A low permeability reservoir rock volume expansion acid fracturing process method

Abstract

The present application provides a kind of low permeability reservoir rock volume expansion acid fracturing process method, comprising the following steps, connecting rock volume expansion acid fracturing process construction equipment and pressure test;Test ground stress;Prepare suitable platform space online prepad fluid base fluid, in the way of exceeding formation breakdown pressure, continuously pump into oil-water well;In the way of exceeding reservoir breakdown pressure, pump acid;Using oscillation alternative injection mode, pump displacement fluid, carry out rock volume expansion expansion, inversion.The present application can realize fracturing and fracture creation without continuous mixing, sand mixing car pry, a large number of fracturing fluid tank and other equipment, through acid dissolution, oscillation volume expansion combination, realize the high conductivity of fracture without proppant, greatly reduce the complex procedure and operation cost of reservoir reconstruction under the condition of offshore oil platform space;Overcome the problems of long time repeated high pressure under the condition of low permeability reservoir in conventional rock volume expansion, resulting in downhole string safety risk, improve the safety and efficiency of rock volume expansion.

Description

Technical Field

[0001] This invention belongs to the field of offshore oilfield reservoir stimulation technology, and in particular relates to a method for acid fracturing to expand the rock volume of low-permeability reservoirs. Background Technology

[0002] Rock expansion technology, also known as micro-fracturing technology, can increase reservoir porosity and permeability in oil and water wells, resulting in significant increases in production and injection. Offshore oilfields began researching this technology in 2020 and applied it to production enhancement measures. Field tests of expansion, water injection, and pressure reduction were conducted in eight injection wells in the Bozhong Oilfield, effectively demonstrating that water injection expansion can effectively unblock near-wellbore areas and improve permeability in loose sandstone.

[0003] Acidizing is a process that improves formation permeability by using the chemical dissolution effect of acid and the hydraulic action of squeezing acid into the formation. It is a conventional process for increasing oilfield production and injection. Combining rock expansion and acidizing technologies fully utilizes the physical fracture creation of rock expansion and the chemical fracture dissolution of acidizing to improve the effectiveness of the process. In 2021-2022, offshore oilfields successively applied this combined technology in multiple wells in loose sandstone reservoirs and ultra-low permeability sandstone reservoirs. Based on the combined rock expansion and acidizing technology, a patent application document with patent number 202110010028.3 entitled "A Method for Implementing Deep Acidizing with Rock Expansion" was submitted. This patent application effectively combines rock expansion and acidizing technologies, utilizing the preliminary steps of rock expansion technology to further reduce the difficulty of acidizing injection. Acid is injected through hydraulic oscillation, achieving the dual effects of rock expansion fracture creation and acidizing fracture dissolution. This can effectively reduce the injection pressure of rock expansion and expand the acidizing treatment radius.

[0004] Through further research on this technology, and by comparing its application effects in loose sandstone reservoirs and low-permeability sandstone reservoirs, we found the following problems with this technology and its combination with acidizing in low-permeability reservoirs: First, the high pumping pressure and long operation cycle can easily cause safety risks to downhole tools.

[0005] For example, the paper "Research and Application of Acidizing Enhancement Technology for Ultra-Low Permeability Sandstone Reservoirs at Sea" records that the A2S1 well was constructed with a high pressure of 31 MPa and a depth of 0.5 m. 3 The flow rate was repeatedly impacting the reservoir at a rate of / min for up to 20 cycles. Prior to this, there had been multiple instances where the K344 packer failed due to damage to the external rubber layer caused by repeated high-pressure oscillations, resulting in complete failure of the annular seal under high pressure. All six attempts to run the tubing were halted due to tool damage, making further construction impossible. There were also instances where a large amount of acid was present in the tubing, making it impossible to effectively pump the formation. Under the high-temperature conditions of the reservoir, there was a risk of further exacerbating the corrosion of the downhole tubing.

[0006] Second, due to the difficulty of fluid absorption in low-permeability reservoirs, the surface construction conditions are difficult to meet the long-term requirements of expansion operations.

[0007] Because rock expansion technology requires construction pressure between the minimum principal stress and the formation fracturing pressure, and the formation's fluid absorption capacity is extremely poor under unfractured reservoir conditions, taking well A2S1 as an example, the original design required a fluid volume of 1979 m³. 3 However, under conditions not exceeding the formation fracture pressure, the maximum discharge rate during well construction was only 0.5 m³. 3 The original design required 3958 minutes, but current ground equipment and personnel are insufficient to handle such long-term, high-load operations. Therefore, the final construction distance was only 194 meters. 3 As a result, the construction effect was greatly reduced.

[0008] To address the aforementioned issues, there is an urgent need to further optimize and develop a rock expansion acid fracturing process suitable for low-permeability reservoirs. Summary of the Invention

[0009] The problem to be solved by this invention is to provide a method for expanding the volume of rocks in low-permeability reservoirs by acid fracturing. This method firstly adopts a simplified pre-fracturing method to create fractures in advance, thereby creating conditions for large-volume pumping for subsequent rock expansion and overcoming problems such as the availability of water sources and numerous equipment. Secondly, in order to prevent fracture closure, acid fracturing and rock expansion oscillation are combined to achieve the purpose of expanding the fractures.

[0010] This invention provides a method for acid fracturing to expand low-permeability reservoir rocks, comprising the following steps: S1: The low-pressure end of the equipment is connected to the acid fracturing pump, fracturing fluid buffer tank, acidizing tank, and liquid filler via a low-pressure pipeline and a butterfly valve. The high-pressure end of the equipment is connected to the acid fracturing pump and the platform wellhead via a high-pressure pipeline and a plug valve. A tee is installed on the high-pressure pipeline at the platform wellhead. The tee is connected to the mud pit via the high-pressure pipeline and a needle valve. After all equipment is connected, a process pressure test is performed. S2: Test ground stress; S3: Prepare an online pre-fracturing fluid base suitable for the platform space and continuously pump it into the oil and water wells in a manner exceeding the formation fracturing pressure; S4: Pump acid at pressures exceeding reservoir fracture pressure; S5: The displacement fluid is pumped in an oscillating alternating injection method to expand and invert the rock volume.

[0011] Furthermore, in S1, the number of acidizing fracturing pumps is one or more; the number of fracturing fluid buffer tanks is one, and the capacity of the fracturing fluid buffer tank is 30m³. 3The number of acidification tanks is 1-3, and the capacity of each acidification tank is 30m³. 3 The quantity of liquid added is one; the mud pit is a fixed device on the platform, and the mud pit has a capacity of at least 200 m³. 3 .

[0012] Furthermore, S2 includes the following steps: S21: Sufficient displacement fluid is prepared in the fracturing fluid buffer tank or the acidizing tank; S22: The displacement of the acid fracturing pump is gradually increased from small to large to inject displacement fluid; S23: Analyze the minimum principal stress and formation fracture pressure of low-permeability reservoirs that require modification.

[0013] Furthermore, step S3 includes the following steps: S31: Prepare fracturing fluid base fluid using the mud pit, and pump the fracturing fluid base fluid into the fracturing fluid buffer tank; S32: Start the acid fracturing pump to pump the fracturing fluid base fluid from the fracturing fluid buffer tank, and simultaneously add a crosslinking agent online through the fluid additive to continuously pump the fluid into the oil and water wells at a pressure exceeding the formation fracturing pressure.

[0014] Furthermore, the fracturing fluid base fluid and the matching crosslinking agent are temperature- and salt-resistant seawater-based fracturing fluids.

[0015] Furthermore, S4 includes the following steps: S41: Stop the pump and close the fracturing fluid buffer tank and the butterfly valve of the liquid filler; S42: Open the butterfly valve of the acidizing tank, start the pump, and continuously pump the acid in the acidizing tank at a pressure exceeding the formation fracturing pressure.

[0016] Furthermore, the acid solution is selected from hydrochloric acid / acetic acid or terrestrial acid according to the target reservoir type. If it is terrestrial acid, the number of acidification tanks is 2-3, with at least one acidification tank reserved to store hydrochloric acid / acetic acid as the pre-treatment liquid and post-treatment liquid, and the remaining tanks storing terrestrial acid as the main acid.

[0017] Furthermore, S5 includes the following steps: S51: Stop the pump, close the butterfly valve of the acidizing tank, open the butterfly valve of the fracturing fluid buffer tank, prepare the displacement fluid in the mud pit, and pump the displacement fluid into the fracturing fluid buffer tank; S52: Start the pump and inject the displacement fluid at a pressure between the minimum principal stress of the formation and the formation fracture pressure. Stop the pump after one stage of pumping and return the liquid to the mud tank by slowly depressurizing through the needle valve. S53: Repeat S51 and S52 until all the displacement fluid has been pumped in.

[0018] Furthermore, the displacement fluid mentioned in S2 and S5 is a reservoir protection fluid injected into the well, and the displacement fluid is a combination of anti-swelling agent, demulsifying agent, and waterproofing agent.

[0019] The advantages and positive effects of this invention are: 1. This invention can achieve fracturing without the need for continuous mixing, sand mixing vehicle skids, large number of fracturing fluid tanks, etc. By combining acid dissolution and oscillation expansion, it can achieve high conductivity of fractures without proppant, which greatly reduces the cumbersome procedures and operating costs of reservoir stimulation under the space conditions of offshore oilfield platforms.

[0020] 2. This invention overcomes the problems of conventional rock expansion under low-permeability reservoir conditions, such as long-term repeated high pressure, which can easily lead to safety risks of downhole tubing and unsatisfactory measures, thus improving the safety and efficiency of rock expansion operations. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process flow according to an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the overall structure of an embodiment of the present invention.

[0023] in: 1. Low-pressure pipeline; 2. Butterfly valve; 3. Ten-way valve; 4. Acidizing fracturing pump; 5. Fracturing fluid buffer tank; 6. Acidizing tank; 7. Liquid filler; 8. High-pressure pipeline; 9. Plug valve; 10. Wellhead; 11. Tee; 12. Needle valve; 13. Mud pit. Detailed Implementation

[0024] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The embodiments of the present invention will be further described below with reference to the accompanying drawings: like Figure 1 As shown, the present invention provides a method for acid fracturing to expand low-permeability reservoir rocks, comprising the following steps: S1: Connect and pressure test the ground construction equipment for the rock expansion acid fracturing process. Specifically, such as... Figure 2As shown, the low-pressure end is connected to the acidizing fracturing pump 4, fracturing fluid buffer tank 5, acidizing tank 6, and fluid filler 7 via low-pressure pipeline 1 and butterfly valve 2, respectively. The high-pressure end is connected to the acidizing fracturing pump 4 and platform wellhead 10 via high-pressure pipeline 8 and plug valve 9. A tee 11 is installed on the high-pressure pipeline 8 at the platform wellhead 10, and connected to the mud pit 13 via high-pressure pipeline 8 and needle valve 12. A process pressure test is then performed. Specifically, the acidizing fracturing pump 4 provided in this embodiment is one or more units, meeting the requirements of construction pressure, construction displacement, and hydraulic power to meet the target reservoir stimulation requirements, while also allowing for effective placement within the platform space; the fracturing fluid buffer tank 5 is one unit with a capacity of 30m³. 3 The number of acidification tanks 6 is 1-3, and the capacity of acidification tank 6 is 30m³. 3 The quantity of liquid additive 7 is 1; mud tank 13 is a fixed device on the platform, and the capacity of mud tank 13 is at least 200m³. 3 .

[0026] S2: Test in-situ stress. Specifically, prepare sufficient displacement fluid in fracturing fluid buffer tank 5 or acidizing tank 6, and use acidizing fracturing pump 4 to gradually increase the displacement of displacement fluid to analyze the minimum principal stress and formation fracturing pressure of the low-permeability reservoir that needs to be modified.

[0027] S3: Prepare an online pre-fracturing fluid base suitable for the platform space and continuously pump it into the oil and water wells at a pressure exceeding the formation fracturing pressure. Specifically, prepare the fracturing fluid base in the platform mud pit 13 and pump it into the fracturing fluid buffer tank 5. Start the acidizing fracturing pump 4 to pump the fracturing fluid base in the fracturing fluid buffer tank 5. Simultaneously, add a crosslinking agent online through the fluid additive 7 to continuously pump it into the oil and water wells at a pressure exceeding the formation fracturing pressure. Preferably, the fracturing fluid base and matching crosslinking agent provided in this embodiment are temperature- and salt-resistant seawater-based fracturing fluids that can be prepared using seawater on-site and meet the target reservoir temperature conditions.

[0028] S4: Pump acid at pressures exceeding the formation fracturing pressure. Specifically, stop the pump, close the butterfly valve 2 of the fracturing fluid buffer tank 5 and the fluid filler 7, open the butterfly valve 2 of the acidizing tank 6, start the pump, and continuously pump acid from the acidizing tank 6 at pressures exceeding the formation fracturing pressure. Preferably, the acid can be selected from hydrochloric acid / acetic acid or terrestrial acid, depending on the type of the target reservoir. If it is terrestrial acid, 2-3 acidizing tanks 6 can be selected, with at least one acidizing tank 6 storing hydrochloric acid / acetic acid as a pre-fluid and post-fluid, and the remaining tanks storing terrestrial acid as the main acid.

[0029] S5: Displacement fluid is pumped using an alternating oscillating injection method to expand and invert rock volume. Specifically, the pump is stopped, butterfly valve 2 of the acidizing tank 6 is closed, butterfly valve 2 of the fracturing fluid buffer tank 5 is opened, displacement fluid is prepared in the platform mud pit 13 and pumped into the fracturing fluid buffer tank 5, the pump is started, and the displacement fluid is injected at a pressure between the minimum principal stress of the formation and the formation fracturing pressure. After one stage of pumping, the pump is stopped, and the liquid is slowly depressurized through needle valve 12 to return to the mud pit 13. This step is repeated until all the displacement fluid has been pumped. Preferably, the displacement fluid in S2 and S5 is the reservoir protection fluid injected into the well, and a combination of functional additives such as anti-swelling agents, demulsifiers, and waterproofing agents can be selected.

[0030] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0031] S1: Prepare two SQP2500 horizontal five-cylinder pumps, each with a pump horsepower of 2300, a maximum output pressure of 105MPa, and a construction displacement of 2.46m³ / h. 3 / min; Fracturing fluid buffer tank 5, quantity 1, capacity 30m³ 3 The acidification tanks consist of three units, each with a capacity of 30m³. 3 Liquid additive 7 is 1 unit; mud pit 13 is a fixed device on the platform with a capacity of at least 200m³. 3 Connect via high and low pressure pipelines 1 equipped with valves, and conduct process pressure testing.

[0032] S2: Prepare sufficient displacement fluid in fracturing fluid buffer tank 5 or acidizing tank 6, and use acidizing fracturing pump 4 to gradually increase the displacement of displacement fluid to analyze the minimum principal stress and formation fracturing pressure of the low-permeability reservoir that needs to be modified.

[0033] S3: The fracturing fluid base fluid is prepared in the platform mud pit 13 and pumped into the fracturing fluid buffer tank 5. The acidizing fracturing pump 4 is started to pump the fracturing fluid base fluid into the fracturing fluid buffer tank 5. At the same time, a crosslinking agent is added online through the liquid additive 7, and continuous pumping is carried out into the oil and water wells at a pressure exceeding the formation fracturing pressure. Specifically, the seawater-based fracturing fluid provided in this embodiment is prepared using seawater, with a base fluid viscosity of 90.3 mPa·s and a crosslinking time of 180-240 s at 160℃ and 170 s. -1 The viscosity was 99 mPa·s under 2h conditions, and the viscosity of the rupture liquid was 2.6 mPa·s.

[0034] S4: Stop the pump, close the butterfly valve 2 of the fracturing fluid buffer tank 5 and the fluid filler 7, open the butterfly valve 2 of the acidizing tank 6, start the pump, and continuously pump the acid in the acidizing tank 6 at a pressure exceeding the formation fracturing pressure. Specifically, in this embodiment, there are 3 acidizing tanks 6, of which 1 stores hydrochloric acid / acetic acid as a pre-fluid and post-fluid, and the system composition is hydrochloric acid and reservoir protection additives; 2 store soil acids as the main acid, and the system composition is hydrochloric acid, fluoroboric acid, hydrogen fluoride and reservoir protection additives.

[0035] S5: Displacement fluid is pumped using an alternating oscillating injection method to expand and invert rock volume. Specifically, the pump is stopped, butterfly valve 2 of the acidizing tank 6 is closed, butterfly valve 2 of the fracturing fluid buffer tank 5 is opened, displacement fluid is prepared in the platform mud pit 13 and pumped into the fracturing fluid buffer tank 5, the pump is started, and the displacement fluid is injected at a pressure between the minimum principal stress of the formation and the formation fracturing pressure. After one stage of pumping, the pump is stopped, and the liquid is slowly depressurized through needle valve 12 to return to the mud pit 13. This step is repeated until all the displacement fluid has been pumped. Preferably, the displacement fluid in S2 and S5 is the reservoir protection fluid injected into the well, and a combination of functional additives such as anti-swelling agents, demulsifiers, and waterproofing agents can be selected.

[0036] In summary, this invention achieves fracturing without the need for continuous mixing, sand-mixing skids, or numerous fracturing fluid tanks. Through a combination of acid dissolution and oscillatory expansion, it achieves high conductivity fractures without proppant, significantly reducing the cumbersome procedures and operational costs of reservoir stimulation under the limited space of offshore oilfield platforms. Simultaneously, it overcomes the problems of conventional rock expansion in low-permeability reservoirs, such as the safety risks of downhole tubing and unsatisfactory results due to prolonged and repeated high pressure, thus improving the safety and efficiency of rock expansion operations.

[0037] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for expanding the volume of low-permeability reservoir rock using acid fracturing, characterized in that: Includes the following steps, S1: The low-pressure end of the equipment is connected to the acid fracturing pump, fracturing fluid buffer tank, acidizing tank, and liquid filler via a low-pressure pipeline and a butterfly valve. The high-pressure end of the equipment is connected to the acid fracturing pump and the platform wellhead via a high-pressure pipeline and a plug valve. A tee is installed on the high-pressure pipeline at the platform wellhead. The tee is connected to the mud pit via the high-pressure pipeline and a needle valve. After all equipment is connected, a process pressure test is performed. S2: Test ground stress; S3: Prepare an online pre-fracturing fluid base suitable for the platform space and continuously pump it into the oil and water wells in a manner exceeding the formation fracturing pressure; S4: Pump acid at pressures exceeding reservoir fracture pressure; S5: Displacement fluid is pumped using an oscillating alternating injection method to perform rock volume expansion and inversion. S5 includes the following steps: S51: Stop the pump, close the butterfly valve of the acidizing tank, open the butterfly valve of the fracturing fluid buffer tank, prepare the displacement fluid in the mud pit, and pump the displacement fluid into the fracturing fluid buffer tank; S52: Start the pump and inject the displacement fluid at a pressure between the minimum principal stress of the formation and the formation fracture pressure. Stop the pump after one stage of pumping and return the liquid to the mud tank by slowly depressurizing through the needle valve. S53: Repeat S51 and S52 until all the displacement fluid has been pumped in.

2. The acid fracturing process for expanding low-permeability reservoir rocks according to claim 1, characterized in that: In step S1, the number of acidizing fracturing pumps is one or more; the number of fracturing fluid buffer tanks is one, and the capacity of the fracturing fluid buffer tank is 30m³. 3 The number of acidification tanks is 1-3, and the capacity of each acidification tank is 30m³. 3 The quantity of liquid added is one; the mud pit is a fixed device on the platform, and the mud pit has a capacity of at least 200 m³. 3 .

3. A method for expanding the volume of low-permeability reservoir rock using acid fracturing according to claim 1 or 2, characterized in that: S2 includes the following steps: S21: Sufficient displacement fluid is prepared in the fracturing fluid buffer tank or the acidizing tank; S22: The displacement of the acid fracturing pump is gradually increased from small to large to inject displacement fluid; S23: Analyze the minimum principal stress and formation fracture pressure of low-permeability reservoirs that require modification.

4. A method for expanding the volume of low-permeability reservoir rock using acid fracturing according to claim 1 or 2, characterized in that: S3 includes the following steps: S31: Prepare fracturing fluid base fluid using the mud pit, and pump the fracturing fluid base fluid into the fracturing fluid buffer tank; S32: Start the acid fracturing pump to pump the fracturing fluid base fluid from the fracturing fluid buffer tank, and simultaneously add a crosslinking agent online through the fluid additive to continuously pump the fluid into the oil and water wells at a pressure exceeding the formation fracturing pressure.

5. The acid fracturing process for expanding low-permeability reservoir rocks according to claim 4, characterized in that: The fracturing fluid base and the matching crosslinking agent are temperature- and salt-resistant seawater-based fracturing fluids.

6. A method for expanding the volume of low-permeability reservoir rock using acid fracturing according to claim 1 or 2, characterized in that: S4 includes the following steps: S41: Stop the pump and close the fracturing fluid buffer tank and the butterfly valve of the liquid filler; S42: Open the butterfly valve of the acidizing tank, start the pump, and continuously pump the acid in the acidizing tank at a pressure exceeding the formation fracturing pressure.

7. The acid fracturing process for expanding low-permeability reservoir rock according to claim 6, characterized in that: The acid solution is selected from hydrochloric acid / acetic acid or terrestrial acid according to the target reservoir type. If it is terrestrial acid, the number of acidification tanks is 2-3, with at least 1 acidification tank reserved to store hydrochloric acid / acetic acid as the pre-treatment liquid and post-treatment liquid, and the remaining tanks storing terrestrial acid as the main acid.

8. A method for expanding the volume of low-permeability reservoir rock using acid fracturing according to claim 1 or 2, characterized in that: The displacement fluids mentioned in S2 and S5 are reservoir protection fluids injected into the well, and the displacement fluids are a combination of anti-swelling agent, demulsifier and drainage aid, and waterproofing agent.

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