Page 104 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

92 Reservoir Engineering

additional oil recovery, whenever possible, the use of mud additives that lower
interfacial tension should be avoided. Greater amounts of residual oil are
displaced from cores as the filtrate production rate is increased. Higher API
filter loss or smaller core diameters will generally lead to larger amounts of
flushing, but a key factor in the amount of mobilized residual oil is the spurt
loss (the rapid loss to the formation that occurs before an effective filter cake
is formed). As stated previously, uniformity of the formation being cored will
inf hence the amount of oil that will be displaced. Identical drilling conditions
may yield varying results with changes in lithology or texture of the reservoir.
In particular, drastic differences may be observed in reservoirs that contain both
sandstone and carbonate oil-bearing strata.
Factors Affecting Oil Saturation Changes During Recovery of Cores. Surface oil
saturations should be adjusted to compensate for shrinkage and bleeding [ 1231.
Shrinkage is the term applied to the oil volume decrease caused by a tem-
perature change or by a drop in pressure which causes dissolved gases to escape
from solution. Shrinkage of reservoir fluids is measured in the laboratory by
differentially liberating the samples at reservoir temperature. The formation
volume factors are used to adjust surface oil volume back to reservoir tem-
perature and pressure. Gases coming out of solution can cause some oil to flow
out of the core even though it may have been flushed to residual oil by mud
filtrate. Bleeding is the term applied to this decrease in oil saturation as the
core is brought to the surface. Calculations have been proposed E1231 to account
for shrinkage and bleeding (see the discussion in this chapter entitled “Estima-
tion of Waterflood Residual Oil Saturation”).
Measurement of Fluid Saturations. There are two primary methods of determining
fluid content of cores; these methods are discussed in API RP40: Recommended
Practice for Core-Analysis Procedure [ 1921. In the retort or vacuum distillation
method, a fluid content still is used to heat and vaporize the liquids under
controlled conditions of temperature and pressure. Prior to testing, the gas space
in the core is displaced with water. The fluids produced from the still are
condensed and measured, and the fluid saturations are calculated. Normally the
percent oil and water are subtracted from 100% to obtain the gas saturation;
however, considerable error may be inherent in this assumption. The second
common method is the distillationextraction method in which water in the core
is distilled, condensed, and accumulated in a calibrated receiving tube. Oil in
the core is removed by solvent extraction and the oil saturation is calculated
from the weight loss data and the water content data.
Conventional core samples have oil content determined by atmospheric
distillation. The oil distilled from a sample is collected in a calibrated receiving
tube where the volume is measured. Temperatures up to 1,200”F (about 650’C)
are used to distill the oil from the sample which causes some coking and
cracking of the oil and the loss of a small portion of the oil. An empirically
derived correction is applied to the observed volume to compensate for the loss.
Calibration tests are made on each type of oil.
Whole core samples have oil content determined by vacuum distillation. This
technique is used to remove oil from the sample without destroying the minerals
of the sample. A maximum temperature of’ 450°F is used. The oil distilled from
the sample is collected in a calibrated receiving tube which is immersed in a
cold bath of alcohol and dry ice at about -75°C. This prevents the oil from
being drawn into the vacuum system. As in the atmospheric distillation method,
corrections must be applied to the measured volumes.