Numerical Modeling of Gas Flow in Coal Using a Modified Dual-Porosity Model: A Multi-Mechanistic Approach and Finite Difference Method

Numerical simulation, a cost-effective technique used to describe and predict gas flow behavior in coal, can provide a reliable means of evaluating gas well performance, solving complex engineering problems, and determining how variations in reservoir properties and gas drainage practice affect wellbore performance at the field scale. This paper first presents a multi-mechanistic gas flow model using a modified dual-porosity model by incorporating the effects of multiscale transport mechanisms in coal, effective stress evaluation, and matrix shrinkage. Then, a numerical model and a simulator are derived using the finite difference method to solve the proposed model and are successfully tested against two sets of in situ gas extraction field data. Subsequently, the effect of parametric variations on gas transport behavior in dry coal seams is quantified through a series of simulations. The simulated results imply that (1) a greater initial pore pressure or a lower gas drainage pressure will lead to a higher gas flow rate; (2) the fracture permeability as well as matrix permeability plays significant roles in gas production profiles, and the former mainly influences the early initial depletion stage, while the latter controls gas flow rate and production decay rate at the late stage; (3) the mass transfer rate between matrix blocks and fractures varies with the distance from the matrix block to the free coal surface, and a shorter distance will result in a greater mass transfer rate; and (4) the decrease of the matrix radius increases the gas released from matrix and leads to a greater gas flow rate and a slower decay rate. The results also indicate that size and matrix permeability are two key parameters affecting the overall gas deliverability, and play a critical role in gas production at the late stage.