A. Bryce Cunningham
BP Exploration, Anchorage
Karen L. Jay
BP Exploration, Houston
Abstract
In the summer of 1988 development drilling began in the Eileen West End of the Prudhoe Bay Field. Production tests of delineation wells in this peripheral area had tested essentially dry oil from intervals of high water saturation. Core data from the four cored wells in this area of the field revealed that much of this water is bound in micropores and therefore irreducible. It was hoped that the Nuclear Magnetic Logging Tool (NML) could be used to quantify both the microporosity, from microporous chert and clays, and irreducible water saturation of the formation.
Examination of the NML data after logging revealed that primary calibration procedures for the tool were inadequate. The scale factor (NSF) and corrections for hole size, hole angle, and coil current were each altered significantly based on NML results in this well. It was determined that the earth’s magnetic field frequency (Larmor frequency) has a large effect on the NML tool calibration and that an empirical rather than theoretical correction is required. Since the Larmor frequency in Alaska is significantly different from that in Texas, where the primary calibrations were previously made, calibration facilities were subsequently installed in Airdrie, Alberta to compensate for the frequency differences in the northern latitudes.
Results of the log and core studies show that microporosity from microporous chert is not discriminated from total porosity by the NML within the water leg of Z-02. The NML does respond to changes in microporosity resulting from varying amounts of authigenic clays content within the sandstones. In addition, a significant hydrocarbon effect is apparent upon the NML log which was not predicted given the low viscosity of West End light oil. The shift to lower free fluid index readings in the hydrocarbon leg relative to readings in the water leg appears to result from anomalously short relaxation times of the Z-02 light oil.