Mind the Gaps: Tiny Pores Encourage Methane Formation

One of the big questions in deep carbon research, which helped inspire DCO’s founding and has been championed by the Deep Energy Community, is whether underground methane forms due to the activities of subsurface microbes (biotic) or from physical and chemical reactions between fluids and minerals (abiotic).

One possible source of abiotic methane is serpentinizing systems, such as hydrothermal vents. At these sites, fluids circulate within the cracks of certain mantle rocks to produce hydrogen, which can then react with carbon dioxide to form methane. While serpentinizing systems appear to be rich in abiotic methane, benchtop experiments and thermodynamic calculations have not always supported the idea that this reaction is feasible.  

In a new paper in the journal Scientific Reports [1], Deep Energy Community Members Thu Le, Alberto Striolo (both at University College London, UK), and David Cole (The Ohio State University, USA), investigate the impact of tiny pores in rocks on the generation of abiotic methane in the subsurface. Using computer simulations, the researchers calculated that tiny, nanometer-sized pores attached to the network of fractures in rocks can grab onto water molecules and change the concentration of carbon dioxide and hydrogen in nearby cracks. This phenomenon, called confinement, may help drive abiotic methane production in the subsurface.

To perform these computational experiments, Cole teamed up with Heath Turner, a chemical engineer at the University of Alabama, Tuscaloosa, and an expert in simulating chemical reactions. They used a method called reactive ensemble Monte Carlo (RxMC) simulations, which chemists developed about 20 years ago to predict the results of complex chemical reactions. This approach estimates whether a chemical reaction will proceed or not in a specific environment by modeling the behavior of the molecules that may participate in the reaction.  

“The beauty of this [study] is that we’ve leveraged an approach that has been seldom used to tackle Earth science problems,” said Cole.

“The beauty of this [study] is that we’ve leveraged an approach that has been seldom used to tackle Earth science problems,” said Cole. He thinks that the RxMC simulations could be useful to investigate other complex systems in Earth science research.  

Specifically, the researchers used the RxMC approach to simulate the Sabatier reaction, a chemical reaction believed to occur in the subsurface where carbon dioxide reacts with hydrogen to form methane and water (expressed chemically, as CO2 + 4H2 => CH4 + 2H2O). The researchers predicted how the existence of pores in silica rock measuring one to ten nanometers, which is about the width of a protein, would affect the Sabatier reaction occurring within fractures in the rock.

In their calculations, they varied different characteristics of the pores and applied temperatures ranging from 350–700 °C and pressures of 10 to 50 times the atmosphere at sea level, to better mimic subsurface conditions.

The simulations suggested that these pores, especially smaller ones with rougher surfaces, sequester water molecules. These confinement effects increase the concentration of hydrogen and carbon dioxide molecules within the network of cracks in the rock, and make it more likely that they will react to make methane. These pores exist in the crust near the surface and also occur deeper within the mantle, so confinement effects may impact methane formation throughout the subsurface.

The researchers point out that the silica in their simulation represents a highly simplified rock environment compared to actual serpentinizing systems. “It was a very simple proof of principle approach we took,” said Cole.

In the future, Cole hopes to perform bench-scale experiments to reproduce the conditions used in the RxMC simulations and to compare the results. The group also has plans to borrow techniques from materials science. By using synthetic materials they will be able to accurately control pore size, and fine-tune surface chemistry to see how those changes affect fluid behavior.

 

At hydrothermal vent systems, melted rock from the mantle heats the ocean crust, forcing seawater to circulate through cracks and microfractures in the rock, where serpentinization reactions occur. Credit: Thu Le et al./Scientific Reports

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