Abstract
In California, more than 11,000 oil and gas wells that did not produce economically viable amounts of hydrocarbons (termed "dry holes") are located outside of established oil and gas field boundaries. Many of these wells were drilled and almost immediately abandoned, often leaving boreholes that were partially or completely uncased and unplugged. While in some cases, plugs were placed across critical zones, such as the base of fresh water, significant extents of open, uncased boreholes remain. These open boreholes may act as vertical conduits for fluid migration, potentially connecting deep, saline aquifers with shallow, usable water. This potential is increased with changes to hydraulic head in shallow aquifers caused by nearby groundwater pumping. To assess the potential for fluid migration, drilling, casing, and plug data were collected for all dry holes outside of oil and gas fields in two key regions in California: Kern County (n=1,297) and the area around Sacramento (n=439). These data were then compared to the completed depths of water wells in proximity to the oil and gas wells. Results show these dry holes have long extents of open borehole between plugged intervals. In some cases, these sections are filled with drilling mud, however in many wells, they are simply left open. Some of these open boreholes extend above the deepest perforation of nearby groundwater wells, leaving open the possibility that groundwater pumping could induce migration of higher salinity fluids into fresher waters.