Patent ID: 7134503

Claim:
A wellbore pump for use in a weilbore having cylindrical walls, a lower end and an upper end, a pressurized fluid being adjacent the lower end of the wellbore; the wellbore pump comprising: a plunger having an upper plunger end, a lower plunger end, and cylindrical plunger walls, the cylindrical plunger walls having an external surface facing the cylindrical walls of the wellbore, the plunger being movable within the wellbore from the lower end to the upper end of the wellbore; a passage in the plunger to permit the pressurized fluid to pass from below the plunger to above the plunger within the wellbore; a stem extending through the plunger to an upper stem end and being movable with respect to the plunger from a lower position to an upper position, the upper stem end being above the upper end of the plunger when the stem is in the upper position; an enclosed chamber having a gas therein; a piston movable with respect to the chamber for causing the volume of the chamber to increase and decrease in size and thereby change the pressure of the gas within the chamber; a valve movable with respect to the plunger from an open position permitting the pressurized fluid to pass from below the plunger through the passage to above the plunger and a closed position preventing fluid from passing from below the plunger through the passage to above the plunger; the valve, the piston and the stem being movable in unison between the lower position wherein the valve is in the open position to the upper position wherein the valve is in the closed position; the gas pressure within the chamber resisting movement of the valve, the piston and the stem from the lower position to the upper position and yielding to movement of the valve, the piston and the stem from the lower position to the upper position in response to a predetermined force of the fluid acting in an upward direction on the piston.