Research On Hydrodynamic Parameters In Rock Reservoir And Optimization Of Geothermal System With THM Coupling

As one kind of geothermal resources,hot dry rock(HDR)is widely distributed and has large reserves,due to its characteristics of clean,stable and renewable,it has caused extensive attention of the world recently.Because of its burial depth and lacking of fluid due to low porosity and permeability,enhanced geothermal system(EGS)has been the most effective mean for heat mining from HDR.The main process of flow and heat transfer in the fractured reservoir involves a multiple field coupling problems,which is called thermal-hydraulic-mechanical coupling effects.When the change of reservoir hydrodynamic parameters and EGS thermal stability could be assessed or predicted accurately,it will play a great significance for the design and operation of EGS project.Based on the above reasons,this paper conduct a numerical study for the flow and heat transfer process in the fractured reservoir,which considers the THM coupling effects comprehensively.The main contents are as follows.(1)According to the basic equations of seepage,heat transfer and rock mechanics in porous media,a fully coupled THM model is established with temperature,pressure and average normal stress as initial variables.Then,three simulations are conducted respectively for solving the problems of: 1-D consolidation,1-D heat conduction and 2-D Mandel-Cryer effect in deformable porous media,the results are in good agreement with analytical solutions.The evolution of temperature,stress and porosity are simulated when cold water injected into a fractured reservoir,the conclusions are in conformity with literature.The great agreement confirmed the feasibility and accuracy of this coupling model.(2)Based on reservoir logging data at songliao basin,a fractured geothermal reservoir model is established.Then the reservoir permeability and porosity are assumed to be a constant,and a comparative study between horizontal and vertical development program is carried out.The results indicate that: the temperature distribution are affected by the fluid density difference and the temperature extended surface has a downward tendency along the deepness;thermal breakthrough has a great negative influence on the thermal stability of EGS,and both the temperature at production points and the thermal power of EGS are reduced rapidly;compared with vertical wells,the EGS reservoir with horizontal wells has a longer seepage path and gets a better thermal compensation effects from base rock,also it can delay the onset of thermal breakthrough and improve the thermal stability of geothermal system.(3)Using the similar fractured reservoir model with horizontal wells,the thermal stability of EGS and the evolution of the reservoir hydrodynamic parameters,such as porosity and permeability,are simulated combined with the THM coupling effects.Moreover,the influence of the uncertainty of the injection rate,injection temperature,initial reservoir permeability and Biot coefficient are compared on the coupled model.The results indicate that: near the injection points,the porosity and permeability are increased rapidly and then they keep as a constant,while near the production points,the porosity and permeability are first decreased gradually,and then increased slightly after thermal breakthrough take place,totally the changing near injection region are more significant;the change of hydrodynamic parameters are influenced by both the variation of temperature and pressure,but it's more sensitive for temperature change;the enhancement of the reservoir permeability speed up the occurrence of thermal breakthrough and the heat generation rate are reduced slightly;increasing the injection rate,reducing the injection temperature and increasing the initial reservoir permeability are benefit for improve the thermal power of EGS,but meanwhile it will promote the occurrence of thermal breakthrough.