Effects of Chemical Structure of Anionic Surfactants on the Wettability of a Carbonate System

Sandeep Gupta, Louisiana State University and Agricultural and Mechanical College


Past studies of surfactants for enhanced oil recovery by wettability improvement, have often categorized surfactants as either non-ionic, anionic or cationic. This research has been done in an attempt to study different surfactants under the same group in greater detail by varying their structures and to classify them on the basis of their abilities to alter reservoir wettability. Two different families of surfactants, both anionic, namely alkyl alkoxy sulfates and alkyl ether carboxylates, have been studied. Surfactants tested in each of these groups are tuned individually by the level of hydrophobicity that they offer by varying the nature of the anionic head group and the number of ethylene oxide and propylene oxide units that are present in their chemical structures. The experimental study is based on the Dual Drop Dual Crystal (DDDC) technique of dynamic contact angle measurement. Ten different surfactants, designed, manufactured and supplied by Sasol, were tested in a Yates oil – Limestone – Yates synthetic brine system at ambient conditions. The concentration levels of the surfactants were kept very low. This enabled substantially increased reaction time for the system to interact with the surfactant. It also helped isolate the effect of interfacial tension reduction and study the wettability alteration, if any, with clarity. Interfacial tension measurements as a factor of time were also conducted to determine its effect over extended periods of time, as opposed to effects of wettability alteration of the system. The surfactant structures were tested in order of decreasing hydrophobicity. Initial experimental results using reasonable concentration levels showed no varying effects between the individual surfactants. However upon considerable reduction of the surfactant concentration, each surfactant showed a variable effect on the oil droplet in terms of the measured dynamic contact angle measured as well as a factor of time. The dimensionless Bond numbers calculated for these surfactants helped quantify the rock fluids interactions by taking into consideration both possible wettability alteration as well as the reduced interfacial tension. It was found that no two extremes of a surfactant in terms of hydrophobicity or hydrophilicity were ideal for the system. The challenge of trying to classify surfactants that are so similar in structure also made way for an alteration in the way results obtained from the DDDC technique are conventionally interpreted. Surfactants have always been a popular choice in the field of enhanced oil recovery. However, no systematic means of classification yet exists that links the structure of a surfactant to its ability to alter reservoir wettability, especially when trying to classify surfactants that all belong to one singular family. This study is a step forward in that direction, to try and create a means of quantifying the effect that a particular structural variance could have on the potential recovery from a reservoir via altered wettability.