Hydraulic fracture interactions with mineral grains

Hydraulic fractures often turn or branch, interacting with pre-existing discontinuities (e.g. natural fracture, grain boundary). Such fracture complexities, especially in the proximity of borehole, impact the subsequent well conductivity. When a fracture finds a discontinuity, it either penetrates or deflects depending supposedly on the in-situ stress and the discontinuity geometry. However, our hydraulic fracture experiments on carbonates show that the fractures deflected more frequently at a grain boundary as they propagated farther away from the borehole. In other words, the fracture complexity consistently increases with the propagation distance. In this study, using energy release rate analyses, we show that the energy dissipation of a penetrating fracture increases with the distance away from the borehole. This means, the farther away the hydraulic fracture propagates, the more easily it deflects at a grain boundary from the energetic point of view. This tendency was also confirmed by numerical hydraulic fracture simulations based on a successive energy minimization approach. Our findings challenge the conventional hydraulic fracture penetration/deflection criteria based only on the in-situ stress and the discontinuity geometry.