Influence of dynamic surface tension on foams

Abstract

In natural gas production, along with gas, a small amount of liquid is produced. Towards the end of a reservoir life, the gas velocity reduces due to a decline in pressure. A low gas velocity in a gas well causes undesirable liquid accumulation (loading) in the production tubing. One of the ways to postpone liquid loading is by injecting surfactants in the well. The agitation by gas/liquid flows causes the surfactants to foam. The actual mechanism of how the surfactant injection which causes deliquification is poorly understood. There is an impetus from the gas industry to develop a mechanistic foam flow model that can predict the flow in the well for different surfactants. This requires identification of the surfactant properties that influence the flow characteristics. Based on a literature study it was concluded that the dynamic surface tension (DST) and equilibrium surface tension (ST) are few characteristic properties of a surfactant that affects the foaming ability. There is no general agreement about the relation between the foamability and the DST. This thesis is formulated to investigate the influence of the DST and the ST on the foaming ability in a customized setup. In gas well deliquification, foams serve the purpose of removing liquids. Therefore, in this work foamability is defined as the liquid content of the created foam. The foamability of different surfactants is tested in a modified Bikerman setup, in which foam is generated by sparging N2 through the surfactant solution. The weight of the produced foam was measured in time. High speed movies were also recorded and analyzed to determine bubble sizes and their velocities in the foam as well as in the bulk liquid. The calculated foam density includes the weight and the foam velocity. DST is measured by the maximum bubble pressure method in a time range of 1 ms to 100 s and the equilibrium surface tension is measured using the du Noüy ring method. The equilibrium surface tension was compared through the corresponding surface pressure, whereas, the DST was compared through the Rosen parameter and the dynamic surface excess concentration. Dynamic surface excess concentration is defined as the surface excess concentration at the time scale of foaming. In the experiments, Sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and polyoxyethylene - 4 lauryl ether (Brij 30) are used as the pure surfactant. In addition a commercial proprietary surfactant successfully applied to deliquify actual gas wells, Trifoam Block 820 (TB820) is also used. The influence of salt is investigated by varying NaCl concentration in a solution with fixed SDS concentration. In the pre-micellar region the foam density increases with the Rosen parameter. However in the micellar region this trend was not consistent for all the surfactants used. An overall comparison showed a logarithmic dependance of the Rosen parameter on the foam density, whereas a linear dependance of the dynamic surface excess concentration on the foam density was observed. In order to obtain a dense foam, the Rosen parameter and the dynamic surface excess concentration should be high. More experiments are needed to determined if these trends are general. This dependance of foam density on DST could assist in choosing the ideal surfactant for a particular gas well deliquification application. A denser foam would potentially be more effective in removing liquids (owing to higher liquid content). The mechanistic foam flow model being developed for real life applications should benefit from this qualitative trend.