Abstract: A casing vacuum system which applies a controlled vacuum to the space within the casing of an oil well in order to maintain the gas pressure in the well within a certain range and thereby improve the recovery of petroleum from the well. A preferred embodiment of the casing vacuum system utilizes a vacuum pump (10) driven by a motor (18) to provide a vacuum. A water accumulator (14) protects the vacuum pump from water damage. A condensate accumulator (16) collects the gas drawn from the well and condenses a portion of it into liquid hydrocarbons. A mercury vacuum switch (30) maintains the gas pressure in the well within the optimum range for recovery of petroleum by stopping the vacuum pump when the pressure is near the lower end of the optimum range and starting the vacuum when the pressure is near the higher end of the optimum range.