Increase in greenhouse gas emissions, especially carbon dioxide (CO2, is thought to be a major cause of climate change. Sequestration of CO2 in brine aquifers is considered a promising method of reducing CO2 concentration in the atmosphere. 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 project leader and is collaborating with many national laboratories and private institutions. BEG reviewed many brine aquifers throughout the U.S. to find an ideal formation and location for the Frio Brine Pilot Experiment site: 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. An existing well was used as the observation well and a new injection well was drilled 100 ft away 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 estimate lithology and porosity, as well as to confirm structural dip using the borehole imaging tool. The 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 through change of thermal neutron absorption cross-section. 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. We started injection of CO2 on October 4, 2004, and 1,600 metric tons of CO2 was injected for 11 days. Breakthrough of CO2 was observed on the third day. Both capture and C/O logs were run to monitor increase of CO2 which was noted within the porous section (6 ft of the perforation interval. At the end of the injection phase, CO2 saturation of up to 80% was accounted in the observation well. Even 4½ months after the injection experiments, CO2 saturation of up to 40% was found in the observation and the injection wells, along with temperature anomalies. These results are used to help us understand crosswell tomography and seismic data. Results have significant relevance regarding how we monitor CO2 in reservoirs and develop methods to contain migration. This paper is concerned primarily with the evaluation of the CO2 migration and accumulation around the injection and monitoring wells.
Monitoring Saturation Changes for CO2 Sequestration: Petrophysical Support of the Frio Brine Pilot Experiment
Sakurai, Shinichi, Ramakrishnan, T.S., Boyd, Austin, Mueller, Nadja, and Susan Hovorka. "Monitoring Saturation Changes for CO2 Sequestration: Petrophysical Support of the Frio Brine Pilot Experiment." Paper presented at the SPWLA 46th Annual Logging Symposium, New Orleans, Louisiana, June 2005.
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