Research On Conductivity Of Artificial Fractures In Dry Hot Rocks Under Injection And Production Conditions

The enhanced geothermal system(EGS)is the main development method of dry hot rock resources.Through the fracturing and transformation of dry hot rock reservoirs,a dry hot rock thermal storage with artificial fracture networks connecting injection and production wells is formed.During the development of injection and production,the thermal-hydraulic-mechanical coupling flow mechanism in artificial fractures is the key to optimizing the development plan and achieving the economic and efficient development of dry hot rock resources.Aiming at this key scientific problem,this article conducts the following researches by carrying out laboratory experiments and numerical simulation analysis methods: The experimental researches on the artificial fracture conductivity of dry and hot rocks under the conditions of injection and mining are carried out.The research focuses on the seepage characteristics of dry hot rock artificial fractures under different injection speeds,injection temperatures,confining pressures and mineral particle sizes;the dry hot rock artificial fracture thermal-hydraulic-mechanical coupling model,under injection and mining condition is established,and the control equation of the conductivity is obtained.The three-dimensional model of experimental rock sample is established and used to verify and analyze the accuracy of the model;the numerical simulation analysis of the dry hot rock reservoir in the Qinghai Gonghe Basin is conducted to study the effects of injection parameters,artificial fracture attributes,and rock attributes the conductivity of artificial fractures and heat extraction performance of the reservoir.The main results and understandings obtained are:(1)Increasing the injection rate is conducive to improving the conductivity of artificial fractures,and the conductivity has an exponential relationship with the injection rate.(2)Increasing the injection temperature will reduce the conductivity of artificial fractures,and the injection temperature and the conductivity are in a fractional function relationship.(3)The higher the confining pressure,the smaller the change of fracture conductivity with injection parameters(4)Numerical simulation results of the thermal-hydraulic-mechanical coupling model of dry hot rocks artificial fractures under injection and mining conditions show that the changes in conductivity,injection pressure and fracture width are consistent with experimental data.Among them,the averaged error of the conductivity is 7.04% compared to experimental data;The increasing the roughness of the fracture surface helps to improve the conductivity.(5)The thermal stress generated by the artificial fracture surrounding rock is tensile stress,which will increase the fracture width and permeability,and help improve the conductivity.In the early stage of production,the increase in conductivity is mainly affected by pore pressure,and in the later period,it is mainly affected by thermal stress.In this paper,the maximum contribution ratio of thermal stress to the improvement of conductivity is 0.87.(6)The initial permeability and initial fracture width of artificial fractures,the elastic modulus of rock and thermal expansion coefficient have a positive correlation with the conductivity of the artificial fracture.Under these experimental conditions,the composition and particle size of granite minerals have no effect on the conductivity of the artificial fracture.(7)The increase of the artificial fracture conductivity will shorten the time for stable production and increase the heat production power.The research revealed the thermal-hydraulic-mechanical coupling flow mechanism in the artificial fractures of dry hot rocks during production development process,and is expected to provide the necessary theoretical basis for optimizing the development plan of dry hot rocks.