Molecular dynamics simulations of brine-surfactant lamellas surrounded by nitrogen at different reservoir conditions

In this work the stability of a thin brine lamella (13% by weight sodium chloride in water) is studied when surrounded by surfactant (sodium olefinsulfonate), under different burial depths and in the presence of nitrogen. We are assuming a temperature gradient of 30 K/km and a pressure gradient of 150 atm/km and covering up to 6 km. As a reference, the same system without surfactant is also studied. The density profiles obtained for the case without surfactant show that nitrogen easily penetrates the lamella at depths greater than 1.0 km. Conversely, the system having the surfactant keeps nitrogen out of the lamella for depths up to ≃5.0 km. We found that there appears a separation of charges along the direction perpendicular to the interfaces which, we think, promotes the lamella affinity for water and hinders its drying. This process, acting at the molecular space time level, may compete with the large scale gravitational draining. At 6 km of burial depth, we observed nitrogen getting into the protected lamella, which in turn provokes a dramatic decrease of the surface tension. We think this points out a superior limit for the withstand of the lamellar structure, and hence, the foam stability.