Carbon storage just doesn’t hold water

CCS which stands for carbon capture and storage or sequestration has been the great hope of middle-of-the-road characters, people other than radical ‘greenies’ or their ‘right-wing’ adversaries. Oil companies and governments have jumped on this and it has figured prominently in many energy and climate scenaria by think-tanks and universities.

In some ways it has acted as an ‘indulgence’ certificate. The logic goes that no matter what one’s position is on anthropogenic global warming, we have a way to paper over the debate. Engineers, and in particular petroleum engineers, the ones accused of destroying the planet, have a solution. Let’s literally bury the problem by re-injecting the offending gas back into the ground. Targets can be old oil and gas reservoirs but, more mentioned are deep saline aquifers of which there are plenty.

Several oil companies tout CCS, and some petroleum engineers, who ought to know better had they done simple calculations, have jumped on the bandwagon. The stakes are high and the rewards for those that promote the idea are lucrative. There is plenty of government and even company money for this. As usual, some researchers, instead of studying the very feasibility of the process, already taken for granted in some circles, have started working on peripheral aspects, such as solubility of CO₂ in water and even mineralization, things that take tens to hundreds of thousands of years to mature. Funding is plentiful.

So, the capture and subsequent geologic sequestration of CO₂ has been central to plans for managing CO₂ produced by the combustion of fossil fuels. Most agree that the magnitude of the task is overwhelming in both physical needs and cost, and it entails several components including capture, gathering and injection. But in the current political environment how can anybody assess what an appropriate cost is if saving the planet is at stake?

The reality is a lot different. What have rarely been calculated, although talked about, are the rate of injection per well and the cumulative volume of injection in a particular geologic formation which are clearly critical elements of the process.

Readers of this column are probably more familiar with my interests in energy geopolitics but many also know that I am a professor of petroleum engineering with a substantial experience in addressing the technical dimensions of energy issues. In a paper published recently with the smarter Economides, my wife, Texas A&M University Prof Christine Ehlig-Economides (Sequestering carbon dioxide in a closed underground volume, Paper SPE 124430, presented at the Annual Technical Conference & Exhibition of the Society of Petroleum Engineers, New Orleans, 4-7 October 2009) we addressed the feasibility of sequestering CO₂ as a means of emissions management. The conclusions are quite negative and, in fact, sobering.

Earlier published reports on the potential for sequestration fail to address the necessity of storing CO₂ in a closed system. Our calculations suggest that the volume of liquid or supercritical CO₂ to be disposed cannot exceed more than about 1% of pore space. This will require from 5 to 20 times more underground reservoir volume than has been envisioned by many, and it renders geologic sequestration of CO₂ a profoundly non-feasible option for the management of CO₂ emissions. Kyoto Protocol or successor accords would imply orders of magnitude larger problem than anything possible as CCS.

Published injection rates, based on displacement mechanisms from EOR experiences, assuming open aquifer conditions, are totally erroneous because they fail to reconcile the fundamental difference between steady state, where the injection rate is constant, and pseudo-steady state where the injection rate will undergo exponential decline if the injection pressure exceeds an allowable value. A limited aquifer indicates a far larger number of required injection wells for a given mass of CO₂ to be sequestered and/or a far larger reservoir volume than the former.

The implications of our work are profound. The work shows that models that assume a constant pressure outer boundary for reservoirs intended for CO₂ sequestration are missing the critical point that the reservoir pressure will build up under injection at constant rate. Instead of the 1-4% of bulk volume storability factor indicated prominently in the literature, which is based on erroneous steady-state modeling, our finding is that CO₂ can occupy no more than 1% of the pore volume and likely as much as 100 times less.

In our work we related the volume of the reservoir that would be adequate to store CO₂ with the need to sustain injectivity. The two are intimately connected. In applying this to a commercial power plant of just 500MW the findings suggest that for a small number of wells the areal extent of the reservoir would be enormous, the size of a small US state. Conversely, for more moderate size reservoirs, still the size of Alaska’s Prudhoe Bay reservoir, and with moderate permeability there would be a need for hundreds of wells. Neither of these bode well for geological CO₂ sequestration and the findings of this work clearly suggest that it is not a practical means to provide any substantive reduction in CO₂ emissions, although it has been repeatedly presented as such by others. OE

By: Prof Michael J Economides
Issue: December 2009

Michael J Economides is a professor at the Cullen College of Engineering, University of Houston, and editor-in-chief of the Energy Tribune. The views expressed in this column do not necessarily reflect OE’s position.