring holes 28 are distributed axially along the constant-diameter rubber sleeve 27 and the filling layer. These holes correspond to various pressure measuring points on the variable-diameter core. One end of the pressure measuring pipeline 29 is placed inside the pressure measuring hole 28, and the other end of the pipeline 29 is connected to the variable-diameter core pressure sensor to measure the pressure at each pressure measuring point on the variable-diameter core.

[0060] The output end of the clamp cylinder 26 is connected to the back pressure control system 5 via an outlet connection assembly. The back pressure control system 5 is connected to the gas-liquid separation system 7. The back pressure control system 5 includes a back pressure pump 51 and a back pressure valve 52 installed on the fluid outlet pipeline. The back pressure pump 51 is connected to the back pressure valve 52 to apply pressure to the back pressure valve 52, thereby controlling the pressure of the fluid flowing out of the back pressure valve 52 from the variable diameter core. One end of the fluid outlet pipeline is connected to the outlet connection assembly of the variable diameter core clamp, and the other end of the fluid outlet pipeline is connected to the gas-liquid separation system 7. The outlet connection assembly includes, from the inside out, a clamp outlet plug 21, a clamp outlet adjusting cap 23, and a clamp outlet pressure cap 25. The clamp outlet adjusting cap 23 is used to adjust the screw-in depth of the clamp outlet plug 21.

[0061] The simulation method based on the above-mentioned variable-diameter core displacement simulation device for simulating near-wellbore subsurface fluid transport includes the following specific steps:

[0062] Step S1: Prepare variable diameter rock cores.

[0063] Variable diameter rock cores can be processed from constant diameter rock cores into conical rock cores using a lathe, or artificial rock cores can be used to directly prepare conical variable diameter rock cores using a conical mold.

[0064] like Figure 4 As shown, the dimensions of the tapered variable-diameter core are designed as the ratio of the radial outflow radius of the actual formation fluid to the flow radius into the near-wellbore zone. The diameter D of the producing end of the cone-shaped variable-diameter core 出 and injection end diameter D 入 The squares of the ratios are equal (i.e.) The design of this tapered variable-diameter core allows for a reduction in the Darcy velocity of the fluid from the core injection end to the production end. times.

[0065] Step S2: Apply a filler layer to make the outer diameter of the coated variable diameter rock core the same. Install an equal diameter rubber sleeve and set a pressure test hole 28 to the pressure test point on the surface of the variable diameter rock core. Connect the pressure test point and the pressure detection component with a pressure test line 29. Place the installed variable diameter rock core inside the clamping cylinder 26.

[0066] Step S3: Set the temperature of the constant temperature chamber 3, preheat the injection fluid and the variable diameter core clamping system 2 to the required experimental temperature, maintain the pressure outside the variable diameter core sleeve at the experimental set pressure through the confining pressure system 4, select the injection fluid and injection method required for the experiment through the injection system, and inject the fluid from the variable diameter core injection end through the inlet connecting component of the injection end of the clamping cylinder 26.

[0067] Step S4: The fluid injection speed is regulated by the injection system, the pressure at each pressure measuring point during the fluid injection process is measured by the pressure detection component, the pressure at each pressure measuring point is collected and recorded by the data processing system 6 as the pressure changes with the injection time, and the pressure of the fluid flowing out of the variable diameter rock core is controlled by the back pressure control system 5.

[0068] Step S5: The fluid flowing out of the back pressure valve 52 flows into the gas-liquid separation system 7 through the outflow fluid pipeline for gas and liquid separation.

[0069] During the experiment, the pressure sensor detects the pressure signal at each pressure measurement point in real time, and the data processing system 6 collects and records the pressure signal transmitted by each pressure sensor in real time, thereby obtaining the pressure or pressure difference distribution data along the flow direction of the conical variable diameter rock core under a fixed volume flow rate.

[0070] Example 2

[0071] The brittleness of low-permeability or carbonate rocks poses a significant challenge to the preparation of tapered variable-diameter cores, particularly due to their susceptibility to fracture during processing. Therefore, to accommodate various rock types for variable-diameter core displacement experiments, a simplified stepped variable-diameter core design can be employed to address the difficulties in preparing tapered variable-diameter cores from brittle rocks. For example... Figure 3 The diagram shows a variable-diameter core clamping system clamping a stepped variable-diameter core. The difference between this embodiment and Embodiment 1 is that the variable-diameter core in this embodiment is a stepped variable-diameter core, which includes at least three different diameter sections. This embodiment uses three diameter sections, such as... Figure 5 As shown, the diameters of the three diameter sections increase sequentially from the injection end to the production end. For stepped variable-diameter cores, they can be a single piece or spliced ​​from equal-diameter cores. For a single stepped variable-diameter core, equal-diameter cores can be machined into stepped variable-diameter cores using a lathe, or artificial cores can be used; a stepped variable-diameter core mold can be directly used to manufacture a single-piece stepped variable-diameter core. For spliced ​​stepped variable-diameter cores, adjacent sections with different diameters are connected by capillary material to increase the continuity of the fluid-solid capillary. The square of the ratio of the maximum diameter D3 to the minimum diameter D1 of the stepped variable-diameter core is equal to the ratio of the actual formation fluid radial outflow radius to the inflow radius into the near-wellbore zone, i.e. Similar to tapered variable-diameter cores, the design of this stepped variable-diameter core size allows for a reduction in the Darcy velocity of the fluid from the core injection end to the production end. The pressure or differential pressure distribution data along the fluid flow direction of the stepped variable diameter core under a fixed volumetric flow rate can also be obtained by following experimental procedures similar to those in Example 1.

[0072] Therefore, this invention provides a variable-diameter core displacement simulation device with the aforementioned structure for simulating near-wellbore subsurface fluid transport. This device successfully simulates the flow conditions of sudden velocity and pressure gradient drops during radial flow of fluid near the wellbore, overcoming the limitation of traditional constant-diameter core displacement devices in simulating sudden changes in velocity and pressure gradients near the wellbore. The variable-diameter core displacement device proposed in this invention can directly and effectively simulate the phase behavior, rheology, seepage patterns, and injection capacity of fluids under conditions of sudden velocity and pressure gradient drops near the wellbore. Furthermore, under a fixed injection volumetric flow rate, variable-diameter core displacement can simultaneously provide pressure gradient data at multiple flow velocities in a single displacement operation, resulting in more efficient data acquisition.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A variable diameter coreflood simulation apparatus for simulating the migration of subsurface fluids in the near wellbore region, characterized by: It includes an injection system, a variable diameter core clamping system, a confining pressure system, a back pressure control system, a data processing system, and a gas-liquid separation system arranged in sequence. The variable diameter core clamping system is equipped with a pressure detection component, which is connected to the data processing system. The variable diameter core clamping system is set inside a constant temperature chamber. The variable-diameter core clamping system includes a clamping cylinder, a constant-diameter rubber sleeve inside the clamping cylinder, a confining pressure system outside the constant-diameter rubber sleeve, a variable-diameter core inside the constant-diameter rubber sleeve, and a filling layer outside the variable-diameter core. The injection end of the clamping cylinder is connected to the injection system through an inlet connecting component; the output end of the clamping cylinder is connected to the back pressure control system through an outlet connecting component, and the back pressure control system is connected to a gas-liquid separation system.

2. The variable diameter core flood simulation device for simulating near wellbore subterranean fluid migration of claim 1, wherein: The variable-diameter core is either a conical variable-diameter core or a stepped variable-diameter core; The diameter of the cone-shaped variable-diameter core gradually increases from the injection end to the production end; The stepped variable diameter core consists of at least three different diameter sections, with the diameter increasing sequentially from the injection end to the production end.

3. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 2, characterized in that: The injection system is used for the injection of different fluids to achieve single-phase fluid injection, two-phase fluid injection, and three-phase fluid injection; Single-phase fluid injection can be gas phase injection, oil phase injection, aqueous phase injection, or aqueous solution injection. Two-phase fluid injection can be either simultaneous injection of gas and liquid phases or alternating injection of gas and liquid phases. Three-phase fluid injection is the simultaneous injection of oil, gas and water. Choose the type and method of injection fluid according to the experimental requirements.

4. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 3, characterized in that: The injection system includes an air injection unit, a water injection unit, and an oil injection unit; The gas injection unit includes a gas cylinder, a gas injection pipeline connected to the gas cylinder, and a gas flow controller. The gas cylinder is equipped with a pressure regulating valve to control the pressure of the gas output from the gas cylinder. The gas flow controller is controlled by an industrial control computer to control the flow rate of the injected gas. The end of the gas injection pipeline is set in a constant temperature chamber, and the end of the gas injection pipeline is equipped with a gas phase preheating coil for the initial heating of the injected gas. The water injection unit includes a constant pressure and constant speed water injection pump and a water injection pipeline. An intermediate water phase container for storing the water phase is installed on the water injection pipeline. The oil injection unit includes an oil injection constant pressure and constant speed pump and an oil injection pipeline. An intermediate oil phase container for storing the oil phase is installed on the oil injection pipeline. The ends of the gas injection line, water injection line, and oil injection line are all connected to one end of the fluid injection line of the variable diameter core holder, and the other end of the fluid injection line is connected to the inlet connection component of the core holder. The gas phase preheating coil, the water phase intermediate container, and the oil phase intermediate container are all placed in a constant temperature chamber. Under the action of the constant temperature chamber, the fluid is heated to the temperature required for the experiment before being injected into the variable diameter rock core.

5. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 4, characterized in that: The inlet connection assembly at the injection end of the clamping cylinder includes, from the inside out, a clamping inlet plug, a clamping inlet adjusting cap, and a clamping inlet pressure cap.

6. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 5, characterized in that: The pressure detection component is connected to the data processing system; The pressure detection assembly includes an inlet pressure sensor, an outlet pressure sensor, and a variable diameter core pressure sensor for collecting pressure from variable diameter cores. Several pressure measuring holes are distributed along the axial direction of the constant diameter rubber sleeve and the filling layer. These pressure measuring holes correspond to various pressure measuring points on the variable diameter core. One end of the pressure measuring pipeline is placed inside the pressure measuring hole, and the other end of the pressure measuring pipeline is connected to the variable diameter core pressure sensor to measure the pressure at each pressure measuring point on the variable diameter core. The confining pressure system includes a ring pressure pump used to maintain the pressure around the core sleeve.

7. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 6, characterized in that: The back pressure control system includes a back pressure pump and a back pressure valve installed on the fluid outlet pipeline. The back pressure pump is connected to the back pressure valve to apply the control pressure of the back pressure valve, so as to control the pressure of the fluid flowing out of the back pressure valve in the variable diameter core. One end of the fluid outlet pipeline is connected to the outlet connection component, and the other end of the fluid outlet pipeline is connected to a gas-liquid separation system.

8. The variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 7, characterized in that: The outlet connection assembly at the output end of the clamping cylinder includes, from the inside out, a clamping outlet plug, a clamping outlet adjusting cap, and a clamping outlet pressure cap.

9. A simulation method based on the variable-diameter core displacement simulation device for simulating near-wellbore subsurface fluid transport as described in claim 8, characterized in that, The specific steps are as follows: Step S1: Prepare variable diameter rock cores; Step S2: Apply a filler layer to make the outer diameter of the coated variable diameter rock core the same. Fit an equal diameter rubber sleeve, set a pressure measuring point through the pressure measuring hole to the surface of the variable diameter rock core, connect the pressure measuring point and the pressure sensor of the variable diameter rock core with a pressure measuring pipeline, and place the installed variable diameter rock core into the holder cylinder. Step S3: Set the temperature of the constant temperature chamber, preheat the injected fluid and the variable diameter core clamping system to the required temperature for the experiment, maintain the pressure outside the variable diameter core sleeve at the set experimental pressure through the confining pressure system, select the injected fluid and injection method required for the experiment through the injection system, and inject the fluid from the variable diameter core injection end through the inlet connecting component of the injection end of the clamping cylinder. Step S4: The fluid injection rate is regulated by the injection system, the pressure at each pressure measuring point during the fluid injection process is measured by the pressure detection component, the pressure at each pressure measuring point is collected and recorded by the data processing system as the injection time changes, and the pressure of the fluid flowing out of the back pressure valve in the core is controlled by the back pressure control system. Step S5: The fluid flowing out of the back pressure valve flows into the gas-liquid separation system through the outflow fluid pipeline for gas and liquid separation.

10. The simulation method of a variable-diameter core displacement simulation device for simulating near-wellbore underground fluid transport according to claim 9, characterized in that: Step S1 is as follows: The variable diameter core size is designed based on the ratio of the actual radial outflow radius of formation fluid to the inflow radius into the near-wellbore zone, as follows: The square of the ratio of the diameter of the producing end to the injection end in a conical variable diameter core is the same as the ratio of the radial outflow radius of the actual formation fluid to the flow radius of the fluid flowing into the near-wellbore zone. The specific formula is as follows: Among them, D 出 D is the diameter of the producing end in a conical variable-diameter core. 入 r is the diameter of the injection end in the tapered variable-diameter rock core. 出 r is the actual radial outflow radius of the formation fluid. 入 The actual radial inflow radius of the formation fluid; the design of the tapered variable-diameter core size is used to reduce the Darcy velocity of the fluid from the injection end to the production end of the variable-diameter core. times; The square of the ratio of the maximum to the minimum diameter in a stepped variable-diameter core is the same as the ratio of the radial outflow radius of the actual formation fluid to the flow radius into the near-wellbore zone. The specific formula is as follows: Where D3 is the diameter of the largest end in the stepped variable-diameter core, D1 is the diameter of the smallest end in the stepped variable-diameter core, and r 出 r is the actual radial outflow radius of the formation fluid. 入 The stepped variable-diameter core size is designed to reduce the Darcy velocity of the fluid from the injection end to the production end, representing the actual radial inflow radius of the formation fluid. times.

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