Wettability Of Reservoir Minerals By Flotation And Correlation With Surfactant Adsorption

Abstract

Abstract Minerals can undergo significant alterations in wettability upon adsorption of surfactants on them. A method that appears promising for measuring wettability is flotation using air or oil extraction. Unlike contact angle, it does not require such drastic treatments as polishing. Good correlation was obtained in the present study between wettability as determined by present study between wettability as determined by flotation and adsorption density. Both air flotation and adsorption exhibit maxima in the same concentration range. Zeta potential of particles measured under the same conditions, however, shows a continuous increase in its negative value with sulfonate concentration. Correlation of these data suggests that sulfonate monomers adsorb in increasing amounts below as well as above the critical micelle concentration but with a reverse orientation. There is however a decrease in measured surface excess, possibly due to such factors as precipitate redissolution and micellar exclusion. precipitate redissolution and micellar exclusion. A combined approach using adsorption, wettability, zeta potential and surface tension shows promise as a valuable method for the study of mechanisms of interaction of surface active agents with minerals. Introduction Information on wetting characteristics of reservoir rocks and changes in them is of much use for studying the performance of oil recovery systems. In micellar flooding systems adsorption or precipitation of surfactants on rock surface can change the wettability of the rock and in turn the oil recovery from it. Wettability is usually determined by means of contact angle techniques. This technique however requires preparation of the coarse mineral sample by means of polishing after the samples are also mounted on plastics and polished and then cleaned using various solvents and leachants. All the above pretreatments of the mineral samples can be expected to affect the surface roughness as well as surface composition of the sample. Smearing of the mineral surface by contaminants during polishing can also be of significant effect. On the other hand, removal of bituminous or organic coatings that have a governing role in imparting oil-wettability to rock surfaces during polishing can also lead to misleading results that are polishing can also lead to misleading results that are not representative of the original samples. Equally important alterations in surface regosity can produce changes in both the equilibrium contact angle and the nature of hysteresis that is important in determining the residual oil saturation. Even though ultralow interfacial tension is often considered as the criterion for the displacement of oil, it is to be noted that one cannot really isolate the ultralow interfacial tension criterion from that of a low contact angle criterion since those two are related to each other by the Young's equation: (1) (2) where the upsilon OS, upsilon WS, and upsilon OW are respectively interfacial tensions of oil/solid, water/solid, and oil/water interfaces and theta is the contact angle measured through the aqueous phase. Furthermore, an oil-displacement mechanism dependent on a "detergency-type" behavior cannot yet be ruled out. Wettability together with interfacial tension plays a major role in achieving the displacement of oil from the pores of the reservoir rocks. Both of the above properties can be expected to be affected by the addition of surfactants. For example, recovery of oil from silica systems is reported to increase through a wettability reversal possibly due to adsorption of amine as well as possibly due to adsorption of amine as well as reduction of oil/water interfacial tension. The governing mechanisms by which surfactants enhance the displacement are not however fully understood. It is known that information on variation of wettability of minerals as a function of surfactant concentration when combined with data for such interfacial properties as adsorption and zeta potential can provide an insight into interfacial mechanisms that govern processes such as imbibition and drainage or displacement. In order to develop an accurate understanding however it is noted that it is necessary to determine the above properties using similar mineral systems. Both adsorption and zeta potential are determined using dilute suspensions of mineral fines. P. 263