Faults as Windows to Monitor Gas Seepage: Application to CO2 Sequestration and CO2-EOR

Monitoring of potential gas seepage for CO2 sequestration and CO2-EOR (Enhanced Oil Recovery) in geologic storage will involve geophysical and geochemical measurements of parameters at depth and at, or near the surface. The appropriate methods for MVA (Monitoring, Verification, Accounting) are needed for both cost and technical effectiveness. This work provides an overview of some of the geochemical methods that have been demonstrated to be effective for an existing CO2-EOR (Rangely, CA, USA) and a proposed project at Teapot Dome, WY, USA. Carbon dioxide and CH4 fluxes and shallow soil gas concentrations were measured, followed by nested completions of 10-m deep holes to obtain concentration gradients. The focus at Teapot Dome was the evaluation of faults as pathways for gas seepage in an under-pressured reservoir system. The measurements were supplemented by stable carbon and oxygen isotopic measurements, carbon-14, and limited use of inert gases. The work clearly demonstrates the superiority of CH4 over measurements of CO2 in early detection and quantification of gas seepage. Stable carbon isotopes, carbon-14, and inert gas measurements add to the verification of the deep source. A preliminary accounting at Rangely confirms the importance of CH4 measurements in the MVA application.