Reservoir modification method for underground thermal storage system to improve cross-seasonal thermal storage efficiency

Abstract

The present invention relates to the field of energy storage and utilization technology, and provides a method for improving the cross-seasonal heat storage efficiency of an underground heat storage system reservoir modification, comprising: conducting indoor experiments through core sampling to determine the physical properties of abandoned oil well reservoirs; conducting a fracturing physical simulation experiment to simulate the crack morphology after reservoir fracturing, and then transporting insulation materials and monitoring their positions; conducting a heat storage effect physical simulation experiment to calculate the heat storage efficiency of different reservoirs, and optimizing the best heat storage reservoir and modification plan; conducting a heat insulation material migration simulation; conducting heat insulation material migration, using a sand-carrying fluid to be placed at the top of the crack, and then using the sand-carrying fluid again to transport the proppant to the middle of the crack; after the fracturing is completed, selecting the time when the formation is about to close but has not closed as a favorable time for fracturing fluid backflow. The present invention can accurately inject insulation materials into the top of the crack, thereby forming an artificial insulation belt, which can effectively reduce the heat dissipation rate of the heat storage system and improve the heat recovery rate.

Description

Technical Field

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

[0002] With the continued growth of global energy demand and the increasingly severe carbon emissions problem, finding clean and efficient energy utilization methods has become a key issue of common concern. Against this backdrop, seasonal thermal energy storage (STES) technology has attracted considerable attention due to its ability to significantly balance energy supply and demand and improve energy efficiency. STES systems efficiently store excess heat underground during the summer and release this heat in the winter or during peak energy demand periods to support heating, cooling, and other energy needs. For example, the geothermal well clustering method for seasonal underground thermal energy storage systems, by establishing clusters of cold and hot wells and optimizing the layout to prevent thermal breakthrough, effectively reduces system construction costs and improves the efficiency of thermal energy storage and utilization. Furthermore, the BTES system (Borehole Thermal Energy Storage) has demonstrated the ability to store heat underground during the non-heating season and extract it for use during the heating season, effectively resolving the temporal and spatial conflicts between energy supply and demand. This technology holds great potential for achieving sustainable energy development.

[0003] Underground Thermal Energy Storage (UTES) is one of the most widely used and efficient inter-seasonal heat storage technologies. Its core principle is to utilize natural underground reservoirs (such as aquifers, sandstone formations, abandoned oil reservoirs, or artificial reservoirs) as thermal energy storage media, transferring heat and storing energy through a heat carrier medium (such as water or air). However, inherent limitations of natural reservoirs, such as insufficient thermal conductivity, high heat loss, and low permeability, limit the system's thermal energy utilization efficiency and long-term operational stability. Therefore, optimizing reservoirs to improve the efficiency of heat storage and release and minimize heat loss during storage have become key challenges in the research and application of UTES technology.

[0004] Existing reservoir modification technologies typically focus on improving reservoir thermal conductivity and reducing heat loss. For example, high-conductivity fillers are used to increase the uniformity of heat transfer within the reservoir, or insulation technology is used to reduce heat loss to the outside world.

[0005] Current research on underground heat storage efficiency, both domestically and internationally, focuses on the impact of reservoir parameters on thermal performance. However, there are few reports on how to modify the reservoir to reduce energy losses during storage and the thermal dissipation rate of the storage system. To address this issue, we have developed a novel reservoir modification method for underground heat storage systems that improves interseasonal thermal storage efficiency, addressing these technical challenges. Summary of the Invention

[0006] The purpose of the present invention is to provide a reservoir transformation method for an underground heat storage system that combines thermal insulation materials and transforms the reservoir through fracturing technology, thereby significantly improving heat storage efficiency and reducing heat loss to improve cross-seasonal heat storage efficiency.

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

[0008] Step 1: Conducting laboratory experiments by core sampling to determine the physical properties of the abandoned oil well reservoir;

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

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

[0011] Step 4: Simulate the migration of thermal insulation materials in the main cracks and branch cracks.

[0012] Step 5: transport the insulation material by using a sand-carrying fluid to 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 yet closed as a favorable opportunity to drain the fracturing fluid.

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

[0015] In step 1, abandoned oil wells are first screened based on regional heating needs, including their location, depth, and reservoir thickness. Production data from the oil production period is then used to assess the reservoir's heat capacity and thermal conductivity. Prioritize abandoned oil wells with intact wellbores, proximity to heat sources, and low conversion costs.

[0016] Secondly, through drilling and core sampling, laboratory tests are carried out on abandoned oil well reservoir rock samples to measure the physical parameters 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 meter 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; secondly, by controlling the gas injection pressure, the porosity of the reservoir rock is measured using a gas porosity meter, the gas diffusion volume is determined, and the porosity is calculated in combination with the known sample volume; for the thermal conductivity of the reservoir rock, the thermal conductivity coefficient of the core sample is tested using a TPS thermal conductivity meter, and the thermal conductivity coefficient 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 to determine the compressive strength and tensile strength of the rock.

[0018] In step 2, based on the physical parameters measured in step 1, an artificial rock block with prefabricated perforations is constructed, and a fracturing physical simulation experiment is conducted using an indoor test. First, a fracturing experiment is carried out using different displacements of different low-temperature fracturing fluids, and an acoustic emission device is used to observe the morphology and direction of the cracks. Second, by injecting fracturing fluids of different viscosities and different insulation material filling amounts, the migration pattern of the insulation material under different injection conditions is analyzed. Finally, CT scanning is used to analyze the crack morphology and the distribution pattern of the insulation material within the fracturing cracks. The physical simulation experiment shows that as the injection flow rate increases, the size of the fracturing cracks first increases and then decreases; and as the injection temperature decreases, the size of the fracturing cracks increases.

[0019] In step 3, based on the experimental results in step 2, physical simulation experiments on heat storage effects are carried out on different reservoirs, and then the heat storage efficiency η corresponding to each reservoir is calculated; the physical simulation experiments show that the larger the fracturing scale, the higher the heat storage efficiency that can be obtained; step 2 determines the relationship between the injection rate, injection temperature and fracture scale during the fracturing process, and then the optimal injection rate and injection temperature range for obtaining large regular fractures can be determined; step 3 determines the relationship between fracture scale and heat storage efficiency; combining steps 2 and 3, a reservoir transformation scheme with higher heat storage efficiency can be obtained, including injection rate and injection temperature, wherein the injection rate is determined by the optimal injection rate during the experiment, the wellbore cross-sectional area in the artificial rock block, and the wellbore cross-sectional area of ​​the on-site construction, and the injection flow rate during on-site construction can be calculated; the injection temperature is determined by the optimal injection temperature during the experiment.

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

[0021]

[0022] in:

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

[0024] Q out — the heat energy actually extractable during the mining phase;

[0025] Q in =m in ×c×ΔT in ;

[0026] in:

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

[0028] c——specific heat capacity;

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

[0030] Q out =m out ×c×ΔT out ;

[0031] in:

[0032] m out —Quality of produced water;

[0033] c——specific heat capacity;

[0034] ΔT out ——The temperature difference between water and reservoir during production.

[0035] In step 4, a simulation experiment is conducted to investigate the effect of fracturing fluid flow rate and viscosity on the migration of thermal insulation materials in the fracture system, and the migration and distribution patterns of thermal insulation materials in the main fracture and branch fractures are analyzed.

[0036] For the main cracks: Due to the diversion effect of the branch cracks, the filling of the thermal insulation material in the main cracks is reduced. Therefore, low-density and small-particle thermal insulation materials are injected first through appropriate displacement, and then high-density and large-particle thermal insulation materials are gradually converted to avoid blockage;

[0037] For branch cracks: The insulation material will gradually accumulate as it migrates in the main cracks, resulting in a reduction in the insulation material entering the branch cracks. Therefore, the injection volume is increased, and the viscosity of the sand-carrying fluid is increased to improve the insulation material migration capacity and ensure that the insulation material can fully fill the branch cracks.

[0038] In step 5, based on the reservoir selected in step 3 and the corresponding optimal transformation plan including injection rate and injection temperature, on-site construction is carried out to prepare fracturing fluid including pre-fluid, sand-carrying fluid and displacement fluid. According to different fracture morphologies, by controlling the displacement and viscosity of the fracturing fluid, a four-stage injection strategy of fracture creation - insulation material filling - proppant laying - displacement is adopted according to the injection rate and injection temperature selected in step 3, and the main fractures and branch fractures are distinguished for insulation material migration and filling.

[0039] In step 5, a pre-fluid is first injected into the reservoir through a high-pressure pump, and the pre-fluid's rock-breaking ability is used to fracture the reservoir and form a large-scale fracture grid. The large-scale fracture grid is obtained by reforming the pre-fluid using a low-temperature fluid fracturing fluid. Subsequently, a sand-carrying fluid is used to carry the insulation material into the fracture. After the 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, a displacement fluid is injected to push the residual proppant in the wellbore into the fracture. After the reformation is completed, the insulation material will remain in the artificial fracture to ensure that the fracture is closed without being affected by the ground stress. The insulation material is a composite silicate. The low-temperature fluid fracturing pre-fluid is liquid nitrogen or liquid carbon dioxide.

[0040] In step 6, the post-fracturing formation closure time is obtained by testing the fracturing pressure drop. When the formation is about to close but has not yet closed, it is used as a favorable opportunity for fracturing fluid return.

[0041] The method for improving the inter-seasonal heat storage efficiency of the underground heat storage system reservoir modification further includes: after step 6, performing a heat injection-heat storage-heat extraction test on the modified reservoir filled with insulation material to evaluate the heat storage efficiency of the modified reservoir.

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

[0043] The reservoir reconstruction method for underground heat storage systems to improve cross-seasonal heat storage efficiency, as described in the present invention, is suitable for the engineering reconstruction of natural reservoirs such as abandoned oil reservoirs, aquifers, and sandstone formations. By constructing an artificial fracture network using low-temperature fluid fracturing technology and synergistically filling it with insulation materials, the method significantly improves heat storage efficiency, reduces heat loss, and enhances the stability of the reservoir during long-term thermal cycles. The reservoir reconstruction method for underground heat storage systems to improve cross-seasonal heat storage efficiency, as described in the present invention, significantly improves heat storage efficiency and reduces heat loss by measuring the physical properties of the target reservoir, combining fracturing technology with the application of insulation materials (composite silicates). During the fracturing process, indoor experiments are used to assess the morphology and direction of the fractures, allowing the insulation material to be precisely placed at the top of the fractures, while also considering the impact of different reservoir insulation material fillings on heat storage efficiency. After fracturing, the fractures naturally close under the action of geostress. At this time, the insulation material is squeezed by the geostress and tightly adheres to the rock surface, forming a highly efficient thermal insulation barrier that effectively prevents heat from diffusing outward. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 A schematic diagram of the relationship between injection temperature and crack size in one embodiment of the present invention;

[0045] Figure 2Schematic diagram of the relationship between injection velocity and fracture size in one embodiment of the present invention;

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

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

[0048] Figure 5 A flowchart of a specific embodiment of the method for improving the cross-seasonal heat storage efficiency of an underground heat storage system according to the present invention;

[0049] Figure 4 In the figure, 1-different reservoirs, 2-fracture creation, 3-interval layers of each reservoir, 4-abandoned wellbore, 5-perforation channels, 6-thermal insulation materials. DETAILED DESCRIPTION

[0050] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

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

[0052] like Figure 5 As shown, Figure 5 This is a flow chart of the method for improving the inter-seasonal heat storage efficiency of an underground heat storage system reservoir modification of the present invention. The method for improving the inter-seasonal heat storage efficiency of an underground heat storage system reservoir modification includes:

[0053] Step 1: Conduct indoor experiments by taking core samples to determine the physical properties of the abandoned oil well reservoir, including permeability, porosity, thermal conductivity and mechanical properties.

[0054] First, based on regional heating needs, abandoned oil wells are screened for parameters such as location, depth, and reservoir thickness. Production data from the oil production period is used to assess the reservoir's heat capacity and thermal conductivity. Prioritize abandoned oil wells with intact wellbores, proximity to heat sources, and low conversion costs.

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

[0056] Step 2: Conduct a physical simulation experiment of fracturing. Construct an artificial rock block containing pores based on the measured physical parameters and perform fracturing operations. Through physical model experiments, simulate the crack morphology after reservoir fracturing, and then transport the insulation material and monitor its position.

[0057] Based on the core parameters determined in Step 1, an artificial rock block with prefabricated perforations was constructed, and fracturing simulations were performed using indoor testing. First, fracturing experiments were conducted using different flow rates of different low-temperature fracturing fluids, and an acoustic emission device was used to observe the fracture morphology and orientation. Secondly, by injecting fracturing fluids of varying viscosities and varying amounts of insulation material, the migration patterns of the insulation material under different injection conditions were analyzed. Finally, CT scanning was used to analyze the fracture morphology and the distribution of the insulation material within the fracture.

[0058] Through physical model experiments, it is found that as the injection flow rate increases, the size of the fracture first increases and then decreases; as the injection temperature decreases, the size of the fracture increases. For example, the experiment uses a rock block with a size of 40cm×40cm×40cm cube. Figure 1 It can be seen that the crack size reaches its maximum when the temperature is below -40℃; Figure 2 It can be seen that the crack scale reaches its maximum when the injection flow rate is 15ml / min-20ml / min, and then gradually decreases.

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

[0060] Through physical simulation experiments, it is concluded that the larger the fracturing scale, the higher the heat storage efficiency. For example, the experiment uses a rock block with a size of 40cm×40cm×40cm cube. Figure 3 It can be seen that as the crack volume increases, the heat storage efficiency increases accordingly. 3 It reaches its maximum value and tends to be stable.

[0061] Furthermore, step two determines the relationship between the injection rate, injection temperature and crack size during the fracturing process, and then determines the optimal injection rate and injection temperature range for obtaining large-scale regular cracks. Step three determines the relationship between crack size and heat storage efficiency. Combining steps two and three, a reservoir transformation plan with higher heat storage efficiency can be obtained, including injection rate and injection temperature. The injection rate is determined by the optimal injection 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, and the injection flow rate during on-site construction can be calculated. The injection temperature is determined by the optimal injection temperature during the experiment.

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

[0063]

[0064] Where V in is the injection flow rate for indoor experiments, u e is the volume flow rate during the experiment, s e is the cross-sectional area of ​​the injection well in the artificial rock block; m is the cross-sectional area of ​​the wellbore during on-site construction, u m Inject volume flow during on-site construction.

[0065] In step 3, the calculation formula of heat storage efficiency η is:

[0066]

[0067] in:

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

[0069] Q out — the heat energy actually extractable during the mining phase;

[0070] Q in =m in ×c×ΔT in ;

[0071] in:

[0072] m in — quality of the injected water;

[0073] c——specific heat capacity;

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

[0075] Q out =m out ×c×ΔT out ;

[0076] in:

[0077] m out —Quality of produced water;

[0078] c——specific heat capacity;

[0079] ΔT out ——The temperature difference between water and reservoir during production.

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

[0081] A simulation experiment was conducted to study the effects of fracturing fluid displacement and viscosity on the migration of thermal insulation materials in the fracture system, and the migration and distribution patterns of thermal insulation materials in the main fractures and branch fractures were analyzed.

[0082] For the main cracks: Due to the diversion effect of the branch cracks, the filling of the thermal insulation material in the main cracks is reduced. Therefore, low-density and small-particle thermal insulation materials are injected first through appropriate displacement, and then high-density and large-particle thermal insulation materials are gradually converted to avoid blockage;

[0083] For branch cracks: As insulation material migrates through the main crack, it gradually accumulates, resulting in less insulation entering the branch cracks. Therefore, increasing the injection rate and the viscosity of the sand-carrying fluid improves the insulation material's migration capacity and ensures that the insulation material can fully fill the branch cracks.

[0084] Step 5: Conduct on-site construction and prepare the fracturing fluid, including pre-pad, sand-carrying, and displacement fluids. After the fracture is formed, the insulation material is first moved, and the sand-carrying fluid is placed at the top of the fracture. The sand-carrying fluid is then used again to move the proppant to the middle of the fracture.

[0085] Based on the reservoir selected in step three and the corresponding optimal transformation plan, including injection rate and injection temperature, on-site construction is carried out to prepare a fracturing fluid consisting of a pre-fluid, a sand-carrying fluid, and a displacement fluid. First, the pre-fluid is injected into the reservoir using a high-pressure pump. The pre-fluid's rock-breaking ability is used to fracture the reservoir and form large-scale fractures. The large-scale fracture grid is created by fracturing the pre-fluid using the low-temperature fluid in the pre-fluid. The sand-carrying fluid is then used to carry the insulation material into the fracture. Once the 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 remaining proppant in the wellbore into the fracture. After the transformation is completed, the insulation material will remain in the artificial fracture, significantly improving the insulation capacity of the fracture end.

[0086] Step 6: After the fracturing is completed, select the time when the formation is about to close but has not yet closed as a favorable opportunity to backflow the fracturing fluid to ensure that the proppant has sufficient formation compaction.

[0087] By measuring the fracturing pressure drop, the post-fracturing formation closure time is determined. When the formation is about to close but has not yet closed, it is used as an opportune time for fracturing fluid flowback. This ensures that the proppant is sufficiently compacted in the formation and is not carried away by the flowback fracturing fluid. At the same time, it ensures that there is no loss of displacement pressure due to the diffusion of pressure after formation closure.

[0088] Step 7: Based on the physical simulation experiments of fracturing, the physical simulation experiments of heat storage effects and the simulation of thermal insulation material migration, a set of standard reservoir transformation technology solutions is formed to guide on-site construction.

[0089] In step one, a steady-state permeability experiment is conducted using a core permeability meter by adjusting different pressure gradients to measure the permeability of the target reservoir. At the same time, the permeability is calculated using Darcy's law to ensure the accuracy of the measured data.

[0090] Secondly, by controlling the gas injection pressure, the porosity of the reservoir rock was measured using a gas porosimeter. The gas diffusion volume was determined and the porosity was calculated based on the known sample volume. To determine the thermal conductivity of the reservoir rock, a TPS thermal conductivity meter was used to test the thermal conductivity of core samples. The thermal conductivity was calculated by measuring the temperature rise over time. Furthermore, to determine the mechanical properties of the reservoir, uniaxial compression tests and Brazilian split tests were conducted to determine the compressive and tensile strengths of the rock.

[0091] In step five, natural fractures are easily connected during reservoir fracturing, forming a complex fracture network. Precisely placing insulation material at the fracture tips presents a challenge. Based on this, this method, through laboratory experiments combined with field operations, employs a four-stage injection strategy of "fracture creation - insulation filling - proppant placement - displacement" based on the injection rate and temperature selected in step three, by controlling the flow rate and viscosity of the fracturing fluid according to different fracture morphologies. This strategy differentiates between primary and branch fractures for insulation material migration and filling.

[0092] When the insulation material is fully filled to the top of the crack, a high-viscosity fracturing fluid is injected at a large flow rate to bring the proppant into the middle section of the crack. The injection flow rate and fracturing fluid viscosity are then reduced to allow the proppant to fully fill the crack, ensuring that the crack is closed without being affected by ground stress.

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

[0094] This method makes full use of abandoned oil reservoirs and transforms them into high-efficiency heat storage systems through reservoir transformation combined with the placement of thermal insulation materials, thereby improving resource utilization 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 recycling.

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

[0097] Step 1. Based on regional heating needs, screen abandoned oil wells for parameters such as location, depth, and reservoir thickness. Evaluate reservoir heat capacity and thermal conductivity using production data from the production period. Prioritize wells with intact wellbores, proximity to heat sources, and low conversion costs.

[0098] Step 2. First, conduct comprehensive physical property measurements on the target reservoir. A core permeability tester is used to conduct steady-state permeability experiments to determine the fluid migration capacity within the reservoir. Subsequently, a gas porosity tester is used to measure the reservoir's porosity, providing a basis for analyzing heat storage capacity. Furthermore, a TPS thermal conductivity tester is used to measure the reservoir's thermal conductivity, thereby clarifying its specific heat transfer characteristics. Uniaxial compression tests and Brazilian split tests are used to determine the reservoir's compressive and tensile strengths.

[0099] Step 3. Construct artificial rock blocks based on reservoir physical parameters and conduct indoor physical model tests. First, fracturing experiments were conducted using different low-temperature fracturing fluids at different flow rates. After fracturing, an acoustic emission device was used to capture the morphology and orientation of the fractures. Furthermore, fracturing fluids of varying viscosities were injected to analyze the migration patterns of the insulation material under different filling conditions. During the experiment, the stress field distribution, permeability changes, and pore structure characteristics were considered, and the effects of different low-temperature fracturing fluids were compared to determine the most suitable reconstruction solution for the target reservoir conditions. The physical model experiments showed that as the injection flow rate increased, the size of the fractures first increased and then decreased; and as the injection temperature decreased, the size of the fractures increased.

[0100] Step 4. Based on the experimental results in step 3, conduct physical simulation experiments on the heat storage effect of different reservoirs, and then calculate the heat storage efficiency η corresponding to each reservoir; through physical simulation experiments, it is concluded 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 rate, injection temperature and fracture size during the fracturing process, and thus the optimal injection rate and injection temperature range for obtaining large regular fractures can be determined. Step 4 determines the relationship between fracture size and heat storage efficiency. Combining steps 3 and 4, a reservoir reconstruction scheme with higher heat storage efficiency can be obtained, including injection rate and injection temperature. The injection rate is determined by the optimal injection rate during the experiment, the wellbore cross-sectional area in the artificial rock block, and the wellbore cross-sectional area during on-site construction, and the injection flow rate during on-site construction can be calculated. The injection temperature is determined by the optimal injection temperature during the experiment.

[0102]

[0103] in:

[0104] Q in — total heat energy input during the injection phase;

[0105] Q out — the heat energy actually extractable during the mining phase;

[0106] Q in= m in ×c×ΔT in ;

[0107] in:

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

[0109] c——specific heat capacity;

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

[0111] Q out= m out ×c×ΔT out ;

[0112] in:

[0113] m out —Quality of produced water;

[0114] c——specific heat capacity;

[0115] ΔT out ——The temperature difference between water and reservoir during production.

[0116] Step 5. Conduct simulation experiments on the effects of fracturing fluid displacement and viscosity on the migration of thermal insulation materials in the fracture system, and analyze the migration and distribution patterns of thermal insulation materials in the main fractures and branch fractures.

[0117] For the main cracks: Due to the diversion effect of the branch cracks, the filling of the thermal insulation material in the main cracks is reduced. Therefore, low-density and small-particle thermal insulation materials are injected first through appropriate displacement, and then high-density and large-particle thermal insulation materials are gradually converted to avoid blockage;

[0118] For branch cracks: As insulation material migrates through the main crack, it gradually accumulates, resulting in less insulation entering the branch cracks. Therefore, increasing the injection rate and the viscosity of the sand-carrying fluid improves the insulation material's migration capacity and ensures that the insulation material can fully fill the branch cracks.

[0119] Step 6. Based on the results of the laboratory experiments, on-site fracturing fluid was prepared, including a pre-flush, a sand-carrying fluid, and a displacement fluid. First, the pre-flush was injected into the reservoir using a high-pressure pump, leveraging its rock-breaking properties to disrupt the reservoir and create fractures. The sand-carrying fluid then carried the insulation material to the top of the fracture. Furthermore, the sand-carrying fluid was used again to carry the proppant to the middle of the fracture, ensuring that the fracture closed without being affected by ground stress. Finally, the displacement fluid was injected to push any remaining proppant in the wellbore into the fracture. After the stimulation was complete, the insulation material remained in the artificial fracture, significantly improving the reservoir's thermal insulation performance.

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

[0121] Step 8. After the reservoir transformation is completed, conduct heat injection-heat storage-heat extraction tests on the transformed reservoir filled with insulation materials to evaluate the heat storage efficiency of the transformed reservoir and develop a standardized reservoir transformation technology plan.

[0122] like Figure 4 As shown, Figure 4 This is a schematic diagram of reservoir fracturing transformation in an inter-seasonal heat storage system, showing the post-fracturing fracture morphology targeting abandoned oil wells. Different reservoirs, fracturing fractures, reservoir interval layers, abandoned wellbores, perforation channels, and insulation materials injected into the top of the fractures can be seen, demonstrating the key steps of using fracturing technology to create fractures in the reservoir and fill them with insulation materials to construct artificial insulation belts.

[0123] The innovation of this invention lies in determining the layer to be transformed in the underground heat storage system, using low-temperature fluid to fracture the layer, and at the same time controlling the migration of the insulation material to accurately inject the insulation material into the top of the fracture, thereby forming an artificial insulation belt, 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 description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art may modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features therein. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

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

Claims

1. A method for improving the storage efficiency of underground heat storage systems across seasons, characterized in that: The method includes: Step 1: Conducting laboratory experiments by core sampling to determine the physical properties of the abandoned oil well reservoir; Step 2: Conduct a fracturing physical simulation experiment to simulate the fracture morphology of the reservoir after fracturing, and then transport the insulation material and monitor its position; Step 3: Conduct a physical simulation experiment on the heat storage effect, calculate the corresponding heat storage efficiency under different reservoir transformation schemes, and select the best heat storage reservoir and transformation scheme; Step 4: Simulate the migration of thermal insulation materials in the main cracks and branch cracks. Step 5: transport the insulation material by using a sand-carrying fluid to 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, the time when the formation is about to close but has not yet closed is selected as a favorable opportunity for fracturing fluid backflow; Among them, in step 5, first, a pre-fluid is injected into the reservoir through a high-pressure pump, and the rock-breaking ability of the pre-fluid is used to fracture the reservoir and form a large-scale fracture grid. The large-scale fracture grid is obtained by using a low-temperature fluid fracturing pre-fluid; then, a sand-carrying fluid is used to carry the insulation material into the fracture. After the 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, a displacement fluid is injected to push the residual proppant in the wellbore into the fracture; after the transformation is completed, the insulation material will remain in the artificial fracture to ensure that the fracture is not affected by the ground stress and is closed. The insulation material is a composite silicate; the low-temperature fluid fracturing pre-fluid is liquid nitrogen or liquid carbon dioxide.

2. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 1 is characterized in that: In step 1, abandoned oil wells are first screened based on their geographic location, well depth, and reservoir thickness based on regional heating needs. Production data from the oil production period is then used to assess the reservoir's heat capacity and heat transfer characteristics. Prioritize abandoned oil wells with intact wellbores, proximity to heat sources, and low conversion costs. Secondly, through drilling and core sampling, laboratory tests are carried out on abandoned oil well reservoir rock samples to measure the physical parameters of different reservoirs, including permeability, porosity, thermal conductivity and mechanical properties.

3. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 2 is characterized in that: In step 1, when measuring the physical properties of the reservoir, first, by adjusting different pressure gradients, a steady-state permeability experiment is conducted using a core permeability meter 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. Secondly, by controlling the gas injection pressure, the porosity of the reservoir rock is measured using a gas porosity meter, the gas diffusion volume is determined, and the porosity is calculated based on the known sample volume. For the thermal conductivity of the reservoir rock, the thermal conductivity coefficient of the core sample is tested using a TPS thermal conductivity meter, and the thermal conductivity coefficient is calculated by measuring the change in temperature rise over time. Furthermore, for the mechanical properties of the reservoir, uniaxial compression tests and Brazilian splitting tests were carried out to determine the compressive strength and tensile strength of the rock.

4. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 1 is characterized in that: In step 2, based on the physical parameters measured in step 1, an artificial rock block with prefabricated perforations is constructed, and a fracturing physical simulation experiment is conducted using an indoor test. First, a fracturing experiment is carried out using different displacements of different low-temperature fracturing fluids, and an acoustic emission device is used to observe the morphology and direction of the cracks. Secondly, by injecting fracturing fluids of different viscosities and different insulation material filling amounts, the migration pattern of the insulation material under different injection conditions is analyzed. Finally, CT scanning is used to analyze the crack morphology and the distribution pattern of the insulation material within the fracturing cracks. The experiment concluded that as the injection flow rate increases, the size of the fracturing cracks first increases and then decreases; and as the injection temperature decreases, the size of the fracturing cracks increases.

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

6. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 5 is characterized in that: In step 3, the calculation formula of heat storage efficiency η is: ； in: Q in — total heat energy input during the injection phase; Q out — the heat energy actually extractable during the mining phase; Q in =m in ×c×ΔT in ; in: m in — quality of the injected water; c——specific heat capacity; ΔT in —The temperature difference between the water and the reservoir during injection; Q out =m out ×c×ΔT out ; in: m out —Quality of produced water; c——specific heat capacity; ΔT out ——The temperature difference between water and reservoir during production.

7. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 1 is characterized in that: In step 4, a simulation experiment is conducted to investigate the effect of fracturing fluid flow rate and viscosity on the migration of thermal insulation materials in the fracture system, and the migration and distribution patterns of thermal insulation materials in the main fractures and branch fractures are analyzed. For the main cracks: Due to the diversion effect of the branch cracks, the filling of the thermal insulation material in the main cracks is reduced. Therefore, low-density and small-particle thermal insulation materials are injected first through appropriate displacement, and then high-density and large-particle thermal insulation materials are gradually converted to avoid blockage; For branch cracks: The insulation material will gradually accumulate as it migrates in the main cracks, resulting in a reduction in the insulation material entering the branch cracks. Therefore, the injection volume is increased, and the viscosity of the sand-carrying fluid is increased to improve the insulation material migration capacity and ensure that the insulation material can fully fill the branch cracks.

8. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 5 is characterized in that: In step 5, based on the reservoir selected in step 3 and the corresponding optimal transformation plan including injection rate and injection temperature, on-site construction is carried out to prepare fracturing fluid including pre-fluid, sand-carrying fluid and displacement fluid. According to different fracture morphologies, by controlling the displacement and viscosity of the fracturing fluid, a four-stage injection strategy of fracture creation - insulation material filling - proppant laying - displacement is adopted according to the injection rate and injection temperature selected in step 3, and the main fractures and branch fractures are distinguished for insulation material migration and filling.

9. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 1 is characterized in that: In step 6, the post-fracturing formation closure time is determined by testing the fracturing pressure drop. When the formation is about to close but has not yet closed, it is used as a favorable opportunity for fracturing fluid flowback.

10. The method for improving the storage efficiency of underground heat storage system across seasons according to claim 1, characterized in that: The method for modifying the reservoir of an underground heat storage system to improve the cross-seasonal heat storage efficiency further includes, after step 6, performing a heat injection-heat storage-heat extraction test on the modified reservoir filled with insulation material to evaluate the heat storage efficiency of the modified reservoir.

11. An underground heat storage system reservoir modification system for improving cross-seasonal heat storage efficiency, characterized by: The underground heat storage system reservoir transformation system for improving cross-seasonal heat storage efficiency adopts the underground heat storage system reservoir transformation method for improving cross-seasonal heat storage efficiency described in any one of claims 1-10 to perform underground heat storage system reservoir transformation.

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