New Experiments Solve the Mystery of Transient and Extreme High Gamma Ray in Vapor-Filled Rock

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.

A series of lab experiments were used to explore the mechanism that generates the high GR signal. These experiments confirm that radon solubility is higher in hexane than distilled water, and they replicate the reversible GR response that occurs with condensation and vaporization. These observations and experiments are at the foundation of a new technology that seeks to monitor vapor properties and observe fluid flow in order to optimize hydrocarbon recovery.