Page 55 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 45

The FWL is the point at which the capillary pressure, P c , in the reser-
voir is zero and below which depth no hydrocarbons will be found within
that pressure system. Often the FWL may be related to the spill point of
the structure, particularly where there is an abundant supply of hydrocar-
bons in the system. On a formation pressure/depth plot, the intersection
between the points of the oil and the water (or gas and water) will fall at
the FWL.
Above the FWL, P c is available to allow the drainage of water by hydro-
carbons. However, particularly in low-permeability rocks, a certain entry
pressure is required before the value of S w can fall below unity. Once this
pressure is reached, hydrocarbons will be found in the rock and one can
be said to be above the OWC or GWC. Note that between the FWL and
the OWC/GWC, pressure points will continue to fall on a waterline.
For an oil/gas reservoir, the pressure will rise above the OWC on a trend
corresponding to an oil gradient (but intersecting the waterline at the
FWL). At the GOC, technically one would expect some kind of similar
FWL/OWC effect to occur with an FOL. However, the situation is not the
same as at the OWC, because one is dealing with three phases
(gas/oil/water) and not two, as before. Hence, it is common practice to
treat the GOC as being the same as the intersection point of the gas and
oil pressure lines. This may be technically incorrect, but I can only say
that it has never caused me any problems during my career as a petro-
physicist. For a gas-only reservoir, the pressure will rise above the GWC
on a trend corresponding to a gas gradient (but intersecting the waterline
at the FWL).
Note that the above considerations have nothing to do with the “tran-
sition zone” that relates to the interval between the OWC or GWC and
the point at which hydrocarbon values start to approach “irreducible”
values. This will be discussed in Chapter 4.
In poor-quality rocks, the effect of entry height can be appreciable (up
to tens of meters). It may have the effect of causing the OWC/GWC to
vary in depth across the field if the reservoir quality is changing.

2.8 PERMEABILITY DETERMINATION

During a typical pretest, the pressure gauge will show a behavior as
shown in Figure 2.8.1.
The behavior of the pressure buildup, analogous to a production-test
buildup, may be used to estimate the properties of the formation. The
mobility (M) of the formation is defined by: