Page 95 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 83

Table 5-18
Typlcal Water-Oil Relative Permeablllty Characterlstics
Strongly
Water-Wet Strongly Oil-Wet
Connate water saturation. Usually greater than Generally less than
20% to 25% PV. 15% PV, frequently
less than 10%.
Saturation at which oil and Greater than 50% Less than 5050
water relative per- water saturation. water saturation.
meabilities are equal.
Relative permeability to Generally less than Greater than 50%
water at maximum 30%. and approaching
water saturation; 100%.
i.e., floodout.
From Reference 133.

However, as mentioned earlier in the section “Wettability and Contact Angle,”
a speckled wettability form of intermediate wetting mimics the relative permeability
characteristics of strongly water-wet conditions [ 1221.

Capillary Pressure Curves. By convention, oil-water capillary pressure, P, is
defined as the pressure in the oil phase, p,, minus the pressure in the water
phase, p,:

pc = Po - P, (5-79)

Depending on wettability and history of displacement, capillary pressure can be
positive or negative. Figure 5-59 presents the effect of wettability on capillary
pressure as related by Killins, Nielsen, and Calhoun [176]. Drainage and
imbibition curves can have similarities, but the capillary pressure values are
positive for strongly water-wet and negative for strongly oil-wet conditions. In
the intermediate wettability case shown in Figure 5-59, the small positive value of
threshold pressure during the drainage cycle suggests the sample was moderately
water-wet [ 138.1. After the drainage cycle, the sample spontmeously imbibed
water until the capillary pressure was zero at a water saturation of 55% . Then,
as water pressure was applied, the maximum water saturation of about 88% was
achieved. As discussed previously, capillary pressure curves can be used as a
criterion of wettability.

Reslstivlty Factors and Saturation Exponents. As shown previously in Table
5-7, Sweeney and Jennings [51] found that the formation resistivity factor
changed when wettability was altered. However, the naphthenic acid they used
to alter wettability may have also reduced porosity which could account for the
increase in the saturation exponent in Equation 5-46. Other investigators
[177,178] have found no significant effect of wettability on formation factors.
Because of the scarcity of data and the difficulty of altering wettability without
affecting other properties, the effect of wettability on formation resistivity
remains unclear.