Back pressure is a negative pressure within a system that “pushes back” the fluid and which must be overcome by the fluid before flow can continue. Back pressure has the potential to reduce fluid flow.
It could arise within a fluid system when there is some form of fluid friction or any form of flow restriction. Fluid friction is created when there is an interaction between the fluid layers and the walls of the pipe. This friction is higher with liquids than with gases since gases can “slide” through the walls of the pipe without clinging to the pipe unlike liquids.
Flow restrictions can come in the form of pipe bends, twists and turns, and any obstruction within the pipe. Any or a combination of these pipe restrictions will exert a back pressure on fluid flow.
Backpressure and Pressure drop
For reservoir fluids to flow from the reservoir, into the production string and all the way to the surface, they must overcome a back pressure exerted by a combination of fluid friction and any other one induced by our choice and configuration of well completions. Pressure will drop each time this back pressure is overcome by the flowing fluid.
There is a relationship between pressure drop and backpressure. See it like the flowing fluid will use up some of its original pressure to overcome the back pressure and thus pressure will drop within the fluid. As pressure drops, it may become necessary to boost the pressure by strategically locating pumps within the flow system.
The presence of back pressure in the flow system will ultimately reduce the maximum flow potential through the pipe or tubing and increase the work that must be done by pumps in pipelines.
Backpressure is not always a disadvantage. Sometimes, sources of back pressure can be deliberately and strategically located within the well completions to serve as a way of controlling sudden pressure surges from produced fluids. These sudden pressure surges could threaten the safety of operations and possibly damage sensitive components. Such wells need a back pressure as a pressure check to control surges.
Artificial lifting is one way of overcoming the effects of backpressure in depleted wells. Backpressure can become a problem in some producing wells nearing the end of their active life (depleted wells). Depleted wells with really low reservoir pressure may not flow at all because of back pressure created in the completion system. Such depleted wells need to be pressure assisted with pumps through a form of artificial lift to overcome the very low pressure and completion imposed backpressure.