Defining Practical Limits of Free LNAPL Recovery by Skimming and High-Vacuum Extraction echniques-a Case Study from the Piedmont of Georgia

摘要

This paper documents the practical limitations of free LNAPL recovery using skimming and high-vacuum extraction at a site in the Piedmont of Georgia. The principal limitation on LNAPL recoverability is attributed to reductions in LNAPL thickness which controls mobility of the free LNAPL. The LNAPL plume area and average thickness in wells declined during a 90-day rising water table period at the test site. Baildown tests were performed at the beginning of the 90-day period to estimate the oil conductivity and transmissivities and estimate multiphase parameters that control fluid saturations, relaive permeabilities, and specific volumes. Using these parameters ,we estimated that 20,000 gal of free LNAPL was smeared into a residual state in response to a 2.3-ft rise in the water-table. During the last third of the 90-day period, two wells were used for 30-day skimming tests, followed by a recovery period, and then 2-hour high-vacuum extraction tests. The production rates for the skimmers were initially commensurate with modeled yields but became ineffective, hence mpractical, after the average LNAPL thicknesses decreased to a fraction of a foot. Recovery of LNAPL by high-vacuum extraction techniques (vacuum truck) produced only about 2.5 gal from two test wells with startintg thicknesses o 0.2 and 1.5 ft. The mobility limitation on skimming is influenced by daily fluctuations in LNAPL thicknesses, which can be quite variable at the end of the smearing cycle. The mobility limitation on vacuum extraction is believed to be adversely influenced by secondary smearing near the well when the water table upcones in resonse to negative pressures above the water table. By testing dueing the rising water-table condition, we could show that the practical limitations on skimming and high-vacuum recovery methods occurred at static equilibrium thicknesses that were significantly larger than the traditional 1/8-inch cleanup criteria.