From Bureau of Economic Geology, The University of Texas at Austin (
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Society of Petrophysicists and Well Log Analysts 46th Annual Logging Symposium
New Orleans, Louisiana, June 26-29, 2005

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

Shinichi Sakurai, T. S. Ramakrishnan*, Austin Boyd*, Nadja Mueller*, and Susan Hovorka


The increase in 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. The Bureau of Economic Geology (BEG) is the leading institution in the project and is collaborating with many national laboratories and private institutes.

BEG reviewed many aquifers in Texas to find an ideal formation and location for the Frio Brine Pilot Experiment site: the South Liberty field, Dayton, Texas. Most wells were 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. The target interval was not cemented, so cementing jobs and bond evaluations were repeated to confirm an acceptable bond behind casing. A new injection well was drilled 100 ft away about 30 ft downdip from the observation well. Conventional cores were cut and indicated 32 to 35 percent porosity and 2,500 md air permeability. Wireline logs were run to confirm the structural dip using the borehole imaging tool and to estimate lithology and porosity. Wireline formation tester was used to sample formation water.

Because of high formation water salinity, along with high porosity, a pulsed neutron tool was selected as the primary log for monitoring saturation changes. The C/O ratio measurement and dipole acoustic tool were also used to estimate saturation changes. Baseline logs of these tools were recorded as preinjection values.

Breakthrough of CO2 was observed on the third day after injection began in early October. Both capture and C/O logs were run to monitor saturation changes on the fourth day, and a significant decrease in sigma was noted within the porous section (6 ft) of the perforation interval. Postinjection logs were compared with baseline logs to determine CO2 distribution, and these results will be used to understand crosswell tomography and seismic data. We will show the changes in CO2 distributions. The results have significant relevance regarding how we monitor CO2 in reservoirs and develop methods to contain migration.