Patent ID: 7874367

Claim:
A downhole pump comprising: a housing; a piston slidably disposed within the housing, the piston dividing a volume within the housing into a first stage chamber and a second stage chamber; a first stage check valve in a first passage connecting the first stage chamber to an intake area below the pump; a second stage check valve in a second passage connecting the first stage chamber and the second stage chamber wherein the second passage passes through the piston and the second stage check valve is carried by the piston; a discharge check valve in a third passage connecting the second stage chamber and a discharge of the pump; a pressure-operated control valve having an inlet in fluid communication with the first stage chamber and configured to discharge fluid from the pump in response to a pressure in the first stage chamber exceeding a threshold pressure; a shaft coupled between the piston and a reciprocating driving mechanism above the pump wherein the shaft passes through the second stage chamber; and a transfer port defined in the piston, the transfer port having a first end opening into the second stage chamber wherein an outlet of the second stage check valve discharges into the transfer port and an inlet of the discharge check valve draws from the transfer port.