Page 335 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

302 Reservoir Engineering

Well Test Analyses

In-situ oil saturations can be estimated by combining relative permeability data
determined in the laboratory with field well test data such as:
Production data
Single well transient tests in producing wells
Pressure buildup
Pressure drawdown
Multiplerate testing
Multiple-well transient tests
Interference tests
Pulse tests

Details of the methods for developing the well test data can be found in the
literature [ M,66,228], and were summarized earlier. If sufficient, good quality
relative permeability data are available from laboratory tests that simulate
downhole conditions, the techniques of correlating well test analyses can give
some indication of the reservoir oil saturation. For these techniques to be
applicable, numerous assumptions are made: the reservoir interval is homo-
geneous, horizontal, and isotropic with a small and constant compressibility; fluid
properties and saturations are uniform in the formation; relative permeabilities
are constant throughout the test area: there are no oil-water or gas-oil contacts;
and the tested wells are not affected by other wells outside the test area [319].
The following discussion applies to waterflooded reservoirs with no free gas
saturation, but could be extended to more complex systems.
Production Data

When the producing oil and water flow rates, formation volume factors, and
viscosities are known, relative permeability ratios can be determined

(5-240)

where k, = relative permeability
q = flow rate
B = formation volume factor
p = viscosity

and the subscripts w and o refer to water and oil, respectively. Thus, the relative
permeability ratio between water and oil can be estimated from the producing
water-oil ratio (WOR).
Relative permeabilities determined in the laboratory may be based on any one
of the following measures of core permeability: air, water at 100% S, or oil at
irreducible water saturations. Laboratory-derived permeability ratios can be used
to find the water saturation at which the field-derived permeability ratio would
occur. In the absence of gas saturation,

so = 1 - S" (5-241)
Provided core analysis data are available, this method is easy and rapid. However,
no information is obtained regarding wellbore damage or specific permeabilities