An important and fascinating debate is emerging in the research/scientific community concerning the ability of saline reservoirs to accept large volumes of CO2. In the first volume of a new journal Greenhouse Gases Science and Technology (wileyonlinelibrary.com), Quanlin Zhou and Jens T Birkholzer of Lawrence Berkeley National Laboratory have continued their significant contribution to the literature with a disciplined, but still highly readable paper (On scale and magnitude of pressure build-up induced by large scale storage of CO2).
As many who are reading know, saline aquifers are believed to have the greatest potential by far to store CO2. A potential problem that might occur is that if the pressure increases to a high enough level it can potentially fracture the seal above the aquifer and push salt water from the aquifer into shallower, possibly fresh water layers. With large-scale injection being considered, this pressure build-up issue needs to be examined closely.
The authors looked at existing oil and gas and groundwater injection and production and the impacts on pressures, and estimated the changes in pressure. Using these results they simulate injection into some aquifers in the US.
They chose two examples for the paper- the Mount Simon Sandstone in the Illinois Basin, considered by the authors to be an 'open' system where there are no major lateral barriers to the movement of fluids (including CO2), and the San Joaquin Basin in California, a partially closed system (which may have some partially sealed compartments). A closed system is where the aquifer is sealed off as a compartment by non-porous sealing rocks above and laterally- generally by faults that seal off the aquifer/reservoir.
They set up 20 hypothetical injection 'projects' about 30 km apart in each area. Using properties consistent for sealing formations in the basins the maximum pressure buildup over 50 year period caused by 5GT of CO2 injection does not breach through the seal and is safely contained. Compared to completely sealed systems, pressure reduction occurs in part due to the salt water already in the aquifer being displaced either over a large area or, in the case of the partly closed San Joaquin system, through water production at an outcrop.
In both cases there is no CO2 escape, and pressures can be maintained if necessary by releasing some salty water through designated wells. Understanding and monitoring pressures will always be important.
They compare their results with a paper published in 2010 by Helix- Economides and Economides, , arguing that the Economides paper unnecessarily restricts its modeling to closed systems and does not take into account pressure reduction through the salt water being displaced. Zhou and Birkholzer see closed systems as relatively rare (for example some fault-bounded oil fields).
However geology in deep saline formations will more commonly be to open or partly closed systems.
I enjoyed reading this paper and recommend it to anyone who has an interest in the science – to read the authors’ work first hand – you don’t have to be a hydrogeologist – or even a geologist to understand. It also addresses other issues that are beyond the scope of this blog. I hope to see more good work from this journal.
The work was funded by the US Office of Sequestration, Hydrogen and Clean Coal Fuels and coordinated by NETL and the US EPA, who will regulate in this area.