In a recently published study, a team of researchers identified a process for managing reservoir pressure that improves the safety of underground carbon storage.
The study team, led by Dr. Paul Ziemkiewicz, director of the West Virginia Water Research Institute at West Virginia University and researchers from the University of Wyoming, and Los Alamos and Lawrence Livermore National Laboratories published their findings in the August 2016 issue of the International Journal of Greenhouse Gas Control.
Efforts to control greenhouse gas emissions from fossil fuel combustion hinge on capturing carbon dioxide (CO2) and permanently storing it. However, finding a permanent home for CO2 is not a simple matter. One option is to store the CO2 deep underground.
Underground storage of CO2 involves pressurizing it to the point where it becomes a liquid and forcing it into porous, deep geological rock formations like sandstone.
These formations have an overlying cap rock formation that will prevent seepage to the surface. Unfortunately, the pore spaces in these deep rock formations are invariably filled with saline water, or brine.
“Water is not very compressible,” said Ziemkiewicz. “If you try to inject carbon dioxide into the formation you need to do so under pressure and then it acts like a piston, transferring that pressurized water to the weakest part of the system. If that pressure is too high it will fracture the cap rock and the CO2 escapes.”
Without pressure management, the best outcome is that the carbon dioxide dissipates gradually through the target formation and remains where it belongs. However, that leaves a lot of uncertainty and restricts the rate at which carbon dioxide can be put into an injection well.
As a result, the Environmental Protection Agency has placed very stringent conditions on carbon storage wells. Regulated as class VI injection wells, the liabilities associated with them are essentially perpetual and few companies are willing to assume that level of financial risk.
Ziemkiewicz pointed out that the carbon storage issue is one of the major factors restricting the adoption of carbon capture technologies “but, if we can manage water in the target formation, we can manage pressure and ultimately, risk.”
In the study, the research team describes a process for controlling reservoir pressure by pumping brine from the target formation prior to carbon dioxide injection.
A single well is used to first withdraw brine then fill the de-watered voids with liquid carbon dioxide. That way, rather than using carbon dioxide to push water out of the way, which can cause unpredictable fracturing, it fills a prepared void and most of the formation’s porous spaces can be used for carbon storage.
This increases reservoir storage capacity and the CO2 never has a chance to build up excessive pressure and stays where it should. The produced brine can be treated for beneficial use.
Dr. Jeri Sullivan Graham, co-author from Los Alamos National Laboratory, points out that the extracted saline water may be a valuable resource if economical desalination can be achieved.
“The water from the formations that we studied in the Tianjin region is brackish-that is, relatively low in salinity. This means that desalination and reuse of the water in this very water-stressed region is highly feasible and could be a game-changer in terms of water resource augmentation.”
Once a zone around a well is filled with carbon dioxide, another well can be developed to repeat the cycle. By replacing withdrawn water with carbon dioxide, the pressure can be returned to the original level, preventing either cap rock fracture or subsidence.
“Another benefit of removing brine prior to storing CO2 is that this removal provides the well-field operators important information about the character of the target formation before any CO2 is stored, which reduces operational risk.” said Dr. Thomas Buscheck, co-author of the study and earth scientist with Lawrence Livermore National Laboratory.
This concept of using multi-purpose wells for reservoir characterization, injection, and withdrawal may be useful in developing other types of underground injection wells where cap rock fracturing and induced seismicity is an issue.
The project was supported by the U.S. Department of Energy’s U.S.-China Clean Energy Research Center’s Advanced Coal Technology Consortium (ACTC). The study is now available online at http://www.journals.elsevier.com/international-journal-of-greenhouse-gas-control/.