Carbon dioxide could be locked underground in hunt for natural gas

Shale gas production, in which natural gas trapped within shale formations is released after a process known as hydraulic fracturing - or fracking - has become an increasingly important source of energy in the US in recent years.

Many countries, including the UK and China, hope also to pursue this unconventional source of natural gas.

But despite around twenty years of commercial shale gas extraction in the US, very little is known about the flow of gas through the ultra-tight porous environment of shale formations, according to Dr Lei Wu at Strathclyde University.

Now Wu is leading an EPSRC-funded project to investigate this process, with the aim of using CO₂ to enhance the recovery of shale gas from geological reservoirs.

After around 10-20 years of shale extraction, when the tail production cannot cover the operational costs, the conventional procedure is to seal the wells forever. However, in a recent field experiment in Tennessee in the US, carbon dioxide was used in a shale reservoir to further increase the production of methane, said Wu.

“They injected CO₂ into the depleted formation, and then closed the well for a few months, allowing CO₂ to diffuse into the shale matrix. They then reopened the well,” he said.

The researchers found that not only did the CO₂ enhance the methane production, but some of the greenhouse gas had been absorbed by the shale matrix, allowing it to be sealed and permanently captured, he said.

“The shale formation prefers CO₂ to methane, so the CO₂ is absorbed and the methane is desorbed,” he said.

To better understand this process, Wu and his colleagues will be developing gas kinetic theory, in which the dynamics of the gas is mimicked by a limited number of “particles” moving from site to site and colliding with each other, to investigate the flow of gas through the shale formation.

Unlike a conventional gas reservoir, pores within a shale matrix can be on the nanometre scale. At this scale, conventional fluid dynamics cannot be used, while full molecular dynamics simulations are too computationally demanding, said Wu.

“We need to know how much CO₂ is needed to replace the methane in the shale formation, how much more methane can be released, and how much CO₂ will be permanently absorbed by the shale matrix."

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