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Effective Stress Controls on Fault Vertical Conductivity: Learnings for Both Carbon Storage and Conventional Hydrocarbon Reservoirs

Naruk, Steve *1; Brandenburg, John P 1; Solum, John G.1; Wolf, David E.1; Origo, Patrick 1
(1) Shell International E&P Inc, Houston, TX.

Fault zones in general may act as either lateral seals, or vertical fluid conduits, or both. The lateral seal capacity of faults has been extensively studied. The potential for faults to leak vertically has not been as extensively studied, but is a critical consideration for development of both CO2 subsurface storage sites, and for conventional reservoirs, particularly those that lie at relatively shallow depths below mudline. Understanding the factors that control vertical leakage is essential for predicting and preventing such leakage for both conventional reservoir development, and subsurface CO2 storage. Our recent comparison of leaking and non-leaking natural CO2 gas accumulations, provides such constraints. We compare and contrast trap configurations, fluid pressures, and stress states for several natural CO2 accumulations from the Colorado Plateau. Extensive surface geologic data is integrated with subsurface data from a large suite of groundwater and hydrocarbon wells. Leakage of CO2 is documented by geochemical surveys and the occurrence of extensive travertine deposits. The leakage occurs exclusively where the total fluid pressure attributable to a combination of hydraulic head and buoyancy of a gas-phase CO2 column reduces the minimum horizontal effective stress to zero, and the tensile strength of the rock is effectively zero due to the presence of pre-existing fractures within the 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 relatively high water injection pressures and ceased when the water injection stopped. The results imply that vertical leakage along fault zones occurs where the fault zones have no tensile strength and the effective normal stress acting on the fault zone is near zero. Prior to development, minimum horizontal effective stresses can be calculated as functions of depth, and mapped onto reservoir surfaces, to identify fault zones with potential to leak fluids to the ground surface or sea floor.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California