Wettability is a key parameter that affects the petrophysical properties of reservoir rocks. The objective of the present work is to investigate the influence of temperature and pressure on the wettability of reservoir rocks. An experimental method for the measurements of contact angle at elevated temperature and pressure has been developed, in which a Pendant Drop Interfacial Tension Cell was modified. Experimental results of contact angle and interfacial tension for two different crude oil-brine-quartz/calcite and mineral oil-distilled water systems over a range of temperatures and pressures are reported. Contact angle for the systems studied increased with pressure, increased with temperature for sandstone system and decreased with temperature for carbonate system. Introduction Wettability has been defined as the relative ability of a fluid to spread on a solid surface in the presence of another fluid. Knowledge of wettability is important for the estimation of oil reserves and for the prediction of production performance. It affects the distribution of water, oil, and gas within a reservoir rock, which in turn affects the displacement behavior and relative permeability characteristics. Wettability is also important to the success of enhanced oil recovery operations. Even though wettability is considered to be a key factor affecting oil production and enhanced oil recovery, almost all of the available data on wettability of core sample, contact angle and interfacial tension for crude oil-brine systems are for room temperature and pressure. The effects of temperature and pressure on floe wettability of reservoir rocks are not well understood. Several methods have been proposed for the measurement of wettability contact angle method is used for measuring floe wettability of a specific surface. Contact angle measurement can be achieved by several different methods, such as tilting plate method, sessile or pendant drop method, vertical rod method, tensiometric method, cylinder method, and capillary rise method (Adamson, Johnson and Dettre, Good, Neumaton and Good, Popiel and McCaffery). Pendant drop method, first suggested by Worthington, was initially used for measuring the interfacial tension between two immiscible fluids. In this method, interfacial tension is calculated using the empirical equations developed by Andreas et al. along with the measurements of drop shape. The apparatus used for interfacial tension measurement by pendant drop method was further developed and modified to measure the contact angle of liquid-rock systems. McCaffery presented an apparatus for measuring interfacial tension and contact angle at elevated temperature and pressure. He reported the measurements of interfacial tension for n-dodecane/water and n-octane/water systems, and contact angle for refined oil/brine/quartz systems. Hjelmeland used this method and reported measurements of interfacial tension and contact angles for stock tank oil, recombined reservoir oil and brine systems. Previous studies about the effects of temperature and pressure on interfacial tension and contact angle indicate that observed trends will depend on the systems studied. This phenomenon has not been well explained until now and the factors influencing wettability of reservoir rocks have not been well understood. P. 117

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