Molecular Dynamics Insights into Bioconversion Induced Matrix Strain

Abstract

ABSTRACT The total annual US consumption of natural gas is expected to surpass 40 trillion cubic feet in the coming years. Microbially enhanced coalbed methane (MECBM) aims to replicate naturally occurring microbial pathways to generate methane from in-situ coal. In a basic gamut of lab-characterization experiments investigating properties of coal as a reservoir, it was revealed microbial treatment of coal results in swelling of the coal matrix. Bio-strains in the matrix result in changes in connected porosity, and its stress-state which governs the flow behavior throughout the life of the producing reservoir. Use of molecular dynamics (MD) enables us to utilize the bio-strain data to understand the dynamic stress-state development in a MECBM reservoir. The Wiser coal molecule was used as the representative molecule, whose reactive potential was minimized using PCFF. The stable MD system enables application of strain, which enables the analysis of internal stresses. Results indicated internal stresses developed during bioconversion exceeded the Von Mises failure criterion for the sample tested in the laboratory under hydrostatic pressure (0.2 MPa). However, the internal stresses for sample under in-situ stress regimes was suppressed, far from the tensile failure conditions. INTRODUCTION AND BACKGROUND There has been a sustained increase in the demand for clean energy sources such as hydrogen and methane, especially as the world works towards meeting sustainable climate goals (Jun et al., 2016). Natural gases like methane have lower carbon footprint, as it generates approximately 50% of the carbon when compared to burning oil and coal for electricity generation (Tollefson, 2012). As such, natural gases are set to go up in production rates to meet environmental demands. Recent data indicates that the world demand for natural gas is expected to increase till 2045, with a significant portion of this demand to be met by increasing production from unconventional resources, such as coalbed methane (CBM) (Birol, 2017; Gonzales, 2021; IEA, 2022). CBM refers to naturally occurring methane extracted from coal and coal seams, which is an unconventional source of natural gas (Haldar, 2018). However, to meet the increasing demand for natural gas, researchers have made efforts to find ways to increase the production of coalbed methane. One such method is to replicate the natural production of methane formed by microbial breakdown of organic components present in coal (Flores et al., 2008; Strapoć et al., 2008; Midgley et al., 2010; Penner et al., 2010). This process of producing microbial methane from coal is referred to as Microbially Enhanced Coalbed Methane (MECBM) (Scott, 1999).