A Numerical Study Of The Exploitation Processes Of CO₂ Plume Geothermal Systems

CO₂-Plume Geothermal System?CPGS?is a geothermal system based on the high permeability natural aquifer with CO₂ as working medium.It may be used to exploit geothermal energy and implement CO₂ geosequestration simultaneously.Aiming at the complex multi-physical field coupling phenomena during the geothermal exploitation process,a physical mathematical model for numerical simulation of the process was established in order to understand geothermal energy exploitation and CO₂ geosequestration.Various influencing factors of CPGS operation were studied systematically.The main work and results are summarized as follows:?1?A non-isothermal,CO₂-Water multiphase porous media flow model was established with a full consideration of variable fluid physical properties.Using this and the geometrical model of the aquifer,a three-dimensional physical mathematical model was proposed to take the thermal compensation effects of the cap and the base rocks of the thermal reservoir.Then the CPGS geothermal exploitation processes that took the thermal compensation into consideration were studied numerically.The influences of thermal compensation and the sensitivity analyses of various geological factors were studied.The results showed that the thermal compensation extends the CPGS production lifetime and stabilizes the system capacity output,and thus gains better heat exploitation performance.The results also showed that the thermal compensation of the base rocks is stronger than that of the cap rocks.Increasing the thermal conductivity and specific heat capacity of both the cap and base rocks helps enhancing the thermal compensation.The increase in the thermal reservoir thickness will directly increase the total heat exploitation quantity,and at the same time weaken the thermal compensation effects.?2?Mechanical model,porosity and permeability dynamic model for thermal reservoir were integrated into the non-isothermal CO₂-Water multiphase seepage model to form the thermal-hydrologic-mechanical?THM?model with a full consideration of the coupling effects of temperature,seepage and stress field.With this model,the CPGS geothermal exploitation process was studied numerically to study the rock deformation effects of various geological parameters.The results showed that the rock significantly shrinkages during the process of the CPGS operation,reduces the volume,and increases the porosity of the reservoir.The deformation also helps increasing the permeability of thermal reservoir,accelerating the rate of exploitation process,and thus enhancing the geothermal exploitation process.With the same initial permeability,low porosity causes large permeability amplification and increases the geothermal exploitation rate.As the rock coefficient of thermal expansion,the sensitivity of rock deformation to temperature also increases,the increase in permeability was more obvious and the rate of geothermal exploitation became faster.Increasing well spacing,the cross-well pressure gradient was decreased.The exploitable geothermal resource was increased and the lifetime was extended.The proportion of the thermal compensation in the total thermal recovery was greater and more energy could be gained from the cap and base rocks.However,the rock deformation became severer and so did the stratum settlement.?3?The influences of cross-well pressure difference and injection temperature on CPGS heat exploitation were investigated numerically using the THM model with consideration of the cap and base rock thermal compensation.The results showed that Increasing the cross-well pressure difference can increase the rate of geothermal exploitation.However,the lifetime of CPGS would be shortened.Reducing the cross-well pressure difference,CPGS would be more stable and sustainable.The variation of the total heat collection with cross-well pressure difference is different for different termination temperature.The larger heat collection could be obtained for the higher termination temperature?say,for example,373.15 K?with large cross-well pressure difference and for the lower termination temperature?say,for example,353.15 K?with the small cross-well pressure difference.The results also disclose that low injection temperature could improve the system heat extraction yield and heat collection greatly,although it has little effect on the lifetime.The heat collection increases significantly as the termination temperature decreases.