Analysis of Fluid Transport Under High-Volume Fluid Injection Involving Abandoned Wells

Abstract

There are over three-million abandoned oil and gas wells in the U.S., the majority of them are unplugged and unrecorded according to an estimation of US Environmental Protection Agency (U.S. EPA). If the abandoned wells present in adjacent to high-volume injection wells, they could allow injected waste fluid to leak from confined formations. Assessment of the fluid transport process when both injection and abandoned wells present could provide insight into the environmental risk and well abandonment regulation. In this study, waste fluid injection is simulated using MODFLOW to evaluate the fluid transport process and the leakage rate in the abandoned wells. A three-layer conceptual model with one injection and one abandoned well is examined in Visual MODFLOW. The abandoned well is simulated as grids with high hydraulic conductivity. Six water budget zones are defined, and the fluid exchange rate among these zones are calculated by simulating the numerical engines under different conditions. These results indicate that fluid leakage is negatively correlated with the distance between two wells. It is also positively correlated with the injection rate, the abandoned well diameter, and the hydraulic conductivity ratio of two geological formations (the injection layer and confining layer). The correlations could be explained by the power function and exponential function with high goodness-of-fit. The two-term fitting functions yield a coefficient of determination greater than 99.5% for any established correlation. On the other hand, if the abandoned well does not have openings in the injection layer, the fluid leakage will be significantly reduced. The results supplement the previous analytical solutions on the leakage problem by identifying extra parameters that would influence the fluid leakage rate and provide more details on the leakage pathways. The research could provide more understanding of the leakage of injected fluids through the abandoned well, which reveal the risk of contamination caused by improper well abandonment and injection.