Page 79 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 67

samples, all of the components with the exception of bentonite made oil-wet
samples significantly less oil-wet. The bland additives that were tested included
bentonite, pregelatinized starch, demid (an organic polymer), drispac (a polyanionic
cellulose polymer), hydroxyethylcellulose, xanthan gum polysaccharide, and CMC.
All of the drilling mud components considered to be bland decreased the
amount of oil imbibed into a core and increased the amount of water imbibed.
Preventing wettability changes in core material, after it has been recovered
at the surface, is equally important so that subsequent laboratory measurements
are representative of reservoir conditions. Changes in wettability of core material
that occur during handling or storage are usually caused by oxidation of the
crude oil, evaporation of volatile components, or decreases in temperature or
pressure which cause the deposition of polar compounds, asphaltenes, or heavy
hydrocarbon compounds [107,125]. Because of the complexity of the mechanisms
involved, the magnitude and direction of changes in wetting conditions, when
reservoir cores are preserved, are not fully understood. Weathering of water-
wet cores has been reported to frequently cause the grain surfaces to become
oil-wet [107]. In other experiments [126] oil-wet cores changed to water-wet upon
contact with air. Therefore it is necessary to preserve core samples at the well-
site to ensure that wettability is not altered by contamination, oxidation, or
evaporation. Two methods of preserving conventional cores, immediately after
they have been removed from the core barrel, will prevent changes in wettability
for several months [107]. One method consists of immersing the core in
deoxygenated formation brine or suitable synthetic brine (Le., drilling mud
filtrate) and keeping the sample in suitable nonmetallic containers that can be
Table 5-17
Empirical Relative Permeability Equations
Oil-gas relative permeabilities
(for drainage cycle relative to oil)

where is the irreducible water saturation.
km k,
A. Unconsolidated sand-well sorted (S*)3.0 (1 - S*)3
B. Unconsolidated sand-poorly sorted (S*)3,5 (1 - S*)2(1 - S"1.5)
C. Cemented sand, oolitic lime, and
vugular lime (574.0 (1 - 5*)2.0(1 - S*2.0)
Water-oil relative permeabilltles
(for drainage cycle relative to water)

where S,, is the irreducible water saturation.
kra kwl
A. Unconsolidated sand-well sorted (1 - S")3.0
8. Unconsolidated sand-poorly sorted {I - S*)2(1 - 5-5) @*)a5
C. Cemented sand, oolitic lime, and
vugular lime (1 - S*)2(1 - s*zq (S74.0
From Reference 20.