Sucker-rod pumps, like all piston pumps working on the positive displacement principle, are designed to pump an incompressible liquid phase. These pumps, therefore, provide very efficient fluid lifting operations in wells where negligible amounts of free gas are present at pump depth. However, the majority of oil wells produce some gas along with the liquid phases, which can exist at the pump intake either as free or as dissolved gas. Gas that is free at suction pressure drastically reduces the pump's volumetric efficiency, because pump valves do not operate properly during the pumping cycle. On the upstroke, the standing valve's opening is delayed. In addition to this, the barrel only partially fills with liquid, since gas occupies some of the barrel space. On the downstroke, the traveling valve is kept closed for a portion of its downward travel by the fluid load from above. All these effects decrease the plunger's effective stroke length, and a considerable reduction of pump displacement occurs. In extreme cases a gas lock can also develop, which completely stops the pumping action.

The detrimental effects of pumping a gaseous mixture with a sucker-rod pump give rise to a multitude of operational problems. These include low pump efficiencies, increased pump and rod failures due to fluid and gas pounding, and losses in liquid production. Accordingly, lifting costs get higher and production economy is considerably decreased. This is why gas interference is considered one of the biggest enemies facing the production engineer working...