Numerical Research Of Multi-well Geothermal System Optimization In Sandstone Geothermal Reservoir

Geothermal energy which is clean and renewable has the characteristics of wide distribution and abundant reserves.With the geothermal energy being paid more attention year by year,the number of geothermal wells in geothermal fields is also rapidly increasing,forming multi-well geothermal system in regional scope.In order to improve the thermal performance,service life and economic benefit of multi-well geothermal system,the optimization of multiwell geothermal system has always been an important research content in the geothermal field.However,due to the difficulties in modeling and data collection of multi-well geothermal system,research on optimization of multi-well geothermal system is insufficient.Based on the sandstone geothermal reservoir in Decheng district,Dezhou city,Shandong province,this paper studies the influencing mechanism of multi-well geothermal system optimization from two aspects of well pattern layout and man-controlled factors.The main research work and conclusions are as follows:(1)Based on the thermal-hydro-mechanical coupling theory,2D thermal-hydro coupling numerical model and thermal-hydro-mechanical coupling numerical model were established to study the influence of stress field on temperature field.The results show that the influence of stress field on temperature field can not be ignored for the sandstone geothermal reservoir studied in this paper.Especially for the deep reservoir,stress field can not only directly affect the distribution of temperature field,but also indirectly affect the temperature field by affecting the flow field.(2)A new staggered well pattern was proposed in this paper,and the thermal-hydromechanical coupling numerical model of the traditional well pattern multi-well geothermal system and the staggered well pattern multi-well geothermal system are established.The dynamic response law of heat extraction rate,production temperature and effective production time under different well pattern is analyzed.The results show that by optimizing well pattern layout.the heat extraction rate of staggered well pattern is always higher than that of traditional well pattern in the early stage of system operation.During the operation of the system,the production temperature and effective production time of staggered well pattern are higher than that of traditional well pattern.Therefore,compared with traditional well pattern,staggered well pattern has obvious advantages in early thermal performance,service life and stable production time.(3)In order to provide reasonable reference of well spacing for the staggered well pattern multi-well geothermal system,3D thermal-hydro-mechanical coupling numerical models were established under 100m,200m,300m and 400m well spacing,and thermal performance and service life under different well spacing are analyzed.The results show that,within a reasonable range of spacing,the greater the well spacing,the greater the heat extraction rate,the smaller the risk of thermal breakthrough,that is,the higher the thermal performance and the longer the service life.For the sandstone geothermal reservoir in Decheng district studied in this paper,staggered multi-well geothermal system needs at least 400m well spacing to have better thermal performance and service life.(4)In order to optimize the thermal performance and service life of multi-well geothermal system from the perspective of human-controllable factors,a three-factor and three-level orthogonal test was designed based on Box-Behnken response surface method.Sensitivity analysis was carried out on the effects of injection temperature,production flow rate and injection flow rate on the thermal performance and service life of mult-well geothermal system,and influence of single factor was analyzed.The results show that the influence of each factor on the heat extraction rate and average production temperature from large to small is:injection flow rate,injection temperature and production flow rate.The higher the injection flow rate,the higher the heat extraction rate,but the lower the average production temperature,the higher the risk of thermal breakthrough.The lower the injection temperature is,the higher the heat extraction rate is,and the lower the average production temperature is,the lower the risk of thermal breakthrough is.The influencet of production flow rate on heat extraction rate and average production temperature is very limited.The above conclusions can provide important theoretical reference and technical support for optimizing thermal performance,increasing service life and improving economic benefits of multi-well geothermal system.