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, including
- PVT analysis for subsequent engineering calculations
- Determination of the components that exist in a particular reservoir to
have an understanding of the economic value of the fluid
- Knowledge 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 necessary
- Knowledge 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
asphaltene
- Determining the contaminating components that affect plant design, such as
the mercury content, sulfur components, and radioactive components
- If 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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
9 June 2006
- Meeting paper published:
11 September 2006
- Revised manuscript received:
12 April 2007
- Manuscript approved:
19 April 2007
- Version of record:
20 December 2007
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