From Bureau of Economic Geology, The University of Texas at Austin (
For more information, please contact the author.

Bureau Seminar, January 7, 2005

Monitoring Saturation Changes for CO2 Sequestration: Petrophysical Support
of the Frio Brine Pilot Experiment

Shinichi Sakurai


Increase of greenhouse gas emissions, especially carbon dioxide (CO2) is thought to be a major cause of global warming. Sequestration of CO2 in underground aquifers is considered a promising method of reducing CO2 concentration. Funded by the U.S. Department of Energy National Energy Technology Laboratory, the Frio Brine Pilot Experiment was begun in 2002. BEG is the leading institution in the project, with the collaboration of many national laboratories.

BEG reviewed many aquifers in Texas to find the proper formation and location for the Frio Brine Pilot Experiment site, and South Liberty field, Dayton, Texas, was chosen. Most wells had been drilled in the 1950’s, oil being produced from the Yegua Formation at ~9,000 ft depth. The fluvial sandstone of the upper Frio Formation in the Oligocene is our target, at a depth of 5,000 ft. We started injection of CO2 on October 4, 2004, and 1,600 tons of CO2 was injected for 15 days.

An existing well was used as the observation well, and after it was plugged below the target zone, cementing confirmed a good bond behind the production tubing. A new injection well was drilled 100 ft away and about 30 ft downdip from the observation well. Conventional cores were cut, revising our reservoir characteristics with a porosity of 32–35 % and air permeability of 2,500 md. Using a borehole imaging tool, we ran wireline logs to confirm structural dip, along with lithology and porosity. A formation tester was run to sample formation water.

Because of high formation porosity and high formation water salinity, the pulse neutron method was used as a primary log for monitoring saturation changes, along with the C/O tool and dipole acoustic tool. Baseline logs were made to record preinjection values. During CO2 injection periods, both TDT and C/O logs were run to check saturation changes. Postinjection logs were also run to check CO2 distribution by comparing them with baseline logs, as well as by comparing crosswell tomography and seismic data. We show results of saturation changes.