Numerical Investigation of Secondary Capture of CO2 Leakage in Permeable Stratigraphic Layers Within Caprock

Abstract:

One of the primary leakage pathways of concern at geologic CO2 sequestration sites is abandoned wellbores. Abandoned wellbores are prevalent at many potential CO2 sequestration sites as a result of fossil energy exploration and extraction. Permeable stratigraphic layers within the caprock above the primary reservoir but below aquifers may provide secondary capture of CO2 leakage along wellbores. These features are prevalent at many potential sequestration sites. In fact, under-pressurized permeable layers encountered during drilling operations are referred to as "thief zones" due to the loss of drilling fluids when they are encountered. While it is unlikely that CO2 leakage will encounter thief zones, unless the CO2 injection operation leads to damage of the caprock, encountering permeable stratigraphic layers along an abandoned wellbore is a likely scenario. The ability for these permeable stratigraphic layers to act as secondary reservoirs is investigated here. We use the FEHM simulator to perform 3D, multi-phase, multi-component, numerical simulations of CO2 and brine leakage along an abandoned wellbore that encounters a potential secondary reservoir. We perform a parametric study of these simulations varying thief zone permeability, wellbore permeability, and CO2 injection rates to investigate the capture provided by the secondary reservoir. The results of this study provide indications of the potential for secondary reservoirs to mitigate leakage along abandoned wellbores during and after CO2 sequestration operations.