A safe extraction system and method for combustible ice based on reservoir modification

Abstract

This invention discloses a safe extraction system and method for combustible ice based on reservoir modification, relating to the technical field of marine natural gas hydrate extraction. The system includes: setting up a safe extraction system for combustible ice; using a first branch production well to extract pore fluid from the overlying sedimentary layer, and using a first branch injection well to inject a liquid carbon dioxide mixture to construct a carbon dioxide hydrate cap layer for geological remediation of the overlying sedimentary layer; using a second branch production well to extract combustible ice from the pores of the combustible ice reservoir, and using a second branch injection well to inject seawater to accelerate the decomposition of the combustible ice; when a third preset condition is met, combustible ice extraction operations are stopped, and a liquid carbon dioxide mixture is injected through the second branch injection well to geologically remediate the combustible ice reservoir; finally, the bottom cavities of the horizontal injection well and the horizontal production well are filled. This invention, through dual-layer geological remediation of the overlying sedimentary layer and the combustible ice reservoir, avoids formation collapse and large-scale methane leakage, achieving safe and efficient development of combustible ice.

Description

Technical Field

[0001] This invention relates to the technical field of marine natural gas hydrate extraction, and more specifically, to a safe extraction system and method for combustible ice based on reservoir modification. Background Technology

[0002] Natural gas hydrate, also known as methane hydrate, is a unique non-stoichiometric crystalline compound and one of the most abundant natural gas resources on Earth. Methane hydrate consists of water molecules linked by hydrogen bonds and gas molecules, primarily CH4, with the majority found in seabed sediments. Due to its vast methane reserves, high energy density, and clean combustion, methane hydrate is considered an important alternative energy source.

[0003] Depressurization extraction of hydrates is considered the most feasible method. However, unlike conventional oil and gas resources, methane hydrate exists in solid form within seabed sediments, providing cementation and skeletal support to sediment particles. Furthermore, the overlying strata of methane hydrate reservoirs are typically non-diagenetic, non-trapped, and highly permeable. During large-scale depressurization extraction, in addition to the potential danger of massive seawater intrusion, there is also the risk of methane hydrate decomposition and phase transformation disrupting the stability of the reservoir and overlying strata, potentially triggering geological and environmental disasters such as formation collapse, submarine landslides, and methane leaks. Due to the vast methane reserves, the instantaneous decomposition of even one-thousandth of methane hydrate in the global oceans could release more than 4% of the total annual global methane emissions. Large-scale methane leaks can lead to ocean acidification and exacerbate global warming. Therefore, maintaining formation stability and preventing large-scale methane leaks are crucial for the safe and efficient development of methane hydrate.

[0004] Compared to other methods of extracting methane hydrate, such as depressurization and thermal shock, CH4-CO2 hydrate replacement development has unique advantages in maintaining marine strata stability and ensuring the safe development of methane hydrate. It not only enables the harvesting of CH4 but also allows for the long-term marine sequestration of greenhouse CO2 gas, achieving a win-win situation. However, this method suffers from serious problems such as low replacement efficiency, small replacement area, and susceptibility to blockage during the replacement process. Summary of the Invention

[0005] To overcome the inherent safety defects in the prior art during the extraction of combustible ice, this invention provides a safe extraction system and method for combustible ice based on reservoir modification. This system can prevent overlying seawater intrusion, maintain formation stability, and prevent methane leakage into the seawater environment during the extraction process, thereby significantly improving extraction efficiency while enhancing the safety of combustible ice development.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] This invention provides a safe extraction system for combustible ice based on reservoir modification, including a horizontal injection unit, a horizontal production unit, and an environmental monitoring unit;

[0008] The horizontal injection unit includes a horizontal injection well, an injection controller, a first branch injection well, a second branch injection well, a first storage tank, a second storage tank, a first injection switch valve, and a second injection switch valve;

[0009] The input end of the horizontal injection well is connected to the first liquid storage tank and the second liquid storage tank respectively, and the output end of the horizontal injection well is connected to the input end of the first branch injection well and the input end of the second branch injection well respectively; the first injection switch valve is set on the input end of the first branch injection well, the second injection switch valve is set on the input end of the second branch injection well, and the control ends of the first injection switch valve and the second injection switch valve are both connected to the injection controller.

[0010] The horizontal production unit includes a horizontal production well, a production controller, a first branch production well, a second branch production well, a first production switch valve, and a second production switch valve.

[0011] The input end of the horizontal production well is connected to the output end of the first branch production well and the output end of the second branch production well, respectively; the first production switch valve is set on the output end of the first branch production well, the second production switch valve is set on the output end of the second branch production well, and the control ends of the first production switch valve and the second production switch valve are both connected to the production controller.

[0012] The environmental monitoring unit is located on the surface of the overlying deposition layer and is communicatively connected to the injection controller and the production controller.

[0013] Preferably, the horizontal injection unit further includes a first wellbore heater, which is evenly distributed on the wellbore wall of the horizontal injection well; the control terminal of the first wellbore heater is connected to the injection controller;

[0014] The horizontal production unit also includes a second wellbore heater, which is evenly distributed on the wellbore wall of the horizontal production well; the control terminal of the second wellbore heater is connected to the production controller.

[0015] The first and second wellbore heaters are used to prevent the formation of solid hydrates in horizontal injection wells or horizontal production wells during the construction of carbon dioxide hydrate caprocks or the extraction of combustible ice, which could lead to wellbore blockage.

[0016] Preferably, the horizontal injection unit further includes a first injection nozzle and a second injection nozzle;

[0017] The input end of the first injection nozzle is connected to the output end of the first branch injection well; the input end of the second injection nozzle is connected to the output end of the second branch injection well.

[0018] Both the first injection nozzle and the second injection nozzle are multi-directional injection nozzles.

[0019] The multi-directional injection nozzle is used to reduce the probability of fluid stagnation caused by excessive resistance due to local high flow rate when injecting fluid into the first branch injection well and the second branch injection well, and to facilitate the transport and diffusion of the injected fluid to the surrounding area.

[0020] Preferably, the first storage tank is used to store a mixture of liquefied carbon dioxide;

[0021] The liquefied carbon dioxide mixture includes liquid carbon dioxide, a carbon dioxide hydrate formation inhibitor, and a carbon dioxide hydrate formation promoter.

[0022] The carbon dioxide hydrate inhibitors include one or more of polyvinylpyrrolidone, polyvinylcaprolactam, and hydroxyl-terminated polycaprolactam.

[0023] The carbon dioxide hydrate promoter includes leucine.

[0024] Preferably, the concentration of the carbon dioxide hydrate formation inhibitor is 300–10000 ppm; and the concentration of the carbon dioxide hydrate formation promoter is 50–1000 ppm.

[0025] Compared to gaseous carbon dioxide, liquid carbon dioxide exhibits stronger flow and diffusion capabilities within sediment pores. Although carbon dioxide hydrate formation inhibitors and promoters have opposite effects on the formation rate of carbon dioxide hydrates, they can be simultaneously extended in terms of nucleation time and loosened in their formation state to increase diffusion range through dosage ratio control. Carbon dioxide hydrate inhibitors are kinetic inhibitors that significantly prolong the nucleation time of liquid carbon dioxide to solid carbon dioxide hydrate, slowing down the formation rate and preventing premature formation of large amounts of hydrate that would hinder transport to distant locations. While carbon dioxide hydrate promoters can slightly accelerate nucleation, they result in a significantly looser state of the formed hydrate, ensuring that even if solid carbon dioxide hydrate is formed during injection, it does not severely clog sediment pores, thus promoting further transport of liquid carbon dioxide hydrate to distant locations. Comparing the concentrations of carbon dioxide hydrate formation inhibitors and carbon dioxide hydrate formation promoters reveals that the concentration of carbon dioxide hydrate formation inhibitors is one order of magnitude that of carbon dioxide hydrate formation promoters. The purpose of using a higher concentration of inhibitors is to achieve better inhibition of carbon dioxide hydrate nucleation, while a lower concentration of promoters is only intended to make the generated carbon dioxide hydrate loose. Otherwise, an excessively high concentration of promoters may cause carbon dioxide hydrate to form earlier, thus weakening the diffusion capacity of carbon dioxide.

[0026] Preferably, the second storage tank is used to store seawater; the temperature of the seawater is 45-65°C.

[0027] The seawater with a temperature of 45-65°C is used to achieve a heat shock effect. If the seawater temperature is too low, it will not have a good heat shock effect, while if the temperature is too high, the heat loss will be serious. By obtaining seawater on site and heating it to 45-65°C, the decomposition of combustible ice can be accelerated while achieving higher heat utilization efficiency.

[0028] Preferably, the system further includes a plurality of pressure sensors;

[0029] The pressure sensors are evenly distributed on the wellbore walls of horizontal injection wells and horizontal production wells located in the overlying sedimentary layer.

[0030] The area around the wellbore is a high-frequency zone for methane leaks. By installing pressure sensors in the vertical direction of the wellbore, the pressure of each layer of sediment in the longitudinal direction can be obtained in real time, preventing wellbore damage and methane leaks caused by excessive pressure differences between layers.

[0031] Preferably, the environmental monitoring unit includes several seabed subsidence monitoring instruments and several methane sensors;

[0032] The seabed subsidence monitoring instrument and methane sensor are evenly arranged on the surface of the overlying sediment layer.

[0033] The seabed subsidence monitoring instrument and methane sensor are used to monitor the occurrence of seabed landslide-like geological events and methane leakage events during the extraction of combustible ice, respectively.

[0034] This invention also provides a method for safe extraction of combustible ice based on reservoir modification. Based on the aforementioned extraction system, the method includes:

[0035] S1: Select the target area for the development of combustible ice deposits and set up the combustible ice safe mining system, including: horizontal injection wells and horizontal production wells sequentially penetrating the seawater layer, the overlying sedimentary layer, and the combustible ice reservoir; the output end of the first branch injection well is set in the overlying sedimentary layer, and the output end of the second branch injection well is set in the combustible ice reservoir; the input end of the first branch production well is set in the overlying sedimentary layer, and the input end of the second branch production well is set in the combustible ice reservoir; a number of seabed subsidence monitoring instruments and methane sensors are evenly arranged on the surface of the overlying sedimentary layer;

[0036] S2: The production controller controls the opening of the first production switch valve, and the first branch production well extracts pore fluid from the sediment for depressurization; the injection controller controls the opening of the first injection switch valve, and the liquefied carbon dioxide mixture in the first storage tank is injected into the overlying sediment layer through the horizontal injection well and the first branch injection well. Driven by the pressure difference between the first branch injection well and the first branch production well, the liquefied carbon dioxide mixture flows in the overlying sediment layer and forms a carbon dioxide hydrate cap layer; when the first preset condition is reached, the production controller controls the closing of the first production switch valve, and the injection controller controls the closing of the first injection switch valve.

[0037] S3: The second production switch valve is opened by controlling the production controller, and the second branch production well extracts pore fluid from the combustible ice reservoir to depressurize the combustible ice. When the second preset condition is met, the second injection switch valve is opened by controlling the injection controller, and the seawater in the second storage tank is intermittently injected into the combustible ice reservoir through the horizontal injection well and the second branch injection well.

[0038] S4: When the third preset condition is met, the second production switch valve is closed by the production controller to end the mining of combustible ice; the liquefied carbon dioxide mixture in the first storage tank is switched by the injection controller and injected into the combustible ice reservoir through the horizontal injection well and the second branch injection well.

[0039] S5: Fill the bottom cavity of horizontal injection wells and horizontal production wells.

[0040] Preferably, in step S2, the injection pressure driving force of the first branch injection well is 1-4 MPa. The injection pressure driving force of the first branch injection well cannot cause stress deformation in the overlying sedimentary layer. The injection pressure driving force refers to the value where the injection pressure is higher than the initial pressure of the overlying layer, that is, the injected liquefied carbon dioxide mixture is in a high-pressure liquid state. By making the injection pressure higher than the pressure of the overlying layer, the fluid can be driven to be transported to a distant location under a local pressure difference.

[0041] Preferably, in step S2, the pressure reduction of the first branch production well is 1–4 MPa. The pressure reduction of the first branch production well is such that it does not cause stress deformation in the overlying sedimentary layer, and is higher than the equilibrium pressure for the decomposition of combustible ice in the combustible ice reservoir.

[0042] The stress deformation of the overlying layer is mainly related to local pressure unevenness. Since there is no other cover layer above the overlying sedimentary layer, stress deformation may lead to reservoir rupture and induce leakage of CH4 free gas in the combustible ice reservoir. Therefore, large deformation should be avoided.

[0043] Preferably, in step S2, the pressure difference between the first branch injection well and the first branch production well is 2 to 8 MPa.

[0044] Preferably, in step S3, the process of depressurizing the combustible ice in the second branch production well is a constant-rate depressurization process; the depressurization rate is 1-2 MPa / day.

[0045] Constant-rate depressurization means controlling the reservoir fluid production rate so that the production pressure decreases at a basically uniform rate. By reasonably adjusting the depressurization rate, it is possible not only to avoid formation deformation and large amounts of sand production caused by excessively rapid or uneven depressurization, but also to improve the production efficiency of combustible ice.

[0046] Preferably, the first preset condition is: the production rate of liquefied carbon dioxide mixture from the first branch production well is 30% of the injection rate of liquefied carbon dioxide mixture from the first branch injection well;

[0047] The second preset condition is: the production pressure of the second branch production well extracting pore fluid from the combustible ice reservoir drops to a constant pressure; the constant pressure is 3-5 MPa;

[0048] The third preset condition is: the average daily extraction rate of combustible ice from the second branch production well is lower than the preset extraction rate for three consecutive days, where the preset extraction rate is 500 m³. 3 / day.

[0049] Under the first preset condition, due to the pressure difference between the first branch injection well and the first branch production well, the liquefied carbon dioxide mixture will continuously flow from the high-pressure injection area to the low-pressure production area. Part of the liquid carbon dioxide will remain in the overlying sedimentary layer to form carbon dioxide hydrate, while another part will flow into the first branch production well and be produced. When the production rate reaches the set value of 30%, the liquid carbon dioxide will basically cover the path from the high-pressure area to the low-pressure area, and a relatively complete carbon dioxide hydrate cap layer can be formed.

[0050] For the third preset condition, since there may be intermittent blockages during the development of combustible ice, the average daily gas production over three consecutive days is used as the criterion to eliminate the impact of intermittent blockages.

[0051] In step S4, the injected liquefied carbon dioxide mixture serves to remediate the methane hydrate reservoir by forming carbon dioxide hydrates within it, thus preventing methane leakage after production ends. Compared to injecting liquefied carbon dioxide during methane hydrate production, injecting it after production is complete reduces the inhibitory effect of carbon dioxide hydrate formation on methane hydrate decomposition, increases methane hydrate production, and improves carbon dioxide hydrate sequestration efficiency.

[0052] Preferably, before step S3, the horizontal injection unit and the horizontal production unit need to be shut down for several days to allow the carbon dioxide hydrate caprock to age and become dense.

[0053] Preferably, in step S5, cement is injected into the horizontal injection well and the horizontal production well to fill the cavity at the bottom of the well. Since the area near the drilling site is a high-risk area for methane leaks, injecting cement into the bottom of the well after repairing the combustible ice reservoir with carbon dioxide hydrate can further prevent methane leaks from occurring.

[0054] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0055] This invention first establishes a safe mining system for combustible ice in the selected target area for combustible ice deposit development. Then, a first branch production well extracts pore fluid from the overlying sedimentary layer, and a liquid carbon dioxide mixture is injected through a first branch injection well to construct a carbon dioxide hydrate cap layer for geological remediation of the overlying sedimentary layer. This provides strong support, prevents collapse of the overlying sedimentary layer, avoids seawater intrusion, maintains formation stability, and prevents methane leakage into the overlying seawater environment. Next, a second branch production well extracts combustible ice from the pores of the combustible ice reservoir. When a second preset condition is met, seawater is injected through the second branch injection well to accelerate the decomposition of the combustible ice. When a third preset condition is met, combustible ice mining operations are stopped, and a liquid carbon dioxide mixture is injected through the second branch injection well to geologically remediate the combustible ice reservoir, preventing methane leakage after mining ends and increasing hydrate replacement efficiency and carbon dioxide seabed sequestration capacity. Finally, the bottom cavities of the horizontal injection well and horizontal production well are filled to further prevent methane leakage. This invention achieves safe and efficient development of combustible ice by performing dual-layer geological remediation on the overlying sedimentary layer and the combustible ice reservoir, thus avoiding formation collapse and large-scale methane leakage. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of the structure of a safe extraction system for combustible ice based on reservoir modification as described in Example 1.

[0057] Figure 2 This is a schematic diagram of a safe extraction system for combustible ice based on reservoir modification, as described in Example 2.

[0058] Figure 3 This is a flowchart of a safe extraction method for combustible ice based on reservoir modification, as described in Example 3.

[0059] In the diagram, 1-Horizontal injection well, 2-Injection controller, 3-First branch injection well, 4-Second branch injection well, 5-First storage tank, 6-Second storage tank, 7-First injection switch valve, 8-Second injection switch valve, 9-Horizontal production well, 10-Production controller, 11-First branch production well, 12-Second branch production well, 13-First production switch valve, 14-Second production switch valve, 15-First injection nozzle, 16-Second injection nozzle, 17-Pressure sensor, 18-Seabed subsidence monitor, 19-Methane sensor. Detailed Implementation

[0060] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0061] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0062] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0063] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0064] Example 1

[0065] This embodiment provides a safe extraction system for combustible ice based on reservoir modification, such as... Figure 1 As shown, it includes a horizontal injection unit, a horizontal production unit, and an environmental monitoring unit;

[0066] The horizontal injection unit includes a horizontal injection well 1, an injection controller 2, a first branch injection well 3, a second branch injection well 4, a first storage tank 5, a second storage tank 6, a first injection switch valve 7, and a second injection switch valve 8.

[0067] The input end of the horizontal injection well 1 is connected to the first liquid storage tank 5 and the second liquid storage tank 6 respectively, and the output end of the horizontal injection well 1 is connected to the input end of the first branch injection well 3 and the input end of the second branch injection well 4 respectively; the first injection switch valve 7 is set on the input end of the first branch injection well 3, and the second injection switch valve 8 is set on the input end of the second branch injection well 4. The control ends of the first injection switch valve 7 and the second injection switch valve 8 are both connected to the injection controller 2.

[0068] The horizontal production unit includes a horizontal production well 9, a production controller 10, a first branch production well 11, a second branch production well 12, a first production switch valve 13, and a second production switch valve 14.

[0069] The input end of the horizontal production well 9 is connected to the output end of the first branch production well 11 and the output end of the second branch production well 12 respectively; the first production switch valve 13 is set on the output end of the first branch production well 11, and the second production switch valve 14 is set on the output end of the second branch production well 12. The control ends of the first production switch valve 13 and the second production switch valve 14 are both connected to the production controller 10.

[0070] The environmental monitoring unit is located on the surface of the overlying deposition layer and is communicatively connected to the injection controller 2 and the production controller 10.

[0071] Example 2

[0072] This embodiment provides a safe extraction system for combustible ice based on reservoir modification, such as... Figure 2 As shown, it includes a horizontal injection unit, a horizontal production unit, and an environmental monitoring unit;

[0073] The horizontal injection unit includes a horizontal injection well 1, an injection controller 2, a first branch injection well 3, a second branch injection well 4, a first storage tank 5, a second storage tank 6, a first injection switch valve 7, a second injection switch valve 8, a first wellbore heater, a first injection nozzle 15, and a second injection nozzle 16.

[0074] The input end of the horizontal injection well 1 is connected to the first liquid storage tank 5 and the second liquid storage tank 6 respectively. The output end of the horizontal injection well 1 is connected to the input end of the first branch injection well 3 and the input end of the second branch injection well 4 respectively. The output end of the first branch injection well 3 is connected to the input end of the first injection nozzle 15, and the output end of the second branch injection well 4 is connected to the input end of the second injection nozzle 16.

[0075] The first injection switch valve 7 is installed at the input end of the first branch injection well 3, and the second injection switch valve 8 is installed at the input end of the second branch injection well 4. The control ends of the first injection switch valve 7 and the second injection switch valve 8 are both connected to the injection controller 2. The first wellbore heaters are evenly distributed on the wellbore wall of the horizontal injection well 1. The control end of the first wellbore heaters is connected to the injection controller 2.

[0076] In this embodiment, both the first injection nozzle 15 and the second injection nozzle 16 are multi-directional injection nozzles;

[0077] The first storage tank 5 is used to store a mixture of liquefied carbon dioxide;

[0078] The liquefied carbon dioxide mixture includes liquid carbon dioxide, a carbon dioxide hydrate formation inhibitor, and a carbon dioxide hydrate formation promoter.

[0079] The carbon dioxide hydrate inhibitors include one or more of polyvinylpyrrolidone, polyvinylcaprolactam, and hydroxyl-terminated polycaprolactam.

[0080] The carbon dioxide hydrate promoter includes leucine.

[0081] The concentration of the carbon dioxide hydrate formation inhibitor is 300–10000 ppm; the concentration of the carbon dioxide hydrate formation promoter is 50–1000 ppm.

[0082] The second liquid storage tank 6 is used to store seawater; the temperature of the seawater is 45-65℃.

[0083] The horizontal production unit includes a horizontal production well 9, a production controller 10, a first branch production well 11, a second branch production well 12, a first production switch valve 13, a second production switch valve 14, and a second wellbore heater.

[0084] The input terminals of the horizontal production well 9 are respectively connected to the output terminals of the first branch production well 11 and the second branch production well 12; the first production switch valve 13 is located on the output terminal of the first branch production well 11, and the second production switch valve 14 is located on the output terminal of the second branch production well 12; the control terminals of both the first production switch valve 13 and the second production switch valve 14 are connected to the production controller 10; the second wellbore heaters are evenly distributed on the wellbore wall of the horizontal production well 9; the control terminals of the second wellbore heaters are connected to the production controller 10.

[0085] The system also includes several pressure sensors 17;

[0086] The pressure sensors 17 are evenly arranged on the wellbore walls of the horizontal injection well 1 and the horizontal production well 9 located in the overlying sedimentary layer.

[0087] The environmental monitoring unit includes several seabed sedimentation monitoring instruments 18 and several methane sensors 19; the seabed sedimentation monitoring instruments 18 and methane sensors 19 are evenly arranged on the surface of the overlying sediment layer and are all communicatively connected to the injection controller 2 and the production controller 10.

[0088] In practical implementation, the first and second wellbore heaters are used to prevent the formation of solid hydrates in the horizontal injection wells or horizontal production wells during the construction of the carbon dioxide hydrate caprock or the extraction of combustible ice, which could lead to wellbore blockage. Multi-directional injection nozzles are used to reduce the probability of fluid stagnation induced by excessive resistance due to high local flow rates during the injection of liquid into the first and second branch injection wells, facilitating the transport and diffusion of the injected liquid in all directions. The area around the wellbore is a high-frequency methane leakage zone. By installing pressure sensors in the vertical direction of the wellbore, the pressure of each layer of sediment can be acquired in real time, preventing wellbore damage and methane leakage due to excessive pressure differences between layers. The seafloor subsidence monitoring instrument and methane sensor are used to monitor the occurrence of seafloor landslide-like geological events and methane leakage events during the extraction of combustible ice, respectively.

[0089] Example 3

[0090] This embodiment provides a safe extraction method for combustible ice based on reservoir modification, using the extraction system described in Embodiment 2, such as... Figure 3 As shown, the method includes:

[0091] S1: Select the target area for the development of combustible ice deposits and set up the combustible ice safe mining system, including: horizontal injection well 1 and horizontal production well 9 sequentially penetrating the seawater layer, the overlying sedimentary layer and the combustible ice reservoir; the output end of the first branch injection well 3 is set in the overlying sedimentary layer, and the output end of the second branch injection well 4 is set in the combustible ice reservoir; the input end of the first branch production well 11 is set in the overlying sedimentary layer, and the input end of the second branch production well 12 is set in the combustible ice reservoir; a number of seabed subsidence monitoring instruments 18 and methane sensors 19 are evenly arranged on the surface of the overlying sedimentary layer;

[0092] S2: The production controller 10 controls the opening of the first production switch valve 13, and the first branch production well 11 extracts pore fluid from the sediment for depressurization; the injection controller 2 controls the opening of the first injection switch valve 7, and the liquefied carbon dioxide mixture in the first storage tank 5 is injected into the overlying sediment layer through the horizontal injection well 1 and the first branch injection well 3. Driven by the pressure difference between the first branch injection well 3 and the first branch production well 11, the liquefied carbon dioxide mixture flows in the overlying sediment layer and forms a carbon dioxide hydrate cap layer; when the first preset condition is reached, the production controller 10 controls the first production switch valve 13 to close, and the injection controller 2 controls the first injection switch valve 7 to close.

[0093] Among them, the stress deformation of the overlying layer is mainly related to local pressure unevenness. Since there is no other cover layer above the overlying sedimentary layer, stress deformation may lead to reservoir rupture and induce leakage of CH4 free gas in the methane hydrate reservoir. Therefore, it is necessary to avoid large deformation.

[0094] The injection pressure driving force of the first branch injection well is 1-4 MPa. This injection pressure driving force cannot cause stress deformation in the overlying sedimentary layer. The injection pressure driving force refers to the value where the injection pressure is higher than the initial pressure of the overlying layer, meaning the injected liquefied carbon dioxide mixture is in a high-pressure liquid state. By making the injection pressure higher than the overlying layer pressure, fluid can be driven to be transported to distant locations under local pressure differences.

[0095] The pressure reduction range of the first branch production well is 1-4 MPa; the pressure reduction of the first branch production well cannot cause stress deformation of the overlying sedimentary layer, and is higher than the decomposition equilibrium pressure of combustible ice in the combustible ice reservoir.

[0096] The pressure difference between the first branch injection well and the first branch production well is 2–8 MPa;

[0097] In this embodiment, the liquefied carbon dioxide mixture includes liquid carbon dioxide, a carbon dioxide hydrate formation inhibitor, and a carbon dioxide hydrate formation promoter;

[0098] The carbon dioxide hydrate inhibitors include one or more of polyvinylpyrrolidone, polyvinylcaprolactam, and hydroxyl-terminated polycaprolactam.

[0099] The carbon dioxide hydrate promoter includes leucine.

[0100] The concentration of the carbon dioxide hydrate formation inhibitor is 300–10000 ppm; the concentration of the carbon dioxide hydrate formation promoter is 50–1000 ppm.

[0101] Compared to gaseous carbon dioxide, liquid carbon dioxide exhibits stronger flow and diffusion capabilities within sediment pores. Although carbon dioxide hydrate formation inhibitors and promoters have opposite effects on the formation rate of carbon dioxide hydrates, they can be simultaneously extended in terms of nucleation time and loosened in their formation state to increase diffusion range through dosage ratio control. Carbon dioxide hydrate inhibitors are kinetic inhibitors that significantly prolong the nucleation time of liquid carbon dioxide to solid carbon dioxide hydrate, slowing down the formation rate and preventing premature formation of large amounts of hydrate that would hinder transport to distant locations. While carbon dioxide hydrate promoters can slightly accelerate nucleation, they result in a significantly looser state of the formed hydrate, ensuring that even if solid carbon dioxide hydrate is formed during injection, it does not severely clog sediment pores, thus promoting further transport of liquid carbon dioxide hydrate to distant locations. Comparing the concentrations of carbon dioxide hydrate formation inhibitors and carbon dioxide hydrate formation promoters reveals that the concentration of carbon dioxide hydrate formation inhibitors is one order of magnitude that of carbon dioxide hydrate formation promoters. The purpose of using a higher concentration of inhibitors is to achieve better inhibition of carbon dioxide hydrate nucleation, while a lower concentration of promoters is only intended to make the generated carbon dioxide hydrate loose. Otherwise, an excessively high concentration of promoters may cause carbon dioxide hydrate to form earlier, thus weakening the diffusion capacity of carbon dioxide.

[0102] The first preset condition is: the production rate of liquefied carbon dioxide mixture from the first branch production well is 30% of the injection rate of liquefied carbon dioxide mixture from the first branch injection well. Due to the pressure difference between the areas of the first branch injection well and the first branch production well, the liquefied carbon dioxide mixture will continuously flow from the high-pressure injection area to the low-pressure production area. Part of the liquid carbon dioxide will remain in the overlying sedimentary layer to form carbon dioxide hydrate, while another part will flow into the first branch production well and be produced. When the production rate reaches the set value of 30%, the liquid carbon dioxide will essentially cover the path from the high-pressure area to the low-pressure area, forming a relatively complete carbon dioxide hydrate cap layer. After the carbon dioxide hydrate cap layer is formed, the horizontal injection unit and the horizontal production unit need to be shut down for several days to allow the carbon dioxide hydrate cap layer to age and become dense.

[0103] This embodiment constructs a carbon dioxide hydrate caprock with a wider coverage and greater strength in the overlying sedimentary layer, giving it strong support capabilities to prevent the overlying sedimentary layer from collapsing. In the subsequent development of combustible ice, it effectively avoids the intrusion of overlying seawater, maintains the stability of the formation, and prevents methane leakage into the overlying seawater environment, thereby improving the safety of combustible ice development and significantly increasing extraction efficiency.

[0104] S3: The production controller 10 controls the second production switch valve 14 to open, and the second branch production well 12 extracts pore fluid from the combustible ice reservoir to depressurize the combustible ice. When the second preset condition is met, the injection controller 2 controls the second injection switch valve 8 to open, and the seawater in the second storage tank 6 is intermittently injected into the combustible ice reservoir through the horizontal injection well 1 and the second branch injection well 4.

[0105] Among them, the process of depressurizing combustible ice extraction by the second branch production well is constant-rate depressurization; the depressurization rate is 1-2 MPa / day.

[0106] The second preset condition is: the production pressure of the second branch production well extracting pore fluid from the combustible ice reservoir drops to a constant pressure; the constant pressure is 3-5 MPa;

[0107] Constant-rate depressurization means controlling the reservoir fluid production rate so that the production pressure decreases at a basically uniform rate. By reasonably adjusting the depressurization rate, it is possible not only to avoid formation deformation and large amounts of sand production caused by excessively rapid or uneven depressurization, but also to improve the production efficiency of combustible ice.

[0108] In this embodiment, the temperature of the seawater is 45-65°C. The purpose of having seawater at 45-65°C is to achieve a heat shock effect. If the seawater temperature is too low, it will not have a good heat shock effect, while if the temperature is too high, the heat loss will be serious. By obtaining seawater on site and heating it to 45-65°C, it is possible to accelerate the decomposition of combustible ice while achieving higher heat utilization efficiency.

[0109] S4: When the third preset condition is met, the production controller 10 controls the second production switch valve 14 to close, ending the mining of combustible ice; the injection controller 2 controls the switching of the liquefied carbon dioxide mixture in the first storage tank 5 to be injected into the combustible ice reservoir through the horizontal injection well 1 and the second branch injection well 4.

[0110] In order to eliminate the impact of intermittent blockage during the development of combustible ice, a third preset condition is set: if the average daily extraction volume of combustible ice from the second branch production well is lower than a preset extraction volume for three consecutive days, the extraction of combustible ice will be terminated. The preset extraction volume is 500 m³. 3 / day.

[0111] The injected carbon dioxide mixture serves to remediate the methane hydrate reservoir by forming carbon dioxide hydrates, thus preventing methane leakage after production ceases. Compared to injecting liquefied carbon dioxide during methane hydrate production, injecting it after production is complete reduces the inhibitory effect of carbon dioxide hydrate formation on methane hydrate decomposition, increases methane hydrate production, and improves carbon dioxide hydrate sequestration efficiency.

[0112] S5: Fill the bottom cavity of horizontal injection well 1 and horizontal production well 9; specifically, inject cement into the horizontal injection well and horizontal production well to fill the bottom cavity. Since the drilling area is a high-risk area for methane leakage, injecting cement into the bottom of the well after repairing the combustible ice reservoir with carbon dioxide hydrate can further prevent methane leakage.

[0113] In the specific implementation process, the first and second wellbore heaters are installed to prevent the formation of solid hydrates in the horizontal injection well or horizontal production well during the construction of the carbon dioxide hydrate caprock or the extraction of combustible ice, which would cause wellbore blockage and further improve the extraction efficiency. Multi-directional injection nozzles are used to reduce the probability of fluid stagnation caused by excessive resistance due to high local flow rates during the injection of fluid in the first and second branch injection wells. This facilitates the transport and diffusion of the injected fluid in all directions. Combined with adjusting the dosage ratio of carbon dioxide hydrate formation inhibitors and carbon dioxide hydrate formation promoters in the liquefied carbon dioxide mixture, the diffusion capacity of the carbon dioxide mixture is synergistically enhanced, increasing its diffusion distance and depth, forming a carbon dioxide hydrate caprock with a wider coverage area. Subsequently, through a multi-day aging process of the carbon dioxide hydrate caprock, the caprock becomes denser. Compared with the single carbon dioxide hydrate caprock construction method, in this method, the carbon dioxide hydrate in the overlying sedimentary layer is both an intermediate target product and a second guarantee for the safe and efficient development of methane hydrate. When used as an intermediate target product, it mainly serves the methane hydrate reservoir exploitation process and the safe and stable completion of the methane hydrate reservoir repair process after exploitation. It can effectively prevent the intrusion of overlying seawater, maintain formation stability, and prevent methane leakage into the overlying seawater environment. This embodiment, based on the construction of a carbon dioxide hydrate caprock, conducts a combined depressurization-thermal shock extraction operation for methane hydrate. Compared to simple depressurization or thermal shock extraction, this reduces water production and improves extraction efficiency and economic benefits. Furthermore, this invention overcomes the problem of low CH4-CO2 hydrate replacement efficiency by first decomposing the methane hydrate and then injecting carbon dioxide into the original methane hydrate reservoir, significantly increasing hydrate replacement efficiency and carbon dioxide seabed sequestration capacity.

[0114] The same or similar labels correspond to the same or similar parts;

[0115] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0116] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A safe extraction system for combustible ice based on reservoir modification, characterized in that, It includes a horizontal injection unit, a horizontal production unit, and an environmental monitoring unit; The horizontal injection unit includes a horizontal injection well (1), an injection controller (2), a first branch injection well (3), a second branch injection well (4), a first storage tank (5), a second storage tank (6), a first injection switch valve (7), and a second injection switch valve (8). The input end of the horizontal injection well (1) is connected to the first liquid storage tank (5) and the second liquid storage tank (6) respectively. The output end of the horizontal injection well (1) is connected to the input end of the first branch injection well (3) and the input end of the second branch injection well (4) respectively. The first injection switch valve (7) is set on the input end of the first branch injection well (3), and the second injection switch valve (8) is set on the input end of the second branch injection well (4). The control ends of the first injection switch valve (7) and the second injection switch valve (8) are both connected to the injection controller (2). The horizontal production unit includes a horizontal production well (9), a production controller (10), a first branch production well (11), a second branch production well (12), a first production switch valve (13), and a second production switch valve (14). The input end of the horizontal production well (9) is connected to the output end of the first branch production well (11) and the output end of the second branch production well (12), respectively; the first production switch valve (13) is set on the output end of the first branch production well (11), and the second production switch valve (14) is set on the output end of the second branch production well (12); the control ends of the first production switch valve (13) and the second production switch valve (14) are both connected to the production controller (10); The environmental monitoring unit is set on the surface of the overlying deposition layer and is communicatively connected to the injection controller (2) and the production controller (10); The first storage tank (5) is used to store a mixture of liquefied carbon dioxide; The liquefied carbon dioxide mixture includes liquid carbon dioxide, a carbon dioxide hydrate formation inhibitor, and a carbon dioxide hydrate formation promoter. The carbon dioxide hydrate inhibitors include one or more of polyvinylpyrrolidone, polyvinylcaprolactam, and hydroxyl-terminated polycaprolactam. The carbon dioxide hydrate promoter includes leucine; The concentration of the carbon dioxide hydrate formation inhibitor is 300~10000 ppm; the concentration of the carbon dioxide hydrate formation promoter is 50~1000 ppm. By adjusting the dosage ratio, the carbon dioxide hydrate nucleation time can be extended and the carbon dioxide hydrate formation state can be loosened to increase the diffusion range. The pore fluid of the overlying sedimentary layer is extracted using the first branch production well (11), and liquid carbon dioxide mixture is injected using the first branch injection well (3) to construct a carbon dioxide hydrate cap layer to geologically repair the overlying sedimentary layer, giving it strong support capacity, preventing the collapse of the overlying sedimentary layer, avoiding the intrusion of overlying seawater, maintaining formation stability, and preventing methane leakage into the overlying seawater environment; then, the combustible ice in the pores of the combustible ice reservoir is extracted using the second branch production well (12), and when the second preset condition is reached, seawater is injected through the second branch injection well (4) to accelerate the decomposition of combustible ice; when the third preset condition is reached, the combustible ice mining operation is stopped, and liquid carbon dioxide mixture is injected through the second branch injection well (4) to geologically repair the combustible ice reservoir, avoiding methane leakage in the later stage after mining, while increasing the hydrate replacement efficiency and carbon dioxide seabed storage capacity.

2. The safe extraction system for combustible ice based on reservoir modification according to claim 1, characterized in that, The horizontal injection unit also includes a first wellbore heater, which is evenly distributed on the wellbore wall of the horizontal injection well (1); the control end of the first wellbore heater is connected to the injection controller (2); The horizontal production unit also includes a second wellbore heater, which is evenly distributed on the wellbore wall of the horizontal production well (9); the control terminal of the second wellbore heater is connected to the production controller (10).

3. The safe extraction system for combustible ice based on reservoir modification according to claim 1, characterized in that, The horizontal injection unit also includes a first injection nozzle (15) and a second injection nozzle (16). The input end of the first injection nozzle (15) is connected to the output end of the first branch injection well (3); the input end of the second injection nozzle (16) is connected to the output end of the second branch injection well (4); Both the first injection nozzle (15) and the second injection nozzle (16) are multi-directional injection nozzles.

4. The safe extraction system for combustible ice based on reservoir modification according to claim 1, characterized in that, The second liquid storage tank (6) is used to store seawater; the temperature of the seawater is 45~65℃.

5. The safe extraction system for combustible ice based on reservoir modification according to claim 1, characterized in that, The system also includes several pressure sensors (17). The pressure sensors (17) are evenly arranged on the wellbore walls of the horizontal injection well (1) and the horizontal production well (9) located in the overlying sedimentary layer.

6. The safe extraction system for combustible ice based on reservoir modification according to claim 4, characterized in that, The environmental monitoring unit includes several seabed subsidence monitoring instruments (18) and several methane sensors (19). The seabed subsidence monitoring instrument (18) and methane sensor (19) are evenly arranged on the surface of the overlying sediment layer.

7. A method for safe extraction of combustible ice based on reservoir stimulation, characterized in that, Based on the mining system of claim 6, the method includes: S1: Select the target area for the development of combustible ice deposits and set up the combustible ice safe mining system, including: horizontal injection well (1) and horizontal production well (9) sequentially penetrating the seawater layer, the overlying sedimentary layer and the combustible ice reservoir; the output end of the first branch injection well (3) is set in the overlying sedimentary layer, and the output end of the second branch injection well (4) is set in the combustible ice reservoir; the input end of the first branch production well (11) is set in the overlying sedimentary layer, and the input end of the second branch production well (12) is set in the combustible ice reservoir; several seabed subsidence monitoring instruments (18) and methane sensors (19) are evenly arranged on the surface of the overlying sedimentary layer. S2: The production controller (10) controls the opening of the first production switch valve (13), and the first branch production well (11) extracts the pore fluid in the sediment for depressurization; the injection controller (2) controls the opening of the first injection switch valve (7), and the liquefied carbon dioxide mixture in the first storage tank (5) is injected into the overlying sediment layer through the horizontal injection well (1) and the first branch injection well (3). Driven by the pressure difference between the first branch injection well (3) and the first branch production well (11), the liquefied carbon dioxide mixture flows in the overlying sediment layer and forms a carbon dioxide hydrate cap layer; when the first preset condition is reached, the production controller (10) controls the first production switch valve (13) to close, and the injection controller (2) controls the first injection switch valve (7) to close. S3: The second production switch valve (14) is opened by the production controller (10), and the second branch production well (12) extracts pore fluid in the combustible ice reservoir to depressurize the combustible ice. When the second preset condition is met, the second injection switch valve (8) is opened by the injection controller (2), and the seawater in the second storage tank (6) is intermittently injected into the combustible ice reservoir through the horizontal injection well (1) and the second branch injection well (4). S4: When the third preset condition is met, the second production switch valve (14) is closed by the production controller (10) to end the mining of combustible ice; the liquefied carbon dioxide mixture in the first storage tank (5) is switched by the injection controller (2) and injected into the combustible ice reservoir through the horizontal injection well (1) and the second branch injection well (4); S5: Fill the bottom cavity of the horizontal injection well (1) and the horizontal production well (9).

8. The method for safe extraction of combustible ice based on reservoir modification according to claim 7, characterized in that, The first preset condition is: the production rate of liquefied carbon dioxide mixture from the first branch production well (11) is 30% of the injection rate of liquefied carbon dioxide mixture from the first branch injection well (3); The second preset condition is: the second branch production well (12) extracts pore fluid from the combustible ice reservoir to reduce the production pressure to a constant pressure; the constant pressure is 3~5MPa; The third preset condition is: the average daily collection of combustible ice from the second branch production well (12) is lower than the preset collection amount for three consecutive days, and the preset collection amount is 500 m3 / day.

9. The method for safe extraction of combustible ice based on reservoir modification according to claim 7, characterized in that, Before step S3, the horizontal injection unit and the horizontal production unit need to be sealed for several days to allow the carbon dioxide hydrate caprock to age and become dense.

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