skim pit
An open-air pit into which a mixture of oil and water is pumped and left to separate so that the oil can be extracted.
skim pit: gravity separator for oil-water mixtures
A skim pit is a shallow, open excavation or constructed basin used on drilling sites and in oil extraction operations to separate crude oil from water and solids. When drilling fluid returns to surface carrying entrained oil and formation water, the mixed stream is directed into the pit where gravity, time, and surface tension allow the oil phase to separate and float. Workers or mechanical skimmers then remove the recovered oil from the top while the water layer drains or is pumped away.
The pit itself is typically an earthwork basin with dimensions ranging from 10 to 40 metres on a side, depending on site production rates and the viscosity of the recovered oil. Depth is usually 1 to 3 metres. The bottom is often lined with clay or synthetic membrane to prevent seepage into groundwater, and sloped gently toward a sump point for water removal. In fixed installations, concrete or steel construction replaces earthwork, with proper drainage and containment to meet environmental regulations.
Separation mechanics and variables
The residence time required for adequate separation depends on the oil-water interfacial tension, temperature, and the presence of emulsifying agents in the drilling fluid. Heavier crude oils may separate in 4 to 6 hours; lighter, more volatile fractions may require 12 to 24 hours. Water temperature, salinity, and pH all influence the rate. Surfactants or polymers in drilling mud can slow separation significantly, sometimes requiring chemical treatment or heating to accelerate it.
Skim pits are labor-intensive and have environmental limitations. Residual oil saturation in the water layer can range from 5 to 15 percent by volume even after settling. Volatile organic compounds evaporate from the pit surface, creating air quality and worker safety concerns. Modern operations increasingly replace open skim pits with closed separator vessels and centrifuges, which achieve higher oil recovery, eliminate fugitive emissions, and occupy less real estate on the wellsite.
The term 'skim' comes from the skimmer or skimming ladle, the hand tool or mechanical rake used to remove oily material from the surface. In older terminology, skim pits were also called 'slush pits' or 'reserve pits', though modern usage distinguishes them: a slush pit specifically receives cuttings and spent drilling fluid, while a skim pit is dedicated to oil recovery from returns.