Summary
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.
Introduction
Reservoir-fluid samples are obtained for a number of reasons, includingPVT analysis for subsequent engineering calculationsDetermination of the components that exist in a particular reservoir to have an understanding of the economic value of the fluidKnowledge of the contents of certain components that exist in the reservoir fluid for further planning and future drilling programs, such as the content of sulfur compounds and carbon dioxide, and the corrosiveness of the fluid. This will have an impact on the material used for casing, tubing, and surface equipment that may be necessaryKnowledge of the fluid's ability to flow through production tubing, pipelines, and other flow lines, and possible problems that may arise because of viscosity changes because of precipitation of solids such as wax and/or of asphalteneDetermining the contaminating components that affect plant design, such as the mercury content, sulfur components, and radioactive componentsIf the saturation pressure is approximately equal to the reservoir pressure then a second phase may be present. This is particularly relevant for gas reservoirs, where further drilling may discover an oil or condensate leg.
Mostly the samples are required to obtain a better knowledge of a combination of these effects; however, it must be kept in mind that often the sample is not required to resolve all of these issues.
Well-conditioning for variable coefficient second order boundary value difference systems