A method for controlling hydraulic gradient to accelerate salt release in weakly permeable bed of brine exploitation

Abstract

This invention relates to the fields of hydrogeological engineering and salt mine resource development, specifically a hydraulic gradient control method for accelerating salt release from weakly permeable aquifers during brine extraction. It aims to address the problems of insufficient vertical driving force, high geological structural safety risks, and lack of sustained displacement capacity in existing brine extraction technologies. The method includes the following steps: pumping wells are arranged in a ring within the overlying aquifer of the weakly permeable layer, and injection wells are arranged in the underlying aquifer. Through the combined action of the negative pressure generated by pumping and the positive pressure generated by injection, a controllable vertical hydraulic gradient is created within the weakly permeable layer. This transforms the salt release mode from simple molecular diffusion to a combined convection-diffusion displacement, significantly improving salt release efficiency. This method can be implemented by modifying existing extraction wells, has low engineering costs, and is suitable for the efficient and safe extraction of salt resources from coastal weakly permeable aquifers, showing broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of hydrogeological engineering and salt mine resource development, and specifically relates to a hydraulic gradient control method for accelerating the release of salt from weakly permeable layers during brine mining. Background Technology

[0002] Coastal underground brine is an important salt resource in my country, rich in useful components such as sodium chloride, potassium chloride, and bromine, and is a crucial raw material source for industries such as salt chemical industry and bromine extraction. Coastal brine deposits typically consist of alternating layers of highly permeable aquifers and low-permeability, weakly permeable layers. Long-term large-scale mining has led to the gradual depletion of recoverable brine in the aquifers, increasing the pressure on resource supply. Exploration data shows that the pores of the weakly permeable layers contain large amounts of high-salinity brine, with a salinity 1.2 to 2 times that of adjacent aquifers, totaling hundreds of millions of tons, representing an important potential source of salt. However, the weakly permeable layers are mainly composed of clay and silt layers, with permeability coefficients typically below 0.1 m / d, even as low as 0.001 m / d. Salt release mainly relies on molecular diffusion, and the salt migration rate is extremely slow in its natural state, making it difficult to achieve efficient industrial-scale mining, resulting in a large amount of "dead reserves." To overcome this bottleneck, existing mining technologies have explored various intervention methods, but still have the following three shortcomings: First, the radial flow generated by traditional single-well pumping preferentially converges along highly permeable aquifers, offering extremely limited vertical driving force to weakly permeable layers, thus failing to effectively utilize the salt resources contained within these layers. Second, while chemical release methods increase salt solubility by dissolving salt rocks, they pose safety risks such as damaging the formation framework and triggering ground subsidence or collapse, making large-scale engineering applications difficult. Third, existing water injection methods employ constant pressure injection, causing pore water in weakly permeable layers to tend towards a new equilibrium state under constant pressure. Salt migration efficiency continuously declines with prolonged operation, lacking effective means to maintain efficient salt release.

[0003] Therefore, there is an urgent need to develop a method for efficient extraction of salt from permeable aquifers that can overcome the limitations of low permeability, provide effective and continuous driving force, and ensure the safety of the formation structure. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a hydraulic gradient control method for accelerating the release of salt from weakly permeable layers during brine extraction. This method solves the problems of insufficient vertical driving force, high geological structural safety risks, and lack of continuous displacement capacity in existing technologies.

[0005] To achieve the above objectives, the present invention provides a method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction, specifically comprising the following steps: (1) Confirm the distribution, thickness and permeability coefficient of the overlying aquifer, weakly permeable layer and underlying aquifer at the target site, and determine the initial chloride ion concentration in the pore water; (2) At least one pumping well shall be installed in the aquifer above the weakly permeable layer, and at least one injection well shall be installed in the aquifer below the weakly permeable layer. (3) Start the pumping well to carry out pumping operation, and at the same time inject fluid into the injection well to apply injection pressure. Through the synergistic effect of the negative pressure generated by pumping and the positive pressure generated by injection, a vertical hydraulic gradient is formed in the weak permeable layer, driving the salt in the pore water of the weak permeable layer to migrate directionally to the overlying aquifer. (4) Monitor the head pressure of the overlying aquifer where the pumping well is located and the head pressure of the underlying aquifer where the injection well is located in real time, and calculate the vertical hydraulic gradient value accordingly; if the gradient value meets the preset driving conditions, switch to the periodic pressurized water injection mode after stable operation for a preset time; if it does not meet the conditions, perform the control operation.

[0006] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention creates a controllable vertical hydraulic gradient by placing pumping wells in the aquifer above the weakly permeable layer and injection wells in the underlying aquifer, thereby changing the salt release mode of the weakly permeable layer from simple molecular diffusion to a combined convection-diffusion displacement. The salt release efficiency is increased by 3 to 5 times compared with the natural state, effectively solving the key technical problem of the inefficient release of salt in weakly permeable layers. 2. The present invention adopts a periodic pressurized water injection mode, which continuously breaks the dynamic balance of fluid in the weakly permeable layer through pressure oscillation disturbance. Compared with the constant pressure water injection mode, the salt migration efficiency is further improved by 15% to 20%, overcoming the problem of salt migration efficiency decaying over time under constant pressure. 3. This invention is equipped with an emergency pressure relief valve and adopts a linkage control strategy of "prioritizing water pumping and supplementing with pressurization" to keep the water injection pressure below 80% of the formation fracturing pressure. This effectively improves the salt release efficiency while ensuring the safety of the formation structure and avoids the risk of the chemical release method damaging the formation structure. 4. This method can be implemented by modifying existing mining wells or abandoned wells. It has low construction costs, is easy to operate, and is suitable for large-scale application of saline resources in coastal weak permeable layers. Attached Figure Description

[0007] The invention will now be further described with reference to the accompanying drawings.

[0008] Figure 1 This is a flowchart of a method according to an embodiment of the present invention; Figure 2 This is a cross-sectional schematic diagram of the layout of pumping wells and injection wells in an embodiment of the present invention; Figure 3This is a plan view of the layout of pumping wells and injection wells in an embodiment of the present invention. Detailed Implementation

[0009] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0010] Example 1

[0011] This embodiment uses a coastal brine extraction area as an example. The geological structure of the target site, from top to bottom, consists of: an overlying aquifer, a weakly permeable layer, and an underlying aquifer. The vertical thickness of the weakly permeable layer is... m.

[0012] like Figure 1 As shown, the present invention provides a method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction, which specifically includes the following steps: S1: Site Survey The parameters of the above-mentioned strata were confirmed through hydrogeological drilling and electrical logging (see...). Figure 2 The overlying aquifer is approximately 8m thick with a permeability coefficient of [missing information]. m / s; the thickness of the weakly permeable layer is approximately 5m, and the permeability coefficient is... m / s, initial pore water chloride ion concentration approximately 180 g / L; underlying aquifer thickness approximately 12 m, permeability coefficient... m / s. Simultaneously, the formation fracturing pressure was determined to be 0.12 MPa using the on-site stepped pressurization method. Based on this, the upper limit of the water injection pressure was determined to be 0.096 MPa (80% of the formation fracturing pressure); the emergency pressure relief valve trigger threshold was set to... MPa, which is higher than the upper limit of water injection pressure but lower than the formation fracturing pressure, serves as a safety warning boundary.

[0013] S2: Well Group Deployment S2.1: Layout of pumping wells Four pumping wells were arranged in a ring around the overlying aquifer (see...). Figure 3), with a circular radius of 5 m. Each pumping well is evenly distributed on the circumference, and the distance between adjacent wells is about 7.9 m. The buried depth from the ground surface to the top plate of the overlying aquifer is about 5 m, and the well depth reaches the bottom of the overlying aquifer, with a total well depth of about 13 m. PVC well pipes (pipe diameter 110 mm) are used. Each pumping well is equipped with a variable-frequency submersible pump (rated flow 10 m³ / h, power 1.5 kW), and the pumping flow is precisely controlled by adjusting the output frequency of the frequency converter (range 20 - 50 Hz). After the installation of the pumping wells is completed, water is injected into the well pipes to the ground surface and kept under pressure for 30 minutes. If the water level drops by no more than 5 cm, the sealing is determined to be qualified.

[0014] S2.2: Layout of injection wells In the underlying aquifer, 1 injection well is arranged directly below the center of the circle of the 4 pumping wells (see Figure 2 ). The well depth reaches about 3 m below the top of the underlying aquifer. The total well depth starting from the ground surface is about 21 m (the buried depth from the ground surface to the top plate of the overlying aquifer is about 5 m, the thickness of the overlying aquifer is 8 m, the thickness of the weakly permeable layer is 5 m, and the penetration in the underlying aquifer is 3 m, totaling 21 m). Galvanized steel pipes (pipe diameter 75 mm) are used. The injection well is connected to a pressure-bearing water storage tank (volume 2 m³, rated pressure 0.3 MPa) and a self-acting regulating valve (regulation range 0 - 0.15 MPa). The pressure-bearing water storage tank and the self-acting regulating valve are both installed on the ground injection water pipeline. After the pipeline installation is completed, water is filled and pressurized to 0.12 MPa (1.2 times the rated injection pressure), and the pressure is maintained for 10 minutes. If the pressure drop does not exceed 0.006 MPa (5% of the rated pressure), the pressure test is determined to be qualified. The injected fluid is seawater with a salinity of about 35 g / L, and sodium polyaspartate scale inhibitor is added, with an initial concentration set at 20 mg / L; thereafter, the scale inhibitor is replenished every 24 hours according to the real-time conductivity detection results to maintain the concentration within the range of 15 - 25 mg / L.

[0015] S3: Establishment of vertical hydraulic gradient S3.1: Establishment of initial conditions Start the pumping well group. The initial pumping flow is set at 4 m³ / h, increasing by 1 m³ / h every 2 hours and gradually increasing to 8 m³ / h, causing the water level in the overlying aquifer to drop by about 0.8 m, corresponding to a hydraulic gradient of about 0.16 (i.e., 0.8 m / 5 m), which is significantly greater than the natural state hydraulic gradient (about 0.01 - 0.02). It is determined that an effective directional hydraulic gradient has been formed. At the same time, start the injection well, with the initial injection pressure set at 0.03 MPa, and gradually increase it to the target value of 0.05 MPa through the self-acting regulating valve, and operate under stable pressure.

[0016] S3.2: Calculation and regulation of vertical hydraulic gradient Calculate the vertical hydraulic gradient in real time according to the formula where, is the vertical hydraulic gradient; This refers to the fluid pressure difference between the overlying aquifer and the underlying aquifer. The vertical thickness of the weakly permeable layer.

[0017] The lowest vertical hydraulic gradient is determined by the permeability coefficient of the weakly permeable layer. and the target minimum seepage velocity It is confirmed that the calculation formula is as follows: ; in, To meet the minimum vertical hydraulic gradient required for the driving conditions; The target minimum seepage velocity; The permeability coefficient of the weakly permeable layer; when the measured vertical hydraulic gradient... Greater than or equal to When the preset driving conditions are met, it is determined that the conditions are met.

[0018] In this embodiment, the water level of the overlying aquifer drops by 0.8m, resulting in a pressure difference of approximately 7.84kPa; the injection pressure of the underlying aquifer is set at 0.05MPa (i.e., 50kPa). The combined pressure difference forms the total pressure difference. kPa. Given that the thickness of the weakly permeable layer is L = 5m, the actual vertical hydraulic gradient is calculated as follows: ; In this embodiment, the permeability coefficient of the weakly permeable layer m / s, target minimum seepage velocity m / d, then the minimum vertical hydraulic gradient is calculated as: ; Convert to pressure gradient (multiplied by the specific weight of water) ): ; Based on the above theoretical calculations, the target driving condition set in this embodiment is as follows: kPa / m. The measured gradient is 11.6 kPa / m, which is greater than the set target driving condition, thus satisfying the driving condition.

[0019] If monitoring reveals insufficient measured gradient, the control strategy is as follows: prioritize increasing the pumping flow rate; when the pumping flow rate has reached the rated upper limit (10 m³ / h) and still cannot meet the gradient requirements, then simultaneously increase the injection pressure, and strictly control the injection pressure within 0.096 MPa (i.e., 80% of the formation fracturing pressure).

[0020] S3.3: Periodic pressurized water injection After 48 hours of stable operation, the water injection mode was switched to a periodic pressurized water injection mode. This mode uses 0.05 MPa as the pressure baseline and sets a complete cycle of 12 hours: during the first 2 hours of the cycle, the injection pressure was increased to 0.07 MPa (40% overpressure, within the recommended range of 25%–50% of the baseline); the pressure was then reduced back to the 0.05 MPa baseline for the following 10 hours. By cyclically executing this process, a pressure oscillation disturbance field was constructed, continuously enhancing salt migration.

[0021] S3.4: Emergency Response An emergency pressure relief valve was installed above the bottom of the permeable layer, with a trigger threshold set at 0.10 MPa. When the measured pressure at the bottom of the permeable layer reached this threshold, the valve automatically opened, releasing the pressure to 0.05 MPa before automatically closing, and simultaneously issuing a warning to the operator. To ensure the valve functioned correctly, a functional check was performed every 7 days during the test.

[0022] S4: Parameter Monitoring and Technical Effectiveness After 30 days of continuous operation, the main monitoring results are as follows: the chloride ion concentration in the pumping well water increased from approximately 120 g / L initially to approximately 195 g / L, an increase of approximately 56% compared to the natural diffusion control group (concentration approximately 125 g / L at the same time) without artificial hydraulic gradient; the salt release flux of the weakly permeable layer increased by approximately 3.8 times compared to the natural state. On the 7th day after switching to the periodic pressurization mode, the salt migration efficiency increased by approximately 18% compared to the constant pressure mode, verifying the superiority of the periodic pressurization mode. Throughout the process, monitoring showed that the ground subsidence was less than 2 mm, the geological structure remained intact and stable, and the emergency pressure relief valve was not triggered at any point.

[0023] Example 2

[0024] This embodiment is basically the same as Embodiment 1, except for the geological conditions and water injection operation parameters. In this embodiment, the target site has a weakly permeable layer thickness of approximately 8m and a permeability coefficient of... With a flow rate of m / s and a formation fracturing pressure of 0.18 MPa, the upper limit of the water injection pressure is determined to be 0.144 MPa, and the emergency pressure relief valve trigger threshold is set to 0.15 MPa. Due to the relatively high permeability coefficient of the weakly permeable layer, the target driving gradient is appropriately reduced and set to [value missing]. kPa / m. The baseline injection pressure was set at 0.08 MPa, and the pressure during the periodic overpressure phase was 0.11 MPa (overpressure range of approximately 38%). The remaining operating steps were the same as in Example 1. After 30 days of continuous operation in this example, the chloride ion concentration in the pumping well water increased from approximately 150 g / L initially to approximately 222 g / L, an increase of approximately 48%. The formation structure remained stable, verifying the applicability of this method under different permeability and weak permeability conditions.

[0025] Example 3

[0026] This embodiment is essentially the same as Embodiment 1, except for the arrangement of the pumping wells. The site exhibits significant stratigraphic anisotropy, with the salinity of the weakly permeable layer being significantly higher in one direction than in others. To address this, the pumping wells are arranged in a non-uniform ring pattern: two pumping wells are densely placed in the direction of dominant salinity, and one well is placed in each of the other directions, for a total of five pumping wells, to enhance the directional displacement effect on salinity in the dominant direction. The remaining operational steps are the same as in Embodiment 1, verifying the flexible applicability of this method under anisotropic stratigraphic conditions.

[0027] Example 4

[0028] This embodiment is basically the same as Embodiment 1, except for the well location layout scheme. For scenarios with limited space or small-scale mining, this embodiment provides a simplified "one extraction, one injection" vertically corresponding layout method.

[0029] Specifically, a pumping well is installed in the overlying aquifer, and a corresponding injection well is installed directly below the pumping well in the underlying aquifer. The two wells are coaxial and coincident in the vertical direction, thus forming a direct vertical hydraulic channel between the upper and lower aquifers.

[0030] Under this deployment scheme, the hydraulic gradient control method remains unchanged: negative pressure is created by lowering the overlying water level through pumping wells, while positive pressure is applied through injection wells directly below. Although this scheme does not form a ring-shaped horizontal converging flow field as described in Example 1, the vertical pressure difference between the two wells... Still following the formula Calculations and adjustments were performed. Theoretical and experimental results show that, under the same pressure difference setting, this "one-injection-one-extraction" scheme can form a more concentrated vertical high gradient zone in the local area between the two wells, and can also effectively drive the release of salt in the weakly permeable layer, thus fully verifying the feasibility and applicability of this method in single-well mode.

[0031] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction, characterized in that, Includes the following steps: S1: Confirm the distribution, thickness, and permeability coefficient of the overlying aquifer, weakly permeable layer, and underlying aquifer at the target site, and determine the initial chloride ion concentration in the pore water; S2: At least one pumping well is installed in the aquifer above the weakly permeable layer, and at least one injection well is installed in the aquifer below the weakly permeable layer. S3: Start the pumping well to carry out pumping operation, and at the same time inject fluid into the injection well to apply injection pressure. Through the synergistic effect of the negative pressure generated by pumping and the positive pressure generated by injection, a vertical hydraulic gradient is formed in the weak permeable layer, driving the salt in the pore water of the weak permeable layer to migrate directionally to the overlying aquifer. S4: Real-time monitoring of the head pressure of the overlying aquifer where the pumping well is located and the head pressure of the underlying aquifer where the injection well is located, and calculation of the vertical hydraulic gradient value accordingly; if the gradient value meets the preset driving conditions, the system will switch to the periodic pressurized water injection mode after a preset stable operation time; if not, the system will perform control operations.

2. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, When the number of pumping wells in S2 is greater than one, the multiple pumping wells are evenly arranged in a ring in the overlying aquifer to guide the salt in the weakly permeable layer to converge towards the center of the ring; each pumping well is equipped with a variable frequency submersible pump, and the pumping flow rate is controlled by adjusting the output frequency of the frequency converter.

3. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 2, characterized in that, The injection well in S2 is located in the underlying aquifer directly below the center of the annulus; the injection well is connected to a pressurized water storage tank and a self-regulating valve, and the injection pressure is stably output through the self-regulating valve; the injection pressure is controlled at 60% to 80% of the formation fracturing pressure.

4. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, When there is one pumping well in S2, the injection well is located in the underlying aquifer directly below the pumping well. The pumping well is equipped with a variable frequency submersible pump, and the pumping flow rate is controlled by adjusting the output frequency of the frequency converter. The injection well is connected to a pressurized water storage tank and a self-regulating valve, and the injection pressure is stably output through the self-regulating valve. The injection pressure is controlled at 60% to 80% of the formation fracturing pressure.

5. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The pumping well pipe in S2 is made of PVC, and the injection well pipe is made of galvanized steel pipe. After the pumping well is installed, water is injected into the pipe to the ground surface and the pressure is stabilized for 30 minutes. If the water level drops by no more than 5 cm, it is considered to be a qualified seal. After the injection pipeline is installed, it is filled with water and pressurized to 1.2 times the rated injection pressure and held for 10 minutes. If the pressure drop does not exceed 5% of the rated pressure, it is considered to be a qualified pressure test.

6. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The fluid in S3 contains sodium polyaspartate scale inhibitor, the concentration of which is maintained in the range of 15-25 mg / L; the scale inhibitor is added every 24 hours based on the real-time conductivity test results.

7. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The formula for calculating the vertical hydraulic gradient value in S4 is as follows: ； in, The vertical hydraulic gradient; This refers to the fluid pressure difference between the overlying aquifer and the underlying aquifer. The vertical thickness of the weakly permeable layer.

8. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The preset driving condition in S4 is: measured vertical hydraulic gradient. Greater than or equal to the target minimum vertical hydraulic gradient ; The target minimum vertical hydraulic gradient is determined by the permeability coefficient of the weakly permeable layer. and the target minimum seepage velocity The calculation formula is as follows: ； in, The target is the lowest vertical hydraulic gradient; The target minimum seepage velocity; The value represents the permeability coefficient of the weakly permeable layer.

9. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The duration of each complete cycle of the periodic pressurized water injection mode in S4 is 12 hours, with the overpressure phase lasting 2 hours and the baseline pressure phase lasting 10 hours.

10. The method for controlling the hydraulic gradient to accelerate the release of salt from weakly permeable layers during brine extraction according to claim 1, characterized in that, The control operation in S4 is as follows: prioritize increasing the pumping flow rate; when the pumping flow rate has reached the rated upper limit of 10m³ / h and still cannot meet the gradient requirements, then simultaneously increase the injection pressure, and strictly control the injection pressure within 80% of the formation fracturing pressure.

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