Page 154 - Hydrocarbon Exploration and Production Second Edition
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Reservoir Description 141
Height
irreducible water
saturation
OWC
FWL
0.0 1.0
Water Saturation
Figure 6.31 Saturation^height with capillaries.
many tens of metres. When taking pressure samples with a FPT to construct a
pressure–depth plot, it is advisable to obtain pressures outside the transition zone,
where the gradients are truly representative of the single fluid, rather than of a
mixture of the two fluids (oil and water in this example).
The change of saturation with height above a FWL can have a significant effect
on the volume of hydrocarbons. It is therefore important to accurately model this
change mathematically in order to estimate the correct volume of hydrocarbons in
the reservoir. A saturation–height function is the mathematical term that is derived
to model this change and is calibrated using log data and special core analysis (SCAL)
tests which are discussed further in Section 6.3.2.
6.3. Data Gathering
Introduction and Commercial Application: Data gathering is an activity which
provides the geologist and engineer with the information required to estimate the
volume of the reservoir, its fluid content, productivity and potential for
development. Data gathering is not only carried out at the appraisal and
development planning stage of the field life cycle, but continues throughout the
field life. This section will focus on the data gathered for field development
planning; data gathering for managing the field during the production period is
discussed in Chapter 16.
The timely acquisition of static and dynamic reservoir data is critical for the
optimisation of development options and production operations. Reservoir data
enable the description and quantification of fluid and rock properties. The amount
and accuracy of the data available will determine the range of uncertainty associated
with estimates made by the subsurface engineer.
6.3.1. Classification of methods
The basic data gathering methods are direct methods, which allow visual inspection
or at least direct measurement of properties, and indirect methods whereby we infer