a pump having a semi - rigid flexible outer tube with a sealed top and a sealed bottom , check valves at the top and bottom such that the outer tube is in fluid communication with the working fluid , an inner tube concentrically disposed within the outer tube , and a controlled source that applies pressure to expand the inner tube to apply a force to the working fluid in the annulus defined by the inner and outer tubes is used to pump viscous materials and / or slurries from the bottom of small diameter wells . the tubes are made from nonmetallic , elastic materials and can be inexpensively manufactured .