A comparative study on permeability evolution of hot dry granite under different cooling rates
Injecting low-temperature water into high-temperature reservoirs during geothermal exploitation of deep hot dry rocks will change the permeability of reservoir rocks,which will directly affect the heat recovery efficiency of the enhanced geothermal system(EGS).To explore the variation characteristics and evolution mechanism of the permeability of hot dry granite after water cooling,permeability tests were conducted on hot dry granites under different cooling rates through a fully automatic gas permeability test system,based on which the influence of confining pressure and loading path on the permeability of granite was analyzed.The microstructures of hot dry granite were observed by polarization microscope and computed tomography(CT),so that the two-dimensional and three-dimensional microstructure change characteristics can be discussed,and the permeability evolution mechanism can be revealed.The experimental results show that:(1)The permeability and seepage flow rate of hot granite gradually increase with the increase of temperature under different cooling rates,and the permeability and seepage flow rate of granite under the condition of water cooling are always greater than those under the condition of air cooling,while the minimum seepage pressure showing the opposite trend;(2)The two-dimensional microcrack density,average width,and three-dimensional porosity of granite under different cooling rates continuously increase with temperature,and the values of microcrack density,average width,and porosity under water-cooling conditions are always greater than those under nature cooling conditions;(3)The changes of granite permeability under different cooling rates are mainly related to the evolution of microcracks inside the rocks,which are mainly caused by the non-uniform mineral expansion and internal physical and chemical reactions.Water cooling further promotes the increase of permeability of hot dry granite;(4)As the confining pressure increases,the permeability of hot dry granite under different cooling rates shows a negative exponential decrease trend,and the permeabilities of granite under unloading conditions are much lower than those under loading conditions.The experimental results are expected to reveal the permeability evolution mechanism in the process of deep hot dry rock geothermal exploitation and provide reliable parameters for EGS thermal recovery simulation.
rock mechanicspermeabilitygranitecooling ratemicrostructureunloadinggeothermal energy
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