| With the development of society and the consumption of fossil fuels,environmental pollution and other problems become increasingly prominent,how to develop and utilize geothermal energy scientifically and reasonably has become the focus of social attention.In order to study the practical problems of determining the optimal well spacing in the water pumping and reinjection of sandstone thermal storage fluid geothermal resources,this paper takes the typical water pumping and reinjection geothermal well system project in the Guanzhong Basin as an example and constructs the three-dimensional numerical model to simulate the optimal well spacing in the water pumping and reinjection of middle and deep sandstone thermal storage in the Guanzhong area under multiple scenarios.The hydraulic linkage of the sandstone thermal storage water pumping and reinjection system is studied by building the model,and the changes in groundwater level under different pumping schemes are compared and analysed to propose the optimal pumping scheme to solve the current practical situation of the groundwater level decreasing year by year.Finally,the response of the thermal reservoir temperature and flow fields to the operational,hydraulic and thermal parameters of the geothermal water pumping and reinjection system is analysed based on sensitivity analysis,and the order of response of each influencing parameter is determined,which can provide priority attention for the control and avoidance of thermal breakthroughs in the actual water pumping and reinjection project,and has certain practical significance for the initial design and later operation of the water pumping and reinjection project.The main findings and conclusions of this paper are as follows:（1）Based on the collection,collation and analysis of the basic data of the study area,the practical difficulties in the development and utilization of the middle-deep sandstone geothermal resources in the Guanzhong Basin are analyzed.The Guanzhong Basin is located in the middle of Shaanxi Province.It is a Cenozoic tectonic depression and belongs to continental rift sedimentary basin.Its topographic distribution is mainly controlled by the differential movement of step normal faults and secondary fault blocks.At present,in the process of geothermal resource exploitation in Guanzhong Basin,there are some problems,such as the decrease of formation heat storage pressure,the decrease of exploitation well water and the lack of energy carrier.For the rational development and sustainable utilization of geothermal resources,it is necessary to implement strict requirements for the reinjection of geothermal tail water in the same layer.The determination of reasonable well spacing of pumping and reinjection wells is one of the key problems in the practice of geothermal tail water reinjection.In this paper,by using numerical simulation analysis to explore the optimal well spacing without thermal breakthrough under normal pumping and reinjection activities.By simulating the influence of parameter changes on the flow field,temperature field and optimal well spacing of thermal reservoir.（2）Analyse the profile of a typical reinjection project,construct a mathematical and numerical model and fit the model for validation.The typical pumping and reinjection well project selected in the study is located in the northern part of Xi’an Depression.The stratum of the model pumping and reinjection section is the Neogene Pliocene Lantian-Bahe Formation（N2l+bh）,and the regional groundwater flows roughly from northwest to southeast.Firstly,the mathematical equation of underground hot water flow and the differential equation of underground hot water heat migration are constructed.Then,the plane range of the model in the study area is determined by the contour map of underground hot water,and the model is meshed and vertically layered.According to the existing measured data and parameters,the initial condition assignment,boundary condition assignment,mesh discretization and time discretization of the model are carried out.Finally,the model is verified by the measured data,which shows that the model has high simulation.Through the combination of mathematics and numerical model,the integration of theory and practice can be achieved,which makes the model analysis more scientific and can more accurately reflect the operation status of geothermal system.（3）Analysis of the hydraulic connection between water pumping and reinjection wells in a typical sandstone thermal storage project,analysis of the optimum well spacing on the basis of no thermal breakthrough,and proposal of an optimum pumping scheme.Using the constructed analysis model,the key engineering problems such as the optimal well spacing of the sandstone thermal storage and reinjection system are analyzed in depth.The results of the simulations show that the optimal well spacing between the wells is200-350m for a simulated harvesting and reinjection system in the Guanzhong area with a volume of 2400m3/d,100%reinject and only operating during the heating period（120d）each year,while ensuring the hydraulic connection.A significant inverse correlation was found between the groundwater level drop and the reinjection rate.The high reinjection rate can slow down the extent of groundwater level decline,and in actual production activities,the reinjection efficiency of geothermal tailwater should be increased as much as possible,which can alleviate the trend of rapid groundwater level decline to a certain extent.（4）The degree of influence of changes in each parameter of the numerical model on the temperature and flow fields is analysed by means of sensitivity analysis.Eight parameter variables were selected to simulate the response of the model temperature and flow fields to different parameters of the target aquifer for 15 years of operation（5475 d）at a well spacing of 200m.The simulation results show that:（1）The temperature field and the optimal well spacing are the most responsive to the tailing water reinjection temperature,followed by the amount of recovery and reinjection.The temperature field and the optimal well spacing also have a certain response to the specific heat of medium skeleton volume of the target aquifer,but the response to other parameters is very weak,and the influence of tailing water recharge temperature and reinjection volume on the temperature field is dominant.The optimal well spacing is positively correlated with pumping and reinjection volume,longitudinal thermal dispersion,permeability coefficient and thermal conductivity of medium skeleton,and inversely correlated with tailing water reinjection temperature,effective porosity,unit water storage coefficient and volume specific heat of medium skeleton.（2）The response analysis of the model thermal reservoir flow field to different parameters shows that the flow field has the strongest response to the permeability coefficient,followed by the amount of drainage and reinjection.The flow field also has a certain response to the unit water storage coefficient,and other parameters have no obvious influence on the flow field.The influence of permeability coefficient and reinjection amount on temperature field is dominant.It can be seen from the analysis results that the amount of pumping and reinjection is the only parameter to which both temperature field and flow field have strong responsiveness.In the actual process of pumping and reinjection,attention should be paid to the influence of the amount of pumping and reinjection on the geothermal system,and a reasonable plan and amount of pumping and reinjection should be formulated comprehensively to ensure the reasonable,scientific and sustainable operation of the geothermal water pumping and reinjection system. |
