Effects of fluid diffusivity on hydraulic fracturing processes using visual analysis

Hydraulic fracturing arises as a method to enhance oil and gas production, and also as a way to recover geothermal energy. It is, therefore, essential to understand how injecting a fluid inside a rock reservoir will affect its surroundings. Hydraulic fracturing processes can be strongly affected by the interaction between two mechanisms: the elastic effects caused by the hydraulic pressure applied inside fractures and the poro-mechanical effects caused by the fluid infiltration inside the porous media (i.e. fluid diffusivity); this, in turn, is affected by the injection rate used. The interaction between poro-elastic mechanisms, particularly the effect of the fluid diffusivity, in the hydraulic fracturing processes is not well-understood and is investigated in this paper. This study aims to experimentally and theoretically comprehend the effects of the injection rate on crack propagation and on pore pressures, when flaws pre-fabricated in prismatic gypsum specimens are hydraulically pressurized. In order to accomplish this, laboratory experiments were performed using two injection rates (2 and 20 ml/min), applied by an apparatus consisting of a pressure enclosure with an impermeable membrane in both faces of the specimen, which allowed one to observe the growth of a fluid front from the pre-fabricated flaws to the unsaturated porous media (i.e. rock), before fracturing took place. It was observed that the fracturing pressures and patterns are injection-rate-dependent. This was interpreted to be caused by the different pore pressures that developed in the rock matrix, which resulted from the significantly distinct fluid fronts observed for the two injection rates tested.