Gamma ray (GR) logs from infill wells in heavy oil development projects frequently exceed 1000 GAPI, but only through the hot vapor cloud that develops as injected steam displaces heavy oil. GR values in the same sands that are liquid-filled, and immediately below the vapor-filled rock, are typically less than 100 GAPI. Previous work (O’Sullivan, 2008) shows that high GR values are caused by drilling-related cooling of vapor-filled rock. GR is thought to increase when water- and hydrocarbon-molecules, with solubilized radon atoms attached, are concentrated by 100 times or more as they approach the dew point and condense around a chilled well. After the chilled well begins to re-heat and equilibrate with the hot reservoir (36 hours or less) GR returns to normal levels. An experiment demonstrated that the cycle of GR increase and decrease can be repeated indefinitely, simply by chilling the well and then allowing it to warm back.Samples of the condensed vapor have not been acquired, nor has condensed vapor gamma (“CVG”) been generated in a lab under controlled conditions, so much remains to be learned about the nature of CVG, and how it can be used to understand reservoir processes.To put the condensed vapor gamma (CVG) effect into context, logs from thousands of heavy oil development wells from two large oil fields of the San Joaquin Valley, California were systematically reviewed. GR logs through vapor-filled rock for reservoirs in Midway-Sunset Field show that CVG amplitude is higher (≈ 2000 GAPI) in poorly-sorted rocks than in well-sorted clean sands (≈ 200 GAPI). The difference is driven by higher residual oil saturation in poorly-sorted rocks. Higher radon solubility and vapor pressure for oil, compared to water, lead to higher CVG values.GR logs through well-sorted sands in Belridge Field, were anticipated to be low, and similar to those for well-sorted sands in Midway-Sunset Field. Instead, the CVG amplitude is unexpectedly high, exceeding 10,000 GAPI. A cross section of seven closely-spaced wells, drilled within an eight-year time span, shows that these very high GR values strongly correlate within a 60-foot interval. For the entire field, maps that track the year-by-year onset of high GR show interesting, but unexplained patterns that are restricted to limited areas and time intervals.The difference between the CVG responses in the two fields may be explained by the observation that, for Belridge Field, the very high GR values occurred years after the steam flood on this reservoir peaked, during the time when development of a deeper reservoir containing light hydrocarbons was accelerating. CVG amplitude may have increased when the heavy oil vapor cloud was overprinted with light hydrocarbon from the deeper reservoir. With the light hydrocarbon, vapor pressure increases and improves the efficiency of radon absorption.The observations suggest that, under certain conditions, it is possible to develop a method for in situ evaluation of vapor properties. Although the condensation-induced gamma signal has only been documented to occur in wells drilled in heavy oil steam floods, the effect could occur in any reservoir containing condensable vapor, provided that the vapor can be cooled to the dew point. Applications include evaluation of vapor composition, identification of barriers, and time-lapse monitoring of changes in vapor properties as an indicator of enhanced recovery process efficiency.Controlled generation of CVG in the laboratory is a logical next step toward improved understanding of this phenomena. Continuous in situ observation and monitoring of CVG is also recommended, in order to explore the linkage between CVG and development activities.
展开▼
