From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, March 26, 2010
The Rock Physics Basis for 4D Seismic Monitoring of CO2 Fate:
Are We There Yet?
Link to streaming video: available 03.26.2010 at 1:55pm
Dr. Tiziana Vanorio
Monitoring, verification, and accounting (MVA) of CO2 fate are three fundamental needs in geological sequestration. The primary objective of MVA protocols is to identify and quantify (1) the injected CO2 stream within the injection/storage horizon and (2) any leakage of sequestered gas from the injection horizon, providing public assurance. Thus, the success of MVA protocols based on seismic prospecting depends on having robust methodologies for detecting the amount of change in the elastic rock property, assessing the repeatability of measured changes, and interpreting and analyzing the detected changes to make quantitative predictions of the movement, presence, and permanence of CO2 storage, including leakage from the intended storage location.
Having the appropriate rock-physics model is generally a key element for time-lapse seismic monitoring, both to infer the significance of detectable changes (i.e. qualitative interpretation) and to convert them into actual properties of the reservoir rocks (i.e. quantitative interpretation). Nevertheless, because of the peculiar ability of CO2-rich water to promote physicochemical imbalances within the rock, we must address whether traditional rock-physics models can be used to invert the changes in geophysical measurements induced in porous reservoirs by the injection of CO2, making it possible to ascribe such changes to the presence or upward migration of CO2 plumes.
In this seminar, I will first illustrate the most common pitfalls of geophysically mapping CO2 distributions, particularly when CO2 signatures are masked by ambiguities in the scales of CO2-water mixing and by changes to the host rock due to dissolution and/or precipitation (i.e. change in porosity, changes at the grain contacts, salt precipitation, and/or mineral transformation). Then, I will show recent experimental results pertaining to (1) the acoustic properties of H2O-CO2 solution under subcritical conditions and (2) the changes in velocity induced within rock samples upon injection of CO2-rich brine. Results show that the seismic response of CO2-water-rock systems is far from being a pure fluid-substitution problem. Apparently, the magnitude and location of the physical changes rather depend on the specific fluid-rock coupling.