Page 337 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

504 Reservoir Engineering

is generally less accurate than saturations determined from field relative per-
meability data and is normally regarded as an approximation.
Applicability

Because of the rigid requirements of the assumptions made, and the problems
with interpreting the field data, oil saturations obtained from well test analyses
are considered rough estimates. The saturation estimate is an overall average
for the region of the reservoir influenced by the test. If permeability variations
or other conditions cause a variation in the vertical saturation distribution, these
techniques will not yield meaningful data. For these techniques to be considered
for oil saturation determinations, good laboratory core analysis data are essential.
However, because of the low costs and relative ease in conducting the tests, plus
the additional important information obtained, well test analyses should be
developed along with the other more direct methods of determining residual
oil saturations.

Coring and Core Testing
Well Coring

Well coring is the process of obtaining representative samples of the produc-
tive formation. The choice of depth at which to begin coring can often be a
problem, Cores from the regions of interest may not be obtained because of
unexpected changes in stratigraphy. There is also the possibility that the region
cored will be a nonproductive region which did not contain significant hydro-
carbon content initially. However, analysis and testing of core samples continues
to be an important method of determining residual oil [322,323].
Various techniques are used to obtain core samples: conventional diamond-
bit coring, rubber-sleeve coring, pressure coring, sidewall coring, and recovery
of cuttings generated from the drilling operation. The last two methods are not
used for residual oil measurements. Conventional coring is normally done in
competent formations to obtain fulldiameter cores. In unconsolidated, or poorly
consolidated formation, a core barrel containing a rubber sleeve is used. The
core sample is held together by the sleeve and its properties during laboratory
tests remain reasonably representative of conditions in the formation [322].
Two main problems in coring for determination of residual oil are that further
flushing of oil to below-normal waterflood residuals can take place around the
core bit, and that loss of oil occurs, due mainly to gas expansion, as the core is
lifted to the surface.
Flushing During Coring. For a condition where the in-place oil saturation is
at its waterflood residual value, no more oil can be produced at normal flow
rates. During the coring operation, it is important to avoid extreme flushing
conditions that could cause part of the residual oil to be mobilized [194]. Some
of the variables that control the amount of residual oil flushed from a core by
mud filtrate are: borehole-to-formation differential pressure (overbalance),
interfacial tension, wettability (the following discussion applies principally to
water-wet rocks), core permeability, coring penetration rate, core diameter, type
of drill bit, drilling mud composition (including particle size distribution), depth
of invasion of mud particles into the core, rate of filtrate production (both spurt
loss and total fluid loss), permeability of the formation, and nature of the
reservoir (uniformity, texture etc.) [324]. In one type of system investigated in