Underground heat storage system reservoir transformation method for improving cross-seasonal heat storage efficiency

Abstract

The invention relates to the technical field of energy storage and utilization, and provides an underground heat storage system reservoir transformation method for improving cross-seasonal heat storage efficiency, which comprises the following steps: measuring physical property parameters of an abandoned oil well reservoir through rock core sampling and indoor experiment; carrying out a fracturing physical simulation experiment, simulating the fracture form of the reservoir after fracturing, transporting the thermal insulation material, and monitoring the position of the thermal insulation material; carrying out a heat storage effect physical simulation experiment, calculating the heat storage efficiency of different reservoirs, and optimizing an optimal heat storage reservoir and a transformation scheme; carrying out thermal insulation material migration simulation; transferring the thermal insulation material, throwing the sand-carrying fluid to the top end of the crack, and then transferring the propping agent to the middle of the crack by using the sand-carrying fluid again; and after fracturing is finished, the time when the stratum is to be closed but not closed is selected as a favorable opportunity for reverse drainage of the fracturing fluid. The heat insulation material can be accurately injected into the top end of the crack, then an artificial heat insulation zone is formed, the heat dissipation rate of a heat storage system can be effectively reduced, and the heat recovery rate is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage and utilization, and particularly to a method for reservoir transformation of an underground heat storage system for improving cross-seasonal heat storage efficiency. Background Art

[0002] With the continuous growth of global energy demand and the increasingly severe carbon emission problem, finding clean and efficient ways of energy utilization has become a key topic of common concern. In this context, the cross-seasonal heat storage technology (Seasonal Thermal Energy Storage, STES) has attracted much attention because it can significantly balance energy supply and demand and improve energy utilization efficiency. The cross-seasonal heat storage system (STES) stores excess heat energy underground efficiently in summer and releases this heat in winter or during peak energy demand periods to support heating, cooling, and other energy needs. For example, the layout method of geothermal well groups in the cross-seasonal underground heat storage system can prevent the phenomenon of heat breakthrough by establishing clusters of cold wells and hot wells and optimizing the layout, effectively saving the system construction cost and improving the efficiency of heat energy storage and utilization. In addition, the BTES system (Borehole Thermal Energy Storage) also demonstrates the ability to store heat underground in the non-heating season and extract and utilize it in the heating season, effectively solving the contradiction between energy supply and demand in terms of time and space. This technology shows great potential in achieving sustainable energy development.

[0003] The underground heat storage system (Underground Thermal Energy Storage, UTES) is one of the widely used and efficient cross-seasonal heat storage technologies at present. Its core is to use natural underground reservoirs (such as aquifers, sandstone layers, abandoned oil reservoirs or artificial reservoirs) as heat energy storage media, and transfer heat through heat carrier media (such as water or air) to achieve energy storage. However, due to inherent defects such as insufficient thermal conductivity, large heat loss, and low permeability of natural reservoirs, the thermal energy utilization efficiency of the system and the stability of long-term operation are limited. Therefore, how to optimize the reservoir to improve the efficiency of heat storage and release and reduce heat loss during storage has become the core challenge in the research and application of underground heat storage technology.

[0004] In the aspect of reservoir transformation in the prior art, it usually focuses on improving the thermal conductivity of the reservoir and reducing heat loss. For example, high-thermal-conductivity filling materials are used to increase the uniformity of heat transfer in the reservoir, or adjacent insulation layer technology is used to reduce heat dissipation to the outside.

[0005] Currently, most domestic and foreign research on underground heat storage efficiency focuses on the impact of reservoir parameters on heat storage effects, and there are few reports on relevant research on how to transform the reservoir to reduce energy loss during heat storage and the heat dissipation rate of the heat storage system. Therefore, we have invented a new method for transforming the reservoir of an underground heat storage system to improve cross-seasonal heat storage efficiency, which solves the above technical problems. Summary of the Invention

[0006] The object of the present invention is to provide a method for transforming the reservoir of an underground heat storage system to improve cross-seasonal heat storage efficiency, which combines heat insulation materials, transforms the reservoir through fracturing technology, significantly improves heat storage efficiency, and reduces heat loss.

[0007] The object of the present invention can be achieved by the following technical measures: A method for transforming the reservoir of an underground heat storage system to improve cross-seasonal heat storage efficiency, the method comprising:

[0008] Step 1: Through core sampling, conduct indoor experiments to measure the physical properties of the abandoned oil well reservoir.

[0009] Step 2: Conduct a fracturing physical simulation experiment to simulate the fracture morphology after reservoir fracturing, and then transport the heat insulation material and monitor its position.

[0010] Step 3: Conduct a heat storage effect physical simulation experiment, calculate the corresponding heat storage efficiency under different reservoir transformation schemes, and optimize the best heat storage reservoir and transformation scheme.

[0011] Step 4: Conduct a simulation of the migration of the heat insulation material in the main fracture and branch fractures.

[0012] Step 5: Conduct the migration of the heat insulation material, use the sand-carrying fluid to place it at the top of the fracture, and then use the sand-carrying fluid again to transport the proppant to the middle of the fracture.

[0013] Step 6: After the fracturing is completed, select the time when the formation is about to close but has not closed as the favorable opportunity for the reverse drainage of the fracturing fluid.

[0014] The object of the present invention can also be achieved by the following technical measures:

[0015] In Step 1, first, according to the regional heating demand, screen parameters such as the geographical location, well depth, and reservoir thickness of the abandoned oil well, and evaluate the heat capacity and heat conduction characteristics of the reservoir in combination with the production data during the oil production period. Preferentially select abandoned oil wells with intact wellbores, close to heat sources, and low transformation costs.

[0016] Secondly, through drilling and core sampling, conduct laboratory tests on the reservoir rock samples of the abandoned oil well to measure the physical properties of different reservoirs, including permeability, porosity, thermal conductivity, and mechanical properties.

[0017] In step 1, when measuring the physical properties of the reservoir, first, by adjusting different pressure gradients, a steady-state permeability experiment is carried out using a core permeability tester to measure the permeability of the target reservoir. At the same time, the permeability is calculated in combination with Darcy's law to ensure the accuracy of the measured data. Second, by controlling the gas injection pressure, a gas porosity meter is used to measure the porosity of the reservoir rock, measure the gas diffusion volume, and calculate the porosity in combination with the known sample volume. For the thermal conductivity of the reservoir rock, a TPS thermal conductivity meter is used to test the thermal conductivity of the core sample, and the thermal conductivity is calculated by measuring the change in temperature rise over time. Further, for the mechanical properties of the reservoir, uniaxial compression tests and Brazilian splitting tests are carried out respectively to measure the compressive strength and tensile strength of the rock.

[0018] In step 2, based on the physical property parameters measured in step 1, artificial rock blocks containing prefabricated perforations are constructed, and a physical simulation experiment of fracturing is carried out using indoor tests. First, fracturing experiments are carried out with different displacement rates and different low-temperature fracturing fluids, and an acoustic emission device is used to observe the fracture morphology and trend. Second, by injecting fracturing fluids with different viscosities and different filling amounts of heat insulation materials, the migration law of heat insulation materials under different injection conditions is analyzed. Finally, CT scanning is used to analyze the fracture morphology and the distribution law of heat insulation materials in the fracturing fractures. Through the physical simulation experiment, it is obtained that as the injection flow rate increases, the scale of the fracturing fracture first increases and then decreases; as the injection temperature decreases, the scale of the fracturing fracture increases.

[0019] In step 3, based on the experimental results in step 2, a physical simulation experiment of heat storage effect is carried out on different reservoirs, and then the heat storage efficiency η corresponding to each reservoir is calculated. Through the physical simulation experiment, it is obtained that the larger the fracturing scale, the higher the heat storage efficiency that can be obtained. Step 2 determines the relationship between the injection flow rate, injection temperature and fracture scale during the fracturing process. Furthermore, the optimal injection flow rate and injection temperature range that can obtain large-scale fractures can be determined. Step 3 determines the relationship between the fracture scale and the heat storage efficiency. Combining step 2 and step 3, the reservoir transformation plan with higher heat storage efficiency includes the injection flow rate and injection temperature. Among them, the injection flow rate can be calculated by the optimal injection flow rate during the experiment, the cross-sectional area of the wellbore in the artificial rock block, and the cross-sectional area of the wellbore during the on-site construction. The injection temperature is the optimal injection temperature during the experiment.

[0020] In step 3, the calculation formula for the heat storage efficiency η is:

[0021]

[0022] Where:

[0023] Q in ——The total heat energy input during the injection stage;

[0024] Q out ——The heat energy that can be actually extracted during the production stage;

[0025] Q in =m in ×c×ΔT in ;

[0026] Where:

[0027] m in ——Mass of the injected water;

[0028] c——Specific heat capacity;

[0029] ΔT in ——Temperature difference between the water and the reservoir during injection;

[0030] Q out =m out ×c×ΔT out ;

[0031] Where:

[0032] m out ——Mass of the produced water;

[0033] c——Specific heat capacity;

[0034] ΔT out ——Temperature difference between the water and the reservoir during production.

[0035] In step 4, conduct a simulation experiment on the influence of the displacement and viscosity of the fracturing fluid on the migration of the heat insulation material in the fracture system, and analyze the law of the migration distribution of the heat insulation material in the main fracture and the branch fractures;

[0036] For the main fracture: Due to the diversion effect of the branch fractures, the filling of the heat insulation material in the main fracture is reduced. Therefore, inject low-density and small-particle-size heat insulation materials preferentially at an appropriate displacement, and gradually change to high-density and large-particle-size heat insulation materials to avoid blockage;

[0037] For the branch fractures: The heat insulation material will gradually accumulate during migration in the main fracture, resulting in a decrease in the heat insulation material entering the branch fractures. Therefore, increase the injection displacement and at the same time increase the viscosity of the sand-carrying fluid to improve the migration ability of the heat insulation material and ensure that the heat insulation material can fully fill the branch fractures.

[0038] In step 5, according to the reservoir selected in step 3 and the corresponding optimal transformation plan including the injection flow rate and injection temperature, carry out on-site construction to prepare a fracturing fluid including: preflush fluid, sand-carrying fluid and displacement fluid; According to different fracture morphologies, by controlling the displacement and viscosity of the fracturing fluid, adopt a four-stage injection strategy of fracture formation - heat insulation material filling - proppant placement - displacement according to the injection flow rate and injection temperature selected in step 3, and distinguish between the main fracture and the branch fractures for heat insulation material migration and filling.

[0039] In step 5, first, the preflush fluid is injected into the reservoir through a high-pressure pump. The preflush fluid is used to fracture the reservoir with its rock-breaking ability and form a large-scale fracture network, and the large-scale fracture network is obtained by transforming with a low-temperature fluid fracturing preflush fluid; subsequently, the heat-insulating material is carried into the fracture by the proppant-carrying fluid. After the heat-insulating material reaches the top of the fracture, the proppant-carrying fluid is used again to carry the proppant to the middle of the fracture; finally, the displacement fluid is injected to push the residual proppant in the wellbore into the fracture; after the transformation is completed, the heat-insulating material will remain in the artificial fracture to ensure that the fracture is not closed under the influence of in-situ stress. The heat-insulating material is a composite silicate; the low-temperature fluid fracturing preflush fluid is liquid nitrogen or liquid carbon dioxide.

[0040] In step 6, the post-fracture formation closure time is obtained by testing the pressure drop during the fracturing. When the formation is about to close but has not closed yet, it is an opportune time for the fracturing fluid to flow back.

[0041] The method for reservoir transformation of the underground heat storage system for improving the cross-seasonal heat storage efficiency further includes: after step 6, conducting heat injection - heat storage - heat extraction tests on the reservoir filled with the heat-insulating material after transformation to evaluate the heat storage efficiency of the reservoir after transformation.

[0042] The object of the present invention can also be achieved by the following technical measures: A reservoir transformation system for an underground heat storage system for improving the cross-seasonal heat storage efficiency, characterized in that the reservoir transformation system for the underground heat storage system for improving the cross-seasonal heat storage efficiency uses the method for reservoir transformation of the underground heat storage system for improving the cross-seasonal heat storage efficiency to conduct reservoir transformation of the underground heat storage system.

[0043] The method for reservoir transformation of the underground heat storage system for improving the cross-seasonal heat storage efficiency in the present invention is applicable to the engineering transformation of natural reservoirs such as abandoned oil reservoirs, aquifers, and sandstone formations. By constructing an artificial fracture network through low-temperature fluid fracturing technology and collaborating with the filling of heat-insulating materials, the heat storage efficiency is significantly improved, the heat loss rate is reduced, and the stability of the reservoir during long-term heat cycles is enhanced. The method for reservoir transformation of the underground heat storage system for improving the cross-seasonal heat storage efficiency in the present invention, through the determination of the physical property parameters of the target reservoir, combined with the application of fracturing technology and heat-insulating materials (composite silicate), greatly improves the heat storage efficiency and reduces heat loss. During the fracturing process, the fracture morphology and trend are evaluated through laboratory tests, and the heat-insulating material is accurately placed at the top of the fracture, and the influence of the filling of different reservoir heat-insulating materials on the heat storage efficiency is considered. After the fracturing is completed, the fracture naturally closes under the action of in-situ stress. At this time, due to the extrusion of in-situ stress, the heat-insulating material closely adheres to the rock surface, forming an efficient heat-insulating barrier to effectively prevent the outward diffusion of heat. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the relationship between the injection temperature and the fracture scale in an embodiment of the present invention;

[0045] Figure 2Schematic diagram of the relationship between injection flow rate and fracture scale in an embodiment of the present invention;

[0046] Figure 3 Schematic diagram of the relationship between heat storage efficiency and fracture volume in an embodiment of the present invention;

[0047] Figure 4 Schematic diagram of the basic fracturing implementation process of the present invention taking a disused oil well as an example;

[0048] Figure 5 Flow chart of a specific embodiment of the reservoir transformation method of the underground heat storage system for improving cross-seasonal heat storage efficiency of the present invention;

[0049] Figure 4 In the figure, 1 - different reservoirs, 2 - fracturing to create fractures, 3 - interlayer of each reservoir, 4 - disused wellbore, 5 - perforation channels, 6 - thermal insulation material. Detailed implementation mode

[0050] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0051] It should be noted that the terms used herein are only for describing specific implementation modes and are not intended to limit the exemplary implementation modes according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0052] As Figure 5 shown Figure 5 is the flow chart of the reservoir transformation method of the underground heat storage system for improving cross-seasonal heat storage efficiency of the present invention. The reservoir transformation method of the underground heat storage system for improving cross-seasonal heat storage efficiency includes:

[0053] Step 1: Through core sampling, conduct laboratory experiments to measure the physical properties of the reservoir of the disused oil well, including permeability, porosity, thermal conductivity, and mechanical properties.

[0054] First, according to the regional heating demand, screen parameters such as the geographical location, well depth, and reservoir thickness of the disused oil well. Evaluate the heat capacity and heat conduction characteristics of the reservoir in combination with the production data during the oil production period. Preferentially select disused oil wells with intact wellbores, adjacent heat sources, and low transformation costs.

[0055] Secondly, through drilling and core sampling, conduct laboratory tests on the reservoir rock samples of the disused oil well to determine the permeability, porosity, thermal conductivity, and mechanical properties of different reservoirs.

[0056] Step 2: Conduct a fracturing physical simulation experiment. Construct artificial rock blocks with pores according to the measured physical property parameters, perform fracturing operations, and through the physical simulation experiment, simulate the fracture morphology after reservoir fracturing, and then transport the thermal insulation material and monitor its position.

[0057] Based on the core parameters measured in Step 1, construct artificial rock blocks containing prefabricated perforations and conduct fracturing simulations using indoor tests. First, conduct fracturing experiments with different displacement rates and different low-temperature fracturing fluids, and use an acoustic emission device to observe the fracture morphology and trend; second, analyze the migration law of the thermal insulation material under different injection conditions by injecting fracturing fluids with different viscosities and different filling amounts of the thermal insulation material; finally, use CT scanning to analyze the fracture morphology and the distribution law of the thermal insulation material in the fracturing fractures.

[0058] Through the physical simulation experiment, it is obtained that: with the increase of the injection flow rate, the scale of the fracturing fracture first increases and then decreases; with the decrease of the injection temperature, the scale of the fracturing fracture increases. For example: the size of the rock block used in the experiment is a 40cm×40cm×40cm cube, and by Figure 1 it can be seen that the fracture scale reaches the maximum at temperatures below -40°C; by Figure 2 it can be seen that the fracture scale reaches the maximum at an injection flow rate of 15 ml / min - 20 ml / min and then gradually decreases.

[0059] Step 3: Based on the experimental results in Step 2, conduct physical simulation experiments on the heat storage effects of different reservoirs, and then calculate the corresponding heat storage efficiency η of each reservoir.

[0060] Through the physical simulation experiment, it is obtained that: the larger the fracturing scale, the higher the heat storage efficiency that can be obtained. For example: the size of the rock block used in the experiment is a 40cm×40cm×40cm cube, and by Figure 3 it can be seen that with the increase of the fracture volume, the heat storage efficiency increases accordingly, and reaches the maximum value at 6000 cm 3 and tends to be stable.

[0061] Furthermore, Step 2 determines the relationship between the injection flow rate, injection temperature and fracture scale during the fracturing process, and then the optimal injection flow rate and injection temperature range that can obtain large-scale regular fractures can be determined. Step 3 determines the relationship between the fracture scale and the heat storage efficiency. Combining Step 2 and Step 3, the reservoir transformation plan with higher heat storage efficiency includes the injection flow rate and injection temperature. Among them, the injection flow rate can be calculated by the optimal injection flow rate during the experiment, the cross-sectional area of the wellbore in the artificial rock block, and the cross-sectional area of the wellbore during on-site construction to obtain the injection flow rate during on-site construction, and the determination of the injection temperature is the optimal injection temperature during the experiment.

[0062] Indoor experiment - on-site construction flow conversion formula:

[0063]

[0064] Among which, V in is the injection flow rate in the laboratory experiment, and u e is the volume flow rate during the experiment, and s e is the cross-sectional area of the injection wellbore in the artificial rock block; s m is the cross-sectional area of the wellbore during on-site construction, and u m is the injection volume flow rate during on-site construction.

[0065] In Step 3, the calculation formula for the heat storage efficiency η is:

[0066]

[0067] Among which:

[0068] Q in ——The total heat energy input during the injection stage;

[0069] Q out ——The heat energy that can be actually extracted during the production stage;

[0070] Q in =m in ×c×ΔT in ;

[0071] Among which:

[0072] m in ——The mass of the injected water;

[0073] c——Specific heat capacity;

[0074] ΔT in ——The temperature difference between the injected water and the reservoir;

[0075] Q out =m out ×c×ΔT out ;

[0076] Among which:

[0077] m out ——The mass of the produced water;

[0078] c——Specific heat capacity;

[0079] ΔT out ——The temperature difference between the produced water and the reservoir.

[0080] Step 4: Conduct a simulation of the migration of the thermal insulation material, that is, conduct a simulation experiment on the influence of the displacement and viscosity of the fracturing fluid on the migration of the thermal insulation material in the fracture system, for the thermal insulation material in the main fracture and the branch fractures.

[0081] Carry out simulation experiments on the influence of the displacement and viscosity of fracturing fluid on the migration of thermal insulation materials in the fracture system, and analyze the laws of the migration and distribution of thermal insulation materials in the main fractures and branch fractures.

[0082] For the main fractures: Due to the diversion effect of the branch fractures, the filling of thermal insulation materials in the main fractures decreases. Therefore, low-density and small-particle-size thermal insulation materials are preferentially injected at an appropriate displacement, and then gradually changed to high-density and large-particle-size thermal insulation materials to avoid blockage.

[0083] For the branch fractures: The thermal insulation materials will gradually accumulate during migration in the main fractures, resulting in a reduction in the thermal insulation materials entering the branch fractures. Therefore, the injection displacement is increased, and at the same time, the viscosity of the sand-carrying fluid is increased to enhance the migration ability of the thermal insulation materials and ensure that the thermal insulation materials can fully fill the branch fractures.

[0084] Step Five: Carry out on-site construction to prepare fracturing fluid including preflush fluid, sand-carrying fluid and displacement fluid. After the fractures are formed, first carry out the migration of thermal insulation materials, and use the sand-carrying fluid to place them at the top of the fractures. Subsequently, use the sand-carrying fluid again to transport the proppants to the middle of the fractures.

[0085] According to the reservoir selected in Step Three and the corresponding optimal stimulation plan including injection flow rate and injection temperature, carry out on-site construction to prepare fracturing fluid including preflush fluid, sand-carrying fluid and displacement fluid. First, inject the preflush fluid into the reservoir through a high-pressure pump, and use the rock-breaking ability of the preflush fluid to fracture the reservoir and form large-scale fractures. The large-scale fracture network is obtained by fracturing and reforming with low-temperature fluid in the preflush fluid. Subsequently, use the sand-carrying fluid to carry the thermal insulation materials into the fractures. After the thermal insulation materials reach the top of the fractures, use the sand-carrying fluid to carry the proppants to the middle of the fractures again. Finally, inject the displacement fluid to push the residual proppants in the wellbore into the fractures. After the transformation is completed, the thermal insulation materials will remain in the artificial fractures, greatly improving the thermal insulation ability at the ends of the fractures.

[0086] Step Six: After the fracturing is completed, select the time when the formation is about to close but has not yet closed as the favorable opportunity for fracturing fluid backflow, and ensure that the proppants have sufficient formation compaction.

[0087] Obtain the post-fracture formation closure time by testing the fracturing pressure drop. When the formation is about to close but has not yet closed, it is used as the favorable opportunity for fracturing fluid backflow. This not only ensures that the proppants have sufficient formation compaction and are not carried out by the backflow fracturing fluid, but also ensures that there is no loss of displacement pressure due to the diffusion of pressure after the formation closure.

[0088] Step Seven: Based on the fracturing physical simulation experiments, heat storage effect physical simulation experiments and thermal insulation material migration simulations, form a set of standard reservoir stimulation technical solutions to guide on-site construction.

[0089] In Step 1, by adjusting different pressure gradients, a steady-state permeability experiment is carried out using a core permeability tester to measure the permeability of the target reservoir. At the same time, the permeability is calculated in combination with Darcy's law to ensure the accuracy of the measured data.

[0090] Secondly, by controlling the gas injection pressure, a gas porosity meter is used to measure the porosity of the reservoir rock, measure the gas diffusion volume, and calculate the porosity in combination with the known sample volume. For the thermal conductivity of the reservoir rock, a TPS thermal conductivity meter is used to test the thermal conductivity of the core sample, and the thermal conductivity is calculated by measuring the change of temperature rise over time. Further, for the mechanical properties of the reservoir, uniaxial compression tests and Brazilian split tests are carried out respectively to measure the compressive strength and tensile strength of the rock.

[0091] In Step 5, natural fractures are easily connected during the reservoir fracturing process to form a complex fracture network, and how to accurately place the thermal insulation material at the top of the fracture becomes a problem to be solved. Based on this, this method combines indoor experiments and on-site construction. According to different fracture morphologies, by controlling the displacement and viscosity of the fracturing fluid, a four-stage injection strategy of "fracture creation - thermal insulation material filling - proppant placement - displacement" is adopted according to the injection flow rate and injection temperature selected in Step 3, and the main fractures and branch fractures are distinguished for the migration and filling of the thermal insulation material.

[0092] When the thermal insulation material is fully filled to the top of the fracture, a high-viscosity fracturing fluid is injected at a large displacement to bring the proppant into the middle section of the fracture. Subsequently, the injection displacement and the viscosity of the fracturing fluid are reduced to ensure that the proppant is fully filled in the fracture and prevent the fracture from closing under the influence of in-situ stress.

[0093] The above thermal insulation material uses composite silicate; the low-temperature fluid fracturing preflush uses liquid nitrogen or liquid carbon dioxide.

[0094] This method makes full use of abandoned oil reservoirs. Through reservoir transformation combined with the placement of thermal insulation materials, it is transformed into an efficient heat storage system, improving resource utilization rate and reducing environmental impact.

[0095] The present invention further improves the sustainability of underground heat storage technology in terms of heat storage efficiency, stability and resource reuse.

[0096] In a specific embodiment of applying the present invention, the reservoir transformation method of the underground heat storage system for improving cross-seasonal heat storage efficiency includes the following steps:

[0097] Step 1. According to the regional heating demand, screen parameters such as the geographical location, well depth, and reservoir thickness of abandoned oil wells. Evaluate the heat capacity and heat conduction characteristics of the reservoir in combination with the production data during the oil production period. Preferentially select oil wells with intact wellbores, close to heat sources and low transformation costs.

[0098] Step 2. First, conduct a comprehensive determination of physical property parameters for the target reservoir. Use a core permeability tester to conduct a steady-state permeability experiment to measure the migration ability of fluids in the reservoir. Subsequently, use a gas porosity meter to measure the porosity of the reservoir to provide a basis for analyzing the heat storage capacity. In addition, use a TPS thermal conductivity meter to measure the thermal conductivity of the reservoir to clarify the specific characteristics of its heat transfer. Through uniaxial compression tests and Brazilian split tests, measure the compressive strength and tensile strength of the reservoir.

[0099] Step 3. Construct artificial rock blocks based on the reservoir physical property parameters and conduct indoor physical model tests. First, conduct fracturing experiments with different displacement rates and different low-temperature fracturing fluids; after fracturing, use an acoustic emission device to obtain the morphology and trend of the fractures after fracturing; further, inject fracturing fluids with different viscosities and analyze the migration law of thermal insulation materials under different filling amounts. During the experiment, consider the stress field distribution, permeability changes, and pore structure characteristics, and determine the transformation plan most suitable for the target reservoir conditions by comparing the effects of different low-temperature fracturing fluids. It is obtained from the physical model experiment that as the injection flow rate increases, the scale of the fracturing fractures first increases and then decreases; as the injection temperature decreases, the scale of the fracturing fractures increases.

[0100] Step 4. Based on the experimental results in Step 3, conduct physical simulation experiments on the heat storage effects of different reservoirs, and then calculate the corresponding heat storage efficiency η of each reservoir; it is obtained from the physical simulation experiment that the larger the fracturing scale, the higher the heat storage efficiency that can be obtained.

[0101] Furthermore, Step 3 determines the relationship between the injection flow rate, injection temperature, and fracture scale during the fracturing process, and thus the optimal injection flow rate and injection temperature range that can obtain large-scale regular fractures can be determined. Step 4 determines the relationship between the fracture scale and the heat storage efficiency. Combining Step 3 and Step 4, the reservoir transformation plan with a higher heat storage efficiency includes the injection flow rate and injection temperature. Among them, the injection flow rate can be calculated based on the optimal injection flow rate during the experiment, the cross-sectional area of the wellbore in the artificial rock block, and the cross-sectional area of the wellbore during on-site construction to obtain the injection flow rate during on-site construction, and the injection temperature is the optimal injection temperature during the experiment.

[0102]

[0103] Where:

[0104] Q in —— The total thermal energy input during the injection stage;

[0105] Q out —— The thermal energy that can actually be extracted during the recovery stage;

[0106] Q in= m in ×c×ΔT in ;

[0107] Wherein:

[0108] m in —— Mass of the injected water;

[0109] c —— Specific heat capacity;

[0110] ΔT in —— Temperature difference between the water and the reservoir during injection;

[0111] Q out= m out × c × ΔT out ;

[0112] Wherein:

[0113] m out —— Mass of the produced water;

[0114] c —— Specific heat capacity;

[0115] ΔT out —— Temperature difference between the water and the reservoir during production.

[0116] Step 5. Conduct a simulation experiment on the influence of the displacement and viscosity of the fracturing fluid on the migration of the thermal insulation material in the fracture system, and analyze the law of the migration and distribution of the thermal insulation material in the main fracture and the branch fractures.

[0117] For the main fracture: Due to the diversion effect of the branch fractures, the filling of the thermal insulation material in the main fracture is reduced. Therefore, inject low-density and small-particle-size thermal insulation materials preferentially with an appropriate displacement, and gradually change to high-density and large-particle-size thermal insulation materials to avoid blockage;

[0118] For the branch fractures: The thermal insulation material will gradually accumulate during migration in the main fracture, resulting in a reduction in the thermal insulation material entering the branch fractures. Therefore, increase the injection displacement and at the same time increase the viscosity of the sand-carrying fluid to improve the migration ability of the thermal insulation material and ensure that the branch fractures can be fully filled with the thermal insulation material.

[0119] Step 6. Based on the results obtained from the laboratory experiments, carry out on-site construction to prepare a fracturing fluid including a preflush fluid, a sand-carrying fluid, and a displacement fluid. First, inject the preflush fluid into the reservoir through a high-pressure pump, and use the rock-breaking ability of the preflush fluid to fracture the reservoir and form fractures; subsequently, use the sand-carrying fluid to carry the thermal insulation material to the top of the fractures. Further, use the sand-carrying fluid again to carry the proppant to the middle of the fractures to ensure that the fractures are not closed under the influence of in-situ stress; finally, inject the displacement fluid to push the residual proppant in the wellbore into the fractures. After the transformation is completed, the thermal insulation material will remain in the artificial fractures, greatly improving the thermal insulation performance of the reservoir.

[0120] Step 7. After fracturing, accurately obtain the post-fracture formation closure time, and select the time when the formation is about to close but has not yet closed as the favorable timing for fracturing fluid flowback.

[0121] Step 8. After the reservoir transformation is completed, conduct heat injection - heat storage - heat extraction tests on the reservoir filled with heat insulation materials after transformation, evaluate the heat storage efficiency of the reservoir after transformation, and form a set of standardized reservoir transformation technical solutions.

[0122] As Figure 4 shown, Figure 4 As shown in the figure, it is a schematic diagram of reservoir fracturing transformation in a cross - seasonal heat storage system, presenting the fracture morphology after fracturing targeting abandoned oil wells. It can be seen the different reservoirs, fracture - creating for fracturing, each reservoir interlayer, abandoned wellbore, perforation channels, and heat insulation materials at the top of the injection fractures, demonstrating the key link of creating fractures in the reservoir by fracturing technology and filling heat insulation materials to construct an artificial heat - insulating zone.

[0123] The innovation of the present invention lies in determining the layer to be transformed in the underground heat storage system, using low - temperature fluid to fracture the transformation layer, and at the same time, by controlling the migration of heat insulation materials, accurately injecting heat insulation materials to the top of the fractures, thereby forming an artificial heat - insulating zone, which can effectively reduce the heat dissipation rate of the heat storage system and improve the heat recovery rate.

[0124] Finally, it should be noted that the above - mentioned are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

[0125] Except for the technical features described in the specification, the rest are known technologies to those skilled in the art.

Claims

1. A method for reservoir transformation of an underground heat storage system for improving cross-seasonal heat storage efficiency, characterized in that, The method includes: Step 1: Through core sampling, conduct indoor experiments to measure the physical properties of the reservoir of abandoned oil wells. Step 2: Conduct a physical simulation experiment of fracturing to simulate the fracture morphology after reservoir fracturing, and then transport the thermal insulation material and monitor its position. Step 3: Conduct a physical simulation experiment of heat storage effect, calculate the corresponding heat storage efficiency under different reservoir transformation schemes, and optimize the best heat storage reservoir and transformation scheme. Step 4: Conduct a simulation of the migration of the thermal insulation material in the main fracture and branch fractures. Step 5: Conduct the migration of the thermal insulation material, use the sand-carrying fluid to place it at the top of the fracture, and then use the sand-carrying fluid again to transport the proppant to the middle of the fracture. Step 6: After the fracturing is completed, select the time when the formation is about to close but has not closed as the favorable opportunity for the backflow of the fracturing fluid.

2. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 1, characterized in that In Step 1, first, according to the regional heating demand, screen the geographical location, well depth, and reservoir thickness of the abandoned oil wells, and evaluate the heat capacity and heat conduction characteristics of the reservoir in combination with the production data during the oil production period. Preferentially select the abandoned oil wells with intact wellbores, close to heat sources, and low transformation costs. Secondly, through drilling and core sampling, conduct laboratory tests on the reservoir rock samples of the abandoned oil wells to measure the physical properties of different reservoirs, including permeability, porosity, thermal conductivity, and mechanical properties.

3. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 2, wherein, In Step 1, when measuring the physical properties of the reservoir, first, by adjusting different pressure gradients, conduct a steady-state permeability experiment using a core permeability tester to measure the permeability of the target reservoir, and at the same time calculate the permeability in combination with Darcy's law to ensure the accuracy of the measured data; secondly, by controlling the gas injection pressure, use a gas porosity tester to measure the porosity of the reservoir rock, measure the gas diffusion volume, and calculate the porosity in combination with the known sample volume; for the thermal conductivity of the reservoir rock, use a TPS thermal conductivity tester to test the thermal conductivity of the core sample, and calculate the thermal conductivity by measuring the change of temperature rise with time. Furthermore, for the mechanical properties of the reservoir, conduct uniaxial compression tests and Brazilian splitting tests respectively to measure the compressive strength and tensile strength of the rock.

4. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 1, characterized in that In Step 2, based on the physical property parameters measured in Step 1, construct artificial rock blocks containing prefabricated perforations and conduct a physical simulation experiment of fracturing using indoor tests; first, conduct fracturing experiments with different displacement and different low-temperature fracturing fluids, and use an acoustic emission device to observe the fracture morphology and trend; secondly, by injecting fracturing fluids with different viscosities and different filling amounts of thermal insulation materials, analyze the migration law of the thermal insulation materials under different injection conditions; finally, use CT scanning to analyze the fracture morphology and the distribution law of the thermal insulation materials in the fracturing fractures; through the experiment, it is found that: with the increase of the injection flow rate, the scale of the fracturing fracture first increases and then decreases; with the decrease of the injection temperature, the scale of the fracturing fracture increases.

5. The method for reservoir reconstruction of the underground heat storage system for improving the cross-seasonal heat storage efficiency according to claim 1, characterized in that In step 3, based on the experimental results in step 2, physical simulation tests on the heat storage effect are carried out for different reservoirs, and then the corresponding heat storage efficiency η is calculated for different reservoir transformation schemes; it is obtained through physical model experiments that the larger the fracturing scale, the higher the heat storage efficiency that can be obtained; step 2 determines the relationship between the injection flow rate, injection temperature and fracture scale during the fracturing process, and then determines the optimal injection flow rate and injection temperature range that can obtain large-scale fractures. Step 3 determines the relationship between the fracture scale and the heat storage efficiency. Combining step 2 and step 3, the reservoir transformation scheme with higher heat storage efficiency includes the injection flow rate and injection temperature. The determination of the injection flow rate is to calculate the injection flow rate during on-site construction through the optimal injection flow rate during the experiment, the cross-sectional area of the wellbore in the artificial rock block, and the cross-sectional area of the wellbore during on-site construction. The determination of the injection temperature is the optimal injection temperature during the experiment.

6. The method for reservoir transformation of the underground heat storage system for improving the cross-seasonal heat storage efficiency according to claim 5, characterized in that In step 3, the calculation formula for the heat storage efficiency η is: Wherein: Q in —— The total thermal energy input during the injection stage; Q out —— The actually extractable heat energy during the mining stage; Q in = m in × c × ΔT in ; Where: m in —— mass of the injected water; c——specific heat capacity; ΔT in —— Temperature difference between water and reservoir during injection; Q out = m out × c × ΔT out ; Where: m out —— mass of produced water; c——specific heat capacity; ΔT out —— Temperature difference between water and reservoir at production.

7. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 1, characterized in that, In step 4, a simulation experiment on the influence of the displacement and viscosity of the fracturing fluid on the migration of the heat insulation material in the fracture system is carried out to analyze the law of the migration distribution of the heat insulation material in the main fracture and the branch fractures; For the main fracture: due to the diversion effect of the branch fractures, the filling of the heat insulation material in the main fracture is reduced. Therefore, the low-density and small-particle-size heat insulation material is preferentially injected through an appropriate displacement, and gradually changed to the high-density and large-particle-size heat insulation material to avoid blockage; For the branch fractures: the heat insulation material will gradually accumulate during the migration in the main fracture, resulting in a reduction in the heat insulation material entering the branch fractures; therefore, the injection displacement is increased, and at the same time, the viscosity of the sand-carrying fluid is increased to enhance the migration ability of the heat insulation material to ensure that the heat insulation material can fully fill the branch fractures.

8. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 5, characterized in that In step 5, according to the reservoir selected in step 3 and the corresponding optimal transformation scheme including the injection flow rate and injection temperature, on-site construction is carried out to prepare a fracturing fluid including a preflush fluid, a sand-carrying fluid and a displacement fluid; according to different fracture morphologies, by controlling the displacement and viscosity of the fracturing fluid, a four-stage injection strategy of fracture formation-insulation material filling-proppant placement-displacement is adopted according to the injection flow rate and injection temperature selected in step 3, and the heat insulation material migration filling is carried out by distinguishing the main fracture and the branch fractures.

9. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 8, characterized in that, In step 5, first, the preflush fluid is injected into the reservoir through a high-pressure pump, and the rock-breaking ability of the preflush fluid is used to fracture the reservoir and form a large-scale fracture network. The large-scale fracture network is obtained by transforming the preflush fluid with low-temperature fluid; then, the sand-carrying fluid is used to carry the heat insulation material into the fracture. After the heat insulation material reaches the top of the fracture, the sand-carrying fluid is used again to carry the proppant to the middle of the fracture; finally, the displacement fluid is injected to push the residual proppant in the wellbore into the fracture; after the transformation is completed, the heat insulation material will remain in the artificial fracture to ensure that the fracture will not close under the influence of in-situ stress. The heat insulation material is a composite silicate; the low-temperature fluid fracturing preflush fluid is liquid nitrogen or liquid carbon dioxide.

10. The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 1, characterized in that, In step 6, the post-fracture formation closure time is obtained by testing the fracturing pressure drop. When the formation is about to close but has not closed, it is an opportune time for the fracturing fluid to flow back.

11. The method for reservoir transformation of an underground heat storage system for improving cross-seasonal heat storage efficiency according to claim 1, characterized in that, The method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency further includes, after step 6, performing heat injection - heat storage - heat extraction tests on the reservoir filled with heat insulation materials after transformation to evaluate the heat storage efficiency of the reservoir after transformation.

12. Underground heat storage system reservoir transformation system for improving cross-seasonal heat storage efficiency, characterized in that, The reservoir transformation system of the underground heat storage system for improving cross-seasonal heat storage efficiency uses the method for reservoir transformation of the underground heat storage system for improving cross-seasonal heat storage efficiency described in any one of claims 1 - 11 to perform reservoir transformation of the underground heat storage system.

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