PERMEABILITY ESTIMATES FOR THE SOUTH ELLWOOD FAULT

Fault‑zone permeability kf has been estimated by independent methods from two offshore wells in close proximity to the South Ellwood fault at Platform Holly, Santa Barbara channel, California. This fault runs parallel to the axis of the structure of the South Ellwood field, a Monterey Formation with a mean permeability of about kf ~10 md (10‑14 m2) about1 km beneath the sea bed. The pressure has been 70% of hydrostatic since 1984. Tidal frequency pressure variation from well #13 indicates that sea water is moving down the fault and entering the reservoir near well #13. From the pressure build‑up data and an estimate of the fault zone dimensions adjacent to the well, we calculate kf = 19 md with respect to water for the fault. Our other permeability estimate comes from well #7, completed about 5.2 km southeast of well #13. Overlying well #7 on the sea bed are steel tents that monitor gas seeping from the damage zone of the South Ellwood fault. When well #7 is shut down, seepage production increases at a constant rate of 45.3 m3 day/day to 31.2 m3 day/day (over shut down periods ranging from 21 days in 2003 to 45 days in 2005, respectively. Using these changes in flow rate and the pressure differences between the seep tents and the perforation intervals in the well, we have calculated kf = 30 md with respect to gas for the approximate one‑kilometer long fracture/fault flow path with an average cross section of 1860 m2 from the Darcy equation. Our two independent fault‑zone permeability estimates are similar and indicate that, at least locally, the damage zones of faults can have surprisingly high kf values over kilometer length scales.