Instances of CO 2 leakage from naturally occurring reservoirs serve as analogues for the potential release of CO 2 from geologic storage sites. This paper summarizes, compares, and contrasts the features, events, and processes (FEPs) that can be identified from these analogues which include both naturally occuring releases, and those asociated with industrial processes such as drilling.The following conclusions are drawn: (1) Carbon dioxide can accumulate beneath, and be released from, primary and secondary shallower reservoirs with capping units located at a wide range of depths; (2) Many natural releases of CO 2 are correlated with a specific event that triggered the release; (3) Unsealed fault and fracture zones may act as direct flow paths for CO 2 from depth to the surface; (4) Improperly constructed or abandoned wells can rapidly release large quantities of CO 2 ; (5) The types of CO 2 release at the surface vary widely between and within different leakage sites; (6) The hazard to human health was small in most cases due to implementation of post-leakage public education and CO 2 monitoring programs; (7) While changes in groundwater chemistry were related to CO 2 leakage, waters often remained potable.Lessons learned for risk assessment associated with geologic carbon sequestration are discussed.

A controlled field pilot has been developed in Bozeman, Montana, USA, to study near surface CO 2 transport and detection technologies. A slotted horizontal well divided into six zones was installed in the shallow subsurface. The scale and CO 2 release rates were chosen to be relevant to developing monitoring strategies for geological carbon storage. The field site was characterized before injection, and CO 2 transport and concentrations in saturated soil and the vadose zone were modeled. Controlled releases of CO 2 from the horizontal well were performed in the summers of 2007 and 2008, and collaborators from six national labs, three universities, and the U.S. Geological Survey investigated movement of CO 2 through the soil, water, plants, and air with a wide range of near surface detection techniques. An overview of these results will be presented.

A new field facility was used to study CO2 migration processes and test techniques to detect and quantify potential CO2 leakage from geologic storage sites. For 10 days starting 9 July 2007, and for seven days starting 3 August 2007, 0.1 and 0.3 t CO2 d−1, respectively, were released from a ∼100‐m long, sub‐water table (∼2.5‐m depth) horizontal well. The spatio‐temporal evolution of leakage was mapped through repeated grid measurements of soil CO2 flux (FCO2). The surface leakage onset, approach to steady state, and post‐release decline matched model predictions closely. Modeling suggested that minimal CO2 was taken up by groundwater through dissolution, and CO2 spread out on top of the water table. FCO2 spatial patterns were related to well design and soil physical properties. Estimates of total CO2 discharge along with soil respiration and leakage discharge highlight the influence of background CO2 flux variations on detection of CO2 leakage signals.

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[1] We evaluate a comprehensive soil CO 2 survey along the San Andreas fault (SAF) in Parkfield, and the Calaveras fault (CF) in Hollister, California, in the context of spatial and temporal variability, origin, and transport of CO 2 in fractured terrain. CO 2 efflux was measured within grids with portable instrumentation and continuously with meteorological parameters at a fixed station, in both faulted and unfaulted areas. Spatial and temporal variability of surface CO 2 effluxes was observed to be higher at faulted SAF and CF sites, relative to comparable background areas. However, d13 C (À23.3 to À16.4%) and Á 14 C (75.5 to 94.4%) values of soil CO 2 in both faulted and unfaulted areas are indicative of biogenic CO 2 , even though CO 2 effluxes in faulted areas reached values as high as 428 g m À2 d À1. Profiles of soil CO 2 concentration as a function of depth were measured at multiple sites within SAF and CF grids and repeatedly at two locations at the SAF grid. Many of these profiles suggest a surprisingly high component of advective CO 2 flow. Spectral and correlation analysis of SAF CO 2 efflux and meteorological parameter time series indicates that effects of wind speed variations on atmospheric air flow though fractures modulate surface efflux of biogenic CO 2 . The resulting areal patterns in CO 2 effluxes could be erroneously attributed to a deep gas source in the absence of isotopic data, a problem that must be addressed in fault zone soil gas studies.