Experimental method and system for evaluating sealing capacity of wellbore annulus cement stone microcrack

Abstract

The invention discloses an experimental method and system for evaluating the sealing capability of a microcrack of wellbore annulus cement stone. The method comprises the following steps: step 1, preparing cement paste; 2, curing the cement paste to obtain a set cement sample; 3, a saline solution is prepared according to the formation water mineralization degree; 4, placing the saline solution and CO2 in a piston type intermediate container; 5, the set cement sample is saturated with saline water; step 6, pressurizing the cement stone sample saturated with the saline water to form a crack, so as to obtain the initial permeability of the cement stone sample; step 7, putting the cement stone sample subjected to fracture forming into a core holder, and monitoring the pressure of the inlet end of the core holder; when the pressure change of the inlet end of the core holder reaches an expected rising and falling trend, collecting data of the pressure of the inlet end to obtain an end point permeability; 8, if the terminal permeability is lower than the initial permeability, it shows that the set cement sample has the micro-crack self-sealing capacity. The experimental method and system simulate and measure the change of the sealing performance in the shaft direction.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas, and specifically, to an experimental method and system for evaluating the sealing ability of microcracks in wellbore annulus cement stone. Background Art

[0002] In the oil industry, CCUS (Carbon Capture, Utilization and Storage) technology mainly injects the collected CO2 into depleted oil and gas reservoirs and saline aquifers underground through wellbores for storage, or it can also be used for oil displacement. However, due to the production and construction process of the CCUS well cement sheath, microcracks or microannular gaps are generated in the cement body due to cement curing shrinkage and external load effects, or stress changes caused by casing pressure testing and temperature field fluctuations, and formation compaction and slippage, etc., resulting in the generation of microcracks inside the cement sheath, thereby reducing the wellbore sealing performance. For CCUS wells, when the injected CO2 dissolves in formation water, carbonic acid is formed. When the pH value is lower than 4, saturated CO2 brine begins to diffuse or penetrate through the cement sheath of the well. If there are microcracks in the cement stone body, the corrosion process will be accelerated. This process will seriously affect the integrity of the CCUS wellbore, most likely resulting in CO2 leakage and annulus pressure buildup, and even leading to safety accidents such as blowouts.

[0003] Currently, most of the research on the sealing ability of CCUS annulus cement stone reflects the corrosion resistance of the cement stone through corrosion depth measurement and compressive strength testing. However, in actual production and operation, the overall sealing performance along the longitudinal direction of the wellbore needs to be considered. In the case of microcracks existing in the annulus cement sheath, the quality of the annulus cement sheath sealing performance is the key to directly determining whether the CCUS wellbore can operate stably in the long term.

[0004] In view of the above problems, researching a simulation and evaluation method for the overall sealing ability along the wellbore direction after microcracks exist in the CCUS annulus cement stone is of great significance for the actual production and operation of CCUS wells. Summary of the Invention

[0005] To solve at least one of the above technical problems, embodiments of the present invention propose an experimental method and system for evaluating the sealing ability of microcracks in wellbore annulus cement stone; specifically, an experimental method for evaluating the sealing ability of the annulus cement sheath along the wellbore direction in the case of damage to the CCUS annulus cement stone, which can be used in the evaluation of the integrity of the CCUS wellbore.

[0006] An experimental method for evaluating the sealing ability of microcracks in wellbore annulus cement stone disclosed in embodiments of the present invention, the method includes the following steps:

[0007] Step 1: Preparation of cement slurry;

[0008] Step 2: Pour the cement slurry into the mold and cure it under formation temperature and confining pressure to obtain a cement stone specimen;

[0009] Step 3: Prepare a brine solution with corresponding components and ion concentrations according to the formation water salinity;

[0010] Step 4: Place the brine solution obtained in Step 3 in a piston-type intermediate container, and inject CO2 under pressure into the piston-type intermediate container at room temperature;

[0011] Step 5: Immerse the cement stone specimen obtained in Step 2 in the brine solution obtained in Step 3 and saturate the cement stone with brine under vacuum conditions;

[0012] Step 6: Apply pressure to the cement stone specimen saturated with brine to create fractures. When microcracks appear in the cement stone specimen, stop applying pressure to obtain the initial permeability of the cement stone specimen;

[0013] Step 7: Place the cement stone specimen after creating fractures in Step 6 in a core holder, control the outlet-end pressure, confining pressure and temperature of the core holder, inject the brine solution in the piston-type intermediate container into the core holder through the inlet-end of the core holder, and monitor the inlet-end pressure of the core holder; When the change in the inlet-end pressure of the core holder reaches the expected rising and falling trend, collect the data of the inlet-end pressure to obtain the end permeability;

[0014] Step 8: Compare the end permeability and the initial permeability. If the end permeability is lower than the initial permeability, it indicates that the cement stone specimen has the ability of microcrack self-sealing; If the end permeability is higher than the initial permeability, it indicates that the cement stone specimen does not have the ability of microcrack self-sealing.

[0015] In some embodiments, in Step 2: Stir the cement slurry evenly and pour it into a cylindrical mold for curing to make a cylindrical cement stone specimen; The curing conditions are to select the formation temperature and formation confining pressure at a certain depth, and cure for 28 days to fully hydrate the cement stone; After the curing is completed, take out the cement stone specimen, clean it and seal it in absolute ethanol.

[0016] In some embodiments, in Step 4: Inject the brine solution obtained in Step 3 into the experimental medium cavity at the upper part of the piston-type intermediate container, and inject CO2 under pressure through a booster injection pump into the piston-type intermediate container at room temperature and keep the pressure of the piston-type intermediate container always at 8 MPa - 10 MPa to saturate CO2 for more than 4 hours.

[0017] In some embodiments, the process of creating cracks under pressure in step 6 is as follows: Place the cylindrical cement stone specimen saturated with brine horizontally on a uniaxial compressor, apply pressure to create cracks at a rate of 1.0 kN / s to 2.0 kN / s. Stop applying pressure when there are three or more microcracks with a gap of 0.1 mm or more along the axial load direction on the surface of the cement stone specimen.

[0018] In some embodiments, in step 7: Place the cement stone specimen after creating cracks in step 6 into a core holder. Control the pressure at the outlet end of the core holder to be 4 - 6 MPa lower than the confining pressure of the core holder through a backpressure valve. Use a pressure sensor to monitor the pressure in real time at the inlet end and collect pressure data; select the formation confining pressure for the confining pressure of the core holder and the formation temperature for the experimental temperature.

[0019] In some embodiments, in step 7: Use a peristaltic pump as the power source of the displacement system. Inject the displacement medium into the lower displacement medium chamber of the piston - type intermediate container. Displace the displacement medium in the piston - type intermediate container at a constant rate of 0.01 mL / min, so that the brine solution in the upper experimental medium chamber of the piston - type intermediate container is pushed out and injected into the core holder. Preferably, the displacement medium is water.

[0020] In some embodiments, the expected rising and falling trend in "when the pressure change at the inlet end of the core holder reaches the expected rising and falling trend" in step 7 includes: The pressure at the inlet end of the core holder rises to a peak, suddenly drops, and then rises again, and the above process is repeated 2 times or more.

[0021] The embodiments of the present invention also disclose an experimental system used for the experimental method of evaluating the micro - crack sealing ability of wellbore annulus cement stone as described above. The experimental system includes a peristaltic pump, a piston - type intermediate container, a core holder, and a confining pressure pump;

[0022] The piston - type intermediate container is divided into an upper experimental medium chamber and a lower displacement medium chamber by a piston driven to move reciprocally. An experimental medium inlet / outlet is provided at the upper end of the piston - type intermediate container, and a displacement medium inlet / outlet is provided at the lower end;

[0023] The core holder is provided with an inlet end, an outlet end, and a confining pressure interface;

[0024] The peristaltic pump is connected to the displacement medium inlet / outlet of the piston - type intermediate container through a pipeline. The experimental medium inlet / outlet at the upper end of the piston - type intermediate container is connected to the inlet end of the core holder through a pipeline;

[0025] The confining pressure interface of the core holder is connected to the confining pressure pump through a pipeline; a backpressure valve is provided at the outlet end of the core holder to control the pressure at the outlet end of the core holder.

[0026] In some embodiments, the core holder is selected from the TY-2C type core holder that is resistant to high temperature and CO2 corrosion.

[0027] In some embodiments, the pipeline is a 316L stainless steel pipeline with an inner diameter of 4 - 6 mm and a maximum pressure resistance of 40 MPa.

[0028] Adopting the above technical solutions, the present invention has at least the following beneficial effects:

[0029] The experimental method for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the present invention adopts a comparison method of permeability, enriching the method of evaluating the sealing ability of the cement sheath only through the changes in corrosion depth and compressive strength after the corrosion of the annulus cement sheath in the existing CCUS wells;

[0030] The experimental method for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the present invention uses a core holder to simulate the wellbore environment and the CO2 corrosion environment, and measures the change in the sealing performance along the axial direction of the core holder (i.e., the brine flow direction) when there are microcracks in the cement sheath, that is, simulates the sealing performance in the wellbore direction when there are microcracks in the annulus cement sheath of the CCUS well;

[0031] The experimental method for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the present invention directly reflects the change in the sealing performance when there are microcracks in the annulus cement sheath of the CCUS well by real-time monitoring and collecting the change in the pressure at the inlet end of the core holder, converting the pressure value into a change in permeability and comparing it with the initial permeability;

[0032] The experimental method and system for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the present invention are simple and easy to implement, and the data is more real, vivid and intuitive. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0034] Figure 1 is a flowchart of the experimental method for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the embodiments of the present invention;

[0035] Figure 2 is a schematic diagram of the experimental system for evaluating the microcrack sealing ability of the annulus cement sheath in the wellbore of the embodiments of the present invention.

[0036]

LIST OF DRAWING REFERENCE NUMBERS

[0037] 1 - Advection pump;

[0038] 2 - Piston - type intermediate container; 201 - Upper experimental medium cavity; 202 - Lower displacement medium cavity; 203 - Piston;

[0039] 3 - Core holder; 301 - Inlet end; 302 - Outlet end; 303 - Confining pressure interface;

[0040] 4 - Confining pressure pump;

[0041] 5 - Valve. Specific implementation mode

[0042] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further elaborates on the embodiments of the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0043] In this embodiment, "up" refers to the upper part of the view based on the orientation of Figure 2 and "down" refers to the lower part of the view based on the orientation of Figure 2

[0044] In this embodiment, "axial direction" refers to the extension direction of the core holder.

[0045] In this embodiment, "longitudinal direction" refers to the length extension direction of the cylindrical cement stone sample.

[0046] The experimental method design for evaluating the micro - crack sealing ability of wellbore annulus cement stone disclosed in the embodiments of the present invention includes the preparation of cement stone specimens with micro - cracks, the CO2 corrosion method under simulated formation conditions, and the evaluation method of the sealing ability of cement stone. The objective of the present invention is to solve the evaluation method of the sealing performance along the longitudinal direction of the wellbore under the conditions of considering the existence of micro - cracks in the annulus cement sheath and CO2 corrosion in CCUS wells, and has the characteristics of novel method, simple and convenient method, and strong practicability.

[0047] Figure 1 An experimental method for evaluating the micro - crack sealing ability of wellbore annulus cement stone is shown. The method includes the following steps:

[0048] Step S100: Prepare cement slurry with reference to "GB / T 19139 - 2012 Test Methods for Oil Well Cement";

[0049] Step S200: Pour the cement slurry into a mold and cure it under formation temperature and confining pressure to obtain cement stone specimens;

[0050] Step S300: Configure a brine solution with corresponding components and corresponding ion concentrations according to the formation water salinity;

[0051] ​Step S400: Place the brine solution obtained in Step 3 in a piston-type intermediate container, and inject pressurized CO2 into the piston-type intermediate container at room temperature;

[0052] Step S500: Immerse the cement stone specimen obtained in Step 2 in the brine solution obtained in Step 3 and saturate the cement stone with brine under vacuum conditions;

[0053] Step S600: Apply pressure to create cracks in the cement stone specimen saturated with brine. When microcracks appear in the cement stone specimen, stop applying pressure to obtain the initial permeability of the cement stone specimen. Among them, the prepared brine solution is used as the permeability measurement medium for the cement stone specimen, and the permeability test is carried out according to the petroleum and natural gas industry standard "SY / T6385-1999 Determination Method for Rock Porosity and Permeability under Overburden Pressure", which is recorded as the initial permeability.

[0054] Step S700: Place the cement stone specimen with cracks created in Step 6 into a core holder, control the outlet-end pressure, confining pressure, and temperature of the core holder, inject the brine solution in the piston-type intermediate container into the core holder through the inlet end of the core holder, and monitor the inlet-end pressure of the core holder; when the change in the inlet-end pressure of the core holder reaches the expected rising and falling trend, stop the experiment, collect the data of the inlet-end pressure to obtain the end permeability; among them, the collected data is used to calculate the permeability through the linear Darcy formula. The experimental duration depends on the actual situation required, and the preferred experimental duration is more than 24 hours.

[0055] Step S800: Compare the end permeability and the initial permeability. If the end permeability is lower than the initial permeability, it indicates that the cement stone specimen has the microcrack self-sealing ability; if the end permeability is higher than the initial permeability, it indicates that the cement stone specimen does not have the microcrack self-sealing ability.

[0056] The experimental method for evaluating the microcrack sealing ability of the wellbore annulus cement stone of the present invention adopts a comparison method of permeability, which enriches the method of only evaluating the sealing ability of the cement stone through the changes in corrosion depth and compressive strength after the corrosion of the annulus cement stone body in the existing CCUS well.

[0057] In some embodiments, in Step S200: Stir the cement slurry evenly and pour it into a cylindrical mold for curing to make a cylindrical cement stone specimen; the curing conditions are to select the formation temperature and formation confining pressure at a certain depth, and cure for 28 days to fully hydrate the cement stone; after the curing is completed, take out the cement stone specimen, clean it and place it in anhydrous ethanol for sealing. Prepare for the subsequent CO2 corrosion experiment.

[0058] The selection of a cylindrical mold for the preparation of the cement stone specimen is to simulate the cylindrical shape of the wellbore and ultimately to simulate the sealing ability of the cement stone along the extension direction of the wellbore. Therefore, using a cylindrical cement stone specimen is more representative.

[0059] In some embodiments, in step S400: The brine solution obtained in step S300 is injected into the upper experimental medium chamber of the piston-type intermediate container. At room temperature, CO2 is pressurized and injected into the piston-type intermediate container through a booster injection pump, and the pressure of the piston-type intermediate container is maintained at 8 MPa - 10 MPa of saturated CO2 for more than 4 hours.

[0060] Using a piston-type intermediate container can ensure that the CO2 in the brine solution is always injected under pressure.

[0061] In some embodiments, in step S500: The cement stone specimen in anhydrous ethanol is taken out and naturally air-dried in indoor air. The air-dried cement stone specimen is placed in the prepared brine solution, and vacuum is applied to saturate the cement stone specimen with brine for a continuous time of more than 8 hours.

[0062] This allows the cement stone specimen to be fully impregnated with the brine solution, preparing for further simulating the downhole corrosion state.

[0063] In some embodiments, the process of pressure-induced fracturing in step S600 is as follows: The cylindrical cement stone specimen saturated with brine is horizontally placed on a uniaxial compressor, and pressure-induced fracturing is carried out at a rate of 1.0 kN / s - 2.0 kN / s. When three or more microcracks with a resolution visible to the human eye (gap greater than 0.1 mm) appear on the surface of the cement stone specimen along the axial load direction, the pressure application is stopped.

[0064] Performing pressure-induced fracturing on the cylindrical cement stone specimen along the axial load direction on its surface is also to simulate the compressive cracking corrosion along the wellbore extension direction.

[0065] In some embodiments, in step S700: The cement stone specimen after fracturing in step S600 is placed in a core holder. The pressure at the outlet end of the core holder is controlled by a backpressure valve to be 4 - 6 MPa lower than the confining pressure of the core holder. The inlet end is monitored in real time using a pressure sensor to collect pressure data; the confining pressure of the core holder is selected as the formation confining pressure, and the experimental temperature is the formation temperature.

[0066] The experimental method for evaluating the microcrack sealing ability of the cement sheath in the wellbore annulus of the present invention uses the cement stone placed in a core holder to simulate the wellbore environment and the CO2 corrosion environment, and measures the change in the sealing performance along the axis of the core holder (i.e., the brine flow direction) when there are microcracks in the cement stone, that is, simulates the sealing performance measurement in the wellbore direction when there are microcracks in the cement sheath of the CCUS well annulus.

[0067] In some embodiments, in step S700: An advection pump is used as the power source of the displacement system. The displacement medium is injected into the lower displacement medium chamber of the piston-type intermediate container. The displacement medium of the piston-type intermediate container is displaced at a constant rate of 0.01 mL / min, so that the saline solution in the upper experimental medium chamber of the piston-type intermediate container is ejected and injected into the core holder.

[0068] In some embodiments, the expected rising and falling trend in "when the pressure at the inlet end of the core holder changes to reach the expected rising and falling trend" in step S700 includes: The pressure at the inlet end of the core holder rises to a peak, suddenly drops, and then rises again, and the above process is repeated 2 times or more.

[0069] Figure 2 The schematic diagram of the experimental system used in the experimental method for evaluating the microcrack sealing ability of the wellbore annulus cement stone as described above is shown. The experimental system includes an advection pump 1, a piston-type intermediate container 2, a core holder 3, and a confining pressure pump 4. The piston-type intermediate container 2 is divided into an upper experimental medium chamber 201 and a lower displacement medium chamber 202 by a reciprocating piston 203 driven by a drive. An experimental medium inlet and outlet is provided at the upper end of the piston-type intermediate container 2, and a displacement medium inlet and outlet is provided at the lower end. The core holder 3 is provided with an inlet end 301, an outlet end 302, and a confining pressure interface 303. The advection pump 1 is connected to the displacement medium inlet and outlet of the piston-type intermediate container 2 through a pipeline. The experimental medium inlet and outlet at the upper end of the piston-type intermediate container 2 is connected to the inlet end 301 of the core holder 3 through a pipeline. The confining pressure interface 303 of the core holder 3 is connected to the confining pressure pump 4 through a pipeline; a back pressure valve is provided at the outlet end 302 of the core holder 3 to control the pressure at the outlet end 302 of the core holder 3. Valves 5 for on-off are provided between the core holder 3 and the piston-type intermediate container 2, between the piston-type intermediate container 2 and the advection pump 1, and between the core holder 3 and the confining pressure pump 4, so as to facilitate controlling whether each component is connected.

[0070] In some embodiments, the core holder 3 is selected from the TY-2C type core holder that is resistant to high temperature and CO2 corrosion.

[0071] In some embodiments, the pipeline is a 316L stainless steel pipeline with an inner diameter of 4-6 mm and a maximum pressure resistance of 40 MPa.

[0072] The experimental method and system for evaluating the microcrack sealing ability of the wellbore annulus cement stone of the present invention are simple and easy to implement, and the data are more real, vivid, and intuitive.

[0073] Embodiment

[0074] The cement slurry used for well cementing has the following composition: Class G oil well cement, 2% fluid loss reducer, 0.2% retarder, 0.3% defoamer, and water. The water-cement ratio of the cement slurry is 0.35, and the density of the cement slurry is 1.88 kg / m 3 .

[0075] Referring to the "Test Methods for Oil Well Cement - GB / T 19139-2012", the above-mentioned cement slurry for well cementing is stirred evenly and poured into a cylindrical steel mold with an inner diameter of 25 mm and a height of 50 mm for curing to make a cylindrical cement stone specimen. The curing conditions are formation temperature and formation confining pressure. Here, the formation temperature of 90 °C and the confining pressure of 20 MPa are selected from a 2000-meter formation, and the cement stone is cured for 28 days to allow full hydration. After the curing is completed, the cement stone specimen is taken out, cleaned, and sealed in absolute ethanol for subsequent CO2 corrosion experiments.

[0076] Preparation of saturated carbonate water: Prepare the corresponding brine solution according to the formation water salinity. Determine the proportion of mineral components containing the corresponding ions according to the ion composition (taking Table 1 as an example).

[0077] Table 1 Ion composition of formation water simulating a salinity of 8000 mg / L and reagent addition amounts corresponding to preparing 1 L of formation water

[0078]

[0079] Pour the prepared brine solution into a piston-type intermediate container. At room temperature, inject CO2 into the intermediate container through a booster injection pump and keep the pressure of the piston-type intermediate container always at 8 MPa - 10 MPa for saturation for more than 4 h.

[0080] Take out the cement stone specimen from absolute ethanol and let it air-dry naturally in the indoor air for 40 min. Place the air-dried cement stone in the prepared brine solution, evacuate it to make the cement stone saturated with brine, and the saturation time lasts for more than 8 h.

[0081] After the above process is completed, place the cement stone specimen saturated with brine horizontally on a uniaxial compressor and apply pressure to create cracks at a rate of 1.0 kN / s - 2.0 kN / s. Stop applying pressure when there are more than three microcracks on the cement stone specimen that reach the human eye resolution (size above 0.1 mm). Use the prepared brine solution as the permeability measurement medium for the cement stone specimen and conduct permeability tests according to the petroleum and natural gas industry standard "SY / T6385-1999 Determination Methods for Rock Porosity and Permeability under Overburden Pressure", which is recorded as the initial permeability.

[0082] Place the cement stone specimen in the existing TY-2C type core holder that is resistant to high temperature and CO2 corrosion. Control the pressure at the outlet end of the core holder within a range that is 4 - 6 MPa lower than the confining pressure through the backpressure valve. Use a pressure sensor at the inlet end for real-time computer monitoring and collect pressure data. Select the formation confining pressure for the confining pressure of the core holder, that is, select a confining pressure of approximately 20 MPa corresponding to a formation depth of 2000 meters; the experimental temperature is the formation temperature, that is, select an experimental temperature of approximately 90 °C corresponding to a formation depth of 2000 meters. Use a piston pump as the power source for the lower displacement medium chamber of the piston-type intermediate container. Inject water into the lower displacement medium chamber of the piston container and displace the displacement medium of the piston-type intermediate container at a constant rate of 0.01 mL / min, so that the brine solution in the experimental medium chamber above the piston-type intermediate container is pushed out and injected into the core holder.

[0083] When it is monitored that the pressure at the inlet end of the core holder shows the phenomenon that the pressure rises to a certain value, suddenly drops, and then rises again twice within 24 hours, the experiment stops, and the collected pressure data is used to calculate the end-point permeability through the linear Darcy formula.

[0084] Evaluate the sealing ability with the permeability as the evaluation criterion, compare the end-point permeability after corrosion and the initial permeability, and evaluate the self-sealing ability of the microcracks in the cement stone of this embodiment.

[0085] It should be particularly noted that each component or step in the above-mentioned various embodiments can be crossed, replaced, added, or deleted with each other. Therefore, the combinations formed by these reasonable permutations and combinations should also fall within the protection scope of the present invention, and the protection scope of the present invention should not be limited to the above-mentioned embodiments.

[0086] The above are exemplary embodiments disclosed by the present invention. The order of disclosure of the above embodiments of the present invention is only for description and does not represent the superiority or inferiority of the embodiments. However, it should be noted that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Various changes and modifications can be made without departing from the scope defined by the claims. The functions, steps, and / or actions of the method claims according to the disclosed embodiments here do not need to be executed in any specific order. In addition, although the elements disclosed in the embodiments of the present invention can be described or claimed in individual form, they can also be understood as multiple unless explicitly limited to the singular.

[0087] Those of ordinary skill in the art should understand that: the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope (including the claims) disclosed by the embodiments of the present invention is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other variations in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the embodiments of the present invention shall be included within the protection scope of the embodiments of the present invention.

Claims

1. An experimental method for evaluating the microcrack sealing ability of wellbore annulus cement stone, characterized in that, The method includes the following steps: Step 1: Preparation of cement slurry; Step 2: Pour the cement slurry into a mold and cure it under formation temperature and confining pressure to obtain a cement stone specimen; Step 3: Configure a brine solution with corresponding components and corresponding ion concentrations according to the formation water salinity; Step 4: Place the brine solution obtained in Step 3 in a piston-type intermediate container, and inject CO2 into the piston-type intermediate container under normal temperature by pressure boosting; Step 5: Immerse the cement stone specimen obtained in Step 2 into the brine solution obtained in Step 3 and saturate the cement stone with brine under vacuum conditions; Step 6: Apply pressure to create cracks in the cement stone specimen saturated with brine. When microcracks appear in the cement stone specimen, stop applying pressure to obtain the initial permeability of the cement stone specimen; Step 7: Place the cement stone specimen after crack formation in Step 6 into a core holder, control the outlet end pressure, confining pressure and temperature of the core holder, inject the brine solution in the piston-type intermediate container into the core holder through the inlet end of the core holder, and monitor the inlet end pressure of the core holder; When the change in the inlet end pressure of the core holder reaches the expected rising and falling trend, collect the data of the inlet end pressure to obtain the end permeability; Step 8: Compare the end permeability and the initial permeability. If the end permeability is lower than the initial permeability, it indicates that the cement stone specimen has the ability of microcrack self-sealing; If the end permeability is higher than the initial permeability, it indicates that the cement stone specimen does not have the ability of microcrack self-sealing.

2. The experimental method for evaluating the microcrack sealing ability of the cement sheath in the wellbore annulus according to claim 1, wherein In Step 2: Stir the cement slurry evenly, pour it into a cylindrical mold for curing to make a cylindrical cement stone specimen; The curing conditions are to select the formation temperature and formation confining pressure at a certain depth, and cure for 28 days to fully hydrate the cement stone; After the curing is completed, take out the cement stone specimen, clean it and seal it in anhydrous ethanol.

3. The experimental method for evaluating the microcrack sealing ability of the cement sheath in the wellbore annulus according to claim 1, characterized in that, In Step 4: Inject the brine solution obtained in Step 3 into the upper experimental medium chamber of the piston-type intermediate container, inject CO2 into the piston-type intermediate container under normal temperature by pressure boosting through a pressure boosting injection pump and keep the pressure of the piston-type intermediate container always at 8MPa - 10MPa and saturate CO2 for more than 4 hours.

4. The experimental method for evaluating the microcrack sealing ability of the cement sheath in the wellbore annulus according to claim 2, wherein The process of applying pressure to create cracks in Step 6 is as follows: Place the cylindrical cement stone specimen saturated with brine horizontally on a uniaxial compressor, apply pressure to create cracks at a rate of 1.0kN / s - 2.0kN / s. When more than three microcracks with a gap of more than 0.1mm appear on the surface of the cement stone specimen along the axial load direction, stop applying pressure.

5. The experimental method for evaluating the microcrack sealing ability of the wellbore annulus cement sheath according to claim 1, wherein In Step 7: Place the cement stone specimen after crack formation in Step 6 into a core holder, control the outlet end pressure of the core holder to be 4 - 6MPa lower than the confining pressure of the core holder through a back pressure valve, use a pressure sensor to monitor the inlet end in real time and collect pressure data; The confining pressure of the core holder is selected as the formation confining pressure, and the experimental temperature is the formation temperature.

6. The experimental method for evaluating the microcrack sealing ability of the wellbore annulus cement sheath according to claim 5, wherein In Step 7: A peristaltic pump is used as the power source for the lower displacement medium chamber of the piston-type intermediate container. The displacement medium is injected into the lower displacement medium chamber of the piston-type intermediate container, and the displacement medium of the piston-type intermediate container is displaced at a constant rate of 0.01 mL / min, so that the saline solution in the experimental medium chamber at the upper part of the piston-type intermediate container is ejected and injected into the core holder.

7. The experimental method for evaluating the microcrack sealing ability of the cement sheath in the wellbore annulus according to claim 5, wherein The expected rising and falling trend in "when the pressure change at the inlet end of the core holder reaches the expected rising and falling trend" in step 7 includes: The pressure at the inlet end of the core holder rises to a peak, suddenly drops, and then rises again, and the above process is repeated 2 times or more.

8. An experimental system used in the experimental method for evaluating the microcrack sealing ability of wellbore annulus cement stone according to any one of claims 1-7, characterized in that, The experimental system includes a peristaltic pump, a piston-type intermediate container, a core holder, and an confining pressure pump; The piston-type intermediate container is divided into an upper experimental medium chamber and a lower displacement medium chamber by a piston that reciprocates under drive. An experimental medium inlet and outlet is provided at the upper end of the piston-type intermediate container, and a displacement medium inlet and outlet is provided at the lower end; The core holder is provided with an inlet end, an outlet end, and a confining pressure interface; The peristaltic pump is connected to the displacement medium inlet and outlet of the piston-type intermediate container through a pipeline, and the experimental medium inlet and outlet at the upper end of the piston-type intermediate container is connected to the inlet end of the core holder through a pipeline; The confining pressure interface of the core holder is connected to the confining pressure pump through a pipeline; a back pressure valve is provided at the outlet end of the core holder to control the pressure at the outlet end of the core holder.

9. The experimental system according to claim 8, characterized in that, The core holder is selected from the TY-2C type core holder that is resistant to high temperature and CO2 corrosion.

10. The experimental system according to claim 8, characterized in that The pipeline is a 316L stainless steel pipeline with an inner diameter of 4-6 mm and a maximum pressure resistance of 40 MPa.

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