The influence mechanism of the discontinuous rock on the propagation path of hydraulic fractures in granite

Abstract

The development of hot dry rock(HDR) provides a zero-carbon and negative-carbon solution for China's energy industry. Hydraulic fracturing is one of the main means for building the geothermal reservoirs. Different from sedimentary rocks such as coal and shale, granitic hot dry rock is igneous rock, in which the geologically discontinuous rock, such as veins and rock interfaces are developed. Therefore, it is difficult to predict and control the propagation geometry of the hydraulic fractures when intersecting with these discontinuous rocks. Based on XFEM, the investigations were conducted on the influence mechanism of the discontinuous rock on the propagation path of hydraulic fractures in granite. The result show that: The greater the difference in tensile strength and Young's modulus between rock matrix and the discontinuous rocks, the hindrance effect on the propagating hydraulic fracture is more obvious. A large difference in Poisson's ratio is beneficial for hydraulic fractures to penetrate the discontinuous rock. For the granite with dense matrix, a smaller injection rate is beneficial for the hydraulic fracture to penetrate the discontinuous rock. When the maximum in-situ stress is consistent with the direction of hydraulic fracture initiation, a larger in-situ stress difference coefficient is beneficial for hydraulic fractures to penetrate the discontinuous rock. Under the same geological and engineering parameters, hydraulic fractures are more likely to penetrate the rock interface rather than rock veins. At the same time, the difference in the cementation degree of the discontinuous rock can also affect the propagation path of hydraulic fractures in granite.

Keywords

granite / hydraulic fracture / the discontinuous rock / extended finite element method