Abstract
In California, oil and gas wells that did not produce economically viable amounts of hydrocarbon were usually left uncompleted and quickly abandoned. Over 11,000 of these wells, termed "dry holes," are outside of oil and gas fields. Many of them are in close proximity to shallow groundwater wells. Completion and abandonment practices in these dry holes varied widely. In some cases, the wells were only partially cased, or had casing pulled during abandonment, leaving uncased borehole filled only with drilling mud. Cement plugs were generally placed during the abandonment process, but often limited to critical zones above the hydrocarbon zone and across the base of fresh water (3,000 ppm total dissolved solids (TDS)). Together, these practices have the potential for leaving extents of uncased borehole acting as vertical pathways connecting deeper saline aquifers with shallower and potentially usable groundwater aquifers (10,000 ppm TDS). While the presence of conduits does not necessary lead to migration of fluids, changes in hydraulic head, for example by pumping from shallow groundwater aquifers, could induce migration of higher TDS fluids into fresher waters. Here we investigate the extent of these open borehole present in dry holes outside of oil and gas fields in the San Joaquin Valley of California. For a selection of wells we collect the casing and abandonment history from publicly available well histories to map out the depth extent of uncased boreholes. We then compare the wells to stratigraphy and groundwater TDS estimated by Archie's equation determined from oil and gas borehole geophysical logs. We hope this preliminary study will help groundwater managers assess the risk of these dry holes to groundwater resources.