MONITORING, VERIFICATION, AND ACCOUNTING OF GAS MICROSEEPAGE TO THE SURFACE IN A LARGE-SCALE CARBON DIOXIDE-EOR PROJECT

The success of geologic carbon dioxide sequestration is dependent on long-term storage and monitoring, verification, and accounting (MVA) of injected carbon dioxide and other gases that might be entrained in the reservoir fluid. This study of the mature Rangely, Colorado EOR project determined carbon dioxide and methane gas flow rates to the atmoshpere, verified a deep-source for the methane, a shallow environmental source for carbon dioxide and a first-order accounting of the microseepage rates through the surface into the atmosphere. The geochemical processes operating were further verified using stable isotopes of carbon and carbon-14.

Seasonal surface flux measurements and shallow soil gas measurements were made in initial surveys, which were used to select locations of 10-m deep holes for the determination of shallow concentration gradients of gases. The accounting of gas microseepage estimated deep-sourced carbon dioxide seepage to the surface at <170 tonnes/year over the 78 km² area of the field. The microseepage of deep-sourced methane was estimated at 400 and 700 tonnes/year, respectively, by two different methods. The 23 Mt (2002) or 28 Mt (2010) of carbon dioxide that is stored in the Rangely reservoir is as solution sequestration with 99% as aqueous carbon dioxide and 1% as bicarbonate ion.