Adding carbon dioxide (CO2) instead of water to the fluids used in fracking could improve the efficiency of shale gas extraction, reports a paper in Nature Communications. The findings could also point to an effective means of storing CO2 underground.

Hydraulic fracturing, or ‘fracking’, is a way of recovering fossil fuels (such as methane) trapped in reservoir rocks by injecting fluids underground at high-pressure, thereby creating tiny fractures through which shale gas can escape. Typically the fluids used in fracking consist of water mixed with sand and other additives. Although supercritical CO2 - a high-pressure state where CO2 is neither a liquid nor a gas - has also been employed in the past, associated costs have limited its use.

Benoit Coasne and colleagues use computer simulations and statistical modelling to study shale gas reservoirs at the nanoscale. They model the effects of different types of fracking fluids on interactions between molecules of methane and the pore structure of rocks in a reservoir. They then show that methane recovery is hampered when water is used as a fracking fluid because wet pore surfaces in the rocks surrounding the reservoir act as energy barriers preventing methane molecules from being removed. They find that this leads to a rapid decline in methane production rate, and note that these results from modelling are in agreement with previous field measurements. Replacing water with supercritical CO2 eliminates the energy barrier, and no rapid decline in production rates is observed, the authors find.

They also show that CO2 naturally replaces methane within the pores, raising hopes that CO2 could be stored in the ground during fracking, minimising carbon footprints and possibly reducing structural damage to the underlying rock formations.