The effect of surfactants on upward air-water pipe flow at various inclinations

In this work, we extend our previous efforts on the effect of surfactants on air-water flow in a vertical pipe by also considering pipe inclinations between 20° (with respect to horizontal) and vertical. For air-water flow, independent of the inclination, there is a regular annular flow at large gas flow rates, and an irregular churn or slug flow at low gas flow rates. Closely related to the transition between regular and irregular flow, although not necessarily coinciding with it, there is a minimum in the pressure gradient as a function of the gas flow rate. In gas wells, surfactants are used to shift this minimum to lower gas flow rates, which allows a stable gas production up to lower reservoir pressures. In this work, we investigate how the pipe inclination affects air-water flow without and with surfactants. Surfactants generate foam, which decreases the density and increases the thickness of the film at the pipe wall. For vertical flow, we previously established that surfactants increase the pressure gradient at high gas flow rates, decrease the pressure gradient at low gas flow rates, shift the minimum in the pressure gradient to lower gas flow rates, and shift the transition between regular and irregular flow to lower gas flow rates. The new results described in this paper show that for large gas flow rates, both the flow with and without surfactants is unaffected by the inclination. At low gas velocities, however, in inclined pipes the surfactants are much less effective at shifting the transition between irregular flow and regular flow and at shifting the minimum in the pressure gradient than in vertical pipes. The foam causes a regular film morphology at the top wall of the pipe, but is unable to make the morphology of the bottom liquid film regular. As a result, at low gas flow rates the relative decrease of the pressure gradient due to surfactants is smaller for smaller inclinations from horizontal. This larger relative decrease for vertical flow compared to inclined flow is related to an increased foam formation and therefore a smaller mass density of the film in vertical flow.