VALIDATION OF AS-RECEIVED OIL-BASED-CORE WATER SATURATIONS FROM PRUDHOE BAY

Determining connate-water saturations is a key factor in calculating o riginal hydrocarbon pore volume (HCPV) for the Ivishak reservoir, the largest productive horizon in the Prudhoe Bay field. As-received water saturation, S-w, measurements on oil-based-mud cores (OBC's) from 27 wells were used as the core standard for S-w determination. Measured v alues ranged from < 1 to > 56% PV, which raised questions as to the va lidity of these values as being representative of in-situ conditions. Studies of the effects of the cutting, surfacing, and handling of thes e cores were conducted. Some cores were recovered with centers uninvad ed by mud filtrate. S-w values in these sections were the same as thos e in mud-filtrate-invaded sections of other portions of the same core. Pressure and conventional cores had similar S-w values in adjacent in tervals and to nearby oil-based-mud-cored wells. Rigsite S-w were meas ured and found to be the same as those determined routinely by Core La boratories in Anchorage. These data offered convincing proof that, abo ve the oil/water transition zone, water was not flushed from the core during the coring process, expelled or evaporated during surfacing, or evaporated while in transit to Anchorage for routine measurements. A large, alternative, centrifuge capillary pressure database was develop ed to provide an independent check of the OBC results. Development of this database required selection of samples statistically representati ve of rock types in the reservoir and use of laboratory techniques tha t produced reliable results. Comparing S-wPc with S-wobc values requir ed the consideration of the burial and hydrocarbon accumulation histor ies and the variations in hydrocarbon properties within the reservoir.