Sampling Procedures for Oil Reservoir Fluids

The problem of determining the productive capabilities and ultimate yield of oil reservoirs is approached through use of various engineering calculations utilizing basic data. These data are concerned with the formation itself, the fluids contained therein, and the fluid-flow relations between the formation and the contained fluids. The first and last of these are obtained from core samples and are, in the main, based on an accumulation of data which can be evaluated and applied in a statistical manner. The fluid properties, on the other hand, are usually obtained from one sample and are applied to the reservoir as a whole. While considerable effort has been expended in developing the procedures for laboratory analysis, little his been done toward perfecting procedures for obtaining the samples from which all data are obtained and upon which, subsequently, all calculations are based. It is readily apparent that no laboratory procedures, however intensive, can result in sound engineering data from unreliable, possibly non-representative, fluid samples: therefore, it is important that every practical effort be made to obtain representative samples through proper well conditioning and sampling. Many of the difficulties encountered in sampling reservoir fluids can be avoided, or minimized, if samples are collected as soon as possible in the productive life of a reservoir. Some difficulties are inherent, however, and the following discussion is directed toward a better understanding of the problems involved and of the procedures available for collecting reliable fluid samples.