Radioactive Tracer Logs - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 84

Carbon Dioxide Injection
The injection of carbon dioxide (CO 2 ) into porous formations is now a common
practice. The primary reason is for enhanced oil recovery (EOR) projects. These
include secondary and/or tertiary recovery programs to recover residual hydrocar-
bons left in a reservoir after primary production and/or water flooding has run its
course. The source for the CO 2 in these cases is normally a naturally occurring
deposit generated by volcanic activity over geologic time. The gas percolates along
fractures up into a porous rock that traps sizeable accumulations. In such deposits
some of the gas may be free and some dissolved in an aquifer, if present. An exam-
ple of such a deposit is the Bravo Dome in New Mexico.
A secondary reason is for the purposeful sequestration of CO 2 released by, for
example, electricity generating plants that burn fossil fuels. In either case the injec-
tion well operators need to know if the CO 2 injectant is going where it is supposed
to go. Two principle logging methods are used to monitor where the CO 2 ends up
after it has left the cased borehole and entered the target formation.
The first method involves running compensated thermal neutron logs at regular
time intervals and comparing the surveys run after injection has taken place with a
base log run before the injection was started. Since thermal neutron logs respond
mainly to the hydrogen content of a formation the displacement of water and hydro-
carbons (both have hydrogen atoms in their molecular makeup) by CO 2 causes a
marked change in the logging tool response in the zones where the CO 2 injection
has taken place successfully.
A second method involves adding a radioactive isotope to the CO 2 injectant and
then logging the injection well with a gamma ray logging tool. Short-lived iodine
isotopes are frequently used for this purpose. Depths in the wellbore where gamma
radiation levels are higher than the naturally occurring background radiation levels
indicate the zones where the CO 2 injection has been successful.
Bibliography
Interpretative methods for production well logs. second edition, Dresser Atlas (1982), section 4.
Production log interpretation. Schlumberger (1973) 20-23.