Page 206 - Hydrocarbon Exploration and Production Second Edition

Field Appraisal 193

well 1 well 2

dip angle fault
15 deg.
OWC

Figure 8.2 Partially appraised structure.

It should be noted that the RF for a reservoir is highly dependent on the
development plan, and that initial conditions alone cannot be used to determine this
parameter.
In determining an estimate of reserves for an accumulation, all of the
above parameters will be used. When constructing an expectation curve for
STOIIP, GIIP or UR, a range of values for each input parameter should be used, as
discussed in Section 7.2, Chapter 7. In determining an appraisal plan, it is necessary
to determine which of the parameters contributes most to the uncertainty in
STOIIP, GIIP or UR.
Take an example of estimating GRV, based on seismic data and the results of two
wells in a structure (Figure 8.2). The cross-section has been generated, and a base
case GRV has been calculated.
The general list of factors influencing the uncertainty in the GRV included the
shape of structure, dip of flanks, position of bounding faults, position of internal
faults and depth of fluid contacts (in this case the OWC). In the above example,
the OWC is penetrated by two wells, and the dip of the structure can be
determined from the measurements made in the wells, which in turn will allow
calibration of the 3D seismic. The most significant sources of uncertainty in GRV
are probably the position and dip of the bounding fault, and the extent of the
field in the plane perpendicular to this section. By looking at the quality of the
seismic data, an estimate may be made of the uncertainty in the position of
the fault, and any indications of internal faulting which may affect the
volumetrics. The determination of geological uncertainties requires knowledge
of the environment of deposition, diagenesis and the structural pattern of the
field. The quantification often starts with a subjective estimate based on regional
knowledge of the geology. In cases where little data is available, ‘guesstimates’ may
need to be supplemented with data or reservoir trends observed in neighbouring
fields.