Permeability evolution of granite under cyclic hydraulic fracturing and its deterioration during in-situ stress retention in geothermal engineering
Minghui Li, Jun Lü, Wenchong Zhang, Zhouqian Wu, Cunbao Li, Heping Xie
Abstract
Hot Dry Rock (HDR) geothermal resources are a promising source of renewable energy with significant exploitation potential. HDR is typically exist at depths exceeding 2500 m, which presents challenges in terms of high in-situ stress and low permeability of the rock. To overcome these challenges and reduce the risk of fracking induced earthquakes, a scheme for cycling fluid injection has been proposed. In addition, the influence of geo-stress on the re-closure of rock fractures, as well as its effect on granite permeability, are important considerations in HDR exploitation. To study this, the GCTS triaxial testing system has been modified to simulate the hydraulic fracturing process of the deep hot rock reservoir, and permeability tests were conducted simultaneously. A protocol-cyclic fluid injection has been applied to fracture granite specimens under various fluid pressures. The results of the experiments showed that the permeability of the rock was significantly improved after fracturing, with an increase of 1–2 orders of magnitude compared to before. Cyclic hydraulic fracturing techniques can significantly reduce the granite breakthrough pressure and create more complex fracture networks. The permeability of the specimens displayed a decreasing trend during stress retention and tended to stabilize at around 1000 min. The maximum decrease was 90 %, which highlights the importance of considering the impact of stress aging on permeability in engineering. Additionally, the axial and radial strains of the specimens also decreased during stress retention. The mechanism for the change in rock permeability was attributed to the closure of newly formed fractures from hydraulic fracturing. Moreover, the experiments also showed that the smaller the injection fluid pressure, the faster the permeability of the specimens stabilized after fracturing. These findings can provide a theoretical basis for the effect of stress aging on granite permeability and offer reference for secondary fracturing time intervals in practical engineering applications. • The fracture characteristics of granite under flexible fracturing were studied. • The permeability characteristics of granite fractures under different flexible fractures were investigated. • The characteristics of fracture closure induced by long time in-situ stress in granite geothermal reservoir was quantitatively studied. • A new permeability model considering the closure effect of granite fractures is proposed.