Exploration system, exploration method and application of middle-deep buried pipe groundwater seepage field

Abstract

The present application belongs to the technical field of geothermal energy development and utilization, and relates to a medium-deep layer geoburied pipe underground water seepage field exploration system, an exploration method and application. The system comprises a target medium-deep layer coaxial sleeve geoburied pipe, a distributed optical fiber temperature measurement system and a temperature difference fluid injection system. The optical fiber is attached to the outer pipe wall or annulus of the target medium-deep layer coaxial sleeve geoburied pipe. The injection system pipeline is connected to the ground circulation system interface of the target medium-deep layer coaxial sleeve geoburied pipe. The exploration method comprises background field monitoring, temperature difference fluid injection, well shut-in monitoring and thermal front tracking, data inversion and seepage field analysis. The present application makes full use of existing geoburied pipe infrastructure and distributed optical fiber temperature measurement systems, uses injected temperature difference fluid to form underground thermal disturbance, and inverts seepage field parameters by monitoring the spatiotemporal evolution of the temperature field, thereby avoiding expensive special hydrological drilling and complex monitoring network construction, achieving "one well with multiple uses", and reducing the cost of underground water seepage field exploration.

Description

Technical Field

[0001] This invention belongs to the field of geothermal energy development and utilization technology, specifically relating to the exploration system, exploration method and application of groundwater seepage field in medium and deep buried pipes. Background Technology

[0002] Accurately understanding the groundwater seepage field (velocity and direction) is crucial for predicting and optimizing the long-term heat extraction performance of deep coaxial casing underground pipe (DBHE) systems. Currently, obtaining site-scale seepage field information mainly relies on traditional hydrogeological exploration methods. Commonly used methods include pumping / injection tests, geophysical exploration methods (such as resistivity methods and electromagnetic methods), and steady-state thermal testing. In the design of existing DBHE systems, the lack of reliable seepage field data often forces the adoption of conservative assumptions, such as ignoring seepage or assuming isotropic uniform seepage. This leads to significant deviations between numerical simulation predictions and actual operational performance, affecting system design optimization and economic efficiency.

[0003] The specific disadvantages of existing seepage detection methods are as follows: High cost and significant disturbance: Traditional pumping tests or tracer tests require drilling specialized hydrological wells, which involves huge investments, long construction periods, and may alter the original hydrogeological conditions of the site.

[0004] Limited information dimensions and insufficient accuracy: Existing methods struggle to obtain vertical seepage profile information along the entire heat exchange section of the DBHE well (up to 2000-3000 meters), and also find it difficult to obtain vector information of the seepage direction economically and efficiently. Traditional methods obtain regionally averaged or point-like parameters, which cannot meet the requirements of DBHE's refined design for describing the non-uniformity of the seepage field.

[0005] Environmental risks and monitoring difficulties: Some chemical tracers are persistent in the environment, which may pose long-term pollution risks. Concentration monitoring relies on discrete sampling, which is cumbersome and has low spatiotemporal resolution.

[0006] Disconnected from DBHE operating conditions: Existing test methods are conducted under static or specific hydrological conditions, failing to closely integrate with the DBHE's "heat extraction without water extraction" operating characteristics (there is a heat-fluid coupling effect), resulting in biases in the test results when used to predict the long-term thermal performance of the DBHE.

[0007] Ignoring the influence of temperature on the physical properties of porous media: Traditional seepage inversion models typically assume that the porosity and permeability of soil and rock are constant and do not change with temperature. However, during medium-deep geothermal extraction, the injected low-temperature (or high-temperature) fluids significantly alter the temperature field of the surrounding soil and rock, causing thermal expansion and contraction of the pore skeleton, changes in fluid viscosity, and micro-deformation of the pore structure, resulting in dynamic changes in porosity and permeability (thermal-pore-permeability coupling effect). Ignoring this effect introduces significant systematic errors, causing the inverted seepage parameters to deviate from the true values.

[0008] Therefore, a precise detection method for groundwater seepage fields that considers the thermal-pore-permeability coupling effect, is low-cost, low-disturbance, and has high information dimensions is needed to solve the above-mentioned technical problems. Summary of the Invention

[0009] This invention aims to transform the buried pipe of a medium-deep coaxial casing into an "in-situ thermal pulse detection platform." It utilizes a temperature difference fluid (cold or hot) as an artificial thermal pulse source and combines it with a distributed fiber optic thermometry (DTS) system. By comparing the spatiotemporal evolution differences of the temperature field along the well depth before and after the application of the thermal pulse, and considering the functional relationship between formation porosity / permeability and temperature, it achieves high-precision inversion of the groundwater seepage field (velocity, direction, porosity, and thermal-pore-permeability coupling parameters).

[0010] This invention provides the following technical solution: a system for exploring the seepage field of groundwater in medium-deep buried pipes, comprising: The target is a deep coaxial casing buried pipe, used as an exploration object for the groundwater seepage field.

[0011] Thermodynamic fluid injection system is used to inject fluid (cold or hot) with a temperature difference from the original temperature of the formation into the deep coaxial casing buried pipe of the target, forming a controllable artificial thermal pulse.

[0012] A distributed fiber optic temperature measurement system is used to monitor the background field of the groundwater seepage field (before thermal pulse injection), the thermal pulse injection process, and the temperature field recovery process after well shut-in.

[0013] The distributed optical fiber temperature measurement system includes distributed temperature sensing optical fibers, which are attached to the outer wall of the deep coaxial sleeve buried pipe of the target, or the distributed temperature sensing optical fibers are laid in the annulus of the deep coaxial sleeve buried pipe of the target; the laying range of the distributed temperature sensing optical fibers extends from the surface to the bottom of the well.

[0014] The thermo-fluid injection system includes: an injection pump, flow meter, thermometer and pipelines that can precisely control the flow rate and pressure; and an interface of the injection system pipeline connected to the ground circulation system of the target deep coaxial casing buried pipe.

[0015] Preferably, the host accuracy of the distributed fiber optic temperature measurement system is at least ±0.01°C, and the spatial resolution is not less than 0.5 meters.

[0016] Preferably, the temperature difference between the thermal fluid and the original formation temperature is in the range of 5-15℃.

[0017] This invention also discloses a method for exploring the seepage field of groundwater in medium-deep buried pipes. This method uses the above-mentioned exploration system and includes the following steps: Step S1, Background Field Monitoring: At least 72 hours before the injection of the thermoelectric fluid, stop the heating operation of the target buried pipe and start the distributed fiber optic temperature measurement (DTS) system to continuously monitor the original temperature background field of the wellbore and surrounding formation. Record the steady-state temperature distribution along the well depth (mainly reflecting the geothermal gradient and weak thermal anomalies caused by natural seepage).

[0018] Step S2, Thermostatic Fluid Injection: The prepared thermostatic fluid is injected at a constant flow rate into the deep coaxial casing buried pipe of the target well, forming a short-term closed loop. During the injection process, a distributed fiber optic temperature measurement system continuously monitors the wellbore temperature changes caused by the thermostatic fluid injection. Details are as follows: Start the injection pump to inject the prepared fluid, which has a significant temperature difference from the original formation temperature (typically cooled by 5-15°C), at a constant flow rate. (e.g., 2-5 m³ / h) is injected into the annulus of the DBHE. Fluid flows down the annulus to the bottom of the well, enters the channel between the inner tube and the annulus through the bottom opening, and then returns to the surface from the inner tube, forming a short-term closed loop. The injection duration is... (For example, 6-24 hours). During the injection process, DTS continuously monitors and records the rapid drop in wellbore temperature caused by the injection of cold fluid. The temperature difference fluid establishes a "thermal disturbance source" inside the wellbore, and exchanges heat with the formation through the well wall, forming a temperature disturbance zone in the surrounding rock and soil mass dominated by heat conduction.

[0019] Step S3, Well Shutdown Monitoring and Thermal Front Tracking: After injection is completed, stop circulation and close the wellhead valve. During the monitoring period... The DTS system maintains continuous monitoring for several days to several weeks. During this time, the recovery of the temperature field in the wellbore and surrounding formation is influenced by two dominant processes: a) Heat conduction recovery: The formation transfers heat to the cooled wellbore and surrounding soil and rock, and the temperature gradually rises.

[0020] b) Seepage-driven thermal front upwelling-diffusion migration: Groundwater seepage carries thermal fronts, which migrate in the same direction as the groundwater seepage, and the migration velocity is proportional to the seepage velocity. DTS can capture weak localized spatiotemporal temperature anomalies caused by thermal fronts passing through specific depth locations, distinct from pure heat conduction recovery modes. .

[0021] Step S4, Data Inversion and Seepage Field Analysis: Establish an inversion numerical model that considers the coupling of heat, pores and seepage. By fitting the difference between the measured background field and the thermal impulse response field, invert the seepage velocity vector, porosity, permeability and their temperature sensitivity parameters.

[0022] Preferably, in step S4, establishing the inversion numerical model specifically includes: Existing methods neglect the effect of temperature on the properties of porous media. This invention introduces the following coupling relationship: 1. Porosity changes with temperature (thermal expansion effect):

[0023] in Reference temperature Porosity below is the volumetric thermal expansion coefficient (1 / K) of the soil and rock skeleton.

[0024] 2. Permeability versus porosity (Kozeny-Carman relationship):

[0025] in The initial permeability (m²) is represented by the exponent. Usually, 2-3 is chosen.

[0026] 3. Fluid viscosity changes with temperature (Arrhenius type or exponential type):

[0027] in The dynamic viscosity at the reference temperature. It is the activation energy.

[0028] The above relationships together constitute a complete coupling of heat, pores, and permeation, enabling the inversion model to accurately reflect the dynamic response of the permeation field under the action of thermal pulses.

[0029] An axisymmetric or three-dimensional coupled transient model of "wellbore-formation" is established based on an open-source multiphysics simulation platform. The governing equations include: Groundwater seepage equation:

[0030]

[0031] in, Porosity is a dimensionless quantity. The fluid density is expressed in kg / m³; t represents time in seconds. For gradient operators, The Darcy velocity vector is in m / s; This represents the permeability tensor, in m². Represents pore pressure, in Pa; is the acceleration due to gravity, m / s²; z represents the depth, m (positive downwards); For source and sink terms, it represents the mass flow rate of fluid injected (positive) or extracted (negative) into a unit volume of rock and soil, kg / (m³·s); Expresses the fluid dynamic viscosity, Pa·s; Heat transfer equation:

[0032] in, The effective volumetric heat capacity of rock and soil, J / (m³·K), is expressed by... calculate; denoted by the effective thermal conductivity tensor, W / (m·K); For heat source sinks, it represents the amount of heat exchange per unit volume of rock and soil per unit time (such as heat extraction or injection caused by buried pipe heat exchange), W / m³; Indicates temperature, °C or K; The specific heat capacity at constant pressure of a fluid is expressed in J / (kg·K). The flow and heat transfer process inside the wellbore is simplified to a one-dimensional pipe flow model coupled with the formation model through the borehole wall boundary conditions.

[0033] More preferably, in step S4, the parameter inversion solution specifically includes: The background temperature field obtained from DTS measurements in steps S1-S3 and thermal pulse response temperature field The target fitting data is used for inversion. The parameter set to be inverted is defined as follows:

[0034] in For initial porosity and permeability, These are the parameters for the thermal-pore-permeability coupling.

[0035] Construct a joint objective function to simultaneously fit the background field and the dynamic response field:

[0036] in These are the background field weighting coefficients. This represents the simulated steady-state background temperature.

[0037] Optimization algorithms (such as Levenberg-Marquardt algorithm or genetic algorithm) are used to automatically adjust the parameter set P and iteratively run the numerical model until the objective function is minimized. The output includes: Seepage velocity profile along well depth

[0038] Dominant seepage direction

[0039] initial porosity Penetration rate

[0040] Thermal-pore-permeability coupling coefficient

[0041] in, This represents the temperature value (°C) calculated by the numerical model at the corresponding depth and time given a parameter set P. This indicates the depth obtained through DTS measurements. ,time Temperature value, °C; These are the weighting coefficients. , These represent the depth point coordinates and the time point value, respectively.

[0042] An optimization algorithm is used to automatically adjust the parameter set P and iteratively run the numerical model until the objective function is achieved. Minimizing the parameters yields the inversely derived actual seepage field and related formation parameters of the site.

[0043] Preferably, in step S3, the temperature field recovery of the wellbore and surrounding strata is affected by at least two dominant processes, including: thermal conduction recovery: the strata transfer heat to the cooled wellbore and surrounding soil and rock, and the temperature gradually rises; thermal front flow-diffusion migration: groundwater seepage carries the thermal front and migrates in the same direction as the groundwater seepage, and the migration speed is proportional to the seepage speed; the distributed fiber optic temperature measurement system captures a weak local temperature spatiotemporal anomaly signal caused by the thermal front passing through a specific depth position, which is different from the pure thermal conduction recovery mode.

[0044] This invention also discloses the application of a method for exploring groundwater seepage fields in medium-deep buried pipes, including: Model calibration: Input the seepage field parameters and thermal-pore-permeability coupling parameters obtained from the inversion into the long-term performance prediction model to improve the prediction accuracy.

[0045] Design optimization: Optimize the pipe group layout according to the seepage direction (e.g., pipe rows perpendicular to the main seepage direction), and consider the impact of temperature changes on formation permeability, dynamically adjusting the operation strategy.

[0046] System assessment: Evaluate the long-term stability of existing single wells or pipe groups under real seepage and thermal coupling conditions.

[0047] The beneficial effects of this invention are: 1. This invention can significantly reduce detection costs and complexity: This invention makes full use of existing DBHE infrastructure and DTS system, avoiding expensive specialized hydrological drilling and complex monitoring network construction, and realizing "one well for multiple uses".

[0048] 2. This invention can obtain high-dimensional and detailed seepage field information: This invention obtains vertical seepage profile, direction, porosity and thermal-pore-seepage coupling parameters at one time, providing unprecedented detailed input data for DBHE design and solving the problem of insufficient information dimension in the prior art.

[0049] 3. This invention enables in-situ, low-disturbance, and environmentally friendly detection: The detection process of this invention is carried out under conditions close to the actual operating conditions of DBHE, with only a small amount of fluid with a temperature difference from the formation injected, resulting in minimal disturbance, more representative results, no risk of chemical pollution, and in line with the concept of green exploration.

[0050] 4. This invention can significantly improve the accuracy of numerical simulation prediction: This invention incorporates the heat-pore-permeability coupling effect into the inversion framework, providing the model with one of the most realistic and critical boundary conditions. It solves the inversion bias caused by neglecting the temperature effect in traditional methods, enabling the prediction of long-term heat extraction performance, life assessment and optimization design of DBHE to move from "empirical guessing" to "data-driven" accurate scientific calculation, which helps to reduce investment risk and improve project economy.

[0051] 5. This invention promotes the intelligent development of geothermal resources: The method of this invention closely integrates monitoring, diagnosis and design optimization, which is a key technical link in building "geological transparency" and "digital twin geothermal fields", representing the direction of refined and intelligent development of geothermal resources. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the thermal-tracing combined injection stage of the exploration system, exploration method and application of the deep buried pipe groundwater seepage field in this invention. Figure 2 This is a flowchart of the data inversion solution of the present invention. Detailed Implementation

[0053] The relevant technologies of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0054] like Figures 1-2 As shown, the application method of the biodegradable thermodynamic fluid in seepage detection of medium-deep buried pipes according to this embodiment is described in detail below: 1. System Composition and Preparation: Target DBHE: Select a medium-deep coaxial casing buried pipe (depth H, e.g., 2600 meters) as the test object.

[0055] Distributed Fiber Optic Temperature Measurement (DTS) System: One or more distributed temperature sensing optical fibers are tightly bound to the outer wall of the DBHE or placed within the annulus. The DTS main unit must have an accuracy of ±0.01°C, a spatial resolution better than 0.5 meters, and a temperature measurement range covering the area from the surface to the bottom of the well. This system can also be used for monitoring the soil and rock temperature field during the DBHE's regular heating operation.

[0056] Fluid preparation and injection system: Prepare a certain volume ( For example, a fluid with a temperature difference of 10-20 m³. To enhance the thermal signal, the temperature of this solution can be adjusted to have a significant temperature difference from the original formation temperature. (Typically cooled by 5-15°C). Equipped with an injection pump, flow meter, thermometer, and piping for precise flow and pressure control, and interfaced with DBHE's ground circulation system.

[0057] 2. Operation and testing procedures: Step S1: Background Field Monitoring. At least 72 hours before thermoelectric fluid injection, DBHE suspends conventional heating operation, starts the DTS system, and continuously monitors and records the temperature background field of the wellbore and surrounding formation. Where z represents depth and t represents time. This background field is used for subsequent data comparison.

[0058] Step S2: Thermodynamic Fluid Injection (Artificial Thermal Pulse). Start the injection pump and inject the prepared fluid, which has a significant temperature difference with the original formation temperature (typically 5-15°C cooling), at a constant flow rate. (e.g., 2-5 m³ / h) is injected into the annulus of the DBHE. Fluid flows down the annulus to the bottom of the well, enters the channel between the inner tube and the annulus through the bottom opening (or through a specific channel design), and then returns to the surface from the inner tube, forming a short-term closed loop. The injection duration is... (For example, 6-24 hours) Ensure a certain volume of temperature-differential fluid enters and diffuses into the aquifer surrounding the wellbore. During injection, DTS continuously monitors and records the rapid temperature drop around the well caused by the injection of cold fluid. .

[0059] During injection, the thermo-fluid establishes a "temperature-tracer dual-marker" initial disturbance source within the wellbore. This disturbance source exchanges heat with the formation through the wellbore wall, forming a temperature disturbance zone in the surrounding soil and rock mass dominated by heat conduction. When groundwater seepage occurs, the seepage effect has a significant "dragging" and "deformation" effect on the spatial distribution and evolution of this temperature disturbance zone. This effect is recorded by the DTS system in the form of high-precision temperature field data. Therefore, this invention utilizes the thermo-fluid within the wellbore to mark the fluid state within the wellbore, while using DTS to monitor the thermal response outside the wellbore to inversely extrapolate the formation seepage field. The thermo-fluid itself does not leave the wellbore, but the precise initial conditions it provides are crucial to ensuring the uniqueness and accuracy of the inversion results.

[0060] In other words, in a closed loop, the thermofluid does not physically enter the formation, but rather acts as a "marker" for the low-temperature fluid within the wellbore. When the fluid temperature inside the wellbore changes due to injection, the migration speed of this temperature front in the formation is significantly affected by groundwater seepage. The temperature field changes monitored by DTS are actually tracking this temperature disturbance signal, which is "excited by the cold fluid in the wellbore and modulated by the formation seepage field." The presence of the thermofluid (even if only within the wellbore) ensures that DTS can accurately identify the source and characteristics of the "cold front."

[0061] Therefore, this method is an innovative application that utilizes the heat-fluid coupling effect to invert the seepage field while strictly adhering to the principle of "extracting heat but not water." It does not involve cross-boundary fluid exchange, but rather achieves the perception of formation fluids through thermal signals.

[0062] Technical Description: This thermal pulse generates a localized temperature disturbance zone in the surrounding soil and rock mass. Since both porosity and permeability change with temperature (thermal expansion effect), the evolution of this disturbance zone is controlled not only by the seepage field but also by the modulation of the physical properties of the porous medium by temperature. This invention utilizes this "thermal-porous-permeable" coupling effect to invert seepage parameters and coupling coefficients by analyzing subtle differences in the temperature field response.

[0063] Step S3: Well Shutdown Monitoring and Thermal Front Tracking. After injection is complete, stop circulation and close the wellhead valve ("Well Shutdown" state). Monitoring will continue for several days to weeks thereafter (monitoring period). Within this area, the DTS system maintains continuous automatic monitoring. At this time, the temperature field recovery of the wellbore and surrounding formation is influenced by two dominant processes: a) Thermal conduction recovery: The formation transfers heat to the cooled wellbore and surrounding soil, and the temperature gradually rises. b) Thermal front flow-diffusion migration: Groundwater seepage carries the thermal front (which is also a weak "temperature anomaly" because its temperature still differs from the formation) and migrates. The migration direction is consistent with the groundwater seepage direction, and the migration velocity is proportional to the seepage velocity. DTS can capture weak localized spatiotemporal temperature anomaly signals, distinct from the pure thermal conduction recovery mode, caused by the thermal front passing through a specific depth location. .

[0064] During the temperature field reconstruction process, the thermal front migration velocity recorded by DTS not only reflects the groundwater flow velocity but also implicitly contains information about porosity changes with temperature. For example, pore contraction in low-temperature zones leads to decreased permeability, thereby slowing down thermal front migration; conversely, pore expansion in high-temperature zones accelerates migration. These characteristics are incorporated into the inversion model.

[0065] Step S4: Data inversion and seepage field analysis. This is the core technical step of this method.

[0066] Establish an inversion numerical model: Based on an open-source multiphysics simulation platform (such as OpenGeoSys), establish an axisymmetric or three-dimensional coupled transient model of the wellbore-formation. The governing equations mainly include: (1) Groundwater seepage equation (Darcy's law):

[0067]

[0068] in, Porosity For fluid density, This is the Darcy velocity vector (core parameter to be inverted). For the permeability tensor, Pore ​​pressure, This is the acceleration due to gravity.

[0069] (2) Heat transfer equation (considering convection and conduction):

[0070] in, For effective volumetric heat capacity, For effective thermal conductivity, For heat sources and sinks (including the effects of injecting cold fluid).

[0071] The flow and heat transfer process inside the wellbore can be simplified as a one-dimensional pipe flow model coupled with the aforementioned formation model through the borehole wall boundary conditions.

[0072] Parameter inversion solution: The spatiotemporal temperature field data obtained from DTS measurements in steps S1-S3 are used to solve the problem. The target fitting data is used for inversion. The parameter set to be inverted is set as follows: ,in Let v be the components of the seepage velocity in the three directions (which can be simplified to the magnitude v and the direction angle θ of the velocity in the main seepage direction). For vertical and horizontal dispersion. Construct the objective function, such as in least squares form:

[0073] in, This represents the temperature field calculated by the numerical model under given parameters P. These are the weighting coefficients.

[0074] An optimization algorithm (such as the Levenberg-Marquardt algorithm or genetic algorithm) is used to automatically adjust the parameter set P and iteratively run the numerical model until the objective function is achieved. Minimization means achieving an optimal match between the simulated and measured temperature fields. The resulting parameter set P* represents the inverted true seepage field and related formation parameters of the site.

[0075] Example of output result: Seepage velocity: The direction is 25° north of east.

[0076] initial porosity penetration rate .

[0077] coefficient of thermal expansion Kozeny-Carman Index .

[0078] Output results: The inversion results can be output as a seepage velocity profile along the well depth direction. Dominant seepage direction (e.g., 30° north of east), average seepage velocity, effective porosity, etc.

[0079] 3. Application of results: Model calibration: The accurate seepage field parameters obtained from the inversion are directly input into the "thermal-fluid coupling" numerical model (such as the TH model in OpenGeoSys) used for long-term performance prediction of DBHE, which greatly improves the accuracy and reliability of model prediction.

[0080] Design optimization: Optimize the layout of subsequent pipe groups based on the seepage direction. For example, arrange the pipe rows perpendicular to the main seepage direction to maximize the heat transport capacity of groundwater, improve the overall heat extraction performance of the pipe group, and suppress heat accumulation.

[0081] System assessment: Evaluate the long-term operational potential and stability of existing single wells or pipe groups under real seepage conditions.

[0082] The difference between this invention and the prior art lies in: (1) Innovation of detection platform: The existing or planned DBHE is directly used as the carrier for the integration of "injection" and "monitoring" wells, without the need for additional drilling of special hydrological holes, which greatly reduces costs and construction disturbance.

[0083] (2) Integration of monitoring methods: The high-precision distributed optical fiber temperature measurement (DTS) system integrated or installed in DBHE is used to sense the weak thermal signal anomalies caused by the thermal difference fluid transport, so as to realize automatic, real-time and high-resolution monitoring of the continuous spatiotemporal temperature field (i.e. thermal difference fluid transport field) along the entire well depth, replacing the traditional discrete sampling.

[0084] (3) Greening of temperature difference fluid: The temperature difference fluid that can be naturally degraded within a predetermined time is used, and there is no risk of environmental residue after the detection task is completed, which is environmentally friendly.

[0085] (4) Inversion model coupling: A composite mass and heat transfer inversion model is established that couples heat conduction, convection (thermal difference fluid transport), thermal difference fluid degradation kinetics and DBHE wellbore heat exchange. By fitting the spatiotemporal temperature field data measured by DTS, multiple parameters such as seepage velocity magnitude, direction, vertical distribution (profile) and formation porosity are inverted at once.

[0086] Alternatives to this implementation include: Alternative thermal disturbance methods: In addition to injecting cold water, hot water can also be injected to create thermal disturbance, and the temperature field changes caused by the temperature rise can be used for inversion. Tap water or water with chemical properties similar to formation water can be used as the injected fluid to avoid adverse effects on the formation.

[0087] Alternatives to signal monitoring methods: In addition to relying on DTS to monitor the temperature field, if a distributed acoustic sensing (DAS) system is integrated into DBHE, micro-vibration signals caused by thermal disturbances can be monitored and cross-verified with the temperature field inversion results.

[0088] Alternative injection methods: In addition to injection through the DBHE’s own circulation system, a shallow auxiliary well can be drilled near the DBHE to inject the temperature difference fluid into the target aquifer. The DTS in the DBHE is still used as the monitoring well, which is suitable for situations where it is necessary to study the seepage field over a larger horizontal range.

[0089] Simplification and replacement of inversion model: For sites where seepage is dominant, the model can be simplified by ignoring the heat conduction term and establishing a thermal front transport inversion model with the convection equation as the core. The seepage velocity can be estimated directly by using the "thermal front" transport velocity monitored by DTS (extracted through methods such as cross-correlation analysis).

[0090] Combination with other detection methods: This method can be used for rapid preliminary screening. For critical areas or complex seepage fields, the inversion results of this method can be used as initial values ​​to guide subsequent small-scale, more refined traditional pumping tests or high-density resistivity method exploration, forming an exploration system that combines "surface-point" approaches.

[0091] In summary, this invention provides an innovative system and method for exploring the seepage field of medium-deep underground water through buried pipes, possessing significant technical advantages and application value. By organically combining distributed fiber optic thermometry technology, biodegradable thermofluids, and a buried pipe heat exchange system, high-precision, low-disturbance, and environmentally friendly in-situ monitoring of the underground seepage field is achieved. This method not only breaks through the reliance on specialized drilling and complex monitoring networks in traditional hydrogeological exploration but also significantly improves the dimensionality and accuracy of data acquisition, providing strong technical support for the scientific development of geothermal resources.

[0092] Practical applications of this invention demonstrate its unique advantages in several aspects. First, in terms of detection efficiency, continuous seepage profile information along the well depth can be obtained through a single thermal-tracing combined injection operation, significantly shortening the exploration cycle. Second, in terms of economy, it makes full use of existing buried pipe infrastructure, avoiding additional drilling costs while reducing construction risks and environmental impact. Furthermore, the numerical simulation results based on the inversion model can directly serve engineering design, providing a reliable scientific basis for the optimized layout and long-term performance prediction of buried pipe networks.

[0093] From a technological development perspective, this invention represents a significant advancement in the field of geothermal resource exploration and development. By combining modern sensing technology, numerical simulation methods, and optimization algorithms, this invention successfully transforms complex underground seepage processes into quantifiable and analyzable datasets, opening new avenues for subsequent scientific research and engineering practice.

[0094] In summary, this invention, with its unique technical concept and outstanding application effects, provides a brand-new solution for the sustainable development of medium-deep geothermal resources, and has broad prospects for promotion and important social significance.

[0095] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A system for exploring the seepage field of groundwater in medium-deep buried pipes, characterized in that, include: The target is a deep coaxial casing buried pipe, which is used as an exploration object for the groundwater seepage field; The temperature difference fluid injection system is used to inject fluid with a temperature difference from the original temperature of the formation into the deep coaxial casing buried pipe of the target, thereby creating underground thermal disturbance; A distributed fiber optic temperature measurement system is used to monitor the background field of the groundwater seepage field and to monitor the process of sudden rise or fall in well perimeter temperature caused by the injection of temperature difference fluid. The distributed optical fiber temperature measurement system includes a distributed temperature sensing optical fiber, which is attached to the outer wall of the deep coaxial sleeve buried pipe of the target, or the distributed temperature sensing optical fiber is laid in the annulus of the deep coaxial sleeve buried pipe of the target. The distributed temperature sensing fiber is laid from the ground surface to the bottom of the well. The thermodynamic fluid injection system includes: an injection pump capable of precisely controlling flow rate and pressure, a flow meter, a thermometer, and pipelines; The injection system pipeline is connected to the ground circulation system interface of the deep coaxial casing buried pipe of the target.

2. The exploration system for groundwater seepage field in medium-deep buried pipes according to claim 1, characterized in that, The main unit accuracy of the distributed fiber optic temperature measurement system is at least ±0.01°C, and the spatial resolution is not less than 0.5 meters.

3. The exploration system for groundwater seepage field in medium-deep buried pipes according to claim 1, characterized in that, The temperature difference between the fluid and the original formation temperature ranges from 5 to 15°C.

4. A method for exploring the groundwater seepage field in medium-deep buried pipes, characterized in that, The exploration method employs the exploration system described in any one of claims 1 to 3, and the exploration method includes the following steps: Step S1, Background Field Monitoring: At least 72 hours before the injection of the temperature difference fluid, stop the heating operation of the target deep coaxial casing buried pipe and start the distributed fiber optic temperature measurement system to continuously monitor the temperature background field of the wellbore and the surrounding formation. Step S2, Thermostatic Fluid Injection: The prepared thermostatic fluid is injected into the deep coaxial casing buried pipe of the target at a constant flow rate to form a short-term closed loop. During the injection process, the distributed fiber optic temperature measurement system continuously monitors the temperature change around the well caused by the thermostatic fluid injection. Step S3, Well Shutdown Monitoring and Thermal Front Tracking: After the thermo-fluid injection is completed, circulation is stopped and the wellhead valve is closed; during the monitoring period, the distributed fiber optic temperature measurement system maintains continuous monitoring; Step S4, Data Inversion and Seepage Field Analysis: Establish an inversion numerical model, perform parameter inversion solution and output the results; the output results include: seepage velocity profile along well depth, dominant seepage direction, average seepage velocity, and effective porosity.

5. The method for exploring the groundwater seepage field of a medium-deep buried pipe according to claim 4, characterized in that, In step S4, establishing the inversion numerical model specifically includes: An axisymmetric or three-dimensional coupled transient model of "wellbore-formation" is established based on an open-source multiphysics simulation platform. The governing equations include: Groundwater seepage equation: in, Indicates porosity. Let t represent fluid density and time. For gradient operators, For Darcy velocity vector, Represents the permeability tensor. Indicates pore pressure, z represents gravitational acceleration; z represents depth. This indicates the mass flow rate of fluid injected or extracted per unit volume of rock or soil. Indicates the dynamic viscosity of the fluid; Heat transfer equation: in, Indicates the effective volumetric heat capacity of the rock and soil mass. Represents the effective thermal conductivity tensor. It represents the amount of heat exchanged per unit volume of rock and soil per unit time. Indicates temperature. This indicates the specific heat capacity at constant pressure of a fluid; The flow and heat transfer process inside the wellbore is simplified to a one-dimensional pipe flow model coupled with the formation model through the borehole wall boundary conditions.

6. The method for exploring the groundwater seepage field of a medium-deep buried pipe according to claim 5, characterized in that, In step S4, the parameter inversion solution specifically includes: The spatiotemporal temperature field data obtained from steps S1 to S3 are used as the reference. As the target fitting data for inversion, the parameter set to be inverted is set as follows: ,in The components of the seepage velocity in the three directions are: For longitudinal and lateral dispersion; The objective function is constructed as follows: in, This represents the temperature value calculated by the numerical model at the corresponding depth and time, given a parameter set P. This indicates the depth obtained through DTS measurements. ,time Temperature value, These are the weighting coefficients; , These represent the depth point coordinates and the time point value, respectively. An optimization algorithm is used to automatically adjust the parameter set P and iteratively run the numerical model until the objective function is achieved. Minimizing the parameters yields the inversely derived actual seepage field and related formation parameters of the site.

7. The method for exploring the groundwater seepage field of a medium-deep buried pipe according to claim 4, characterized in that, In step S3, the temperature field recovery of the wellbore and surrounding strata is affected by at least two dominant processes, including: thermal conduction recovery: the strata transfer heat to the cooled wellbore and surrounding soil and rock, and the temperature gradually rises; thermal front flow-diffusion migration: groundwater seepage carries the thermal front and migrates in the same direction as the groundwater seepage, and the migration speed is proportional to the seepage speed; the distributed fiber optic temperature measurement system captures a weak local temperature spatiotemporal anomaly signal caused by the thermal front passing through a specific depth position, which is different from the pure thermal conduction recovery mode.

8. An application of a method for exploring the groundwater seepage field in medium-deep buried pipes, characterized in that, The exploration method employs the method described in any one of claims 4 to 7, and the application includes: Model calibration: The seepage field parameters obtained by inversion are directly input into the "thermal-fluid coupling" numerical model used for long-term performance prediction of medium-deep coaxial casing buried pipes to improve the accuracy and reliability of model prediction. Design optimization: Based on the seepage direction, optimize the layout of the subsequent pipe group to increase the heat transport capacity of groundwater, improve the overall heat extraction performance of the pipe group and suppress heat accumulation. System assessment: Evaluate the long-term operational potential and stability of existing single wells or pipe groups under real seepage conditions.

Images