Fundamental Reservoir-Engineering Principles Used To Explain Lenses of Shoreline Residual Oil 20 Years After the Exxon Valdez Oil Spill

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 141809, ’Fundamental Reservoir-Engineering Principles Explain Lenses of Shoreline Residual Oil 20 Years After the Exxon Valdez Oil Spill,’ by Gary Pope, SPE, University of Texas at Austin; Kim Gordon, SPE, Platt, Sparks & Associates; and Jim Bragg, Creative Petroleum Solutions, prepared for the 2011 SPE Americas E&P Health, Safety, Security, and Environmental Conference, Houston, 21-23 March. The paper has not been peer reviewed. Twenty years after the Exxon Valdez spill, weathered residual oil can be found in small isolated lenses of subsurface sediments of a very small fraction of Prince William Sound (PWS) shorelines. Fundamental reservoir-engineering principles and tools can be applied to understand and explain how oil entered the subsurface, why most (but not all) of it was washed out by tides and occasional storms, and why and under what conditions some of it persists. Introduction In 1989, the Exxon Valdez oil spill released 10.8 million gal of Prudhoe Bay crude oil into PWS that subsequently was dispersed into marine waters or washed onto Alaskan shorelines that included the coastline and small-island shorelines within PWS (Fig. 1). More than 70% of the PWS shoreline consists of steep cliffs and impermeable-bedrock outcrops where oil did not penetrate the subsurface. Following the spill, extensive cleanup efforts included active techniques such as high-pressure washing and physical movement of sediments, including berm relocation, and more-passive techniques including bioremediation. By 2007, the US National Oceanic and Atmospheric Administration (NOAA) estimated that less than 0.3% (27,000 gal) of oil remained in small isolated subsurface deposits. The few sites where oil remains often are associated with freshwater outwashes where highly weathered residual oil is trapped in low-permeability sediment below a more-permeable cobble/gravel/boulder veneer or is sheltered by bedrock outcrops. Current Conditions Natural attenuation by wave action and by chemical and biological processes has removed almost all of the oil from both the surface and subsurface. However, very small amounts of oil residue remain below the surface in a few small patches on unusual beaches along a very small fraction of the total shoreline. Hydraulic factors influencing the persistence of these isolated patches were observed in the field where oil residue remains and where it does not. On shorelines where subsurface oil residue persists, it is sheltered from wave action by outcrops and boulders or it is trapped in heterogeneous low-permeability sediments that often are associated with overlying high-permeability sediments that provide preferential flow for groundwater (i.e., water within sediment pores). Because flow velocities are extremely low within the low-permeability sediments, physical removal of the oil by hydraulic forces does not occur. However, the current highly weathered state of the oil indicates that natural attenuation is ongoing within these low-permeability sediments. This residual oil consists of a large number of components that are either insoluble or only slightly soluble in seawater.