Microbial methane dynamics in groundwater flow systems and their potential contribution to atmospheric emissions

Methane dynamics in groundwater flow systems are critical to understanding underground microbial methane systems. The migration of methane in aqueous solution is understudied, although it can only concentrate in large quantities along longer horizontal groundwater flow paths. This is a necessary condition for the formation of commercial accumulations (as hydrocarbon resources) but also increases the potential amount of gas released to the atmosphere in the discharge areas of groundwater flows.

This study focuses on understanding the fundamental elements of an underground microbial methane system, highlighting the microbial gas generation depth range and key groundwater flow system parameters such as volume discharge, Darcy velocity, pressure, temperature, and salinity. To achieve this, by innovatively integrating hydrogeological and petroleum geological knowledge and methodologies, a Python-based computational model was developed. In addition, extensive methane and carbon dioxide solubility databases containing over 200,000 data points were created considering temperature, pressure and salinity conditions. To address gaps related to methane and carbon dioxide solubility reverse data engineering was applied using Python language.

The model comprises two principal domains: (1) a midline zone where semi-horizontal groundwater flow maintains roughly constant pressure, temperature, and salinity conditions, facilitating microbial gas dissolution, and (2) a discharge zone where upward groundwater flow triggers decrease of these parameters, leading to oversaturation and gas exsolution. Present-day microbial gas generation depth was established based on generation kinetics, while the theoretical regional groundwater flow system was characterized by the basin-scale evaluation of measured hydraulic data. Model input parameters, such as pressure, temperature, salinity, and flow velocity were sourced from existing publications. As a result, the model defines (a) the minimum horizontal migration length necessary for groundwater saturation with methane, (b) the volume of methane transported in solution, (c) the quantity of methane gas released in underground discharge zones that can be trapped, and (d) the quantity of methane gas released to the surface.

When applied to the Central Pannonian Basin, including the largest microbial gas accumulation in Hungary (Hajdúszoboszló field), the model can explain the formation of this accumulation at the end of a horizontal flow converging zone where flow direction turns upward due to the regional flow conditions and a major fault zone. From the gas amount which arrives at the discharge area during 1 million years from a 300 km² charge area, about 226 million m³ released under the surface that could be trapped and about 700 million m³ released to the surface. The latter means 700 m³ gas emission per year which only comes from groundwater discharge. Sensitivity analyses provided further insights into the controlling factors of microbial gas migration and their relationships highlighting the complexity of the system.

Ongoing work is testing the model around another significant microbial gas accumulation in Hungary (Kunmadaras field), where hydrogeological conditions are different, further refining our understanding of methane dynamics in groundwater flow systems.

The research was supported by the Papp Simon Foundation, Hungary.