In an embodiment of the invention, a down-hole pump comprises a hydraulic chamber having a passage for fluid communications with a hydraulic conduit, a produced fluid chamber having an inlet and an outlet, a first check valve associated with the inlet, a second check valve associated with the outlet, a stored energy unit, a piston, having one side exposed to the stored energy unit and a second side exposed to the hydraulic chamber and a displacement member projecting from said piston into the produced fluid chamber. Additional embodiments and aspects, including embodiments for power units as well as system and method aspects, are also disclosed.