REDUCTION OF SURFACTANT ADSORPTION BY ALKALINE SUBSTANCES IN CARBONATE RESERVOIR

Abstract

Surfactant flooding is a chemical enhanced oil recovery process in which small amount of surfactant is added to injected water. The technique is performed to reduce interfacial tension between oil and water to ultra-low condition where oil and water phases are linked together as surfactant molecule composes of both hydrophilic and hydrophobic parts. Oil is then liberated in flowing water in a form of emulsion and residual oil saturation can be further reduced to nearly zero in appropriate conditions. However, surfactant loss due to adsorption onto rock surface is one of the major concerns as surfactant compounds are generally expensive chemical. A polar part of surfactant is attracted by opposite charge of rock. This problem weakens the effectiveness of the injected surfactant slug in reducing interfacial tension between oil-water phases and consecutively causes the process to be uneconomical. However, surfactant adsorption can be minimized. From several studies, alkaline substance can be added in surfactant solution. Alkali which provides strong negative charge will function as a sacrificial agent, competing surfactant in adsorption process and reducing the amount of surfactant adsorption. It is also discovered that surfactant is not permanently adsorbed onto rock surface. Desorption may occur, yielding number of active surfactant monomer to increase. From this study, static adsorption and desorption tests show that sodium carbonate(Na2CO3), which is moderately strong base is recommended to co-inject with sodium dodecyl benzene sulfonate representing surfactant in dolomite sample. Sodium carbonate provides adequately strong charge to compete adsorbing with surfactant onto carbonate surface. At the same time, this base is not too strong to reinforce adsorption of surfactant. It is also found that desorption of surfactant is not a function of adsorbed amount but it is still a function of alkali and surfactant concentrations. Alkali and surfactant concentrations of 0.6% and 0.8% by weight, respectively are discovered to be optimal concentrations. In dynamic adsorption and desorption tests, several parameters related to alkali-surfactant flooding is investigated. The smallest salinity of 10,000 ppm yields the best result as amount of anionic ion is small in the system. The highest injection rate of 0.8 cm3/min, helps preventing adsorption and desorption by reducing retention time between surfactant molecule and rock surface. High temperature of 70oC is more favorable than lower temperature as surfactant is more active and prefers to remain in aqueous phase. Combing the best conditions results in surfactant adsorption of 0.43 mg/g, surfactant desorption of 0.39 mg/g, and surfactant retaining just 0.04 mg/g.