Role of stress field orientation on fault reactivation

Hydraulic fracturing involves injecting water under high pressure into the bedrock formation to enhance permeability of deep geo-reservoirs. Faults are planar discontinuities cutting the reservoirs, and their reactivation during injection depends of their static friction coefficient, the stress field, (orientation and magnitude), and the loading path. Here we propose to study experimentally the effect of the loading path and pore pressure level on fault reactivation and associated hydraulic behaviour (dilatancy). To this end, we conducted triaxial experiments on Rothbach sandstone saw-cut at angles of 30° and 50° and filled with 2 mm thick gouge layer, as representative of normal and reverse faults. Surprisingly, fault orientated at 30 degrees reactivates at lower friction coefficient than fault orientated at 50 degrees in contradiction to the theory of single plane of weakness (Jaeger 1960). Moreover, under load strengthening path, and independently of the fault orientation, the sample undergoes compaction while deforming. On the contrary, under load weakening path sample undergoes dilation. We also demonstrate that pore pressure level (𝜆 = 𝑝𝑓 𝜎𝑣 with pf the pore pressure, and 𝜎𝑣 the vertical stress) have a small effect on fault reactivation. Results suggest that both fault orientation and loading path are important parameters controlling the efficiency of hydraulic stimulation in reservoirs.