Numerical Modeling of Permeability Sensitivities Based on Characteristics of Heterogeneous Coal Structure Reservoirs

Abstract

Reservoir sensitivities play a crucial role in affecting the production efficiency of coalbed methane, relating to adsorption/desorption dynamics, pore–fracture interactions, and pressure characteristics within the reservoir. This study examined changes in permeability sensitivity within a heterogeneous coal structure reservoir by continuously sampling seven coal sections from roof to floor. The samples underwent low-temperature N₂ and isothermal adsorption analysis of CH₄, as well as microscopic observation experiments. Based on the results, a permeability sensitivity evaluation model was proposed. The study revealed that the No. 3 coal seam in the area maintained its integrity, primarily consisting of intact and cataclastic coal structures. The isothermal adsorption experiments and N₂ adsorption analysis indicated that the highest Langmuir V_L (24.03 cm³/g), specific surface area, and mesopore volumes were found in the middle part of the coal seam. Cataclastic coal, influenced by tectonic deformations, formed more micropores than the intact coal. The coal seam exhibited complex vertical variations in macroscopic and microscopic fractures, including differences in aperture, frequency, spacing, connectivity, and mineralization. The model also simulated changes in permeability sensitivities, considering effective stress and volumetric changes in the coal matrix during gas desorption. It was found that the middle part of the coal seam exhibited the most significant stress sensitivity and the highest permeability loss ratio. The study concludes that attention should be focused on the middle part of the coal seam in the Gaohe and adjacent coal fields for the stimulation of the coalbed methane (CBM) reservoir and prevention of gas outburst. This research is instrumental in determining CBM production design and evaluating development risks.