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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMore than 90% of Brazilian oil and gas reserves lie in offshore fields and over 60% of the total reserves are located in deep and ultradeep waters. Some of the important technical problems associated with deep and ultradeep water drilling involve: i) low formation fracture gradients; ii) long choke lines; and iii) low temperatures at the mud line. The well control planning and strategy for drilling exploratory and development wells in those fields should address those technical drawbacks in order to achieve the safety requirements in a cost-effective scenario.The better understanding of gas solubility in synthetic based drilling fluids plays a fundamental role in terms of preventive (kick detection) and corrective (kick circulation out of the hole) well control practices.The present work involves PVT characterization of two organic liquids (n-paraffin and ester) currently applied in drilling fluid systems for deep and ultradeep water drilling, in Campos Basin (Rio de Janeiro, Brazil). The measurement of thermodynamical properties of the methane-liquid mixtures, such as bubble point pressure, solubility, formation volume factor of oil, formation volume factor of gas and liquid density, were performed for 158 o F and 194 o F temperatures. The measurements were conducted in two different apparatuses: an Hg system and an Hg-free PVT system. The results showed that the correct accounting of formation gas solubility in downhole conditions and during the kick circulation is a very important issue for safely drill deep and ultradeep water wells.