In 2012, the Reservoir Analysis Sonde (RAS) pulsed-neutron system was introduced by Hunter Well Science and Allied Wireline Services. Along with Sigma logging and carbon-oxygen (C/O) spectroscopy, the system features an array of three gamma-detectors (referenced as Near, Far and Long) for increased sensitivity to gas and porosity. One of the initial applications of this system was reservoir monitoring of a CO_2 flood in the Permian Basin of west Texas. The Goldsmith-Landreth (San Andres) field of Ector County, Texas was initially discovered in the 1930’s. After initial production, the field went through years of water-flooding and by the mid 1980’s the reservoir was at residual oil saturation. In 2009, Legado Resources began a pilot study on recovering the residual oil by CO_2 flooding. In contrast to gravity-drained miscible flooding, the pilot used a more active flooding in addition to water-flooding to produce the residual oil. This flood also targeted the residual oil below the ancient oil-water contact. The RAS system was used for monitoring CO_2 position and movement, as well as defining the base of the residual oil. This San Andres reservoir is a dolomite reservoir with porosities in the range 6 to 16 percent; the produced water is also the injection water and is in the range 40 Kppm to 50 Kppm sodium chlorides. A quick-look processing recipe was applied using the ratios of the Near and Long detectors to estimate porosity and gas-in-place. Sections of the reservoir with gas-in-place were assigned an ad hoc gas saturation based on literature for west Texas CO_2 floods; the parameters of Sigma-based oil saturation and the porosity calculations were matched to core data. In an effort to measure gas saturation more accurately, a follow-up project with detailed Monte Carlo models that included the specifics on reservoir rocks and fluids were built and processed. In the literature, (Odom 2001, Trcka 2006, Zett 2011) are discussions on using the multi-detector pulsed-neutron (MDPN) data to measure gas saturation independently using the long-spaced response. Given an independent measurement of the gas saturation, in reservoirs with saline formation water, the Sigma interpretation can be used for oil/water saturation. In contrast, previous literature on 3-phase monitoring of floods (Badruzzaman 1998, Harness 1998, Zalan 2004) describes using carbon-oxygen and sigma as the two inputs in a 3-phase fluid solution. These techniques were primarily focused on steam-floods with fresh water, thus sigma drove the gas (steam) saturation and C/O drove the oil saturation. The SIGMA-MDPN recipe is more robust than the SIGMA-C/O model and can be logged in a single logging pass, however, care must be exercised in diagnosing wellbore uncertainty. Several
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