Experimental Analysis and Modeling of PFT and SF6 Transport in Organic Rich Vadose Zones: Implications for Monitoring CO2 Leakage at CCS Sites

Gawey, Marlo R.; Larson, Toti; Romanak, Katherine; Hovorka, Susan; Phelps, Tommy J.

Perfluorocarbon tracer compounds (PFTs) and sulfur hexafluoride (SF6) are commonly proposed tracers for use in carbon capture and sequestration (CCS) leak detection and vadose zone monitoring programs. During a tracer test, tracers are co-injected with supercritical CO2 and monitored in the vadose zone to calculate leakage rates. These calculations rely on the assumption that tracers exhibit "ideal" tracer behavior- i.e. the tracer is conservative, inert and not partitioned into or sorbed onto geologic media. This assumption has been brought into question by lab and field evaluations- e.g. Zhong and Amonette (2012) showed PFT retardation in smectite-rich sediments while Deeds et al. (2000) showed that four perfluorocarbon tracers (C7F14, C8F16, C9F18, C10F18) partitioned differentially into an unidentified LNAPL in contaminated soil in column studies with partition coefficients ranging from 8.8 +/- 0.6 to 71 +/-3. SF6, in contrast, is shown to be an ideal tracer in many different substrates. The objective of this study is to identify substrates and applications where PFT and SF6 behave conservatively. We compare tracer break-through curves for PFT and SF6 in one dimensional flow through columns packed with acid-washed silica beads or calcite plus organic carbon. Based on previous studies, we expect both tracers to behave conservatively in columns packed with silica beads or calcite and but only SF6 to behave conservatively in columns with added organic carbon. Dry nitrogen is used as the carrier gas and constant flow rate (2 mls/min) and temperature (26°) are maintained. A known mass of tracer is injected at the head of the 1 m-long column (1.27 cm diameter) and the effluent analyzed at regular intervals for tracers at picogram levels by a gas chromatograph with an electron capture detector. Preliminary results of laboratory column experiments are consistent with analytical modeling and confirm the conservative behavior of the control tracer, SF6, in columns packed with silica beads, calcite or left empty.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013