Fundamentals of Reservoir Fluids
Authors: Clark Norman J. 
Published: December 2007

Editor's Note: The following paragraphs briefly introduce and summarize material to be found in a five-part series of Technical Articles which will be published in consecutive issues of JOURNAL OF PETROLEUM TECHNOLOGY. This series will cover the fundamentals of sampling, testing, adjusting and interpreting oil and gas sample data for use in reservoir studies. The first installment begins on page 12 of this issue. References, Tables and Figures will be numbered consecutively, but will be published only with the particular installment of the series in which they are first mentioned.

Fundamentals of Reservoir Fluids--Introduction
During the movement of oil and gas to the surface from the reservoir in which they are found, the temperatures and pressures to which they are subjected change significantly; as a result, their physical properties undergo many radical changes. The economic value of produced oil and gas is dependent upon these physical properties, and the operator finds it invaluable to be able to predict handling and producing techniques which will allow him to produce his reserves in a form that will provide a maximum profit. For many years, therefore, investigators have studied the phase behavior of hydrocarbon materials with the goal of fully developing methods for determining answers to problems concerning the physical behavior both of produced hydrocarbons and of those that are left in the reservoir. Written material regarding the fundamental physical concepts governing hydrocarbon behavior and describing the methods employed in practically applying these concepts to solving production problems is scattered throughout the literature. The purpose of this series, therefore, is to bring together these concepts and methods of their application in solving practical, day-to-day reservoir engineering problems. When production starts and pressure is reduced in a hydrocarbon reservoir, both liquid and gas are formed from what was at first only a liquid (as in an oil reservoir) or only a gas (as in a gas reservoir). One or the other of the two phases (liquid and gas) is produced differentially to some degree because of variations in permeability of the rock to gas and oil as saturations change. Therefore, the hydrocarbon analysis of the composite produced materials changes from that of the original material in place in the reservoirs. Obviously, then, the behavior of reservoir fluids during production operations becomes quite complex, and it is necessary to obtain laboratory analyses of the initial reservoir fluids for conditions of change where liquid and gas separate both differentially and under equilibrium conditions. With these data, production problems involving separation under various conditions can then be solved. The practical approach to the study of reservoir fluid behavior is to anticipate pressure and temperature changes to which the reservoir material will be subjected during production operations, both in the reservoir and at the surface; then the changes to reservoir fluid samples, which occur for each of these pressure and temperature conditions, are measured by laboratory tests. The study of oil and gas reservoir performance requires quantitative data on the composition, characteristics and behavior of the oil and gas in the reservoir under original conditions; in addition, similar data must be obtained for the oil and gas under all changing conditions of temperature and pressure-as the materials move to the wellbore, as they are produced to the surface, and as they are either gathered in the stock tank or piped to the consumer. The laws for perfect gases and perfect solutions do not apply, without modification, to the behavior of hydrocarbon mixtures. Although in recent years many investigators have made notable progress toward developing means for quantitatively evaluating hydrocarbon phase behavior, the industry still does not have a reliable theoretical method by which it can accurately and completely calculate the behavior of complex hydrocarbon mixtures under high pressures. Where the production problem deals with changes in the reservoir material under conditions approximating that of equilibrium conditions, such changes may be calculated with reasonable precision. For this purpose, the concept of K-values has been employed. A K-value is the ratio of the mole fraction of a component in the equilibrium gas phase to the mole fraction of the same component in the equilibrium liquid phase. In essence it is a measure of volatility, which is controlled by the balance between molecular forces at the condition of temperature and pressure to which the material is subjected. A complete description of equilibrium calculations, how they are made, their qualifications and application to reservoir engineering problems will be covered in this series.