Seismic imaging has been used very successfully to map the location of injected CO2 at the Sleipner and Weyburn Projects (Chadwick et al., 2005; White, 2005). Theoretical calculations by Myer et al. (2003) suggested that the detection limits for cone-shaped accumulations would be in the range of 10,000 20,000 tonnes of CO2. However, based on actual experience, lower limits for detection of CO2 accumulations at the reservoir depth have been estimated to be in the range of several thousand tonnes of CO2. Recently, a VSP obtained at the Frio Brine Pilot demonstrated the ability to detect as little as 1600 tonnes of CO2.The ability to detect small quantities of CO2 that have unintentionally escaped from the storage formation is essential for providing early warning that the project is not performing as expected. The purpose of this study is to assess, from a theoretical and practical perspective, the minimum amount of CO2 that could be detected using surface seismic imaging as it migrated towards the surface. To this end, calculations have been made to assess the seismic response of accumulations of 1,000 tonnes of CO2 at depths of 1300 m, 1000 m, 800 m, and 500 m below the land surface. Sensitivity studies have been conducted to assess the influence of the accumulation thickness and saturation. Sensitivity studies will also be done to determine if even smaller accumulations could be detected at a depth of 500 m or less. In addition, the influence of multi-layer accumulations will also be assessed.

Historically, the presence of methane hydrate has been inferred on the basis of bottom simulating reflectors (BSRs), which are believed to mark the phase boundary between hydrate and the probable underlying free gas zone. However, in the Gulf of Mexico, laterally continuous BSRs have not been observed. The presence of laterally discontinuous vertical migration pathways may be one possible reason for the absence of BSRs in the Gulf of Mexico. To investigate this hypothesis, we use the data from a seismic survey conducted in water depth of about 1500 m. We perform a three parameter AVO inversion on the portion of the 2D seismic line in the vicinity of a salt tongue where gas hydrate accumulations may be concentrated. We identify a possible hydrate accumulation that extends 3000 m laterally from the salt flank. The hydrate reservoir is estimated to be 40 m in thickness and overlays a 20 m transition layer between hydrate and free gas. The total volume of methane gas in the reservoir is in the range of 126 to 252 billion cubic feet.

Observed deformation is sensitive to a changing stress field within the Earth. However, there are several impediments to a direct inversion of geodetic measurements for changes in stress. Estimating six independent components of stress change from a smaller number of displacement or strain components is inherently non-unique. The reliance upon surface measurements leads to a loss of resolution, due to the attenuation of higher spatial frequencies in the displacement field with distance from a source.We adopt a technique suited to the estimation of stress changes due to the injection and/or withdrawal of fluids at depth. In this approach the surface displacement data provides an estimate of the volume change responsible for the deformation, rather than stress changes themselves. The inversion for volume change is constrained by the fluid fluxes into and out of the reservoir. The distribution of volume change is used to calculate the displacements in the region above the reservoir. Estimates of stress change follow from differentiating the displacement field in conjunction with a geomechanical model of the overburden. We apply the technique to Interferometric Synthetic Aperture Radar (InSAR) observations gathered over a petroleum reservoir in the San Joaquin Valley of California. An analysis of the InSAR range changes reveals that the stress field in the overburden varies rapidly both in space and in time. The inferred stress variations are found to be compatible with the documented failure of a well in the field.

Preliminary geochemical and geophysical exploration has begun at the Fort Bidwell Indian Reservation (FBIR) which will lead to siting a 1500 m slim exploration well in the summer of 2005. Unusually enriched helium isotope values, geothermometry in nearby gradient wells and encouraging results from time-delay SP surveys are favorable indicators in the work to date.