Hot dry rock preconditioning by two-stage fracturing in excavation-enhanced geothermal system

In the newly-proposed excavation-enhanced geothermal system (E-EGS), the massive hot dry rock (HDR) can be preconditioned by hydraulic fracturing to promote spontaneous block caving. A crucial problem in E-EGS is how to obtain controlled hydraulic fractures that intersect the natural fractures to crack the HDR matrix into blocks of suitable size. Here, we proposed a novel two-stage fracturing approach in which first-stage fracturing is used to modulate the in-situ stress state, rather than to introduce additional joints, and create favorable stratigraphic conditions for the second-stage fracturing to create desired fractures. We found that the minimum in-situ stress (σmin) in the region between fractures is significantly rotated even reaching nearly 90° over first-stage fracturing. In this region, the hydraulic fractures propagate approximately parallel to the initial σmin(σy) during second-stage fracturing and intersect with the natural fractures perpendicular to the initial σmin. The optimum in-situ stress state is achieved via the first-stage fracturing with similar fracture length and spacing. Compared to conventional fracturing, the fractures propagate asymmetrically and are shorter and wider in the second-stage fracturing. Two-stage fracturing can effectively control the fracture trajectory. This study supplies new insights for HDR preconditioning and benefits the effective construction of E-EGS reservoirs.