ROLES OF AQUIFER HYDRAULICS IN OPTIMIZING ASR WELL PERFORMANCE

During pilot hole drilling and aquifer isolation zone testing for an intended aquifer storage and recovery (ASR) well in southern California, the existence of a significant, 150-foot-thick perching layer separating two aquifers was discovered. The upper aquifer is unconfined with a static water level depth of 400 feet, and the deeper aquifer is semi-confined with a static water level depth of 700 feet. Based on the data from the borehole, the team generated conceptual designs that would have maximized production from a completed well using either a dual-casing or a discretely sealed and packer-equipped single casing. Either option would have prevented downward flow from the upper to the lower zone. However, due to the high differential in water levels, lower than expected production rates (only 600 gpm), and marginal water quality in some zones, as well as the additional expense over what the local water district had budgeted, the pilot borehole was abandoned.

During the analysis it was determined that, to extract groundwater from the targeted aquifers using a single, unsealed casing completion, pumping water levels would need to be deeper than the lowest static water level in the aquifers. In cases such as this where a large difference in water levels exists, such deep pumping water levels would be detrimental to the long term integrity of the well. In addition to the hydraulic challenges, slightly differing geochemistry may have limited the compatibility of using the well for groundwater recharge.

Unless properly sealed or packed, wells with perforations across multiple aquifer zones under significantly differing pressures may be yielding less water than potentially capable due to the deep aquifers ‘thieving’ water from upper zones. To determine if such a condition exists, and ultimately design and implement a plan to modify wells to optimize recharge or production, a detailed well evaluation including a down-hole ‘spinner’ or ‘flow-meter’ test under static or pumping conditions should be conducted. If a well is found to be completed through two zones such as those found in the pilot borehole of the subject well, sealing, packing, or otherwise isolating the deeper zone could increase injection/production rates in the short term and/or increase the efficiency or longevity of the well in the long term.