SPE 121952 Different Approaches to Understand Mechanism of Wettability Alteration of Carbonate Reservoirs Rezaei Gomari, K. A. / SPE, StatoilHydro Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE EUROPEC/EAGE Annual Conference and Exhibition held in Amsterdam, The Netherlands, 8–11 June 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract Wettability is a key parameter when concerned with the flow of fluids in any porous medium. This paper describes the several possible mechanisms of wettability alteration of carbonate rocks using different approaches. The most crucial parameters which have severe impact on wettability alteration such as oleic phase composition, aqueous phase pH, as well as temperature are discussed. At the oleic/aqueous interface, the interfacial tension and its variation with the structure of active surface species, partitioning of species between phases, and temperature are important signs to track the wettability alteration. The effect of the IFT on wettability alteration is revealed with the contact angle measurements on the carbonate rock surfaces. On the other hand at rock/fluids interface, the paper presents the variations in surface tension and adsorption isotherms of surface active species on the rock surfaces which imply the strength of wettability alteration. The degree of wettability alteration and its reversibility due to influence of aforementioned parameters are explained based on desorption of active species from surfaces justifying by several thermo-gravimetric measurements. Introduction A considerable amount of work in the past has been directed towards understanding the mechanisms responsible for oil reservoirs being oil-wet, water-wet or mixed wet 1-3 . Factors that may affect the reservoir wettability to be altered from one state to another include oil composition, brine chemistry, rock surface characteristics, capillary pressure, and temperature. Among these factors, oil composition is known to be key factor as it includes any wettability-altering components such as polar components. The influence of the other factors is either to upward the effect of oil composition in altering the wetting surface or to downward the effect. It is reported that about 50 % of the world proven oil reserves are contained in carbonate reservoirs 4 . The wetting properties of carbonate reservoirs are fundamental to the understanding of fluid flow in all aspects of oil production, particularly during water flooding. The aging of carbonate rock containing water and oil turns the initially water-wet rocks into mixed-wet or even oil-wet which means that oil can adhere to the surface of carbonate rock and it is therefore harder to produce. Most carbonate reservoirs are believed to have mixed wettability or to be oil-wet. Several methods are available to determine a reservoir’s wetting preference. The common methods include measurement of contact angle, and displacement technique consist of Amott test and USBM methods 5 . These measuring techniques subject to some difficulties when apply in carbonate reservoirs. Practically speaking, the heterogeneity nature of carbonate reservoir which originated from wide spectrum of environments in which carbonate are deposited , causes significant impact on wettability measurements hence residual oil saturation , capillary pressure , electrical properties and last but not least the oil recovery. Aforementioned difficulties in measuring carbonate surface wettability lead us to evaluate carbonate wettability in terms of adsorption/desorption of polar components on the rock surfaces using adsorption isotherms or thermogravimetric techniques. In this paper different approaches are compared which bring a light to understand wettability alteration in carbonate reservoirs.