Comparison of Water Saturation Values Determined from Capillary Pressure Measurements and Old Logs
Authors: Aguilera R. 
Published: March 2010

Abstract
Calculations of water saturation were carried out for the A Sand of the Bridger Lake Field using techniques based on:capillary pressure measurements,old well logs and old conventional interpretation charts, andcross plot and pattern recognition.
It was found that approaches (1) and (3) provided very close results for average water saturations (31.0 and 30.7%). Calculations using method (2) gave misleading results (65 and 100%).
It is concluded that techniques based on capillary pressure measurements and cross plot pattern recognition provide reliable values of average water saturation. The former has the major disadvantage of being susceptible to error in the determination of the volumetric midpoint of the reservoir. The latter has the advantage of accounting for water saturations in different zones, which still allow us to determine an average water saturation for the whole sand.
It is recommended to exercise special care in the use of conventional "cookbook" interpretation charts and computer programs. The in discriminated use of these charts and programs can result in serious error.

Introduction
The Bridger Lake Field is located about 95 miles east of Salt Lake City (Utah) at the northern foot of the Uinta Mountains on the extreme south flank of the Green River Basin. The legal location according to Utah regulations is Range 14 East, Township 3 North. Figure 1 shows the location of the Field.
The A Sand is located in the lowest part of the Cretaceous Dakota formation. The Dakota formation lies between the Mowry formation and the Morrison formation at an average depth of 6,500 ft. below mean sea level. Core analyses of these zones indicated good oil show with very erratic values of mean permeability. The sand grains are basically light brown, and their size varies from fine to medium.
The purpose of this study was to compare the values of water saturation determined from capillary pressure measurements and logs. The former requires knowledge of the geometric mean permeability and the volumetric mid-point of the reservoir. The latter is based on the following relationships(1):
Equation (1) (Available in full paper)
Equation (2) (Available in full paper)
Equation (3) (Available in full paper)
FIGURE 1: Location map of the Bridger Lake Field. (Available in full paper)

Water Saturation From Capillary Pressure Measurements
For the calculation of the average water saturation, capillary pressure measurements were made on 13 samples for different water saturations as shown in Figures 2, 3, 4, and 5. Table 1 summarizes the properties of these samples. The logarithm of permeability for each sample was plotted against water saturation holding capillary pressure constant. This plot yielded approximate linear trends represented by straight lines in Figure 6.
The next step was to calculate geometric mean permeability. This was accomplished by means of the equation(3):
Equation (4) (Available in full paper)
FIGURE 2: Capillary pressure curves, samples 171 A, 172 A, and 179 A. (Available in full paper)
where
kg = geometric mean permeability, millidarcies
Fj = frequency of j interval, fractional
(ka)j = arithmetic average permeability of logarithmic class interval j
n = total number of classified intervals