Carbon Capture and Storage (CCS) - a geological perspective

Background

The process of Carbon Capture and Storage is divided into 3 phases:

• CO2 capture
• Transport to a suitable location
• CO2 storage in subsurface

 As something of a geology fiend, I am particularly interested in how CO2 is stored within rocks. It is this topic that I will address within this post.

Geological requirements

 CO2 can be stored in rocks with the following charecteristics (Bachu 2008):

• A high porosity (large spaces between sedimentary grains) to be able to uptake CO2
• A high permeability (ability for CO2 to travel between pore spaces, so that CO2 can be pumped into the ground at a rate which makes it worthwhile)
• A formation overlain by a cap rock (An impermeable rock which prevents the upward migration of CO2, which has a natural tendency to do so because of its low density)

 The 3 characteristics outlined in the paper are somewhat simplified, particularly because of the effect of geological structures. For example, if the sequestration site was cross-cut by faults (structural planes of weakness), these could act as permeability pathways which would allow CO2 to migrate towards the surface and escape.

Enhanced oil recovery (EOR)

 When I began to research this topic, the same point was mentioned in almost every paper abstract that I read (e.g Steeneveld et al. 2010). That point was that the storage of CO2 within geological formations is already widespread. This is because CO2 has been used as an enhanced oil recovery method for decades, particularly in the US.

 This review paper by Alvarado & Manrique provides an overview of the use of CO2 for enhanced oil recovery. Put simply, CO2 is pumped into oil reservoirs when recovery yields begin to decline. The less dense CO2 displaces the oil present in pore spaces of rocks, forcing it up a well to the surface. In many cases this can increase reservoir yield by 10-15%. The following youtube video from Richland Community College represents the process using a jar of rocks, effervescing tablets, vegetable oil and water.

 It is somewhat misleading to suggest that the act of geologically sequestering CO2 is commonplace, as the ultimate destination of sequestered CO2 isn't the priority of oil companies employing the method; they are driven entirely by oil yield. 

 This statement also ignores the irony of the process, in that CO2 is used as a mechanism to produce more CO2-producing fuels. Because of this, the ability of CO2 to escape geological formations and the geological viability of the process has, until relatively recently, been poorly understood.

What can we learn from EOR?

 Though little information of the sub-surface behaviour of CO2 can be gleaned from EOR, there are valuable lessons to be learnt for CCS. Namely, EOR proves the viability of large infrastructure projects that exist solely to collect CO2 from industrial processes, transport it and deposit it in geological formations (Bachu 2008). 

 Indeed, as a non-flammable and non-toxic gas (until very high concentrations) it is much safer than other commonly exploited gases such as Hydrogen and Ammonia. Bachu (2008) notes that the greatest threat posed by release of CO2 from sequestration infrastructure is actually global warming.

Seismic imaging of CO2

 So, if the oil industry isn't undertaking comprehensive research to understand what happens to CO2 when it is injected into the subsurface, how can we begin to understand how it behaves?

 Ironically, the technique commonly used to image sequestered CO2, seismic imaging, is a product of the evolution of oil exploration. Seismic imaging essentially involves the production of sound waves on the land/sea surface, which propagate down into subsurface rock layers. 

 Depending upon the compressibility of each rock layer (how easy it is to squash) the characteristics of the reflected seismic waves will vary. Using this information, scientists can reconstruct the subsurface geology. The process is explained in this youtube video below from the Queensland Resources Council.

 This technique was created for oil exploration purposes to hunt for oil and gas within the subsurface. As global demand for oil has increased, the technique has evolved and improved. 3D seismic surveys are now commonplace in the oil industry. 

 For the purposes of tracking sequestered CO2, 4D seismic is used. This is essentially 3D data taken at different time intervals. This review paper by Lumley 2010 explains the principles of 4D seismic interpretation for imaging CO2 in the sub-surface. This is a good high level explanation of the process, however the simple statement that the CO2's low compressibility relative to dry rock allows it to be imaged within the subsurface isn't eluded to in the paper.

4D seismic imaging of sequestered CO2 - a case study 

 This paper from Chadwick et al. (2005) explains how CO2 in the North Sea was injected into a saline aquifer (porous body of rock with a high concentration of saline water in its pore spaces) between 1991 and 2001.

 Using 4D seismic imaging the authors were able to view the evolution of the injected CO2 plume over 10 years. They observed that the CO2 remained confined to the targeted rock layers, and that no loss of CO2 occurred. The evolving CO2 plume is shown in the diagram below.

 This study and others (see Alnes et al. 2008) suggest that the risk of CO2 escape from storage is low. Other 4D seismic studies (Verdon et al. 2010) have found that there has been little or no influence upon earthquake activity from CO2 injection.

Conclusion

 Existing EOR projects highlight the safety & viability of infrastructure dedicated to CO2 capture, transport and storage. Seismic imaging has found that the threat of CO2 escape from geological formations is negligible. Other threats, such as earthquakes induced by increased stresses within rocks due to CO2 input, have also been debunked. From a purely geological standpoint, CCS really does make sense.

Further reading

 If this post hasn't satisfied your thirst for CCS knowledge, this lecture from Biondo Biondi further explains the use of 4D seismic imagery to image CO2.