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Influence of the reservoir/caprock interface on caprock integrity and reservoir performance

The reservoir-caprock interface is often considered a no-flow boundary in reservoir models; however, when deformation features are present at the interface, reservoir fluids can potentially use these features as pathways to travel into and through the caprock.  Our fieldwork in the Colorado Plateau identified a number of structural and diagenetic features potentially capable of influencing CO2 and hydrocarbon transmission.  We have focused on the most common of these interface features, zones of deformation bands in reservoir lithologies that transition to opening-mode fractures in caprock lithologies. The sedimentology, diagenesis, and petrophysical properties of interface sites were described in detail to infer the history of fluid flow across the interfaces and allow numerical single-phase and multiphase flow modeling.  The presence of pyrite within the fractures, as well bleached fracture margins, demonstrate that strongly reducing fluids at least partially penetrated the caprocks. Common hydrocarbon inclusions within fracture filling calcite further demonstrate that hydrocarbons at least partially penetrated the sealing lithologies.  The permeability and pore-size distribution of the deformation bands show that they are capable of greatly impeding flow of supercritical CO2.  The deformation band faults have 2 to 4 orders of magnitude lower single-phase permeability than the host sandstones, and mercury porosimetry indicates that they can form a capillary seal to supercritical CO2 and hydrocarbons – supporting up to a 3 m column of CO2 or hydrocarbons for the case of subhorizonatal bands.  Single-phase FEMOC (finite element method of characteristics) modeling demonstrates that the fracture systems are capable of transmitting significant volumes of supercritical CO2 or hydrocarbons, particularly when the fracture network in the caprock is associated with a deformation-band fault zone in the reservoir lithology.  Such flow is enhanced by a much steeper head gradient at the interface near the fracture due to the presence of the low-permeability fault. These relatively common small-scale features have the potential to produce significant seal bypass, in some cases perhaps sufficient to compromise the primary seal of a carbon sequestration site.