Based on our earlier experimental work on the effect of surfactants on air-water flow in vertical pipes with internal diameters of 34 mm, 50 mm, and 80 mm, we create a mechanistic annular flow model for the pressure gradient. The major effect of the addition of surfactants is the formation of foam. We model the formation of foam and its impact on the flow. In the model we consider a gas core and a film at the wall, which consists of a layer of liquid at the wall and a layer of foam between the liquid layer and the gas core. We do not consider entrainment in the model. We developed four closure relations in order to solve the model: (i) for the density of the foam, (ii) for the viscosity of the foam, (iii) for the interfacial friction between the gas and the film, and (iv) for the thickness of the liquid layer at the wall. Subsequently, we solve for the film thickness that yields the imposed liquid flow rate. Comparing the experimental results for the pressure gradient to the results from the model, we observe that in most cases the model can predict the pressure gradient within 25%. Furthermore, the model is able to predict the onset of downwards flow in the film. Therefore, it can predict the transition between annular flow and churn flow. We show that the effect of five different surfactants on the flow is equal, apart from a scaling factor of the concentration, which means that the model can be applied for many different types of surfactants. The scaling factor is an input parameter to the model, which needs to be determined in a small scale experiment.

In this work, we present results of flow visualisation, pressure gradient measurements, and liquid holdup measurements for air-water flow without and with surfactants in vertical pipes with diameters of 34 mm, 50 mm, and 80 mm. The surfactants cause the formation of foam. This foam has a larger volume and a smaller density than the liquid. The larger volume results in a larger pressure gradient at large gas flow rates. At small gas flow rates, the lower density of the foam causes the transition between the regular annular flow regime and the irregular churn flow regime to shift to lower gas flow rates. As a result foam reduces the pressure gradient and the liquid holdup at small gas flow rates. Surfactants are more reduce the pressure gradient more effectively for thinner liquid films at the wall; therefore, they are more effective for small pipe diameters and small liquid flow rates.

From field experience in the gas industry, it is known that injecting surfactants at the bottom of a gas well can prevent liquid loading. To better understand how the selection of the surfactant influences the deliquification performance, laboratory experiments of air/water flow at atmospheric conditions were performed, in which two different surfactants (a pure surfactant, sodium dodecyl sulfate, and a commercial surfactant blend) were added to the water. In the experiments, a high-speed camera was used to visualize the flow, and pressure-gradient measurements were performed. Both surfactants increase the pressure gradient at high gas-flow rates and decrease the pressure gradient at low gasflow rates. The minimum in the pressure gradient moves to lower gas-flow rates with increasing surfactant concentration. This is related to the transition between annular flow and churn flow, which is shifted to lower gas-flow rates because of the formation of an almost stagnant foam substrate at the wall of the pipe. At high surfactant concentration, it appears that the churn flow regime is no longer present at all and that there is a direct transition from annular flow to slug flow. The results also show that the critical micelle concentration, the equilibrium surface tension, the dynamic surface tension, and the surface elasticity are poor predictors of the effect of the surfactant on the flow. ©2016 Society of Petroleum Engineers.

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.

A major problem in the production of natural gas is liquid loading, i.e. the accumulation of liquids at the bottom of a well at low reservoir pressures. To prevent liquid loading, surfactants are injected at the bottom of the well, which changes the multiphase flow in the well tubing such that a smaller gas velocity is required to transport the liquids to the surface. However, no predictive models for the effect of surfactants on gas-liquid pipe flows are available. We present results of systematic experiments on the effect of surfactants on air-water flow in vertical pipes. The surfactants lead to the creation of foam, which has a lower density than water and can be transported by the gas more easily, which leads to a smaller pressure gradient at small gas flow rates. This effect is independent on the type of surfactant used. Furthermore, from the results we obtained a relation between the thickness of the film at the wall of the pipe and the interfacial friction between the gas and the film. Using the knowledge obtained from the experiments, we developed a mechanistic model for air-water flow with surfactants. This model is able to capture the trends in the pressure gradient observed in the experiments. This work is sponsored by NAM, a Dutch subsidiary of Shell and ExxonMobil