High-resolution characterization of a CO₂ plume using crosswell seismic tomography: Cranfield, MS, USA. (Special Section: Midproject assessment of the SECARB early test at Cranfield, Mississippi.)

摘要

We present the results of a high-resolution time-lapse crosswell seismic survey carried out at a large-scale CO₂ injection pilot located in Cranfield, MS, USA. This dataset, spanning an injector and two monitoring wells, provided a detailed view of the boundaries and internal structure of the injection unit, the Tuscaloosa D/E sand. Time-lapse tomographic processing of one well pair revealed the signature of the injected plume, two zones of decreased P-wave velocity which spatially correspond to higher permeability sections of the reservoir unit. We then used White's model for patchy saturation, combined with secondary information from core and well-log measurements, to convert the imaged changes in P-wave velocity into estimates of supercritical (sc) CO₂ saturation in the interwell region. Our estimate is relatively consistent with existing lithologic and core data as well as independent measurements of scCO₂ saturation derived from pulsed neutron logging. While effective in mapping the zone of CO₂ invasion in the interwell region, several sources of uncertainty still exist in quantifying saturation using crosswell seismic methods, including (but not limited to) the degree of mesoscale mixing or "patchiness" at scales below a seismic wavelength.