Hydraulically driven downhole pump

A pump having a reciprocating annular pump member positioned within a pump chamber, the pump member slidably and sealably mounted around a centrally disposed cylindrical stem mounted at each end with an end wall of the pump chamber, the stem being formed with two or more longitudinally disposed conduits therein, a first conduit being positioned so as to convey pressurized fluid to within the pump member so as to operate the pump, a second conduit being positioned so as to convey exhaust fluid from within the pump member, the stem being formed with an enlarged cylindrical section positioned within the pump member and having means to effect a sliding seal with an inner cylindrical surface of the pump member, comprising:

• said stem being formed with a third longitudinally disposed conduit for conveying fluid to be pumped from the pump chamber upwardly toward the wellhead;
• said third conduit having one or more radial ports extending through the stem's surface within the pump chamber and positioned beyond the axial limit of the pump member stroke;
• a tubular shell sealingly mounted around the stem so as to provide an annular outlet chamber around the stem in communication with the third conduit;
• an end wall of the shell being formed with outlet ports in communication with the pump chamber around the shell so as to receive fluid being pumped into the shell;
• an annular outlet valve of metal or the like mounted and formed so as to cooperate with the end wall of the shell and thereby close said outlet ports and intermittently prevent flow from the outlet chamber through said outlet ports during a suction stroke;
• the outlet valve being of such weight and dimension such that fluid being pumped from the pump chamber will move the outlet valve and enter the outlet chamber during a pressure stroke.