NUMERICAL INVESTIGATION OF WELLBORE METHANE LEAKAGE IN DUAL-DOMAIN POROUS MEDIA AND SUBSEQUENT TRANSPORT IN GROUNDWATER

Three-dimensional, multiphase simulations are used to analyze migration of methane leakage from a hydrocarbon wellbore. The objective is to evaluate the relevance and importance of coupling fast advective transport of methane through fractures with slower diffusive transport in the shale matrix below a freshwater aquifer using a numerical dual-domain mass transfer (DDMT) method, Multiple Interacting Continua (MINC) as implemented in TOUGH2. The conceptual model includes a methane gas-phase leak from a wellbore 20-30 m below an aquifer; multiphase, buoyant transport through shale partially saturated with brine; and, after methane leakage reaches groundwater, multiphase transport under varying lateral groundwater flow gradients. Results suggest that DDMT affects the rate of methane reaching groundwater by (i) long-time secondary storage in less-mobile pore space and (ii) larger methane-plume diameters than those predicted by a one-domain advection-diffusion equation. Compared to models without DDMT, these factors combine to increase methane flow rates by an order of magnitude across the base of the aquifer 100 years after leakage begins. In the simulated aquifer, the persistent, larger diameter gas-phase methane plumes do not migrate with the groundwater gradient. Dissolution of these gas-phase plumes leads to a bimodal aqueous-phase methane breakthrough curve in a simulated water well 100 m downstream from leakage, with peak concentration appearing decades after a one-year pulse of leakage. The major implication is that DDMT in the reservoir can explain newly discovered methane concentrations in water wells attributable to older leakage events. Therefore, remediation of abandoned or legacy wells with wellbore integrity loss may be necessary to prevent future incidents of groundwater contamination.