Evaluation of water saturation effect on mechanical properties and hydraulic fracturing behavior of granite

Abstract

Hydraulic fracturing is a combined process of mechanical and hydraulic behavior due to in situ stress, well bore pressure, fluid infiltration in the rock mass. The initial state of surrounding well bore prior to hydraulic fracturing can be subject to either dry or saturated condition. We investigated various mechanical properties and hydraulic fracturing behavior of dry and saturated granite specimens containing pre-existing microcracks. After saturation, P-wave velocity increased by 27%–33% and S-wave velocity increased by 13%–15%. Saturation weakened the material, with a 3%–6% decrease in uniaxial compressive strength, a 3%–9% decrease in Brazilian tensile strength, and 5%–6% decrease in mode-I fracture toughness, but the elastic modulus and Poisson’s ratio varied little. The tensile strength estimated from hydraulic fracturing tests on cylinders, showed a negligible decrease (2%–3%) in saturated specimens compared with initially dry specimens. However, acoustic emission (AE) monitoring analysis results show that water saturation influence hydraulic fracturing initiation and propagation. When comparing the initially saturated specimens to the dry specimens, higher AE hit rates were observed right before and at the breakdown, and the percent of tension correlated AE hit was slightly lower. Moreover, computed tomography observations show that fractures tend to propagate in a wider area in the saturated specimens.