不同冷却速率下干热花岗岩渗透率演化特征对比研究

A comparative study on permeability evolution of hot dry granite under different cooling rates

深部干热岩地热开采过程中向高温储层注入低温水,将会改变储层岩石渗透率,进而直接影响增强型地热系统(enhanced geothermal system,EGS)采热效率。为探究遇水冷却下干热花岗岩渗透率变化规律及演化机制,利用岩石全自动气体渗透率测试系统进行不同冷却速率下干热花岗岩渗透率演化测试,分析围压和加载方式对花岗岩渗透率的影响规律,并采用偏光显微镜和计算机断层扫描进行干热花岗岩细观结构观测,探讨其二维和三维微观结构变化特征,揭示其渗透率演化机制。试验结果表明:(1)不同冷却速率下干热花岗岩渗透率和渗透流量随温度的升高逐渐增大,且遇水冷却条件下花岗岩渗透率和渗透流量总是大于自然冷却条件下的值,而最小渗透压力则呈现相反的趋势。(2)不同冷却速率下花岗岩试样的二维微裂纹密度和平均宽度、三维孔隙率均随温度的升高而不断增加,且遇水冷却条件下微裂纹密度、平均宽度和孔隙率的值均大于自然冷却条件下的值。(3)不同冷却速率下花岗岩渗透率的变化主要与岩石内部微裂纹的演化有关,而岩石内部微裂纹的演化主要是由于矿物非均匀膨胀和内部物理化学反应所引起的,遇水冷却促进其渗透率进一步提高。(4)随着围压的增大,不同冷却速率下干热花岗岩渗透率呈负指减小的趋势,卸荷作用下花岗岩渗透率远小于加载作用下的值。研究结果期望揭示深部干热岩地热开采过程中渗透率演化机制,并为EGS采热模拟提供可靠的参数。

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.