Fault Permeability Estimated From Rate of Sea Water Recharge Into an Underpressured Hydrocarbon Reservoir

Methane has leaked from the offshore South Ellwood fault at least since discovery of the South Ellwood field at Platform Holly. The fault bounds the north side of the field and has 600 meters of normal offset. The reservoir, which is fractured Monterey shale at one kilometer depth, was initially 5% over hydrostatic pressure, but is currently at 25% below hydrostatic pressure. Production fluid in well tubing that connects the platform and reservoir is isolated from the ocean. New data indicate that the ocean is in direct hydraulic communication with the reservoir in the vicinity of the fault. Quartz pressure sensors were installed at about one km depth in five wells during a 15 day production shut down. A well that intersects the fault at reservoir depth (about one km subsea), shows a pressure variation that matches the frequency of the ocean tide. Within +/- 1 minute, there is no lag between the predicted tide signal and the pressure variation in the well. The pressure change is less than predicted from sea heights, which we attribute to compressibility of the gas in the fault zone. The other wells (160m-1 km from the fault) do not show the tidal signal, indicating that pressure change is not a general effect of the tide on the earth's crust. During testing, fluid pressures increased at a rate of 55 Pa/hr (0.008 psi/hr) in the well adjacent to the fault. We conclude that the pressure recovery from sub-hydrostatic conditions is due to sea water flowing down the fault into the under pressured reservoir. From this data we calculate the permeability of the South Ellwood Fault to be about 20 md, a value similar to the overall field permeability in the fractured Monterey reservoir.