Loss of Containment and Leakage Up Fault Zones

Abstract

Loss of containment up fault zones is a significant common risk for conventional oil and gas reservoirs and CO₂ sequestration (CCS) reservoirs. Such leakage can occur naturally as in the South Ellwood Field offshore California, or it can be induced by water injection such as at the Frade Field in deepwater Brazil. In both oil and gas reservoirs and natural analogs of CCS reservoirs, fluid flow up the fault zones occurs when the total fluid pressure in the reservoir equals the minimum horizontal stress. It occurs independent of the shale gouge ratio and clay smear values and juxtaposition seals along the faults, and independent of the shear stress values. In the naturally occurring deepwater oil and gas examples, fluid pressures were determined from MDT data and compared with the minimum horizontal stresses determined from LOT and FIT measurements. In the naturally leaking CO₂ traps, the fluid pressures were calculated from aquifer potentiometric data, CO₂ phase relations, and compared to minimum horizontal stresses calculated from mudweights. In the Frade example, elevated fluid pressures from water injection were transmitted up an uncased borehole to a depth where they equaled the minimum horizontal stress and were further transmitted up a fault zone to the seafloor. In all the studied examples, the fluid flow from the reservoirs up the fault zones occurs where the total fluid pressure equals the minimum horizontal stress and reduces the minimum horizontal effective stress to zero.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018