From field experience in the gas industry, it is known that injecting surfactants at the bottom of the well can prevent liquid loading. To increase understanding of surfactant selection, laboratory experiments of air-water flow at atmospheric conditions were performed, where two different surfactants (Sodium dodecyl sulfate and Trifoam 820 Block) were added to the water. In the experiments, a high-speed camera was used to visualize the flow and pressure drop measurements were performed. Both surfactants increase the pressure drop at high gas flow rates and decrease the pressure drop at low gas flow rates. The minimum in the pressure drop moves to lower gas flow rates with increasing surfactant concentration. This is related to the transition between churn and annular flow, which is shifted to lower gas flow rates due to the formation of an almost stagnant foam substrate at the wall of the pipe. At high surfactant concentration, it appears the churn flow regime is no longer present at all and there is a transition from annular flow to a regular slug flow. The results also show that both the critical micelle concentration and the equilibrium surface tension are poor predictors of the type and concentration of surfactant required to decrease the pressure drop.