Balancing induced seismicity and permeability creation through aseismic deformation

Abstract:

The mitigation of induced seismicity is a challenge for a sustained production of unconventional hydrocarbons involving waste water disposal and of geothermal power production where large volumes of fluid are circulated in the subsurface. Large pore pressure perturbations localize where fluid flow is limited by the pre-existing joint set. It drives deformation that – if it occurs in brittle mode – manifests as seismicity leading to dilation of the reactivated fracture network. Therefore, induced seismicity is a process which is accompanied by permeability creation.

In many geothermal systems, seismicity is deliberately induced to enhance the reservoir. Experience collected at the EGS at Soultz-sous-Forêts, France has revealed evidence for a large proportion of the induced deformation to be aseismic. Indicators are temporally resolved velocity changes, changes of the stress tensor resolved from inversion of focal mechanisms and direct observation of large slip at wellbores. Furthermore, seismic multiplets, i.e. repeated slippage of asperities with considerable slip accumulation, have been observed not only in Soultz-sous-Forêts but also at other similar systems like in Basel, Switzerland and Landau, Germany. Displacement in the order of up to 0.1 m has been inferred from these observations, which is about one order of magnitude larger than what was observed seismically.

To explain this discrepancy we propose a conceptual model of creep-dominated aseismic deformation that is promoted through elevated pore pressures. While few asperities of the pre-existing fracture network experience repeated brittle deformation evidenced as multiple seismic events, the majority of the fault surface is in a subcritical creeping stage. Elevated pore pressure brings them closer to the failure criterion which can enhance the ductile deformation by several orders of magnitude. Relaxation leads to large-scale deformation accompanied by a strong reduction of differential stresses.