Abstract
Observations on tens-of-meter scale experiments of fault activation by fluid injection conducted in shales allow exploring how aseismic and seismic events may jeopardize the integrity of a sealing caprock overlying a CO2 sequestration reservoir. We contrast the behavior of shale faults with another set of experiments conducted in carbonates. Significant fluid leakage occurs along the initially low-permeability shale faults when rupture is activated. Most of the leakage pathway closes when fluid injection ceases and fluid pressure drops. Dilatant slip on the fault plane alone does not explain the observed leakage behavior, which is also caused by fault opening favored by the softness of the shale, and by the structure of the fault zone that prevents fluids from diffusing into the adjacent damage zone. Experiments show a large amount of aseismic deformation. Small-magnitude seismicity (Mw < -2.5) is observed outside the pressurized leakage patch. Stress transferred from this aseismic deformation patch can build up to stress-criticality and favor seismicity. Thus, in terms of fault activation in caprocks, aseismic fault slip leading to increased permeability and a loss of seal integrity is of great concern.
Highlights
For carbon capture and geological storage (CCS) to become a viable emissions reduction and climate mitigation strategy, the world will need to sequester carbon dioxide (CO2) in the deep subsurface at an unprec edented scale
Key findings are that large leakage flow rates have been measured fault activation was limited in time and in injected fluid volume, that the fault rupture was largely aseismic, and that after activation the fault clamped to almost zero permeability, not a complete seal
We demonstrate that the earthquakes triggered by shale fault activation might be of lower magnitude estimated based on rupture size given the importance of aseismic fault movement
Summary
For carbon capture and geological storage (CCS) to become a viable emissions reduction and climate mitigation strategy, the world will need to sequester carbon dioxide (CO2) in the deep subsurface at an unprec edented scale. In situ experiments at the tens-of-meter scale allow controlled activation of pre-existing faults by fluid injection, provide access to the complex architecture of the entire fault, and with high-resolution monitoring in place offer an opportunity for tracking fault slip and induced seismicity close to the nucleation zone. We discuss how such experiments, by bridging labora tory and reservoir scales, may help to assess the risk for caprock leakage from pressure-driven fault slip in industrial CCS projects
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