Transient high gamma ray (THG) is a process that involves cooling a well in order to cause gamma ray (GR) to change. THG is frequently observed in heavy oil development wells drilled through permeable hot sands undergoing thermal recovery. When these sands contain vapor, the drilling process cools the well, causing droplets of condensed vapor to form around it. Radon adsorbs to the surface of these droplets causing GR to increase significantly – often by a factor of 10 or more in clean sands. When the cooled well reheats, GR returns to the intrinsic value. This dynamic aspect of THG is essential. Other processes, for example, precipitation of radioactive scale, can cause GR to change, but the change is not transient.
After drilling a new well and running casing, THG can be regenerated in a reheated well by circulating cool water. The high-amplitude, high-resolution THG signal observed in the cased hole has been shown to precisely replicate open hole THG, unless reservoir conditions have changed. Over a period of hours, variations in THG buildup and decay rates characterize fluid properties and formation permeability as condensate forms and vaporizes. Flow unit boundaries appear to be identified by systematic changes in decay rate.
THG is expected to have application to unconventional reservoirs when reservoir temperature and pressure are in a range that will allow cooling to cause a phase change to occur. While initial reservoir pressure in unconventional reservoirs is often so high that it is impossible to cause condensation to occur by cooling, pressure in maturing reservoirs will eventually drop sufficiently to bring the reservoir fluid into the 2-phase region. When this occurs, THG can provide data that identifies flow unit boundaries and characterizes reservoir fluids. Repeated THG observations over months and years will enable operators to observe the reservoir response to changes in production, injection and operating procedures.
Years of observations of THG in heavy oil reservoirs demonstrate that THG is a repeatable, reservoir-scale process. In one example, seven wells drilled over a period of 8 years and spaced 250 feet apart show an easily correlated GR with amplitude over 100 times greater than normal through a thick sand.