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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
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