back pressure(The pressure against the face of the reservoir rock caused by the control valves at the wellhead, hydrostatic head of the fluid in the hole, Chokes and piping. Maintenance of backpressure reduces the pressure differential between the formation and the borehole so that oil moves into the well with a smaller pressure loss. This results in the expenditure of smaller volumes of gas from the reservoir, improves the gas-oil ratio, and ensures the recovery of more oil. (Bab GPI) Alexander Demidov)
