FAIRBANKS, PAUL E., Amoco Oil Company, Salt Lake City, UT

ABSTRACT: Enhanced Hydrocarbon Migration through Fractures in a Silty Clay

Subsurface hydrocarbon migration is generally limited in silty/clayey soils of the saturated/unsaturated zones, but may be greatly enhanced by fracture and bedding plane structures. This paper presents a case study of enhanced migration of hydrocarbons in a silty clay matrix originating from a pipeline rupture. The migration moved approximately 100 horizontal feet in a time period of less than one week, discharging into an adjacent canal. Upon initiation of preliminary subsurface investigations, hydrocarbons were observed flowing along visible fracture planes. The fractures have subsequently been studied with regard to origin, orientation, and configuration. Gathered data include: (1) observations that typical hydrocarbon staining is limited to fracture walls, (2) near immediate chang in discharge flow of hydrocarbons to the canal following the installation of a cutoff trench, (3) a significant decrease in the rate of recovery during intermittent pumping from a recovery trench and (4) a significant decrease in product thickness in monitoring wells adjacent to the cutoff trench following installation of a continuous pumping system. Based upon this gathered information there appears to be an enhanced flow rate, and a limited available storage capacity within the soil matrix, indicative of fracture flow of the hydrocarbon. This data is invaluable in the determination of remediation strategy. Specifically, we may anticipate (1) Large recovery capacity, with small volume yield over a relatively short period of time, (2) Limited volume of soils matrix to be remediated, alo g the walls of the fractures, a relatively short time frame, and high flow conditions following de-watering of the saturated zone, favorable to current venting technology.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.