Reduction of Surfactant Adsorption: A Study of Dynamic Surfactant Adsorption/Desorption in Shaly-sandstone Reservoir

Chemical Enhanced Oil Recovery (CEOR) has been implemented in many oilfields in several past decades. The techniques may be costly but they yield good benefits to specific type of reservoir. Sirikit (S1) oilfield, located in the north of Thailand, is one of the good examples that could positively response to the techniques based on screening criteria for CEOR. Surfactant flooding, which is one of the CEOR techniques, may be the most suitable for Sirikit oilfield as it can lower the interfacial tension (IFT) between oil and injected water to the ultra-low condition and hence, oil can be liberated as emulsion form. Fluid flow abilities are improved and at the same time, residual oil saturation is greatly reduced. Following the EOR master plan for S1 field, laboratory study has been carried out to determine chemical formulation that is suitable for S1 crude and formation water. It was, however, discovered recently that surfactant loss due to adsorption could turn the surfactant flooding project in Sirikit oilfield ineffective and uneconomic.

From the study, the cause of high surfactant adsorption is revealed. As reservoir formation in Sirikit oilfield is sandstone but it contains high portion of illite and kaolinite, surface charge of rock is found to be positive instead of negative, coming from numbers of calcium and magnesium ions bound with extremely large surface of clays. Depletion of surfactant therefore, occurs through the precipitation of anionic surfactant with these divalent ions. Based on mechanism of surfactant loss, the suggestion to reduce surfactant adsorption is made. Sodium hydroxide which is a strong base is recommended to co-inject with anionic surfactant in this case as strong base can quickly provide negative charge to neutralize positive charge of clay surface. However, higher concentration of strong base will result in electrolytic force that eventually causes surfactant monomers to adhere onto rock surface, causing higher possibility for surfactant adsorption. Form the experiment, reducing concentration from 0.5 to 0.1% by weight helps decreasing retaining surfactant onto rock surface from 0.90 to 0.38 milligram per gram of rock. Sodium carbonate which is a moderately strong base can also reduce surfactant adsorption but due to weaker charge properties compared to sodium hydroxide, surfactant monomers tend to permanently adsorb onto rock surface, resulting in lower degree of desorption and as a consequence, retaining adsorbed surfactant onto rock surface is high.