A well pump having a power fluid that is to be reciprocated in a conduit so as to power a downhole pump by filling the downhole pump and the lower portion of the power conduit with a hydraulic oil having a specific gravity greater than water and filling the rest of the power conduit with a less compressible...http://www.google.fr/patents/US4540348?utm_source=gb-gplus-shareBrevet US4540348 - Oilwell pump system and method
