Isotopic studies have shown that many of the world’s coalbed natural gas plays are secondary biogenic in origin, suggesting a potential for gas regeneration through enhanced microbial activities. The generation of biogas through biostimulation and bioaugmentation is limited to the bioavailability of coal-derived compounds and is considered carbon positive. Here we show that plant-derived carbohydrates can be used as alternative substrates for gas generation by the indigenous coal seam microorganisms. The results suggest that coalbeds can act as natural geobioreactors to produce low carbon renewable natural gas, which can be considered carbon neutral, or perhaps even carbon negative depending on the amount of carbon sequestered within the coal. In addition, coal bioavailability is no longer a limiting factor. This approach has the potential of bridging the gap between fossil fuels and renewable energy by utilizing existing coalbed natural gas infrastructure to produce low carbon renewable natural gas and reducing global warming.

Complex in situ behavior of fluids during a retrograde condensation process is experimentally investigated in a miniature sandstone core sample. Two depletion experiments were conducted with various pressure decline rates using a three-component synthetic gas mixture with a dew point of 3610 psi. A state-of-the-art miniature core-flooding system integrated with a high-resolution micro-computed tomography scanner was employed to acquire pore-scale evidence of condensate nucleation, growth, accumulation, and mobilization in a natural porous medium under different depletion conditions. Analysis of pore-scale fluid occupancy maps demonstrates the formation of discrete nuclei of the condensate in pore throats and crevices as the pressure drops slightly below the dew point. The in situ fluid configurations show that a greater pressure drawdown rate significantly increases the condensate growth and accumulation. The results also illustrate the occurrence of condensate-to-gas imbibition displacements, i.e., snap-off and piston-like events, and the consequent trapping of the gas phase in the pore space. As the pore pressure is reduced, the condensate droplets are found to connect to each other through wetting layers, whereas the large gas clusters are continuously fragmented into smaller globules with reduced hydraulic connectivities. This effect was more pronounced in the case of the high depletion rate experiment. Furthermore, the condensate banking was not completely eliminated (through evaporation) by re-injecting the gas phase. This implies that in the development of a gas condensate reservoir, condensate dropout and banking should be minimized in the first place by, for instance, producing at lower pressure drawdown rates.

No abstract