Review of Crude Oil Properties

The next few sections contain information on crude oil properties, including several correlations that can be used to estimate values for the properties. McCain, Spivey, and Lenn present an excellent review of these correlations Reservoir Fluid Property Correlations.¹⁹ However, these crude oil property correlations are, in general, not as reliable as the correlations that have been presented earlier for gases. There are two main reasons for the oil correlations being less reliable. The first is that oils usually consist of many more components than gases. Whereas gases are mostly made up of alkanes, oils can be made up of several different classes of compounds (e.g., aromatics and paraffins). The second reason is that mixtures of liquid components exhibit more nonidealities than mixtures of gas components. These nonidealities can lead to errors in extrapolating correlations that have been developed for a certain database of samples to particular applications outside the database. Before using any of the correlations, the engineer should make sure that the application of interest fits within the range of parameters for which a correlation was developed. As long as this is done, the correlations used for estimating liquid properties will be adequate and can be expected to yield accurate results. Correlations should only be used in the early stages of production from a reservoir when laboratory data may not be available. The most accurate values for liquid properties would come from laboratory measurements on a bottom-hole fluid sample. Ezekwe has presented a summary of various methods used to collect reservoir fluid samples and subsequent laboratory procedures to measure fluid properties.³