Shear-Slip Characteristics Of The Hot Dry Rock Fracture

Hot dry rock is dense and has developed natural fractures.The critical point to obtain good transformation effect of hot dry rock thermal reservoir lies in the fact that whether it can effectively activate natural fractures,shear-slip network,and improve reservoir connectivity.The key scientific issue is the thermal-fluid-solid coupling mechanism of shear-slip within fracture.In response to this point,taking hot-dry granite as the research object,the following research was carried out using the approaches of pseudo-triaxial fracturing model experiments,theoretical modeling and numerical analysis methods: the shear-slip process of a single inclined crack under the combined action of the in-situ stress,fluid pressure and high temperature.The variation law of fracture slip distance and shear stress along with the key influence factors such as fracture inclination,horizontal ground stress deviation,temperature difference between fracturing fluid and rock and roughness of joint plane are obtained.A shear-slip model of hot dry rock fractures considering the thermal-fluid-solid coupling function was established,which synthesized each kind of in-situ parameters such as the heat exchange between the thermal reservoir and the fracturing fluid,the in-situ stress and the friction coefficient of the fracture plane.The characteristics of shear-slip distance and slip velocity of fracture were acquired,and the influence of important factors such as thermal stress,mechanical properties of thermal reservoir rocks,and fluid pressure within fracture on the limit slip distance is revealed.The result shows that the crack initiation mode of fracture in hot dry granite includes open model initiation and shear model initiation,which are mainly controlled by the ratio of shear stress to normal stress of fracture plane.That is,the smaller the ratio is,the more prone to shear model initiation.Either way,the cracks will immediately shear-slip after crack initiation.According to the law of shear-slip distance of fracture over time,it can be known that the shear-slip process of fracture in hot-dry granite can be divided into four stages: crack initiation stage,accelerated sliding phase,stable sliding phase,deceleration sliding phase.Under the experimental conditions of this paper,it is found that the main factors influencing the shear-slip are the confining pressure,the ground stress deviation and the fracture inclination;the thermal stress caused by the temperature difference between the fracturing fluid and the hot reservoir can help to promote the slip of fracture,and cannot be ignored;long-term low-injection fracturing methods and high-stress differential heat reservoir can obtain longer crack limit slip distances.Finally,the thermal-fluid-solid coupling mechanism of the influence of fluid pressure,thermal stress and rock mechanics properties on the shear-slip of the fracture is revealed.The results of this study are expected to further enrich the hot-dry rock fracture theory and provide the necessary theoretical basis for the design of hot-dry rock fracturing.