Exploring subterranean carbon capture

Figure 1: Schematic of carbon capture and storage showing a typical depth of which CO₂ would be injected. Courtesy of United States Environmental Protection Agency (http://www3.epa.gov/climatechange/ccs/).

Imaging CO₂ injection

In carbon storage, CO₂ from the exhaust stream of a power station is injected deep underground into porous rock.  The physical and chemical properties that determine the movement and long-term fate of injected CO₂ are all controlled by processes at the pore, or micron scale.  The Diamond Manchester Imaging Branchline (I13-2) at Diamond Light Source provides a highly intense beam and extremely fast detectors that combine to enable rapid data collection. The research team took advantage of these capabilities to observe fluid flow under typical conditions. This means measuring at the pore scale, at high temperatures and pressures that are encountered in storage aquifers.

According to Dr Branko Bijeljic, Senior Research Fellow at Imperial College's Department of Earth Science and Engineering, and Principal Investigator on the study: "previously it had been a considerable challenge to mimic subsurface conditions. Using I13-2 allowed us to observe dynamics of CO₂ flow in a carbonate rock formation at conditions similar to those encountered within the Earth's crust, rendered down to a resolution of several microns and time slices at intervals of tens of seconds.”

Figure 2: Experimental flow apparatus used in this experiment. A) High-pressure syringe pumps were connected to the flow cell and reactor using flexible flow lines. B) Detail of the core assembly showing aluminium core wraps to prevent the diffusive exchange of CO₂ across the flouro-elastomer sleeve, a low-permeability porous plate used for flow control, and the PTFE spacer used to monitor the CO₂ –brine interface during system pressurisation. Valves are numbered V₁₋₁₄. Courtesy of Andrew et al, 2015.

Dr Bijeljic’s team focused on the dynamic processes at play during injection of supercritical CO₂, such as multiphase flow and reactive transport at a range of conditions. This has allowed them to observe key events such as drainage (injection into brine-saturated rock, Video 1, Fig. 3) and subsequent displacement by brine as the CO₂ migrates underground, which can lead to a safer storage.

Figure 3: A rendering of a large drainage event. Regions occupied by CO₂ after the jump but not before shown in blue, the regions occupied by CO₂ both before and after the event shown in yellow and the regions occupied before but not after the event shown in red. This shows not only the interface recession resulting from the quasi-static capillary pressure change associated with the drainage event, but also the snap-off of one of the throats occupied by CO₂ prior to the jump. Courtesy of Andrew et al, 2015.

Dr Bijeljic adds, "Wherever the CO₂ moves, it can leave behind a trail of blobs trapped in the pore space.  We have observed the formation of these so-called ganglia for the first time at reservoir conditions.  The work shows that migration and leakage of CO₂ is unlikely, since the CO₂ will become largely or entirely trapped in the rock."

What this means

By demonstrating how injected CO₂ in its own phase is trapped, and how dissolved CO₂ reacts with the host rock, Dr Bijeljic confirms that the team has elucidated the processes that determine the long-term fate of CO₂ in the subsurface: "Our findings help us achieve the goal of safe CO₂ injection and maintenance of storage sites."

He continues: "Flow and reactive transport between CO2 saturated brine and solid rock has provided new insights into how CO₂ behaves underground, including the change in pore space due to dissolution of rock surface by brine equilibrated CO₂." The channel growth due to rock dissolution is presented in Video 2.

"This allows us to visualise and quantify the arrangement CO₂ and brine in selected carbonate and sandstone samples during CO₂ and water injection on a pore by pore basis. Moreover, these pore-scale experiments provide very useful benchmark data for validation of pore-scale models for multi-phase flow and reactive transport in porous media."

The work also promises extensive practical applications that could be of great benefit to society, aside from obvious atmospheric benefits gained from securely capturing CO₂. According to Dr Bijeljic, the team's efforts have expanded further into multiphase flow in oil recovery processes and diffusive processes in contaminant transport.

For more information on the I13 beamline, or to discuss potential applications, contact the Principal Beamline Scientist Professor Christoph Rau: christoph.rau@diamond.ac.uk.

Funding for the study was provided by the Imperial College Consortium on Pore-Scale Modelling, and the Qatar Carbonates and Carbon Storage Group, provided jointly by Qatar Petroleum, Shell, and Qatar Science and Technology Park.

Vidoe 1: Multiphase flow - drainage of supercritical CO2 in brine saturated rock. Red colour represents CO2 drainage in rock sample.

Video 2: Reactive flow - channel growth due to dissolution of rock surface by brine equilibrated CO2. Video shows both 12 minutes and 120 minutes into the experiment.

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