Injection-induced fracturing process in a tight sandstone under different saturation conditions

Abstract

To investigate the influence of local hydraulic condition on the water injection-induced fracture behavior in tight sandstone, injection tests under triaxial compression were conducted on samples collected from the Sichuan Basin, China. By means of acoustic emission (AE) monitoring, the fracturing behaviors of two samples of different water saturation patterns, partly saturated (the middle part was dry) and fully saturated, are investigated. Experiment results indicate that the local hydraulic condition plays a governing role in the mechanical properties, AE productivity, fracture nucleation and the geometry of the shear fracture zone. During axial loading stage, the partly saturated sample demonstrated ~35% higher elastic modulus than the fully saturated sample. During the injection-induced fracturing stage, progressively increasing AE activity and dilatancy, with increasing injected water volume, were observed preceding the dynamic fracture in the partly saturated sample, demonstrating a positive feedback between damage growing and fluid flow. More AEs were located in the dry or partially saturated regions at water front and thus produced dilatancy. However, the fully saturated sample shows very low AE activity (5% of that in partly saturated sample) which was initiated immediately before the dynamic fracture phase. Due to the very low permeability (~0.001 mD), volume of water injected into the fully saturated sample during the loading and creep stages is very limited, indicating the observed large dilatancy is governed by pore pressure increasing due to stress compaction. AE hypocenters demonstrated that an irregular shear fracture zone was created in the partly saturated sample, while a relatively flat shear plane was formed in the fully saturated sample.