Effective Stress Constraints on Vertical Flow in Fault Zones: Learnings from Natural CO2 Reservoirs

Abstract

Understanding the factors that control vertical fault leakage is essential for predicting such leakage for both conventional reservoir development and subsurface CCS. This study compares leaking and non-leaking natural CO2 traps, providing such constraints. We constrain trap configurations, fluid pressures, and stress states for three natural CO2 accumulations from the Colorado Plateau. Surface geologic data are integrated with subsurface data from groundwater and hydrocarbon wells. Leakage of CO2 is documented by soil surveys and the occurrence of travertine deposits. Leakage occurs where the total fluid pressure reduces the effective stress to approximately zero, and the tensile strength of the rock is effectively zero due to the presence of pre-existing fractures within fault damage zones. These results appear to be consistent with induced fault zone leakage from some deepwater hydrocarbon reservoirs, where the leakage has been related to high water injection pressures and ceased when water injection stopped. The results imply that vertical leakage along fault zones occurs where the fault zones have fracture-based damage zones and the effective normal stress acting on the fault zone is low. These criteria can be used to evaluate the potential for vertical fault leakage regardless of the type of fluid contained within a reservoir.