Abstract
Coalbed methane (CBM) reservoirs are naturally fractured formations with cleats surrounding the coal matrix. Analyzing and predicting CBM production performance is challenging because of the complex fracture networks and gas-water two-phase flow, along with the permeability and porosity variation and time-dependent desorption. During depletion, pressure decreases and consequently stress increases, thereby decreasing cleat aperture and permeability, while the matrix shrinkage effect increases cleat width and permeability. The two effects compete with one another, and dominance depends on coal's geomechanical properties and the change in reservoir properties. In addition, though the gas composition from CBM wells predominantly consists of methane, it contains traces of other gases like ethane, carbon dioxide, nitrogen, etc. The combined effects of pressure-dependent diffusivity, time-dependent desorption, and multicomponent sorption in a two-phase flowing CBM well make the flow dynamics in CBM reservoirs complex. A modified semi-analytical mathematical model is developed to characterize the time-dependent desorption and pressure-dependent diffusivity phenomena, also considering the dynamic variation of composition and multicomponent desorption for two-phase flow of CBM wells producing under semi-steady state conditions. The novelty of the model presented in this study lies in manifesting multicomponent sorption effects in the form of an equivalent single-component sorption phenomenon by establishing new transforms. A fully coupled mathematical model is developed that integrates multicomponent sorption effects in CBM reservoirs with pressure-dependent diffusivity and time-dependent desorption effects. The Article presents a new method to intertwine the desorption material balance of individual components with fully coupled flow equations and gives a workflow to predict and evaluate the production performance of a producing CBM well. The study reveals that a multicomponent, two-phase coalbed methane system can be represented in the form of an equivalent single-component single-phase system, thus paving the way for application of CBM flow equations valid for a single phase and single component. The results of the model have been verified using a CMG-GEM numerical simulator as well as in-field production data. This study brings out a novel computational approach for the compositional simulation of the CBM wells, which is unconditionally stable and involves less complexity than commercial simulators, without compromising the accuracy of the results. The findings of this study can help improve the understanding of the complex flow behavior of CBM reservoirs, where multiple phenomena occur simultaneously, like multicomponent sorption, two-phase flow, pressure-dependent diffusion, and time-dependent desorption.