Modeling Transient Leakage Signals from Abandoned Wells with T2Well: Effects of Plug Gap Aperture

Abstract

Both known and unknown (unmapped) plugged and abandoned wells are potential leakage pathways for CO2 from geologic carbon sequestration (GCS) sites. Although many abandoned wells have cement bridge plugs installed to prevent leakage, the seal between the cement and the inner casing wall is subject to failure creating a leakage flow path for formation fluids and CO2. In this study, we carried out detailed T2Well simulations of non-Darcy flow of CO2 and brine leakage up the gap between cement and inner casing wall and into the open fluid-filled column of a prototypical abandoned well. The goal of our study was to understand the expected leakage behavior dynamics, leakage rates, and the temporal signals associated with leakage that can be targets of near-surface monitoring. Simulation results show that the leakage of CO2 and brine upward in this system is transient with interesting phase interference behavior as buoyant CO2 flowing upward displaces brine as it flows through the cement-casing gap, into the column of brine, and upward to the top of the well. Oscillatory flows with varying pressure, temperature, and flow rates of CO2 and brine show strong dependence on gap aperture. The temporal patterns of leakage may be detectable with surface and near-surface monitoring approaches. In addition to helping with detecting and locating leaking wells, these transient leakage signals may provide information on the cause of leakage that could inform effective remediation design and execution approaches.