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Abstract
The early field project of the Southeast Regional Carbon Sequestration Partnership (SECARB) was conducted in Cranfield Field, western Mississippi. Injection was into coarse grained fluvial deposits of the Cretaceous lower Tuscaloosa formation forming a gentle anticline at depths of 3300 m. CO2 injection started in July 2008, growing to ~23 wells with total injection rates greater than one million tons/year. Focused monitoring programs of deep subsurface and near-surface have been implemented at different study areas. Here we present results of the near-surface monitoring program over a 3-year period including shallow groundwater monitoring and soil-gas monitoring. A general methodology for detecting CO2 leakage into shallow groundwater chemistry is proposed. A set of geochemical indicator parameters can be identified based on characterization of groundwater geochemistry over the monitoring area and then further tested and validated with numerical modeling approaches, lab experiments and field experiments. For the soil-gas monitoring, a site (P-site) where there are a plugged & abandoned well, a nearby open pit, and an engineered pad, representing a typical near-surface environment for soil monitoring, was selected for detail study. The site was heavily instrumented with different sensors for measuring soil-gas concentrations at different depths, soil water content, matric potential, and weather information. Three monitoring technologies were assessed: soil CO2 concentration measurements alone, CO2 flux measurements on the land surface, and soil-gas component measurements. The results indicate that soil-gas component measurements provide more reliable information for gas leakage detection. The methodologies of near-surface monitoring developed in this study can be used to improve monitoring CO2 leakage at other carbon dioxide sequestration projects. This early field project is funded by the US Department of Energy, National Energy Technology Laboratory as part of the Regional Carbon Sequestration Partnerships program. SECARB is led by Southern States Energy Board.

1. Introduction
The Southeast Regional Carbon Sequestration (SECARB) partnership is one of seven Regional Carbon Sequestration Partnerships (RCSP) across the United States and portions of Canada The US Department of Energy, began a 10-year phase III program which includes two large volume injection projects in the lower Tuscaloosa Formation, a formation representative of the Gulf Coast wedge: the early test and an anthropogenic test (Hovorka et al., 2011; Litynski et al., 2009; Rodosta et al., 2011). The “Early Test” began in July 2008, led by the Bureau of Economic Geology and linked to a new CO2–EOR project conducted by Denbury Onshore LLC at Cranfield Field in western Mississippi. The Cranfield site is about 15 miles east to Natchez, MS. Tuscaloosa oil and gas production at Cranfield began in 1944 with drilling of wells in the oil rim below a large gas cap at the top of the structure (Mississippi Oil and Gas Board, 1966). By 1966, nearly all the wells were plugged and abandoned and the Tuscaloosa reservoir was idle and in pressure recovery until Denbury began injection for the EOR project.