A method for evaluating the sealing capability of abandoned well cement plugs in oil and gas wells.

Abstract

This invention discloses a method for evaluating the sealing capability of abandoned well cement plugs in oil and gas wells, comprising the following steps: preparing a steel-cased simulated wellbore; preparing cement slurry; thickening the cement slurry; preparing drilling fluid and simulating drilling fluid flushing of the wellbore; injecting pre-prepared cement slurry into the steel casing; simulating formation temperature and pressure to cure the cement plug; testing the anti-channeling pressure of the cement plug; testing the bonding strength between the cement plug and the inner surface of the steel casing; and testing the compressive strength of the cement plug. This method enables a quantitative evaluation of the sealing capability of abandoned well cement plugs by testing the anti-channeling pressure of the cement plug, the bonding strength between the cement plug and the inner surface of the steel casing, and the compressive strength of the cement plug.

Description

Technical Field

[0001] This invention relates to the field of petroleum extraction technology, and in particular to a method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells. Background Technology

[0002] With the continuous development of oil and gas fields, more and more offshore oil and gas fields are facing decommissioning, and the scale is huge. According to the International Maritime Organization's "Guidelines and Standards for the Removal of Offshore Facilities and Structures in the Continental Shelf and Exclusive Economic Zones", offshore oil and gas fields must be abandoned after production is stopped. Domestic and foreign industries have put forward corresponding operational requirements for the abandonment of oil and gas field wells.

[0003] The wellbore barrier unit of abandoned wells mainly includes sealing units such as cement plugs, cement sheaths, casing, packers, hangers, and downhole safety valves. The failure paths of wellbore barriers in permanently abandoned offshore wells can be roughly divided into 7 categories: (1) reservoir cement plug; (2) intermediate cement plug; (3) production casing; (4) technical casing; (5) cement sheath between production casing and technical casing; (6) cement sheath between technical casing and surface casing; and (7) cement cap.

[0004] Cement plugs are crucial barrier units for maintaining the sealing integrity of abandoned wellbores. Currently, the industry has varying requirements for the length of abandoned cement plugs in oil and gas wellbores, and there is a lack of specific differentiation for different formations, leading to the possibility of over- or under-sealing. Therefore, it is of great significance to study a method for testing the sealing capacity of cement plugs in wellbores in order to quantitatively evaluate their sealing ability. Summary of the Invention

[0005] To address the lack of a quantitative evaluation method for the sealing capacity of abandoned well cement plugs in oil and gas wells, this invention provides a method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells. This method uses oil-based drilling fluid, water-based drilling fluid, whether the formation contains gas, cement slurry type, cement plug size, and formation pressure and temperature as variables. By testing the cement plug's resistance to cross-contamination pressure, the bonding strength between the cement plug and the inner surface of the steel casing, and the compressive strength of the cement plug, a quantitative evaluation of the sealing capacity of abandoned well cement plugs can be achieved.

[0006] This application is achieved using the following technical solution.

[0007] A method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells includes the following steps:

[0008] S1. Prepare a steel-cased simulated wellbore;

[0009] S2. Prepare cement slurry;

[0010] S3. Thickened cement slurry;

[0011] S4. Prepare drilling fluid and simulate drilling fluid flushing of the wellbore;

[0012] S5. Inject pre-cast cement grout into the steel sleeve;

[0013] S6. Simulate formation temperature and pressure to cure cement plugs;

[0014] S7. Test the cement plug's resistance to cross-flow pressure;

[0015] S8. Test the bonding strength between the cement plug and the inner surface of the steel sleeve;

[0016] S9. Test the compressive strength of the cement plug.

[0017] Furthermore, in step S1, steel pipes of similar material to the actual casing are used to simulate casings of different diameters. The bottom and top covers of the steel casing are divided into blind plate type and perforated type. The blind plate type bottom and top cover is used to cure cement slurry, and the perforated type bottom and top cover is used to test the cement plug's resistance to cross-flow pressure.

[0018] Furthermore, the drilling fluid, flushing fluid, and cement slurry were prepared according to the methods described in GB / T 19139-2012 Oil Well Cement Test Methods.

[0019] Furthermore, in step S3, the cement slurry is thickened using an atmospheric pressure thickener at a thickening temperature of 80±1℃ and a thickening time of 20±1min.

[0020] Furthermore, in step S4, the steel casing is soaked with different types of drilling fluid, and then the steel casing soaked in drilling fluid is flushed with the corresponding flushing fluid to simulate the effect of different drilling fluids on the sealing ability of cement plugs.

[0021] Furthermore, the drilling fluid is poured into the steel casing and stirred at a speed of 50 to 70 times per minute for 2 to 3 minutes. After the drilling fluid is poured out, the steel casing is flushed with the corresponding flushing fluid for 2 to 3 minutes. After the flushing fluid is poured out, the steel casing is inverted for 5 to 6 minutes to allow the liquid inside the steel casing to drip out.

[0022] Furthermore, in step S5, the height of the cement slurry level inside the steel sleeve is 25–100 mm.

[0023] Furthermore, in step S7, the top and bottom curing covers of the steel sleeve are removed after curing, and the upper and lower anti-cavitation covers are installed. A high-pressure nitrogen source or a water-filled intermediate container is used to simulate gas-containing and non-gas-containing reservoirs. Gas or water pressure is gradually applied from the upper anti-cavitation cover to the cement plug, with a single pressurization range of 0.1 to 0.2 MPa. After each pressurization, the pressure is stabilized for 5 to 6 minutes. The gas and water outflow from the lower anti-cavitation port is observed. When gas or liquid is seen flowing out, the inlet pressure is the anti-cavitation pressure of the cement plug.

[0024] Furthermore, in step S8, a set of cement plugs is re-cured, and the bonding strength between the cement plugs and the inner surface of the sleeve is tested using a pressure testing machine. First, the steel sleeve cement stone is placed in the center of the stage of the pressure testing machine. The cement stone is uniformly loaded through the displacement mold until the pointer of the testing machine reaches the maximum pressure reading and no longer changes. The maximum pressure is then read to obtain the bonding strength between the cement plugs and the inner surface of the sleeve.

[0025] Furthermore, in step S9, a set of cement plugs is re-cured, and the compressive strength of the cement plugs is tested using a pressure testing machine. First, the cement plugs are pushed out of the steel sleeve (when curing the cement plugs, a thin layer of oil is applied to the inside of the steel sleeve to facilitate pushing the cement plugs out of the steel sleeve), and then they are placed in the center of the stage of the pressure testing machine. The cement plugs are uniformly loaded until the pointer of the testing machine reaches the maximum pressure reading and no longer changes, and the maximum pressure is read to obtain the compressive strength of the cement plugs.

[0026] This application has the following beneficial effects.

[0027] The method of this invention uses oil-based drilling fluid, water-based drilling fluid, whether the formation contains gas, cement slurry type, cement plug size, formation pressure and temperature as variables to test the cement plug's anti-channeling pressure, the bonding strength between the cement plug and the inner surface of the steel sleeve, and the compressive strength of the cement plug, and quantitatively evaluates the sealing ability of the cement plug from multiple dimensions. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the method of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] A method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells includes the following steps:

[0031] Step 1) This experiment uses thick-walled stainless steel pipes (referred to as steel sleeves) to simulate sleeves of different diameters. The inner diameters of the stainless steel pipes are 35mm, 45mm, 60mm, and 80mm, with an effective length of 100mm. The material is 316L stainless steel, with a thickness of 8-15mm, a pressure resistance of 20MPa, and a temperature resistance of 300℃. The bottom and top covers of the steel sleeves are divided into blind plate type and perforated type. The blind plate type bottom and top cover is used for curing cement slurry, while the perforated type bottom and top cover is used for testing the cement plug's resistance to cross-flow pressure.

[0032] Step 2) Cement slurry preparation shall be performed in accordance with GB / T 19139-2012 "Test Methods for Cement in Oil Wells". Before adding the cement to the mixing liquid, it shall be weighed and then thoroughly mixed. Place the mixing cup containing the required mass of mixing water and liquid admixture on the agitator base, start the motor, and maintain a speed of 4000 r / min ± 200 r / min. Stir the required mass of mixing water and liquid admixture at this speed before adding the cement to ensure the liquid admixture is completely dispersed in the mixing water. Within 15 seconds, evenly add the cement and solid powder admixture to the mixing cup. After all the solid materials have been added to the mixing water, cover the mixing cup and continue stirring at a speed of 12000 r / min ± 500 r / min for 35 seconds ± 1 second.

[0033] Step 3) Thicken the cement slurry using an atmospheric pressure thickener. Turn on the power and heater of the atmospheric pressure thickener and set the heating temperature to 80℃. Pour the cement slurry into the slurry cup provided with the thickener, wipe the outer surface of the slurry cup clean with a towel, insert the stirring blade, and tighten the slurry cup lid. When the temperature of the thickener reaches 80℃, place the slurry cup filled with cement into the pressure thickener and pre-process for 20 minutes.

[0034] Step 4) Clean and dry the steel casings of different diameters and their upper and lower bottom covers. Install the maintenance bottom cover. Then pour water-based or oil-based drilling fluid into the steel casing. Stir the drilling fluid in the steel casing with a stirring rod at a speed of 50-70 times per minute for 3 minutes. Then pour the drilling fluid in the steel casing into the recovery tank. Rinse the steel casing soaked in drilling fluid with the corresponding water-based or oil-based flushing fluid for 2 minutes. After pouring out the flushing fluid, invert the steel casing for 5 minutes to allow the liquid inside the steel casing to drip out.

[0035] Step 5) Pour the pre-prepared cement slurry into the steel sleeve after the flushing fluid has been treated, and control the liquid level of the cement slurry in the steel sleeve to be 25-100mm. Then cover it with the top cover.

[0036] Step 6) Place the steel sleeve containing the cement slurry into the high-temperature, high-pressure curing autoclave and cover it with the curing lid. Turn on the main power and close the high-pressure valve and overflow valve. Open the water inlet valve; when water overflows from the thermocouple, tighten the thermocouple. Then edit the program, setting the temperature and pressure. Click "Run" and open the overflow valve. The simulated formation temperature and pressure conditions in this experiment are 37℃×4MPa, 85℃×20MPa, and 130℃×20MPa, respectively.

[0037] Step 7) Test the anti-channeling pressure of the cement plug. Remove the steel sleeve from the curing vessel, remove the upper and lower curing bottom covers, and install the upper and lower channeling test covers. Use a high-pressure nitrogen source or a water-filled intermediate container to simulate gas-containing and non-gas-containing reservoirs. Gradually apply air or water pressure to the cement plug from the upper channeling test cover, with a single pressurization increment of 0.1–0.2 MPa. Stabilize for 5 minutes after each pressurization and observe the gas and water outflow from the lower channeling test port. When gas or liquid is observed flowing out, the inlet pressure is the anti-channeling pressure of the cement plug.

[0038] Step 8) Test the bonding strength between the cement plug and the inner surface of the steel sleeve. Do not use the same cement plug that has been tested for resistance to cross-flow pressure to continue testing the bonding strength between the cement plug and the inner surface of the steel sleeve. A new set of cement plugs must be cured to eliminate the influence of the previous test on the bonding surface. Use a pressure testing machine to test the bonding strength between the cement plug and the inner surface of the sleeve. First, place the cement block in the center of the pressure testing machine's stage. Apply uniform loading to the cement block using the displacement mold until the machine pointer reaches the maximum pressure reading and stops changing. Record the maximum pressure. The bonding strength between the cement plug and the inner surface of the steel sleeve can then be calculated using the following formula.

[0039]

[0040] Where P is the bonding strength (MPa), F is the maximum pressure (kN), h is the length of the cement plug (m), and d is the inner diameter of the steel sleeve (m).

[0041] Step 9) To test the compressive strength of the cement plug, a new set of cement plugs needs to be cured to eliminate the influence of the previous test on the strength of the cement plugs. When curing the cement plugs, apply a thin layer of oil to the inside of the steel sleeve to facilitate the removal of the cement plug from the steel sleeve. Use a pressure testing machine to test the compressive strength of the cement plugs. First, remove the cement plug from the steel sleeve, then place it in the center of the stage of the pressure testing machine. Apply uniform load to the cement plug until the pointer of the testing machine reaches the maximum pressure reading and no longer changes. Read the maximum pressure, and the compressive strength of the cement plug can be obtained by converting it using the following formula.

[0042]

[0043] Where P is the compressive strength (MPa), F is the maximum pressure (kN), and r is the radius of the cement plug (m).

[0044] The present invention uses the above method to determine the varying degrees of influence of cement plug diameter, cement plug length, drilling fluid type, cement slurry type, formation temperature and pressure, and reservoir properties on cement plug flow pressure, bonding strength, and compressive strength. The results are shown in Table 1.

[0045] Table 1

[0046]

[0047]

[0048] The above experimental results show that the anti-channeling pressure of cement plugs is inversely correlated with the diameter of cement plugs and positively correlated with the length of cement plugs. The anti-channeling pressure of cement plugs is greater under water-based drilling fluid conditions than under oil-based drilling fluid conditions. The anti-channeling pressure of expanded cement slurry is greater than that of other cement slurries. Formation temperature and pressure have a certain influence on the anti-channeling pressure of cement slurry, but the regularity is not strong. The anti-channeling pressure of cement slurry in gas-bearing reservoirs is lower. The compressive strength of cement plugs is mainly affected by the type of cement slurry and formation temperature and pressure.

[0049] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells, characterized in that: Includes the following steps: S1. Preparation of steel casing simulation wellbore: Steel pipes with the same material as the actual casing are used to simulate casings of different diameters. The bottom and top covers of the steel casing are divided into blind plate type and perforated type. The blind plate type bottom and top cover is used to cure cement slurry, and the perforated type bottom and top cover is used to test the cement plug's resistance to cross-flow pressure. S2. Prepare cement slurry; S3. Thickened cement slurry; S4. Prepare drilling fluid and simulate drilling fluid flushing of the wellbore; S5. Inject pre-cast cement grout into the steel sleeve; S6. Simulate formation temperature and pressure to cure cement plugs; S7. Test the cement plug's resistance to cross-flow pressure: Remove the top and bottom curing covers of the steel sleeve after curing, and install the upper and lower cross-flow test covers; use a high-pressure nitrogen source or a water-filled intermediate container to simulate gas-containing and non-gas-containing reservoirs, and gradually apply air or water pressure to the cement plug from the upper cross-flow test cover, with a single pressurization amplitude of 0.1~0.2MPa, and stabilize for 5~6 minutes after each pressurization. Observe the gas and water outflow from the lower cross-flow test port. When gas or liquid is seen flowing out, the inlet pressure is the cement plug's resistance to cross-flow pressure. S8. Test the bonding strength between the cement plug and the inner surface of the steel sleeve: Re-cur a set of cement plugs and use a pressure testing machine to test the bonding strength between the cement plug and the inner surface of the sleeve. S9. Test the compressive strength of cement plugs: Re-cur a set of cement plugs and test their compressive strength using a pressure testing machine.

2. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: Drilling fluid, flushing fluid, and cement slurry were prepared according to the methods described in GB / T 19139-2012 Oil Well Cement Test Methods.

3. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: In step S3, the cement slurry is thickened using an atmospheric pressure thickener at a thickening temperature of 80±1℃ and a thickening time of 20±1 minutes.

4. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: In step S4, the steel casing is soaked with different types of drilling fluid, and then the steel casing soaked with drilling fluid is flushed with the corresponding flushing fluid to simulate the effect of different drilling fluids on the sealing ability of cement plugs.

5. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 4, characterized in that: Pour drilling fluid into the steel casing and stir it at a speed of 50-70 times per minute for 2-3 minutes. After pouring out the drilling fluid, flush the steel casing with the corresponding flushing fluid for 2-3 minutes. Pour out the flushing fluid and invert the steel casing for 5-6 minutes to allow the liquid inside the steel casing to drip out.

6. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: In step S5, the height of the cement slurry level inside the steel sleeve is 25~100mm.

7. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: In step S8, the steel sleeve cement plug is first placed in the center of the pressure testing machine platform. The cement plug is then uniformly loaded by the displacement mold until the pointer of the testing machine reaches the maximum pressure reading and no longer changes. The maximum pressure is then read to obtain the bonding strength between the cement plug and the inner surface of the sleeve.

8. The method for evaluating the sealing capacity of abandoned well cement plugs in oil and gas wells according to claim 1, characterized in that: In step S9, the cement plug is first ejected from the steel sleeve, and then placed in the center of the stage of the pressure testing machine. The cement plug is uniformly loaded until the pointer of the testing machine reaches the maximum pressure reading and no longer changes. The maximum pressure is then read to obtain the compressive strength of the cement plug.

Images