Well logs from heavy oil development wells in the San Joaquin Valley, California, frequently record high gamma ray (GR) values through intervals of the hot, vapor-filled rock that remains after injected steam at temperatures greater than 250 degF displaces heavy oil. GR values that exceed 20,000 GAPI and are 200 to 400 times greater than those in similar, but liquid-filled, rock have been observed. These high GR values occur on open-hole logs through new wells that intersect a steam chamber, after circulation of mud while drilling temporarily cools a well and causes hot vapor to condense. Days later, after a completed well re-heats, GR decreases to normal levels. In one well, circulation of cool water regenerated the high GR, demonstrating that the response is reversible. The GR energy spectra matches the uranium series and identifies highly mobile, naturally-occurring, radon and its progeny as the GR source.
The amplitude of the transient GR is related to vapor properties because radon is more soluble in hydrocarbon than in water. Reservoir studies show that GR increases with residual oil saturation in the steam chamber. In a surprising anomaly, a well-swept heavy oil reservoir that contains very little remaining oil has extremely high GR. This response is explained by the invasion of light hydrocarbons sourced from a deeper reservoir. The high vapor pressure of volatile hydrocarbons increases the efficiency of radon capture, and therefore, GR is higher in the condensate surrounding the well.
For a mixture of radon and pentane, an experiment confirms that large and reversible GR responses are observed as temperature changes and pentane-tagged with radon condenses and vaporizes. This implies that when naturally-occurring radon and condensable-hydrocarbon vapor are present at surface, containment and continuous monitoring are required to ensure that inhalation of radon-enriched hydrocarbon vapor condensate does not occur.