POTENTIAL SEAL BYPASS FEATURES PRODUCED BY DEFORMATION-BAND FAULT TO OPENING-MODE FRACTURE TRANSITION AT THE RESERVOIR-CAPROCK INTERFACE

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. We identify a number of structural and diagenetic features potentially capable of influencing CO₂ and hydrocarbon transmission in eolian-mudstone systems. We focus 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- and multi-phase flow modeling. The presence of pyrite within the fractures, bleached fracture margins, and the presence of hydrocarbon inclusions within fracture-filling calcite demonstrate that strongly reducing, hydrocarbon-bearing fluids at least partially penetrated the caprocks. The permeability and pore-size distribution of the deformation bands within the reservoir lithologies indicate that they are capable of greatly impeding flow of supercritical CO₂. The deformation band faults have 2 to 4 orders of magnitude lower permeability than the host sandstones. Mercury porosimetry indicates that they are capable of forming a capillary seal – 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 can transmit significant volumes of fluids, particularly when the fracture network in the caprock is associated with a deformation-band fault zone in the reservoir lithology. Such flow into the caprock is enhanced by initial pressure buildup in the sandstone due to the low permeability deformation band. We are investigating the potential impact of such features on the integrity of carbon sequestration sites.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.