EXTENT OF OPEN ANNULAR SPACE IN OIL AND GAS WELLS IN THE COALINGA OIL FIELD, FRESNO COUNTY, CALIFORNIA

Wells drilled for oil and gas production and enhanced recovery injection are generally constructed by placing casing within a borehole, with the open annular space between the casing and the surrounding rock filled with some amount of annular cement to prevent vertical fluid migration. In modern wells, the annular space is filled completely with cement to prevent vertical migration of hydrocarbons into groundwater resources. In contrast, early well completion practices used only a small amount of cement, generally aimed at preventing water infiltration into oil resources, thereby leaving large amounts of uncemented annular space.

Here we present a case study of a cross section in the Coalinga Oil Field, located in Fresno County, California. This field has been classified as a high potential risk to groundwater in a preliminary study in part due to small vertical separation between hydrocarbon reservoirs and groundwater resources. By characterizing the age and extent of annular cement we aim to better understand and identify potential pathways of vertical fluid migration in the field.

To do this we estimated the extent of annular cement from casing and borehole length and diameter, cement and additive volume, and depth of emplacement collected from publicly available well histories. Cement volume was estimated by applying a multiplier to reported volumes of cement and additives. Annular volume was estimated based on the volumetric difference between the borehole and the outer casing. The extent of cement within the annular space was then calculated by volumetric differencing. Calculations were validated against any available operator-reported top of cement calculations. Prior to the mid-1960s, long extents of open annular space were common, and a number of wells completed lacked any annular cement. While there is some variability in wells cemented after the mid-1960s, most are more consistently cemented. This suggests that older wells, including those that are considered plugged and abandoned, may be potential conduits for vertical fluid migration.