Influence of subsurface heterogeneity on the performance of aquifer storage and recovery in the Denver Basin

Abstract

Aquifer storage and recovery (ASR) is a process through which water is injected into an aquifer for storage and recovered for later use. As water demand worldwide increases there is a growing need to evaluate alternative approaches to water storage, including ASR. Increasing our understanding of the fate of injected water and the subsurface conditions in which ASR is being performed can guide operational choices and decisions on the feasibility of ASR in new regions. Previous evaluations of ASR performance have often assumed homogeneity in the subsurface, overlooking the existence of preferential flow paths created by the combination of transmissive and non-transmissive inter-beds. Because these pathways can influence the lateral transport of water away from injection wells, ASR performance may be impacted. In this study a groundwater flow model within the Denver Basin, Colorado were used to evaluate ASR performance in a heterogeneous subsurface environment. Geologic data in the vicinity of Highlands Ranch, Colorado were synthesized to create heterogeneous, three-dimensional aquifer analogs using multiple-point geostatistical simulation. Flow simulation for these aquifer models was performed to evaluate ASR cycles comprised of injection, storage, and extraction phases, and results were compared to a homogeneous aquifer model. Three metrics were used to assess ASR performance: the extent of hydraulic head changes in the aquifer, fate of injected water particles, and recovery efficiency. Results show that the travel distance of injected water particles was influenced by the presence of heterogeneity and that preferential pathways increase both the variability and maximum distance traveled by injected water particles. Predicted recovery efficiency decreased slightly when heterogeneity was incorporated, while head change extent was far less sensitive to the presence of heterogeneous structures. These results demonstrate not only the influence of aquifer heterogeneity on ASR performance, but also the potential for geostatistical analysis and numerical modeling to be used as tools for planning future ASR operations.