Experimental study and modeling of effect of surfactants on liquid loading in vertical pipes

Liquid loading in vertical gas wells with and without surfactant (foam) application is investigated in this study. Critical micelle's concentration, volume of liquid unloaded with time and half-life data are obtained from bench top tests; these are surface tension tests, unloading test and stability tests respectively. Bench top tests were used to infer the optimum concentration of the five surfactants tested namely: anionic, amphoteric I, amphoteric II, sulphonate and cationic surfactants.;The large scale experimental program includes an air-water flow (base case) and an air-water foam flow experiments. Two pipe diameters were considered: 2-in and 4-in. The test facility consists of a storage tank, a mixing section, horizontal inlet line, vertical test pipes with three sections, a return line and two tanks for spent fluid. Pressure gradient and liquid holdup were measured in the middle section of the vertical pipes where the flow is not affected by entry or exit conditions. Visual observation with a high speed camera was used to gain insight into the direction of film fluid flow under annular flow conditions. The direction of the film (upwards or downwards) is an indication of instability (Luo [42]).;Introduction of foam causes the gas velocity at which film reversal occurs to decrease; this shift increases with increasing surfactant concentration and it is more pronounced in 2-in pipe than in 4-in pipe. For both 2-in and 4-in pipes, at superficial gas velocity below 10 m/s, introduction of surfactant lowers the liquid holdup compared to the air-water case, subsequently this results in lower pressure gradient. At superficial gas velocities greater than 10 m/s, the liquid holdup under foam flow in 4-inch pipe is higher than that seen under the air-water case; this is contrary to the observation in 2-in pipe.;In the presence of surfactants, for both 2-in and 4-in pipes, the initiation of liquid loading is delayed. For 2-in pipe, the surfactants reduce the liquid holdup; they also cause an increase in the total pressure loss. A tradeoff is required between the pressure loss the operator is willing to accommodate and the unloading benefit from the use of surfactants. Beyond 10 m/s, pressure drop in 4-inch pipe is lower than that observed in 2-inch pipe; high speed video observation shows that thick roll waves forms on the wall of 2-inch pipe while very thin roll waves form on the wall of 4-inch pipe.;The residual pressure gradient transition criterion of Luo [42] was extended to predict flow regime transition under foam flow. A new transition criterion based on original Barnea [38] transition criterion was developed and successfully used to predict onset of liquid loading under foam flow. Correlations were developed for liquid holdup, foam holdup, fraction of gas trapped in foam and interfacial friction factor under foam flow. These correlations were used in a modified Alves et al. [67] pressure gradient prediction model to predict pressure gradient under foam flow.