Exploring the potential of horizontal wells for Aquifer Storage and Recovery

Aquifer Storage and Recovery (ASR) is a groundwater management technology in which freshwater is infiltrated into the subsurface during periods of abundance and later extracted for use in periods of scarcity. An example application is balancing water supply and demand for agriculture in regions with strong seasonal fluctuations in precipitation or evaporation. Horizontal directionally drilled wells offer advantages over conventional vertical wells. They are suitable in thin aquifers and the drawdown during pumping is spatially distributed and therefore less pronounced. Well fields consisting of multiple vertical wells can be replaced by a single horizontal well, thereby reducing the required above-ground infrastructure. Furthermore, it is expected that horizontal wells are favorable in saline conditions, where the buoyancy effect and dispersion deform the injected freshwater bubble. Such deformations commonly reduce the recovery efficiency, which is the percentage of injected freshwater that can be recovered. There is currently a lack of knowledge on the behavior of a freshwater bubble in a saline aquifer during a horizontal well ASR cycle. An ASR cycle consists of an injection period, a storage period, and a recovery period.

In this investigation, the application of horizontal well ASR is investigated by means of density-dependent, numerical groundwater modeling with SEAWAT. A horizontal well is compared to a system of multiple vertical wells arranged in a line. The concept of Multiple Partially Penetrating Wells is adopted for the vertical wells. The two systems are compared for a variation of hydrogeological conditions and design choices. The investigated hydrogeological conditions include hydraulic conductivity, dispersivity, aquifer thickness, and groundwater salinity. The investigated design choices include pumping rate and well depth in the aquifer. The systems are evaluated based on the recovery efficiency and the maximum pressure induced by pumping. It is demonstrated under what conditions a horizontal well has distinct advantages over a row of vertical wells.