Separation of carbon dioxide from the products of combustion of fossil fuels in large industrial facilites such as electric power plants and injection of the carbon dioxide for permanent storage in suitable geologic formations underground is a practical approach to slowing the accumulation of carbon dioxide in Earth's atmosphere and its contribution to climate change. This is best seen as an interim measure, needed to slow the pace of climate change during the period required for transition to renewable energy and development of the technology and infrastructure for beneficial utilization of carbon dioxide. In addition to its separation from combustion products, capture of carbon dioxide from the atmosphere is also an important component of the interim plan (Kolbert, 2017).
One of the essential features of a geologic formation suitable for carbon dioxide storage is the presence of impermeable seal layers or caprocks above the zone in which the carbon dioxide is stored, to prevent leakage and upward migration of carbon dioxide toward the surface. Leakage could result in contamination of underground sources of drinking water, which are at shallower depths than the geologic reservoirs under consideration for storage of CO2.
The UAB Caprock Integrity Laboratory was established in 2010 with funding through a Contribution Agreement with Southern Company Services. Since its startup, the laboratory and its investigators have participated in successful partnerships on projects with Advanced Resources International, Alabama Power Company, Denbury Resources, Montana State University, the National Energy Technology Laboratory, Oklahoma State University, Southern Company, and the Southern States Energy Board. Properties of both caprocks and reservoir rocks have been the subjects of research in connection with unconventional gas production (Puckette et al., 2016), seal formation by biomineralization (Phillips, 2013), and commercial-scale CO2 storage (Koperna et al., 2017).
A focus of research projects in the Caprock Integrity Laboratory has been on the "minimum capillary displacement pressure," or "breakthrough pressure," of gas through rocks saturated with brine (Hildenbrand et al., 2002, 2004). In the course of those investigations, it was discovered that the effective permeability of a rock to gas could be determined over the entire range from zero, just prior to breakthrough from a fully brine-saturated rock, up to a maximum effective permeability at the minimum brine saturation, with determination of the functional relationship between effective permeability and gas pressure. Knowledge of this relationship provides a useful tool with which to anticipate the evolution of the effective permeability of a saline formation with progress of CO2 injection and the conditions required to prevent seepage of CO2 through caprocks.
Demonstration of the sealing ability of caprocks is not only a key to insuring the long-term retention of injected CO2 but a requirement for regulatory permitting and public acceptance of geologic sequestration. Assuring satisfactory injectivity of a storage reservoir from start to finish is essential to guaranteeing the economic viability and success of a sequestration project.
The following properties of cores from the injection and confining zones of potential geologic storage targets are measured routinely in the Caprock Integrity Laboratory:
- Porosity and grain density
- Permeability to nitrogen, helium, or carbon dioxide as functions of confining pressure and pore pressure
- Minimum capillary displacement pressure (breakthrough pressure) and effective permeability to nitrogen or helium versus pressure, for rock initially saturated with brine.
- Changes in porosity and permeability on exposure to supercritical carbon dioxide
Quantitative knowledge of these rock properties and behaviors increases the reliability with which caprock integrity and reservoir injectivity can be predicted, to reassure the public, provide technical data for permitting, and reliable analysis to the operator of a storage project that carbon dioxide storage will be safe, secure, and cost effective.
Caprock Integrity Laboratory
Location: BEC 167A
Laboratory Director: Dr. Peter Walsh
These facilities are used for research by students in the undergraduate Honors Program. The Mechanical Engineering Department established and equipped (with support from the U.S. Department of Energy/National Energy Technology Laboratory, Alabama Power Company, and Southern Company) a laboratory for evaluation of the properties of seal layers, or caprocks, that will prevent upward migration of carbon dioxide from geologic formations in which it has been sequestered, to underground sources of drinking water and the atmosphere.
A cylindrical plug of caprock is enclosed in a triaxial flow cell equipped with sealed cavities on both ends (Hildenbrand et al., 2002, 2004). The sample is first saturated with water or brine. A high pressure of gas, above the anticipated breakthrough pressure, is then applied to one end of the plug, with the other end initially at atmospheric pressure. After a time lag that can be many days, when the gas breaks through, the difference in pressure between the upstream and downstream cavities asymptotically approaches a residual pressure difference characteristic of the rock, corresponding to the capillary pressure in the conducting pore having the largest effective diameter. This is the "minimum capillary displacement pressure," above which capillary flow will occur through the rock, given sufficient time.
The measurement is an alternative to the traditional method of continuously increasing the pressure of gas on one face of a brine-saturated plug until breakthrough is observed. The traditional method tends to overestimate the breakthrough pressure because there can be a time lag of days or weeks before gas appears on the downstream side of a plug, even at upstream pressures well above the pressure ultimately identified as the breakthrough pressure. Experiments are conducted using both nitrogen and carbon dioxide gases, with brine characteristic of the reservoir under study. The use of nitrogen permits measurements to be made in the absence of changes in permeability caused by dissolution of the rock by the acidic aqueous solution of CO2, or precipitation of dissolved solids from the brine.
References
- Hildenbrand, A., S. Schlömer, and B. M. Krooss, "Gas breakthrough experiments on fine-grained sedimentary rocks," Geofluids, 2002, 2, 3-23.
- Hildenbrand, A., S. Schlömer, B. M. Krooss, and R. Littke, "Gas breakthrough experiments on pelitic rocks: comparative study with N2, CO2 and CH4," Geofluids, 2004, 4, 61-80.
- Kolbert, E., "Can carbon-dioxide removal save the world?," The New Yorker, November 20, 2017.
- Koperna, G., J. Pashin, and P. Walsh, "Commercial Scale CO2 Injection and Optimization of Storage Capacity in the Southeastern United States," Final Scientific/Technical Report, DOE Cooperative Agreement DE-FE00110554, Submitted by Advanced Resources International, Inc., Oklahoma State University, and University of Alabama at Birmingham, October 27, 2017.
- Phillips, A. J., "Biofilm-Induced Calcium Carbonate Precipitation: Application in the Subsurface, Ph.D. Dissertation, Montana State University, Bozeman, October 2013.
- Puckette, J., J. Pashin, P. Clark, S. Mohamed, J. White, D. Kopaska-Merkel, G. Jin, J. Dawson, D. Hills, P. Walsh, and M. Hannon, "Petrophysics and Tight Rock Characterization for the Application of Improved Stimulation and Production Technology in Shale," Final Report DE-AC26-07NT42677, Prepared for the Research Partnership to Secure Energy for America under Contract 11162-23 by Oklahoma State University, Geological Survey of Alabama, and University of Alabama at Birmingham, June 26, 2016.
Contact
For more information, please reach out to Peter Walsh at