Determination of in-situ stress regime in the Koyna seismic zone, India from hydrofrac tests in a 3 km deep scientific borehole: implications for reservoir triggered seismicity

Knowledge of in-situ stress field is crucial to assess the hazards associated with the impoundment of large water reservoirs. Scientific deep drilling to 3 km depth in the Koyna seismic zone, a classical site of recurrent reservoir triggered seismicity (RTS) over the past six decades, provided a rare opportunity to study the in-situ stress regime and its implications for RTS. Hydraulic fracturing (HF) tests were conducted at 9 levels in the crystalline basement between 1600 m and 2400 m. Breakdown pressures, re-frac pressures and shut-in pressures extracted from the pressure-time curves constrain the stress magnitudes S_hmin and S_Hmax while the orientations of the induced fractures are determined from post-frac acoustic images. The results are as follows. (1) Stress-depth profiles for the depth range 1607–2374 m are given by: S_hmin [MPa] = (22.4 ± 1.7) + (0.019 ± 0.003) × (TVD [m] - 1607); S_Hmax [MPa] = (44.3 ± 2.8) + (0.036 ± 0.006) × (TVD [m] - 1607), TVD being true vertical depth. (2) The mean orientation of S_Hmax is N2°E±19°, consistent with stress-induced wellbore failures and earthquake focal mechanisms. (3) The stress magnitudes confirm strike-slip to normal transitional faulting environment and a critically stressed crust. (4) Low shear stress along the Donichawadi fault and lack of evidence for supra-hydrostatic pressure imply that fault slip could be induced by weak minerals such as phyllosilicate-rich fault gouge. (5) Bulk permeability of the order of 10⁻¹⁴ to 10⁻¹⁶ m² is required to induce slip at the hypocentral depths during monsoon and post monsoon seasons.