Field Studies on Occurrence, Recoverability, and Mitigation Strategy for Free-Phase Liquid Hydrocarbon

Stephen M. Testa, Eric A. Hodder

Numerous petroleum handling facilities such as refineries, bulk liquid storage farms, and underground storage tanks and pipelines pose subsurface environmental risks associated with leaked hydrocarbons. The designation of a facility as a potential health hazard reflects petroleum residues from such facilities that migrate through the subsurface resulting in the presence of free-phase liquid hydrocarbon (FPLH) pools as perched zones on the capillary fringe. These pools serve as a source for soil and ground water contamination, and, potentially, hazardous vapors.

Field studies on the occurrence and recoverability of several FPLH pools of leaked refined product beneath an active refinery site has been investigated and evaluated over a period of six years. The refinery site is underlain by unconsolidated, stratified, laterally discontinuous fluvial deposits comprising the Lakewood Formation of southern California. Six separate FPLH pools of dissimilar product types and comprising an estimated 573,000 bbl of refined product in total volume occur under shallow perched and water table conditions. Product types include gasoline, diesel, feed stock, and mixed heavy oil.

Recoverable product volumes are assumed to range up to 30% using conventional techniques. Over 192,000 bbl of FPLH has been recovered to date via two primary but different recovery and mitigation strategies. In relatively high permeability zones, four two-pump recovery systems are employed. Water produced is reinjected untreated into the aquifer from which it was originally withdrawn through a network of injection wells. The injection wells serve to create a barrier to offsite migration, enhance the hydraulic gradient toward a recovery well, and dispose of water that would otherwise require treatment and subsequent discharge. In relatively low permeability areas, a network of one-pump recovery wells are employed in which water and product generated are routed to an oil/water separator, then to on-site waste-water treatment facilities. Two-pump recovery systems, although frequently overused, have enhanced recovery and increased the overall effectiveness of the mitigation effort. Within the same time frame, it is estimated that overall effectiveness would have been significantly reduced with an estimated total volume of only 25,000 bbl of product being recovered without use of two-pump system.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.