Explosion and other gas ventings at the Fairfax District, Los Angeles

by Douglas H. Hamilton Consulting Geologist 2 Bassett Lane, Atherton CA 94027 tel 321-3071 and Richard L. Meehan 777 Hermosa Way, Menlo Park, CA 94025, Adjunct Professor Stanford Univ tel 650-323-0525 rlmeehan@stanford.edu

Methane gas ventings in the, Fairfax District of Los Angeles resulted in the explosion of a Ross Dress for Less department store in 1985, and the evacuation of several buildings in 1989. The Fairfax District overlies part of the old Salt Lake oil field and is about one-half mile from the LaBrea Tar Pits. The oil field, once developed by more than 400 wells, was largely abandoned prior to being redeveloped by slant drilling starting in 1962. Since then production of oil, salt water and gas has been continuous with the water being reinjected into the field since 1980. The disposal reinjection was into a block adjacent to te Third Street fault which projects to the surface near the surface venting sites. Injection was at surface pressures of up to 770 psi  giving rise to a gradient of about 0.7 psi/ft within the subsurface near the point of injection. We conclude that this resulted in episodic fracturing of the Third Street fault.

The fault acting as a valve structure, was temporarily jacked open by the local fluid overpressure and while open served as a conduit for escape of pressurized methane to the near surface. With depressurization the fault conduit would collapse and the venting cease. Similar effects of fault activation by fluid pressure excursions have been demonstrated in connection with the 1963 failure of Baldwin Hills dam in the Inglewood field, with seismicity triggering elsewhere, and also in geological paleoeffects preserved in vein structures.

Clearly the phenomenon of methane venting in the urban environment can be hazardous, especially if no provision has been made to control it at the surface. But we propose that an adequate response to this hazard should include developing an integrated understanding of both surface and subsurface condifions, starting, where oil field activity is involved, with the geology and operations within the producing zone.

Other old oilfields such as the Los Angeles field have not been subject to artificial repressurization and do not present the same explosive and ground rupture hazards which accompanied reinjection of wastes at the Salt Lake and Inglewood fields. Potentially hazardous migration of gas to the surface at these locations would be controlled by diffusive or natural repressurization processes operating in conjunction with primary or secondary permeability characteristics of the upper few thousand feet of rock or via abandoned wells.

Some illustrations follow: