High Gamma Radiation in Heavy Oil Steam Zones: A Condensation-Induced Effect
Terence P. O’Sullivan
Aera Energy LLC, Bakersfield, California, USA
Heavy-oil reservoirs under active steam injection are usually hotter than 250F. For safety, wells that will intersect a steam zone are drilled with chilled mud, and open-hole logs are acquired while the mud is still cool.
In a new well, at a mud temperature of 140F, a steam zone was identified on open-hole logs by density and neutron separation. The gamma ray (GR) log through the steam zone was higher than 500 GAPI. After casing this well, and allowing it to equilibrate with the steam zone, wellbore temperature was over 300F, and GR decreased to less than 80 GAPI.
This remarkable decrease in GR from “cold” open- hole conditions to “hot” cased-hole conditions has been observed many times in heavy oil development wells.
To evaluate the cause and repeatability of the GR change through the steam zone, tubing was run inside blank casing to allow cooling by circulation of water or air. Fiber optic cable on the outside of tubing provided temperature measurements. GR measurements were periodically made from inside tubing.
In cased-hole, at stabilized reservoir temperatures near 300F, GR ranged from 40 to 80 GAPI. After chilling by circulating water at 120F for 5 hours, GR through steam-filled intervals increased to more than 400 GAPI. The chilled GR log was similar, but not identical to, the open-hole log.
GR through the oil zone and non-reservoir rock was unchanged after cooling. GR in the steam zone returned to low levels when the well re-heated. The experiment was repeated with similar results.
Below the steam zone, thin streaks of high GR were seen in intervals of the oil zone, where there is little or no gas separation. At 330F, stabilized temperature through these intervals was higher than it was anywhere in the steam zone, suggesting that steam under pressure was nearby. Small but significant changes in GR were observed in these streaks as temperature changed.
These streaks of high GR are interpreted to contain incipient steam, and the response indicates that the resolving power of induced GR for steam detection may be better that that of density-neutron methods.
Near-wellbore cooling appears to provide the mechanism for the increase in GR. Naturally-occurring radon gas sourced from uranium and thorium escapes to pores and decays with a 3.8-day half-life. Although alpha-emitting 222Rn is inert and non-condensible, its decay products (214Bi and 214Pb) are gamma-emitting charged particles with half-lives less than 30 minutes. These charged particles become associated with water molecules, but the number of water molecules is greatly reduced in steam-filled rock. When near-wellbore cooling condenses steam, the decay products are drawn toward the well and concentrated with the water molecules. The increase in GR occurs because the concentration factor for steam to liquid-saturated vapor or water usually exceeds 100 to 1.
Condensation-induced GR occurs in wells drilled through heavy-oil steam zones and probably in other situations. This response may provide a simple and powerful new method for identifying and monitoring steam and for evaluating remaining oil.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada