Field Demonstration of the Surfactant/Foam Process for Aquifer Remediation

Abstract

Field Demonstration of the Surfactant/Foam Process for Aquifer Remediation G.J. Hirasaki, SPE, C.A. Miller, SPE, R. Szafranski, D. Tanzil, SPE, and J.B. Lawson, SPE, Rice University, H. Meinardus, M. Jin (SPE), J.T. Londergan, and R.E. Jackson, Duke Engineering and Services, and G.A. Pope (SPE) and W.H. Wade, University of Texas Abstract The first field demonstration of the surfactant/foam process for removal of DNAPL from a heterogeneous alluvial aquifer was conducted during the spring of 1997 at Hill Air Force Base in Utah. The surfactant solution was designed to mobilize and solubilize the contaminant, which was located in the lowest part of the aquifer. During the demonstration, air was injected to form an in situ "foam" in the zones of highest permeability, the purpose being to divert surfactant solution to zones of lower permeability and thereby improve the efficiency of the removal process, as compared to continuous surfactant injection without foam generation. The process was successful in reducing the average DNAPL saturation of the swept pore volume to 0.03%. Introduction and Summary The EPA defines a DNAPL (Dense NonAqueous Phase Liquid) site as "a site where DNAPL has been released and is now present in the subsurface as an immiscible phase", i.e., either free-phase and residual DNAPL or simply residual DNAPL alone. Residual DNAPL is that immiscible liquid trapped by capillary forces within the pore spaces of the sand and silt that comprise the aquifer system. In the saturated zone, it consists of discrete drops or ganglia and is immobile. In contrast, by definition, the free-phase DNAPL is present along continuous pathways through the aquifer and is mobile. The accumulation of DNAPL in an aquifer is a persistent source of contamination that cannot be remediated by the traditional method of "pump and treat". Even if it was possible to detect and produce all of the free-phase DNAPL, the aquifer will continue to be contaminated by the residual DNAPL. Thus, complete remediation will require removal of all of the DNAPL, including residual DNAPL and unswept DNAPL due to aquifer heterogeneities. Surfactant-enhanced aquifer remediation (SEAR) is a promising technology for removal of DNAPL because of its history of recovering residual oil that remains after waterflooding. A common problem with surfactant flooding, both for recovery of petroleum and in aquifer remediation, is the effect of heterogeneities on the performance of the process. The effect of heterogeneity is mitigated by application of mobility control. Mobility control in surfactant flooding has been accomplished by addition of polymer, generation of a viscous microemulsion, and in situ generation of foam by injection of gas. The first field demonstration of the surfactant/foam process for removal of DNAPL from heterogeneous alluvial aquifers was conducted during the spring of 1997 at Hill Air Force Base (AFB) in Utah. The surfactant solution was designed to mobilize and solubilize the contaminant, which was located in the lowest part of the saturated zone of an alluvial aquifer contained in a buried paleo-channel eroded into thick clay deposits. The clay provided a capillary barrier to contaminant migration. During the demonstration, air was injected to form an m situ 'foam' in the zones of highest hydraulic conductivity or permeability, the purpose being to divert surfactant solution to zones of lower conductivity and thereby improve the efficiency of the removal process as compared continuous surfactant injection without foam generation. The demonstration was conducted in a 6.1-meter (20-foot) line drive well pattern with three injection and three extraction wells spanning the width of the buried channel (approximately 3.7 meters, or 12 feet). Hydraulic conductivity ranged from 10–4 m/s (permeability: 10 darcy) to more than 10-–3 m/s (permeability: 100 darcy) with the contaminated zones near the bottom of the channel being in the lower portion of this range. The bottom of the buried channel was some 13.7 meters (45 feet) below the ground surface. The contaminant itself contained approximately 70% trichloroethene (TCE), and smaller amounts of other solvents and dissolved greases.