A method and system for increasing carbonate rock reserves by acidification using supercritical carbon dioxide

Abstract

The present invention provides a method and system for carbonate rock acidification and reserve enhancement using supercritical carbon dioxide, and relates to the technical field of resource exploitation. The present invention uses cold water to pre-wash the well, lowers the wellbore temperature, and enables the CO2 generated in the initial stage of acidification and well washing to maintain stable gas phase CO2. The present invention changes the downhole temperature and pressure to make CO2 undergo a periodic two-phase change between the supercritical phase and the gas phase CO2; at the same time, the CO2 phase change drives the water flow to repeatedly flush the cracks that have been acidified and conducted, and all the dust remaining in the cracks is taken out, thereby improving the carbonate rock acidification and reserve enhancement effect. After the acidification and well washing is completed, the present invention again uses the method of injecting cold water to cause bottom well vibration and turbulence, so that the free dissolved CO2 and supercritical CO2 are quickly transformed into the gas phase, and the transformation of CO2 into the gas phase can be accelerated by lowering the temperature in the well, and the gas discharge is accelerated. The vibration effect and flushing action of the water flow bring out the sediment residues in the permeable channel.

Description

Technical Field

[0001] The invention relates to the technical field of resource exploitation, and in particular to a method and system for increasing carbonate rock reserves by acidification using supercritical carbon dioxide. Background Art

[0002] During the positive circulation drilling of medium-deep geothermal wells, the rock cuttings produced by drilling are easily washed to the surroundings of the water injection drilling well under the action of high-pressure water flow. Due to the large difference between the pore pressure in the original fractures and the pressure of the high-pressure water flow, the rock cuttings and powder are compacted again to a certain extent at a certain position, blocking the original fractures of groundwater seepage.

[0003] When pumping water from deep aquifers, the depth reduction is generally small, less than 200m. According to the pressure calculation formula P=ρgh, the actual pressure generated in the deep aquifer is less than 2MPa. In the underground environment, the cracks are mostly multi-conductive, and the pumping test will produce some seepage. The existence of this partial seepage again reduces the pressure inside and outside the original cracks of groundwater seepage. It is difficult to continuously block, compact and dredge the crack channels caused by the drilling process through the pumping test, resulting in the value obtained by the pumping test being less than the groundwater seepage volume before the drilling construction.

[0004] According to the multiple hydrochloric acid titration experiments conducted by the project team, the reaction between low-concentration hydrochloric acid and rock powder is much more intense than the reaction between low-concentration hydrochloric acid and carbonate rock blocks. This phenomenon should be related to the water-rock contact area. The gas produced by the reaction of hydrochloric acid and rock powder (see formula 1) does not directly increase the bottom hole pressure, but it will destroy the friction between the rock powder particles, making the consolidated rock powder loose again.

[0005]

[0006] For medium-deep geothermal wells over 1000m, the average regional geothermal temperature gradient is 30℃ / 1000m, and the general temperature value of the constant temperature zone is 15℃. The heat storage temperature of geothermal wells at this depth is mostly 40℃, and the geothermal fluid pressure at this depth is calculated based on the water density of 0.992g / cm 3 Calculated, the pressure is about 10MPa. According to the supercritical carbon dioxide three-phase diagram ( Figure 1 ), when the pressure is greater than 7.31MPa and the temperature exceeds 31℃, carbon dioxide will enter a supercritical state. Supercritical carbon dioxide has a fluid density and a gaseous viscosity coefficient. If supercritical carbon dioxide cannot be discharged in time, as the acidification well washing continues, the weakly acidic hydrochloric acid solution will gradually reduce its concentration through acid-rock reaction and diffusion with the water body, and the acidification reaction will be reversed again, generating calcium carbonate precipitation again, blocking the permeable channel (cracks) and reducing the well washing effect. Summary of the invention

[0007] In view of this, the purpose of the present invention is to provide a method and system for increasing carbonate rock reserves by acidification using supercritical carbon dioxide. The method provided by the present invention can completely remove the precipitation in the cracks and improve the effect of carbonate rock acidification and increasing reserves.

[0008] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0009] The present invention provides a method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide, comprising the following steps:

[0010] (1) Wash with cold water until the temperature inside and outside the wellbore is ≤25°C; the temperature inside and outside the wellbore refers to the temperature of the wellbore heat storage section and the surrounding influence zone, and the surrounding influence zone refers to the area outside the wellbore 0 to 5 m away from the wellbore heat storage section;

[0011] (2) Hydrochloric acid is injected into the wellbore to initially acidify the wellbore. When the temperature inside and outside the wellbore is ≤30°C, gaseous carbon dioxide is generated, and the gaseous carbon dioxide is discharged along with water. At the same time, the carbon dioxide drives water to flow from the surrounding areas into the wellbore, and most of the blockages in the wellbore are cleared;

[0012] (3) Continuously inject hydrochloric acid into the wellbore for re-acidification until the pressure inside the wellbore is 8-12 MPa. When the temperature inside and outside the wellbore is >30°C, the gaseous carbon dioxide is converted into supercritical carbon dioxide. After the pressure is released to the pressure inside the wellbore ≤7.3 MPa, the supercritical carbon dioxide is converted into gaseous carbon dioxide, which is continuously discharged with water. At the same time, the phase change of carbon dioxide drives water to flow from the wellbore to the surrounding areas;

[0013] (4) Repeat step (1), cool the supercritical carbon dioxide with cold water to change its phase into gaseous carbon dioxide and discharge it, and the oscillation, turbulence and flushing of the water flow will remove the precipitated residues in the cracks.

[0014] Preferably, the temperature of the cold water is 10-16°C.

[0015] Preferably, the initial acidification time is 10 to 20 minutes.

[0016] Preferably, step (4) further includes repeating steps (2) to (3).

[0017] Preferably, the number of times of repeating step (1) and steps (2) to (3) is 1 to 3 times.

[0018] The present invention provides a system used in the method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide as described in the above technical solution, comprising a drilling rig and a drilling well support 1 located above the wellbore;

[0019] A first opening oil casing 2, a second opening oil casing 3 and an acid injection pipe 4 are located in the wellbore, and a pressure gauge 6 is provided on the acid injection pipe 4;

[0020] A steel plate 5 disposed between the oil casing 2 and the drilling rig and drilling well support 1;

[0021] A pressure relief pipe 7 , one end of which passes through the steel plate 5 , and a pressure relief valve 8 is provided on the pressure relief pipe 7 .

[0022] Preferably, a check valve 9 is also provided on the acid injection pipe 4 .

[0023] Preferably, one end of the acid injection pipe 4 is located in the fissure karst section of the wellbore, and the other end is connected to the blowout pool 10 .

[0024] Preferably, the other end of the pressure relief pipe 7 is connected to the spray pool 10 .

[0025] Preferably, it also includes an acid storage device 11 connected to the acid injection pipe 4.

[0026] The present invention uses cold water to pre-wash the well, lowers the wellbore temperature, and allows the CO2 generated in the initial stage of acidizing well washing to maintain a stable gas phase CO2. The present invention can make the CO2 generated by acidizing change periodically between the supercritical phase and the gas phase CO2 by changing the downhole temperature and pressure, thereby causing the bottom hole pressure to change; at the same time, the CO2 phase change drives the water flow to repeatedly flush the cracks that have been acidified and conducted, and all the dust remaining in the cracks is taken out, thereby improving the carbonate rock acidification and reserve increase effect. After the acidizing well washing is completed, the present invention uses the method of injecting cold water again to cause bottom hole vibration and turbulence, so that the free dissolved CO2 (referring to CO2 dissolved in water in the form of gas, etc.) and supercritical CO2 quickly transform into the gas phase, and by reducing the temperature in the well, the transformation of CO2 into the gas phase can be accelerated, and the gas discharge can be accelerated. The vibration effect and flushing effect of the water flow bring out the sediment residue in the permeable channel. Moreover, the method of cooling in advance by cold water in the present invention is highly economical, and is a unique means to promote a more complete phase change of CO2.

[0027] The system for increasing carbonate rock reserves by acidizing with supercritical carbon dioxide provided by the present invention can completely remove sediments in cracks in the wellbore. Moreover, the present invention can significantly improve the stability of an open oil casing in the wellbore by adding a steel plate to the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the three-phase diagram for carbon dioxide;

[0029] Figure 2The schematic diagram of the structure of the system for increasing carbonate rock reserves by acidification using supercritical carbon dioxide, wherein 1 is a drilling rig and a drilling well support, 2 is a first-opening oil casing, 3 is a second-opening oil casing, 4 is an acid injection pipe, 5 is a steel plate, 6 is a pressure gauge, 7 is a pressure relief pipe, 8 is a pressure relief valve, 9 is a check valve, 10 is a blowout pool 10, and 11 is an acid storage device;

[0030] Figure 3 Schematic diagram of CO2 phase change driving water flow to flush fractures at different stages. The left picture shows CO2 phase change driving water flow to flow into the wellbore from all sides, and the right picture shows CO2 phase change driving water flow from the wellbore to all sides.

[0031] Figure 4 Schematic diagram of pressure control in dynamic acid injection cylinder. DETAILED DESCRIPTION

[0032] The present invention provides a method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide, comprising the following steps:

[0033] (1) Wash with cold water until the temperature inside and outside the wellbore is ≤25°C; the temperature inside and outside the wellbore refers to the temperature of the wellbore heat storage section and the surrounding influence zone, and the surrounding influence zone refers to the area outside the wellbore 0 to 5 m away from the wellbore heat storage section;

[0034] (2) Hydrochloric acid is injected into the wellbore to initially acidify the wellbore. When the temperature inside and outside the wellbore is ≤30°C, gaseous carbon dioxide is generated, and the gaseous carbon dioxide is discharged along with water. At the same time, the carbon dioxide drives water to flow from the surrounding areas into the wellbore, and most of the blockages in the wellbore are cleared;

[0035] (3) Continuously inject hydrochloric acid into the wellbore for re-acidification until the pressure inside the wellbore is 8-12 MPa. When the temperature inside and outside the wellbore is >30°C, the gaseous carbon dioxide is converted into supercritical carbon dioxide. After the pressure is released to the pressure inside the wellbore ≤7.3 MPa, the supercritical carbon dioxide is converted into gaseous carbon dioxide, which is continuously discharged with water. At the same time, the phase change of carbon dioxide drives water to flow from the wellbore to the surrounding areas;

[0036] (4) Repeat step (1), cool the supercritical carbon dioxide with cold water to change its phase into gaseous carbon dioxide and discharge it, and the oscillation, turbulence and flushing of the water flow will remove the precipitated residues in the cracks.

[0037] Unless otherwise specified, the materials and equipment used in the present invention are all commercially available products in the art.

[0038] The present invention uses cold water washing until the temperature inside and outside the wellbore is ≤25°C; the temperature inside and outside the wellbore refers to the temperature of the wellbore heat storage section and the surrounding influence zone, and the surrounding influence zone is the area outside the wellbore 0 to 5m away from the wellbore heat storage section.

[0039] In the present invention, the well preferably comprises a geothermal well, wherein the well has a fractured karst section, and a bottom well sedimentation pipe is arranged at the bottom of the well.

[0040] In the present invention, the temperature of the cold water is preferably 10-16°C, and specifically preferably 10°C, 11°C, 12°C, 13°C, 14°C, 15°C or 16°C. In the present invention, the cold water is washed until the temperature inside and outside the wellbore is ≤25°C, and more preferably the temperature inside and outside the wellbore is washed with cold water to 20-25°C; preferably, 10-16°C cold water is used to continuously flush, inject water and exchange heat under the wellbore to reduce the temperature of the wellbore at the bottom of the wellbore and the surrounding of the fracture (the surrounding influence zone with a spacing of 0-5m from the wellbore), wherein the temperature of the geothermal heat storage over 1000m is generally greater than 40°C. Since the temperature of the isothermal zone in North China is about 15°C, water bodies of 10-16°C are easier to obtain. Shallow isothermal layer water can be used in summer, and surface water can be used in winter.

[0041] After cold water washing until the temperature inside and outside the wellbore is ≤25°C, the present invention injects hydrochloric acid into the wellbore for preliminary acidification. When the temperature inside and outside the wellbore is ≤30°C, gaseous carbon dioxide is generated, which is discharged along with water. At the same time, carbon dioxide drives water to flow from all around into the wellbore, and most of the blockages in the cracks in the wellbore are cleared.

[0042] In the present invention, the time of the preliminary acidification is preferably 10 to 20 minutes, more preferably 15 minutes.

[0043] In the present invention, during the initial acidification and re-acidification process, the rock cuttings and powder produced during the drilling of carbonate rocks, the walls of the karst section fissures fully react with hydrochloric acid to generate CO2. Since the bottom hole temperature is ≤30°C, the generated CO2 is in the gas phase and is easy to discharge, and most of the blockage of the fissures by the rock powder and cuttings is cleared. For the water flow and head pressure in the fissure karst channel, the water flow is entrained and discharged by the continuously generated gas phase CO2 at this time. The direction of the water flow at this stage is from the surroundings to the wellbore (see Figure 3 The present invention uses cold water to pre-wash the well, which can reduce the wellbore temperature and keep the CO2 generated in the initial stage of acid well washing stable as gas phase CO2.

[0044] The present invention continuously injects hydrochloric acid into the wellbore for re-acidification until the pressure in the wellbore is 8-12MPa. When the temperature inside and outside the wellbore is greater than 30°C, gaseous carbon dioxide is converted into supercritical carbon dioxide. After the pressure is relieved to a pressure in the wellbore of ≤7.3MPa, the supercritical carbon dioxide is converted into gaseous carbon dioxide. The gaseous carbon dioxide is continuously discharged entrained with water. At the same time, the phase change of carbon dioxide drives water to flow from the wellbore to the surrounding areas.

[0045] In the present invention, the acidizing pressure of 8-12 MPa is adopted based on the oil casing with a length of 500m and the load-bearing capacity of the machine platform. The acidizing pressure can be amplified with special engineering facilities, such as building a professional concrete platform.

[0046] As the acidification reaction continues, the temperature and pressure inside and outside the wellbore rise. When the wellbore temperature rises to more than 30°C and the pressure is 8-12MPa, the CO2 produced by the acidification gradually turns into supercritical CO2 and mixes with the water. Supercritical CO2 has a low viscosity coefficient in the gas state and is more likely to flow and migrate. The CO2 phase change drives the water to flow from the wellbore to the surrounding area (see Figure 3 In the present invention, the pressure is more preferably 9 to 11 MPa, and further preferably 10 MPa. The pressure is preferably determined by the injection rate of hydrochloric acid.

[0047] If only the above method is used to wash the well, H + As the concentration continues to decrease due to diffusion, supercritical CO2 will once again react with HCO3 - The reverse reaction generates precipitated CaCO3, which can easily cause the fissure channel to be blocked again. The present invention releases the pressure in the wellbore to ≤7.3MPa, and the gas generated by acidification is released during the pressure release process. When the pressure is ≤7.3MPa, CO2 changes from the supercritical phase to the gas phase again, and the volume expands. It can be observed that the water body is continuously entrained and gushed out in streams like a water cannon.

[0048] Since the change of CO2 from supercritical phase to gas phase is a slow process, the pressure reduction in the wellbore is a continuous change (see Figure 4 ), at this time, CO2 phase change drives water to flow into the wellbore again from all around ( Figure 3 Left middle picture). Free CO2 dissolved in water, supercritical CO2, and gaseous CO2 together constitute the existence form of CO2.

[0049] After step (3) is completed, the present invention repeats step (1), and the cold water is used to cool the supercritical carbon dioxide so that it changes into gaseous carbon dioxide and is discharged. The vibration, turbulence and flushing action of the water flow remove the precipitated residues in the cracks.

[0050] The present invention injects cold water into the bottom of the well again to lower the wellbore temperature, and can artificially intervene to accelerate the phase transition of CO2. Adding cold water has two functions. First, by injecting cold water, turbulence is generated at the bottom of the well. The generation of turbulence can make the free dissolved CO2 and supercritical CO2 quickly transform into the gas phase. This process is similar to the "Coke effect". Second, by lowering the temperature in the well, the transformation of CO2 from the supercritical phase to the gas phase can be accelerated. When the temperature is less than 30°C, the phase transition of CO2 is accelerated. The pressure relief state can be maintained in this stage to accelerate the discharge of gas. At this time, the CO2 phase change drives the water flow to still flow into the wellbore from all sides ( Figure 3The scouring action of the water flow brings out the sediment residues produced by the acidification reaction or peeled off from the carbonate rock wall after the acidification reaction, and some of the sediments sink into the sedimentation pipe (bottom sedimentation section) of the wellbore.

[0051] When the pressure in the wellbore decreases from a fluctuating value to a linear decrease (see Figure 4 ), it means that most of the free CO2 and supercritical CO2 are discharged and the pressure relief stops.

[0052] After repeating step (1), the present invention preferably further comprises repeating steps (2) to (3). In the present invention, the number of repeating steps (1) to (3) is preferably 1 to 3 times, more preferably 1 to 2 times.

[0053] Inject hydrochloric acid. The hydrochloric acid injection in this stage will further seep in the channel formed by the first acidification and reserve increase. The secondary acid injection is continued to increase the pressure to 8-12MPa. The above process is repeated to make the CO2 in the wellbore undergo repeated phase changes. At the same time, the water flow is driven to repeatedly flush the fractures that have been acidified and opened, bringing out all the dust remaining in the fractures.

[0054] The present invention provides a system used in the method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide as described in the above technical solution. The schematic diagram of the device structure is as follows: Figure 2 As shown below, combined Figure 2 Provide detailed explanation.

[0055] The system provided by the present invention comprises a drilling rig and a drilling well support 1 located above the wellbore.

[0056] The system provided by the present invention comprises a first opening oil casing 2, a second opening oil casing 3 and an acid injection pipe 4 located in the wellbore; the length (L) of the first opening oil casing 2 is preferably 500m; the acid injection pipe 4 is provided with a pressure gauge 6, and is preferably provided with a check valve 9. After the hydrochloric acid is injected, the downhole pressure will increase. The present invention uses the check valve 9 to prevent cold water from entering the blowout pool 10 when injecting cold water, and to prevent hydrochloric acid from entering the blowout pool 10 when injecting acid; one end of the acid injection pipe 4 is preferably located in the fissure karst section of the wellbore, and the other end is preferably connected to the blowout pool 10.

[0057] The system provided by the present invention includes a steel plate 5 arranged between the oil casing 2 and the drilling rig and drilling well support 1; the steel plate 5 is preferably a drilling tower pressed steel plate, and the steel plate 5 is preferably arranged under the sleepers of the drilling rig unit in the drilling rig and drilling well support 1, and welded to the top of the oil casing 2; the shape of the steel plate is preferably a square steel plate. The present invention uses the total weight of the drilling tower and the drilling rig unit to withstand the reverse pressure of acidizing well washing, which can improve the stability of the oil casing 2.

[0058] In the present invention, the well preferably includes a geothermal well, wherein the well has a fissure karst section, and a bottom well sedimentation pipe (bottom well sedimentation section) is arranged at the bottom of the well; the fissure karst section is more preferably a three-opening open hole fissure karst section.

[0059] Conventional cementing of geothermal wells uses cement. Cement may react with acid. During acid well washing, in order to ensure the pressure value in the oil casing, the wellhead must be sealed to maintain the pressure in the wellbore. If the wellbore pressure is too high, the wellbore will become unstable. The present invention preferably calculates the pressure that the wellbore can withstand based on the deadweight of an open oil casing. The calculation formula is shown in Formula 1:

[0060] m 井筒 g+F 摩 >P 压 A 井筒 (Formula 1);

[0061] Among them, m 井筒 is the weight of an open oil casing; g is the acceleration of gravity; F 摩 is the lateral friction between the oil casing and the well wall; P 压 A is the acid injection pressure; 井筒 is the cross-sectional area of ​​the wellbore.

[0062] In fact, for the weight of the wellbore, the lateral friction between the oil casing and the well wall can be ignored, and in order to ensure the stability of the wellbore after acidizing and washing, the lateral friction should not be taken into account.

[0063] In the present invention, according to the size of the first oil casing in the carbonate rock well is 339.7mm×9.65mm (outer diameter and wall thickness), the wall thickness of the acid injection pipe is 114.30mm, and the first oil casing length L=500m is calculated, the parameters are as follows: m 井筒 =81.10kg / m, A 井筒 =8.0616×10 -2 m 2 , g = 9.8 m / s 2 , A 油管 (Cross-sectional area of ​​an open oil casing wall) = 1.026 × 10 -2 m 2 According to the above parameters, the casing can withstand a pressure of <81.10×500×9.8 / (8.0616×10 -2 -1.026×10 -2), that is, the pressure that an open oil casing can withstand is less than 5.648MPa, and the acid well washing generally requires a pressure of 8-12MPa. The weight of an open oil casing can withstand the reverse lift, which is not enough to meet the pressurization requirements of acid well washing. The present invention welds a steel plate on the top of an open oil casing, and the steel plate is placed under the sleepers of the drilling rig unit. The reverse pressure of acid well washing can be withstood by the total weight of the drilling tower and the drilling rig unit.

[0064] The system provided by the present invention comprises a pressure relief pipe 7, one end of which passes through the steel plate 5, and the other end is preferably connected to the blowout pool 10; a pressure relief valve 8 is provided on the pressure relief pipe 7. In the present invention, the blowout pool 10 is used to receive the water and gas discharged from the well to avoid pollution to the environment.

[0065] The system provided by the present invention preferably further comprises an acid storage device 11 connected to the acid injection pipe 4, and the acid storage device 11 preferably comprises an acid injection vehicle.

[0066] Combine the following Figure 2 A method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide is described in detail: (1) cold water is injected into the acid injection pipe 4 for washing until the temperature inside and outside the wellbore is ≤25°C; (2) hydrochloric acid is injected through the acid injection vehicle 11 and the acid injection pipe 4 for preliminary acidification. When the temperature inside and outside the wellbore is ≤30°C, gaseous CO2 is generated. The pressure relief valve 8 is opened, and the gaseous carbon dioxide is discharged from the pressure relief pipe 7 to the blowout pool 10 along with the water. At this stage, the water flow direction is from the surroundings to the wellbore. The pressure relief valve 8 is closed; (3) hydrochloric acid is continuously injected through the acid injection vehicle 11 and the acid injection pipe 4 for re-acidification until the pressure inside the wellbore is The pressure detected by Table 6 is 8-12MPa. When the temperature inside and outside the wellbore rises to more than 30°C, the gaseous CO2 is converted into supercritical CO2. The pressure relief valve 8 is opened, and CO2 continues to be entrained with water and is released into the release pool 10 through the pressure relief pipe 7. When the pressure in the wellbore is ≤7.3MPa, CO2 is converted from supercritical phase to gas phase, and water flows into the wellbore from all sides again. When the pressure monitored by the pressure gauge 6 decreases from a fluctuating value to a linear decrease, it means that most of the free CO2 and supercritical CO2 are discharged, and the pressure relief valve 8 is closed. (4) Repeat steps (1) to (3) 1 to 3 times.

[0067] To further illustrate the present invention, the method and system for acidizing carbonate rocks and increasing reserves by using supercritical carbon dioxide provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0068] Example 1

[0069] use Figure 2 The device uses supercritical carbon dioxide to acidify carbonate rocks to increase reserves, and the specific steps are as follows:

[0070] (1) Use 10-16°C cold water to continuously flush and inject water into the well to reduce the temperature inside and outside the wellbore to below 25°C; the temperature inside and outside the wellbore refers to the temperature of the wellbore heat storage section and the surrounding influence zone, and the surrounding influence zone is the area outside the wellbore 0-5m away from the wellbore heat storage section.

[0071] (2) Hydrochloric acid is injected through the acid injection vehicle 11 and the acid injection pipe 4 for preliminary acidification. Since the bottom hole temperature is ≤30°C, the CO2 generated is gaseous CO2. The pressure relief valve 8 is opened and the CO2 is discharged to the blowdown pool 10 in time through the pressure relief pipe 7. This stage lasts for 10 to 20 minutes. Most of the blockage of the cracks by rock powder and cuttings is cleared, and the pressure relief valve 8 is closed.

[0072] (3) Hydrochloric acid is continuously injected through the acid injection vehicle 11 and the acid injection pipe 4. As the acidification reaction proceeds, the pressure of the pressure gauge 6 is maintained at 8-12 MPa. The heat released by the reaction of hydrochloric acid and soluble rock accelerates the recovery of the temperature inside and outside the wellbore. When the bottom hole temperature is greater than 30°C, the CO2 generated by the reaction gradually changes to supercritical CO2 and mixes with water. Supercritical CO2 has a low viscosity coefficient in the gas state and is more likely to flow and migrate. The phase change of CO2 drives the water to flow from the wellbore to the surroundings (see Figure 3 The pressure relief valve 8 is opened to release the generated gas, which can reduce the downhole pressure. As the pressure imbalance decreases to below 7.3MPa, CO2 changes from the supercritical phase to the gas phase again, and the volume expands. CO2 continues to carry water and flows from the pressure relief pipe 7 to the release pool 10 in streams, similar to a water cannon. Since the supercritical phase changes to the gas phase is a slow process, the pressure reduction in the wellbore is a continuous change (see Figure 4 ), at this time, CO2 phase change drives water to flow into the wellbore again from all around (see Figure 3 (middle left figure), when the pressure monitored by pressure gauge 6 decreases from a fluctuating value to a linear decrease, it means that most of the free state and supercritical phase are discharged. The free CO2 dissolved in water, the supercritical phase CO2 and the gas phase CO2 together constitute the existence form of CO2.

[0073] (4) Inject cold water at 10-16°C into the bottom of the well again to reduce the wellbore temperature to ≤25°C. Use bottom-hole oscillation and turbulence as well as cold water cooling to make the free dissolved CO2 and supercritical CO2 quickly transform into the gas phase. In this stage, keep the pressure relief valve 8 open and the pressure relief pipe 7 discharged. At this time, the CO2 phase change driving water flow still flows into the wellbore from all sides ( Figure 3 The scouring action of the water flow brings out the sediment residues produced by the acidification reaction or peeled off from the carbonate rock wall after the acidification reaction, and some of the sediments sink into the sedimentation pipe (bottom sedimentation section) of the wellbore.

[0074] (5) Close the pressure relief valve 8 and repeat steps (2) to (3). Specifically, inject hydrochloric acid through the acid injection vehicle 11 and the acid injection pipe 4. The hydrochloric acid injection at this stage will further seep into the channel formed by the first acidification and reserve increase. Continue the secondary acid injection, increase the pressure to 8-12 MPa, and repeat the above process, so that the CO2 in the well undergoes repeated phase changes, and at the same time drive the water flow to repeatedly flush the cracks that have been acidified and opened, and take out all the dust remaining in the cracks.

[0075] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for increasing carbonate rock reserves by acidification using supercritical carbon dioxide, comprising the following steps: (1) Wash with cold water until the temperature inside and outside the wellbore is ≤25°C; the temperature inside and outside the wellbore refers to the temperature of the wellbore heat storage section and the surrounding influence zone, and the surrounding influence zone refers to the area outside the wellbore 0 to 5 m away from the wellbore heat storage section; (2) Hydrochloric acid is injected into the wellbore to initially acidify the wellbore. When the temperature inside and outside the wellbore is ≤30°C, gaseous carbon dioxide is produced, which is then discharged along with water. At the same time, the carbon dioxide drives water to flow from the surrounding areas into the wellbore, and most of the blockages in the wellbore are cleared; (3) Continuously inject hydrochloric acid into the wellbore for re-acidification until the pressure inside the wellbore is 8-12 MPa. When the temperature inside and outside the wellbore is >30°C, the gaseous carbon dioxide is converted into supercritical carbon dioxide. After the pressure is released to the pressure inside the wellbore ≤7.3 MPa, the supercritical carbon dioxide is converted into gaseous carbon dioxide. The gaseous carbon dioxide is continuously discharged with water. At the same time, the phase change of carbon dioxide drives water to flow from the wellbore to the surrounding areas; (4) Repeat step (1), cool the supercritical carbon dioxide with cold water to change its phase into gaseous carbon dioxide and discharge it. The vibration, turbulence and flushing of the water flow will remove the precipitated residues in the cracks.

2. The method according to claim 1, characterized in that The temperature of the cold water is 10-16°C.

3. The method according to claim 1, characterized in that The time of the initial acidification is 10 to 20 minutes.

4. The method according to claim 1, characterized in that: After step (4), steps (2) to (3) are repeated.

5. The method according to claim 4, characterized in that The number of times of repeating step (1) and steps (2) to (3) is 1 to 3 times.

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