Reservoir-fluid sampling revisited - A practical perspective

Abstract

Pressure/volume/temperature (PVT) fluid properties are an integral part of determining the ultimate oil recovery and characterization of a reservoir, and are a vital tool in our attempts to enhance the reservoir’s productive capability. However, as the experimental procedures to obtain these are time consuming and expensive, they are often based on analyses of a few reservoir-fluid samples, which are then applied to the entire reservoir. Therefore, it is of utmost importance to ensure that representative samples are taken, as they are fundamental to the reliability and accuracy of a study. Critical to the successful sampling of a reservoir fluid is the correct employment of sampling procedures and well conditioning before and during sampling. There are two general methods of sampling—surface and subsurface sampling. However, within these, there exist different methods that can be more applicable to a particular type of reservoir fluid than to another. In addition, well conditioning can differ depending on the type of reservoir fluid. Sampling methods for each reservoir type will be discussed with an emphasis on scenarios where difficulties arise, such as near-critical reservoir fluids and saturated reservoirs. Methods, including single-phase sampling and isokinetic sampling, which have been used increasingly in the last decade, will also be discussed with some detail, as will preservation of the representatives of other components in the sample including asphaltenes, mercury, and sulfur compounds. The paper presents a discussion aimed at better understanding the methods available, concepts behind the methods, well conditioning, and problems involved in obtaining representative fluid samples.