Well tester apparatus

Claim 1

A well testing apparatus comprising: an inner member movable upwardly and downwardly between spaced longitudinal positions within an outer member, said members defining a fluid passageway, said passageway being provided in part by upper and lower bores in said inner member extending respectively above and below a transverse barrier section disposed intermediate the ends of said inner member and in part by a flow path extending externally of said inner member between locations above and below said barrier section ; seal means for preventing fluid leakage between said members during upward and downward movement; valve means on said members responsive to upward and downward movement of said inner member for opening and closing said second mentioned part of said flow passage; and piston means on said inner member below said barrier section sealingly slidable within cylinder means in said outer member, the upper face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in said passageway below said valve means , the lower face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in the well annulus surrounding said members, the transverse cross-sectional area of said piston means being substantially equal to the transverse area circumscribed by said seal means so that the pressures of fluids within said passageway below said valve means act with substantially equal force in opposite longitudinal directions and said inner member is balanced with respect to such pressures .