Hot Dry Rock Preconditioning by Multi-stage Fracturing in Excavation-based Enhanced Geothermal System

Abstract

Rock caveability is a key factor for caving hot dry rock to stimulate reservoirs in excavation-based enhanced geothermal system (E-EGS). However, how to induce controllable hydraulic fractures to enhance rock caveability has been not fully understood. Thus, we proposed a rock preconditioning method, called multi-stage fracturing, including first-stage fracturing and second-stage fracturing. In the first-stage fracturing, fractures propagate simultaneously to optimize the in-situ stress state; then, second-stage fracturing is used to create the desired fractures based on this optimized in-situ stress state. We found that after first-stage fracturing, the minimum in-situ stress rotated nearly 90º in the region between fractures. During the second-stage fracturing, hydraulic fractures tend to propagate perpendicular to the new minimum in-situ stress and intersect with pre-existing natural fractures. These findings benefit fracture trajectory optimization and rock caveability enhancement for constructing E-EGS reservoirs.