The role of fault gouge properties on fault reactivation during hydraulic stimulation; an experimental study using analogue faults

During the hydraulic stimulation of shale gas reservoirs the pore pressure on pre-existing faults/fractures can be raised sufficiently to cause reactivation/slip. There is some discrepancy in the literature over whether this interaction is beneficial or not to hydrocarbon extraction. Some state that the interaction will enhance the connectivity of fractures and also increase the Stimulated Reservoir Volume. However, other research states that natural fractures may cause leak-off of fracturing fluid away from the target zone, therefore reducing the amount of hydrocarbons extracted. Furthermore, at a larger scale there is potential for the reactivation of larger faults, this has the potential to harm the well integrity or cause leakage of fracturing fluid to overlying aquifers. In order to understand fault reactivation potential during hydraulic stimulation a series of analogue tests have been performed. These tests were conducted using a Bowland Shale gouge in the Angled Shear Rig (ASR). Firstly, the gouge was sheared until critically stressed. Water was then injected into the gouge to simulate pore fluid increase as a response to hydraulic stimulation. A number of experimental parameters were monitored to identify fracture reactivation. This study examined the effect of stress state, moisture content, and mineralogy on the fault properties. The mechanical strength of a gouge increases with stress and therefore depth. As expected, a reduction of moisture content also resulted in a small increase in mechanical strength. Results were compared with tests previously performed using the ASR apparatus, these showed that mineralogy will also affect the mechanical strength of the gouge. However, further work is required to investigate the roles of specific minerals, e.g. quartz content. During the reactivation phase of testing all tests reactivated, releasing small amounts of energy. This indicates that in these basic conditions natural fractures and faults will reactivate during the hydraulic stimulation if critically stressed. Furthermore, more variables should be investigated in the future, such as the effect of fluid injection rate and type of fluid