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Fault/Fracture Permeability Estimated from Response of a Natural Marine Methane Seep to Underlying Hydrocarbon Production

Boles, James 1; Horner, Steve 2
1 Earth Science, Univ Calif, Santa Barbara, CA.
2 Venoco, Inc, Carpenteria, CA.

Natural methane seeps overly hydrocarbon reservoirs in the Santa Barbara channel of southern California. At one locality near Platform Holly, where seepage is monitored from two steel tents on the seabed, seepage rate can be related to wells producing 1 km beneath the tents. The seepage emanates from a fault and fractures in the underlying siliceous shales of the Monterey Formation. There has been a overall drop in production rate of gas from the platform by a factor of two over the past 20 years, whereas the seep tent has correspondingly dropped seepage rates by a factor of eight. The tent seepage follows the platform production drop by about a year. We have used changes in seep rate to quantify the permeability of the flow path from the reservoir to the 1860 m2 tent area on the seabed.

A recently completed well, perforated at 914 m (3000’) beneath the collection tents, directly affects the seepage into the tents. When the well is shut down, seepage production rates increase at a constant rate of 45.3 m3 day/day (1.6 MCF/day/day) to 31.2 m3 day/day (1.1 MCF/day/day) over shutdown periods ranging from 21 days in 2003 to 45 days in 2005, respectively. Using these changes in flow rate, the known pressure differences between the seep tent and the perforation intervals in the well, we have calculated the permeability with respect to gas for the 914 m (3000’) fracture/fault flow path with an average cross section of 1860 m2 from the Darcy equation. We compare this estimate to our earlier estimate of 19 md permeability with respect to water along on a bounding fault to the reservoir (Boles and Horner, 2003).


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009