Abstract

Fractures are considered to be both problems and opportunities for exploration and production from petroleum reservoirs. The main aim of fracturing an oil medium is the stimulation of oil and gas production by inducing highly conductive channels leading to a well, from which hydrocarbons will flow. Many applications of hydraulic fracturing take place in soft sandstones using moderate to low viscosity fluids. Soft sandstones represent the host rock for a large portion of active aquifers and oil and gas reservoirs because their high porosity both enhances storage and facilitates extraction. The transitional nature of sandstones, in particular, presents some challenges to the safety of these operations and the understanding of the mechanical response of these materials under a variety of conditions is still poorly understood.

The motivation of this research was to gain understanding of the fracture mechanics of soft media under low viscosity fluid injection. Synthetic rock specimens were generated via microbially induced carbonate precipitation (MICP), providing virtually limitless quantities and had customisable characteristics, allowing relevant structural parameters to be varied independently, and hence isolating their effects. Fracturing experiments were conducted with both cohesionless sands and bio-cemented samples at various confining conditions, flow rates, fluid viscosities, cementation levels. The main aim was to get an insight into how each individual factor affects the fracture patterns and pressure response. For those experiments, two experimental setups were developed explicitly for this project that were able to apply actively or passively stresses to the medium which was represented in three dimensions. The transitional behaviour of weakly cemented sandstones was clearly seen on both the fracture patterns and pressure responses obtained when conducting the hydraulic fracturing tests. The work conducted in this research bridges previous works on two-dimensional media, where visualisation of the fracture was possible, and three-dimensional media, where only post-test observations could be made.