Construction method and system of salt cavern oil-gas coexistence system

Abstract

The construction method comprises the following steps that S1, oil injection and brine discharge are conducted, specifically, displacement oil with the density lower than that of brine is injected into a salt cavern through an annular space of a salt cavern wellhead, and high-density brine in the salt cavern is discharged out of a center pipe in a displacement mode; s2, spilled oil recovery: after oil injection is stopped, displacement oil spilled due to pressure in the salt cavern is recovered through a pipeline; gas is injected into the salt cavern through the annulus, displacement oil is displaced and discharged from the center pipe, and the gas injection pressure does not exceed 80% of the stress of the stratum at the casing shoe; s4, oil-gas coexistence space formation: when the oil discharge quantity reaches the preset gas storage space volume, gas injection is stopped, the central pipe is closed, and an oil-gas coexistence salt cavern reservoir with oil stored at the lower part and gas stored at the upper part is formed; salt cavern bottom space utilization and oil-gas coexistence are achieved through oil-gas double replacement.

Description

Technical Field

[0001] This application relates to the field of salt cavern underground storage technology, and in particular to a method and system for constructing a salt cavern oil and gas coexistence system. Background Technology

[0002] In the utilization of underground spaces in salt mines, it is necessary to displace the brine and transform the brine space into a gas storage space. The common method is to use gas injection to drain the brine. However, traditional gas injection methods are limited by the formation stress of the casing shoe (the gas injection pressure must be ≤80% of the formation stress), resulting in limited drainage depth and rendering the lower space of the salt cavern unusable, thus failing to overcome the physical depth bottleneck. Therefore, a new method that can fully utilize the lower space of salt caverns is urgently needed. Summary of the Invention

[0003] To address the aforementioned problems, this application provides a method and system for constructing a salt cavern oil-gas coexistence system. Specifically, it is an oil-gas coexistence system and construction method that utilizes the bottom space of a salt cavern through dual oil-gas replacement. The technical solution is as follows: The first aspect of this application provides a method for constructing a salt cavern oil-gas coexistence system, comprising the following steps: S1 Oil injection and brine discharge: Displacement oil with a density lower than that of brine is injected into the salt cavern through the annulus at the salt cavern wellhead, displacing and discharging the high-density brine from the central pipe; S2 Oil overflow recovery: After stopping oil injection, the displacement oil overflowing due to pressure in the salt cavern is recovered through a pipeline; S3 Gas injection and oil discharge: Gas is injected into the salt cavern through the annulus, displacing and discharging the displacement oil from the central pipe, with the gas injection pressure not exceeding 80% of the formation stress at the casing shoe; S4 Formation of oil-gas coexistence space: When the oil discharge reaches the preset gas storage space volume, gas injection is stopped and the central pipe is closed, forming an oil-gas coexistence salt cavern reservoir with oil storage at the bottom and gas storage at the top.

[0004] For example, in one embodiment of the method for constructing the salt cavern oil-gas coexistence system, the density of the displacement oil in step S1 is 0.6-0.9 g / cm³. 3 The density of the brine is 1.1-1.2 g / cm³. 3 .

[0005] For example, in the construction method of the salt cavern oil-gas coexistence system provided in one embodiment, during the gas injection and oil discharge in step S3, the formation stress at the casing shoe is monitored in real time, and the actual gas injection pressure is adjusted to ensure that the actual gas injection pressure is ≤ 80% of the formation stress. The core of this application lies in first utilizing the low density of oil to displace the high-density brine. Although gas is also low-density, it is not suitable because oil can be recovered through pipelines, while high-pressure gas cannot be recovered and is also very dangerous.

[0006] For example, in one embodiment of the construction method of the salt cavern oil-gas coexistence system, the following subsequent operations are also included: oil extraction operation: displaced oil is driven out of the central tube by injecting gas into the annulus; gas extraction operation: displaced oil is injected into the central tube to drive out the cushion gas from the central tube.

[0007] A second aspect of this application provides an oil and gas coexisting salt cavern storage system, including a salt cavern cavity, a wellhead device, a central pipe, and an annular channel. The salt cavern cavity contains displacement oil at its bottom and stored gas at its top. The wellhead device is located at the top of the salt cavern. The central pipe extends from the wellhead device to the bottom of the salt cavern cavity for discharging brine or displacement oil. The annular channel is located outside the central pipe for injecting displacement oil or gas.

[0008] For example, in one embodiment of the construction method of the salt cavern oil and gas coexistence system, the wellhead device includes annular injection valve, central tube control valve and pressure sensor: the annular injection valve is connected to the inlet of the annular channel; the central tube control valve is connected to the outlet of the central tube; and the pressure sensor monitors the formation stress at the casing shoe in real time.

[0009] For example, in one embodiment of the method for constructing the salt cavern oil-gas coexistence system, the density of the displacement oil is 0.6-0.9 g / cm³. 3 The stored gas is natural gas or compressed air.

[0010] For example, in one embodiment of the construction method of the salt cavern oil-gas coexistence system, there is a sediment layer at the bottom of the salt cavern cavity, and the displacement oil fills the voids in the sediment.

[0011] For example, in the construction method of the salt cavern oil and gas coexistence system provided in one embodiment, the salt cavern cavity is a single-well vertical cavity or a multi-well horizontally connected cavity.

[0012] The present application provides a method for constructing a salt cavern oil-gas coexistence system and the beneficial effects of the system: through an oil-gas synergistic replacement mechanism, low-density oil is first used to replace high-density brine to overcome the depth bottleneck, and then gas is used to replace oil to form a gas storage space. By injecting brine to extract oil and injecting oil to extract gas, the oil-gas space can be flexibly converted, which has strong economy, safety and oil stability. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram illustrating the construction method of the salt cavern oil and gas coexistence system of this application. Figure 2 This is a schematic diagram of the oil extraction operation in the construction method of the salt cavern oil-gas coexistence system of this application. Figure 3 This is a schematic diagram of the gas extraction operation for the construction method of the salt cavern oil and gas coexistence system of this application. Detailed Implementation

[0015] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0016] The first aspect of this application provides a method for constructing a salt cavern oil and gas coexistence system, comprising the following steps: S1 Oil Injection and Brine Discharge: Displacement oil with a density lower than that of brine is injected into the salt cavern through the annulus at the salt cavern wellhead, displacing and discharging the high-density brine from the salt cavern through the central pipe. S2 oil spill recovery: After oil injection stops, the replacement oil that overflows due to pressure in the salt cavern is recovered through the pipeline; S3 Gas Injection and Oil Displacement: Gas is injected into the salt cavern through the annulus to displace the oil from the central tube. The gas injection pressure does not exceed 80% of the formation stress at the casing shoe. S4 Oil and Gas Coexistence Space Formation: When the oil discharge reaches the preset gas storage space volume, gas injection is stopped and the central pipe is closed, forming an oil and gas coexistence salt cavern storage tank with oil storage in the lower part and gas storage in the upper part.

[0017] The method for constructing a salt cavern oil and gas coexistence system in this application, such as Figure 1 As shown, by injecting oil and draining brine → recovering spilled oil → injecting gas and draining oil → coexisting oil and gas, the unusable space at the bottom of the salt cavern is transformed into a gas storage space, increasing the storage capacity by 15%-30%. Through sequential oil-gas replacement, theoretically unusable space is transformed into effective gas storage space, overcoming geological constraints and depth limitations: First, by utilizing the physical property that the density of the replacement oil is lower than that of the brine, a stable density difference driving force is formed during the oil injection and brine drainage stage, completely displacing and draining the brine from the bottom of the salt cavern. This solves the problem that the traditional gas injection and brine drainage method cannot utilize the lower space of the salt cavern due to pressure limitations. Safe and controllable: By using intermediate replacement oil as a transition medium, the risk of the gas injection pressure approaching the formation fracture pressure during direct gas injection and brine drainage is avoided, creating safe conditions for subsequent gas injection and oil drainage. Efficient space utilization: During the gas injection and oil drainage stage, by controlling the amount of oil drained, a stable spatial structure of upper gas storage and lower oil storage is precisely formed, increasing the effective volume of the salt cavern by 15%-30%, realizing the three-dimensional utilization of the salt cavern space.

[0018] For example, in one embodiment of the method for constructing the salt cavern oil-gas coexistence system, the density of the displacement oil in step S1 is 0.6-0.9 g / cm³. 3 The density of the brine is 1.1-1.2 g / cm³. 3 According to the above embodiments, by replacing the oil density (0.6-0.9 g / cm³) 3 Compared to the density of brine (1.1-1.2 g / cm³), 3 Formation Δρ≥0.3 g / cm 3 The density difference drives the displacement, generating sufficient buoyancy to allow the displacement oil to sink effectively and push the brine upwards, improving the efficiency and depth of brine removal. Furthermore, it is immiscible with the brine, driving the brine removal depth to expand. The displacement oil within the selected density range is typically an inert oil such as mineral oil, which is immiscible with the brine, preventing emulsification at the two-phase interface and ensuring the clarity and controllability of the displacement process. Oils within this density range are widely available and cost-effective, facilitating large-scale engineering applications.

[0019] For example, in the construction method of the salt cavern oil and gas coexistence system provided in one embodiment, during gas injection and oil discharge in step S3, the formation stress at the casing shoe is monitored in real time, and the actual gas injection pressure is adjusted to ensure that the actual gas injection pressure is ≤80% of the formation stress. According to the above embodiment, by monitoring and controlling the gas injection pressure to ≤80% of the formation stress in real time, dynamic pressure regulation avoids shear or tensile damage to the rock strata around the wellbore during the gas injection process, fundamentally eliminating the risk of geological disasters caused by overpressure. Controlling the gas injection pressure within a safe range helps maintain the structural integrity of the salt cavern cavity, ensures the long-term sealing and operational safety of the gas storage, solves the casing shoe failure risk of traditional methods, and, combined with pressure sensors and control systems, enables automated management of the gas injection process, reduces manual intervention, and improves operational accuracy and response speed.

[0020] For example, in one embodiment of the method for constructing the salt cavern oil and gas coexistence system, subsequent operations are also included: Oil extraction procedure: such as Figure 2 As shown, gas is injected into the annulus to drive the displacement oil out of the central tube; Gas extraction procedure: such as Figure 3 As shown, displacement oil is injected through the central tube to drive the gas from the bottom of the pad out of the central tube.

[0021] According to the above embodiments, through the dynamic operation of brine injection for oil extraction / oil injection for gas extraction, and through the reversible operation of "brine injection for oil extraction" and "oil injection for gas extraction", the dynamic conversion of oil and gas space is realized, enabling the storage facility to flexibly switch between gas storage and oil storage modes to adapt to changes in market supply and demand; the overflowing replacement oil and cushion gas can be recycled and reused, reducing media loss and operating costs, and improving economic efficiency; this design supports uninterrupted injection and production cycles, avoiding the drawbacks of traditional methods that require shutdown for media replacement, and improving the utilization efficiency of the storage facility.

[0022] A second aspect of this application provides an oil and gas coexisting salt cavern storage system, including a salt cavern cavity 100, a wellhead device, a central pipe 200, and an annular channel 300. The salt cavern cavity 100 contains displacement oil at its bottom and stored gas at its top. The wellhead device is located at the top of the salt cavern. The central pipe 200 extends from the wellhead device to the bottom of the salt cavern cavity 100 for discharging brine or displacement oil. The annular channel 300 is located outside the central pipe 200 for injecting displacement oil or gas.

[0023] The oil and gas coexistence salt cavern storage system of this application, through a cavity design of bottom oil storage and top gas storage, forms a stable oil cushion layer to inhibit gas leakage. The oil cushion layer physically isolates the gas from the brine, reduces corrosion, and improves sealing performance, achieving the following effects: Structural stability: The layout of bottom oil storage and top gas storage utilizes the gravity and sealing properties of the oil body to naturally form an "oil cushion layer," effectively inhibiting gas from escaping upwards and enhancing the storage's sealing performance and pressure maintenance capabilities; Integrated process: The central pipe and annular channel are dedicated to discharge and injection functions respectively, realizing process separation of injection and discharge, avoiding cross-contamination of fluids, and ensuring orderly operation; Strong compatibility: The system structure is suitable for most salt cavern geological conditions, providing a standardized template for the upgrading and transformation of traditional salt cavern gas storage facilities.

[0024] For example, in one embodiment of the oil and gas coexisting salt cavern storage system, the wellhead device includes annular injection valve, central tube control valve, and pressure sensor: the annular injection valve is connected to the inlet of the annular channel; the central tube control valve is connected to the outlet of the central tube; and the pressure sensor monitors the formation stress at the casing shoe in real time. According to the above embodiment, by integrating a pressure sensor and dual-valve control in the wellhead device, the dual-valve design allows for independent control of the injection and discharge processes. Combined with real-time feedback from the pressure sensor, precise adjustment of injection pressure and flow rate can be achieved, improving operational safety. The separate valve design isolates the annulus from the central tube system in case of failure, facilitating maintenance without affecting the overall system operation. Pressure monitoring data provides a basis for assessing the storage operation status and providing risk warnings, supporting intelligent management. The injection / discharge process can be adjusted in real time, reducing manual intervention.

[0025] For example, in one embodiment of the oil and gas coexistence salt cavern storage system, the density of the displacement oil is 0.6-0.9 g / cm³. 3 The storage gas is a gas that does not react with the oil, such as natural gas or compressed air. Natural gas and compressed air do not chemically react with the replacement oil, preventing oil deterioration or the formation of corrosive products and extending system life. According to the above embodiments, chemical inertness prevents pipe corrosion and extends system life. Compressed air or dry natural gas is less likely to cause explosions or corrosion, making it suitable for long-term storage. Natural gas can be directly utilized from existing pipeline networks, and compressed air is easy to prepare, both of which help reduce operating costs.

[0026] For example, in one embodiment of the oil and gas coexistence salt cavern storage system, a sediment layer exists at the bottom of the salt cavern cavity, and displacement oil fills the voids in the sediment. According to the above embodiment, by filling the voids in the sediment with displacement oil, the sediment layer, which is traditionally considered waste space, is transformed into oil storage volume, increasing space utilization by more than 20%; the oil filling can solidify the sediment, reduce the wear of suspended slag on the pipeline, and enhance the structural stability of the bottom of the cavity; and it avoids formation pollution caused by brine residue in the sediment layer, meeting environmental protection requirements.

[0027] For example, in one embodiment of the oil and gas coexisting salt cavern storage system, the salt cavern cavity is a single-well vertical cavity or a multi-well horizontally connected cavity. According to the above embodiments, this application supports single-well / multi-well horizontal cavities adapted to various salt mine geological conditions, with strong scalability. The single-well structure is suitable for layered salt rocks, while the multi-well horizontally connected structure is suitable for thick salt domes, expanding the scope of application of the technology. Multi-well connectivity can form a large-scale storage network, significantly improving the total gas storage capacity and meeting regional energy storage needs. The optimal cavity structure can be selected according to the salt mine morphology, reducing the difficulty and cost of construction.

[0028] Application Cases Traditional salt cavern gas storage construction relies on gas injection and brine discharge technology, the physical limits of which are determined by the following formula: Where σ is the formation stress, ρ 卤 To determine the density of the brine, taking a salt mine in Kunming, Yunnan as an example, the salt caverns... The top is approximately 500m high; according to design values, its maximum pressure is 9MPa, ρ 卤 =1.2g / cm 3 h is calculated according to the formula. max =750m, but the actual depth of the salt cavern is 850m, resulting in the bottom 100m of space being unusable. This application uses oil as an intermediate replacement medium, utilizing the density difference ρ between oil and brine. 油 =0.8g / cm ³ Extend the drainage depth to h max=1120m, the effective utilization depth increased from 750m to 850m, the gas storage volume increased by 15%, and the depth bottleneck problem was completely solved.

[0029] This application, through the "oil-gas dual replacement" method and systematic design, has upgraded the salt cavern space from "single gas storage" to "oil and gas coexistence". It not only breaks through the physical depth limitations of traditional gas injection and brine drainage, but also brings significant improvements in safety, operational flexibility and space utilization, providing an innovative solution for the efficient development of salt cavern storage.

[0030] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for constructing a salt cavern oil and gas coexistence system, characterized in that, Includes the following steps: S1 Oil Injection and Brine Discharge: Displacement oil with a density lower than that of brine is injected into the salt cavern through the annulus at the salt cavern wellhead, displacing and discharging the high-density brine from the salt cavern through the central pipe. S2 oil spill recovery: After oil injection is stopped, the replacement oil that overflowed due to the pressure in the salt cavern is recovered through the pipeline; S3 Gas Injection and Oil Displacement: Gas is injected into the salt cavern through the annulus to displace the oil from the central tube. The gas injection pressure does not exceed 80% of the formation stress at the casing shoe. S4 Oil and Gas Coexistence Space Formation: When the oil discharge reaches the preset gas storage space volume, gas injection is stopped and the central pipe is closed, forming an oil and gas coexistence salt cavern storage tank with oil storage in the lower part and gas storage in the upper part.

2. The method for constructing a salt cavern oil and gas coexistence system according to claim 1, characterized in that, The density of the displacement oil in S1 is 0.6-0.9 g / cm³. 3 The density of the brine is 1.1-1.2 g / cm³. 3 .

3. The method for constructing a salt cavern oil and gas coexistence system according to claim 1, characterized in that, During the gas injection and oil discharge process in S3, the formation stress at the casing shoe is monitored in real time, and the actual gas injection pressure is adjusted to be ≤80% of the formation stress.

4. The method for constructing a salt cavern oil and gas coexistence system according to claim 1, characterized in that, This also includes follow-up operations: Oil removal operation: Gas is injected into the annulus to drive the replacement oil out of the central tube; Gas extraction procedure: Displacement oil is injected through the central tube to expel the gas from the cushion from the central tube.

5. An oil and gas coexisting salt cavern storage system, characterized in that, The salt cavern has a bottom that holds displacement oil and a top that holds stored gas. Wellhead device, located at the top of the salt cavern; The central tube extends from the wellhead device to the bottom of the salt cavern cavity and is used to drain brine or replace oil. The annular channel, located outside the central tube, is used to inject replacement oil or gas.

6. The oil and gas coexistence salt cavern storage system according to claim 1, characterized in that, The wellhead device includes an annular injection valve, a central tube control valve, and a pressure sensor: the annular injection valve is connected to the inlet of the annular channel; the central tube control valve is connected to the outlet of the central tube; and the pressure sensor monitors the formation stress at the casing shoe in real time.

7. The oil and gas coexistence salt cavern storage system according to claim 1, characterized in that, The density of the displacement oil is 0.6-0.9 g / cm³. 3 The stored gas is natural gas or compressed air.

8. The oil and gas coexistence salt cavern storage system according to claim 1, characterized in that, The bottom of the salt cavern contains a sediment layer, and the displacement oil fills the voids in the sediment.

9. The oil and gas coexistence salt cavern storage system according to claims 5-8, characterized in that, The salt cavern cavity is either a single-well vertical cavity or a multi-well horizontally connected cavity.

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