Direct reservoir formation water extractions on more than 8,000 plugs from 27 wells cored with oil-base mud were used to determine water saturation (S,) in Prudhoe Bay field, Alaska. A question and answer format is used to review the abundant and compelling evidence from the multicompany research program that established their validity. The main criticism of oil-mud core Sw is that water might be lost while the core is cut and brought to the surface. This criticism is easily asserted but difficult to disprove. New technology coring bits made possible the recovery of cores that have no invasion at their centers. Photographs clearly show these uninvaded centers. Invaded and uninvaded cores were found to have the same S, values. From pressure-retained cores, it was determined that depressurization did not cause any measurable water loss. Like most reservoirs, Prudhoe Bay has a very long transition zone and, in all but the lowest section, the evidence demonstrates that the core S, values are valid. S, values at Prudhoe Bay vary from 1% to more than 50%. The oil-mud core S, values were used to calibrate S, values from the resistivity logs and the Archie equation by adjusting the saturation exponent (n) value. This calculated n varied from 1.7 to 3.1. Three indirect S, evaluation methods (log, capillary pressure, and chemical tracer-test analyses) give results consistent with the core Sw values after painstaking work to understand their many variables. For example, the reservoir water salinity varies from 6 to 45+ g/L of C1–, and Rw varies from 0.1 3 to 0.72 ohm-m at 68"F, when the aquifer is 0.34 ohm-m. Standard laboratory measurements of n have large uncertainties because of the difficulty of reproducing both the reservoir water saturation and its distribution. The uncertainties of S, from oil-mud cores and the Archie equation are evaluated by partial differential analysis. The practical importance of standard log interpretation is not diminished by this work. This paper, and the team's other publications, show that a significantly more accurate S, measurement is available when the extra effort is justified. Subsequent work elsewhere with oil-emulsion mud has provided cores of equal validity to those in Prudhoe Bay field. Every time hydrocarbon-zone cores are cut in oil-base muds of any type it is clearly worthwhile to obtain good quality core S, measurements.

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