Experimental investigation of the variations in hydraulic properties of a fault zone in Western Shandong, China

Abstract

Coal remains the largest contributor to China’s energy structure. However, coal production has frequently been threatened by groundwater inrush incidents induced by fault zones. The variations in the hydraulic properties of fault zones under high water pressure in relation to groundwater inrush risk are poorly understood. This paper presents high-pressure fluid injection tests used to investigate the variations in hydraulic properties of a fault zone at depth. The analysis of experimental results shows that the injection rate and hydraulic conductivity increase as nonlinear functions of injection pressure. The evolution process induced by high water pressure can be divided into three flow phases, including a Darcy flow phase, a flow transition phase and an unstable flow phase. Two thresholds of injection pressure for the onset of nonlinear flow and the onset of substantial fracture dilation and hydraulic fracturing were identified. The hydraulic conductivity of the fault zone is closely related to the water pressure, with an evident increase in permeability resulting from fracture extension and enhancement of fracture connectivity, which were attributed to water injection. Finally, a conceptual model was developed to demonstrate the textural and hydraulic evolution of the fault zone.