Determining connate water saturations was a key factor in calculating original hydrocarbon pore volume for the Ivishak Reservoir, the largest productive horizon in the Prudhoe Bay Field. As-received water saturation (Sw) measurements on oil-base-mud cores from 27 wells were used as the core standard for Sw determination. Measured values ranged from <1 % PV to 56% PV which raised questions as to the validity of these values as being representative of in-situ conditions. Studies of the effects of the cutting, surfacing and handling of these cores were conducted. Some cores were recovered with centers uninvaded by mud filtrate. Sw values in these sections were the same as in mud-filtrate invaded sections of other portions of the same core and of adjacent cored intervals in the same well. Pressure and conventional cores had similar Sw values in adjacent intervals and to nearby wells. Rig-site Sw's were measured and found to be the same as those determined routinely by Core Labs in Anchorage. These data offered convincing proof that water was not flushed from the core during the coring process nor expelled or dried during surfacing or dried while in transit to Anchorage for routine measurements. Further validation of the oil-base core saturations was provided by the close agreement with centrifuge capillary pressure data on 1120 core plugs.
The importance of knowing the original hydrocarbon pore volume in the Ivishak Reservoir of the Prudhoe Bay Field led to the acquisition of a large Sw database. Key to determining original in-situ water saturations were 27 wells, shown in Figure 1, cored with oil-base mud to allow direct measurement of Sw. While oil-base-mud core (OBC) data have long been the industry standard for determining connate water saturations, the as-received values of Sw in these cores, which ranged from less than 1% PV to more than 56% PV, raised questions as to how representative they were of in-situ values.