Abstract
There has been lots of discussion on associating microseismic activity with the reactivation of bedding planes near the hydraulic fracture tip. Yet evidence for bedding-plane slip has not been observed in cores of stimulated rock volumes, and a consensus on how to integrate bedding plane slip in fracture and reservoir models has not been established. The goal of this study is to investigate the mechanics of bedding-plane slip and the microseismic data that supports it. One of the requirements for microseismic bedding-planes is that they slip in shear. For this, two sources of shear force for bedding plane reactivation are considered: 1) the projection of the maximum principal stress onto the bedding plane, and 2) fluid pressure inside a tensile fracture (i.e. net pressure). The shearing resistance of a bedding plane is then calculated for a range of bedding dips with negligible cohesion. In a stress regime where the maximum principal stress is vertical and bedding dips are less than 10 degrees (e.g. Permian basin) a coefficient of sliding friction of ~0.05 would be required for the reactivation of bedding planes. This value is unrealistically small and contrasts with recent physical experiments on shale where bedding-parallel frictions are ~0.5. Moreover, the idea of microseismic bedding planes is commonly suggested in these types of stress regimes (SV >> SH > Sh) where variations in rock brittleness could have also explained the microseismic observations. However, in stress regimes where the maximum principal stress is horizontal (e.g. Neuquén basin), this study shows that the reactivation of the bedding planes can occur within a much wider range of friction values. It is in these types of stress regimes (SH >> SV and Sh) where the integration of bedding plane slip mechanics in fracture and reservoir models may prove invaluable for modelling the reservoir response to stimulation and production as well as optimizing wellbore design.
