Talk about a win-win situation. Compressed carbon dioxide may be more suitable than water for fracturing methane-rich rock ? a finding that could help the growing hydraulic fracturing industry extract more natural gas from spent fields. And because the carbon dioxide is then trapped below ground, the discovery could also spur the development of large-scale carbon sequestration.
Natural gas production has soared worldwide in recent years as a result of hydraulic fracturing, or fracking ? a process of injecting pressurised water into shale formations to fracture the rock and release massive amounts of natural gas trapped inside.
The more extensive the network of fractures created in the shale, the more pathways are available for the gas inside it to escape. Tsuyoshi Ishida at Kyoto University, Japan, and his colleagues have now found a way to greatly extend that network of fractures by replacing pressurised water with liquid or supercritical CO2.
On a broad scale, the pattern of fractures created in rock by conventional hydraulic fracturing is two-dimensional ? the fractures tend to occur along a plane. Ishida's team found, through experiments involving cubes of granite, that compressed CO2 yielded a fracture pattern that was three-dimensional. Ishida expects the compressed CO2 would produce a similar 3D pattern of fracturing in shale too.
It's unclear exactly why pressurised CO2 yields a different fracture pattern from water, but Ishida's team suggest it might be connected to the fact that compressed CO2 is around 10 times less viscous than water.
Viability of CO2
Shahab Mohaghegh, a petroleum and natural gas engineer at West Virginia University in Morgantown, says the study may hasten the development of large-scale carbon sequestration. Mohaghegh is studying the technical and economic viability of using CO2 to remove methane from shale, but from a different perspective.
He says that shale has a greater affinity for CO2 than methane. When CO2 is injected into a depleted shale formation ? even one that has previously been fracked ? the rock will release more methane because pockets of the gas chemically trapped within the shale will be released in favour of the more chemically attractive CO2.
"Shale is an incredible storage source and carbon sequestration can help us release more methane," he says. "If you can also use carbon dioxide to fracture the rock, that would add a third dimension that could be more significant than sequestration or enhanced recovery."
A 2006 study by the US Department of Energy assessed geologic sequestration options in the US Midwest. It found that saline aquifers offer by far the greatest potential carbon storage capacity ? around 470 gigatonnes ? but shale beds that have been fractured for methane production came in second, with the potential to hold 45 gigatonnes.
Mark Zoback, a geophysicist at Stanford University, California, says fractured shale beds may be a safer place to sequester carbon dioxide than saline aquifers where the injection of the waste gas into an already highly pressurised environment could trigger small earthquakes.
A study earlier this year also pointed out that fracking might unintentionally reduce the suitability of some saline aquifers for CO2 storage because it is often shale that forms an impermeable seal above the aquifer, preventing the gas from escaping back to the surface. Fracking the shale compromises the quality of the seal by opening up fractures.
Journal reference: Geophysical Research Letters, DOI: 10.1029/2012GL052788
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