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Observations Of Condensed-Vapor Gamma In California Heavy Oil Reservoirs


Higher than normal gamma ray (GR) values are frequently observed on open-hole logs run in wells drilled through steam- and vapor-filled intervals of heavy oil reservoirs. Previous work demonstrates that these GR values, which can exceed 1000 GAPI, are caused by drilling-related cooling of the wellbore and condensation of radon-enriched vapor in the surrounding rock. With few exceptions, these high GR values only occur through vapor-filled intervals. To learn more about this “Condensed-Vapor Gamma” (CVG) effect, GR logs and other data from hundreds of heavy oil wells were examined. Systematic variations in CVG appear to be related to vapor composition, rock type and remaining oil saturation. Comparison of GR logs between groups of closely-spaced wells shows that the high GR response correlates and that amplitudes are similar, even when the GR is higher than 1000 GAPI. Higher GR values are found in lower-quality reservoirs which are likely to have higher remaining oil saturation. High GR is also observed to occur in intervals located directly above silt barriers where condensed vapor appears to accumulate. The condensation-induced gamma signal has only been documented to occur in wells drilled in heavy oil steamfloods, but the effect should occur in any reservoir containing condensable vapor, provided that vapor can be cooled toward the dew point. Applications include evaluation of vapor composition, identification of barriers, and time-lapse observation of changes in these properties as an indicator of process efficiency.