高温三轴应力下裂隙后期充填花岗岩渗透特性试验研究

Experimental research on permeability of fractured-subsequently-filled granite under high temperature and triaxial stresses

进行花岗岩母岩(I类花岗岩)、热液充填体(II类花岗岩)、A类裂隙后期充填花岗岩(母岩与充填体之间的胶结界面横向贯通试样,III类花岗岩)和B类裂隙后期充填花岗岩(母岩与充填体之间的胶结界面纵向贯通试样,IV类花岗岩)高温三轴应力下的渗透特性研究。得出I,II,III及IV类花岗岩渗透率随温度变化的阈值温度分别为300℃,200℃,300℃和250℃。低于阈值温度时,4类花岗岩渗透率变化不大;高于阈值温度时,4类花岗岩渗透率分别快速提高了1,3,2及3个量级,其中II,IV类花岗岩渗透率量级在450℃以上达到10-1 mD。利用显微光度计观测了裂隙后期充填花岗岩的细观结构及其在高温作用下热致裂缝数量的变化。发现300℃后长度大于200μm的大裂缝的贯通是导致I,III类花岗岩渗透率增加的原因;充填体因溶蚀作用所具有的较低的强度及劣化的力学性能是致使Ⅱ,IV类花岗岩渗透率大幅超过I,III类花岗岩的主要原因。通过水岩热对流模型分析可知,在裂隙后期充填花岗岩内进行储层建造将大幅缩减施工成本、增加储层水岩换热面积及提高热交换效率,为深层干热岩地热开采提供新的技术及理论思考。

The permeability of the parent rock(type I granite),hydrothermal fluid backfill(type II granite),A-type fractured-subsequently-filled granite(cementation interface between the backfill and the parent rock laterally positioned through the specimen,type III granite) and B-type fractured-subsequently-filled(granite cementation interface between the backfill and the parent rock longitudinally positioned through the specimen,type IV granite) under high temperature and triaxial stresses was studied in this paper. The threshold temperatures of permeability change of types I,II,III and IV granite are 300 ℃,200 ℃,300 ℃ and 250 ℃,respectively. When the temperature is lower than the threshold,the permeability of four kinds of granite almost unchanged. When the temperature exceeds the threshold,however,the permeability of four kinds of granite increases rapidly by 1,3,2 and 3 orders of magnitude respectively,and the permeability magnitude of types II and IV granites reaches 10-1 mD at above 450 ℃. The micro-structure and thermally induced fracture number of fractured-subsequently-filled granite under the action of high temperature were observed by a microscope. It is found that the permeability of types I and III granite increases because of the penetration of fractures with a length greater than 200 μm after 300 ℃,and that the low strength and deteriorated mechanical property of the backfill due to dissolution are the main reasons for the permeability of types II and IV granite to be significantly higher than that of types I and III granite. Through the analysis of the model of convection heat transfer between water and rock mass,it can be seen that the reservoir construction in fractured-subsequently-filled granite can greatly reduce the construction cost,increase the water-rock heat exchange area and improve the heat exchange efficiency. The research work provides a new technical and theoretical thinking for deep HDR geothermal exploitation.