Potential seal bypass and caprock storage produced by deformation-band-to-opening-mode-fracture transition at the reservoir/caprock interface

Abstract

We examined the potential impact on CO2 transport of zones of deformation bands in reservoir rock that transition to opening-mode fractures within overlying caprock. Sedimentological and petrophysical measurements were collected along an approximately 5 m × 5 m outcrop of the Slick Rock and Earthy Members of the Entrada Sandstone on the eastern flank of the San Rafael Swell, Utah, USA. Measured deformation band permeability (2 mD) within the reservoir facies is about three orders of magnitude lower than the host sandstone. Average permeability of the caprock facies (0.0005 mD) is about seven orders of magnitude lower than the host sandstone. Aperture-based permeability estimates of the opening-mode caprock fractures are high (3.3 × 107 mD). High-resolution CO2–H2O transport models incorporate these permeability data at the millimeter scale. We varied fault properties at the reservoir/caprock interface between open fractures and deformation bands as part of a sensitivity study. Numerical modeling results suggest that zones of deformation bands within the reservoir strongly compartmentalize reservoir pressures largely blocking lateral, cross-fault flow of supercritical CO2. Significant vertical CO2 transport into the caprock occurred in some scenarios along opening-mode fractures. The magnitude of this vertical CO2 transport depends on the small-scale geometry of the contact between the opening-mode fracture and the zone of deformation bands, as well as the degree to which fractures penetrate caprock. The presence of relatively permeable units within the caprock allows storage of significant volumes of CO2, particularly when the fracture network does not extend all the way through the caprock.