The effect of magmatic intrusions on coalbed methane reservoir characteristics: a case study from the Hoskissons coalbed, Gunnedah Basin, Australia

Magmatic intrusions can deteriorate coalbed methane reservoir quality by precipitating minerals in natural fracture and cleat system. To date, the effect of magmatic intrusions on coal rank and maturity has been studied extensively. However, their impact on fluid flow capacity and gas content is poorly investigated. This study evaluates the impact of magmatic intrusions on reservoir characteristics of the Hoskissons coal interval in the Gunnedah Basin (eastern Australia) where numerous coal-intrusion associations exist. Drill stem test (DST), borehole image logs and core data are used to determine fluid flow characteristics, gas content and quality in 14 wells across the Gunnedah Basin. The integration of borehole image logs and DST data analysis enables us to determine the existence, openness, and hydraulic conductivity of natural fracture and cleat systems in the Hoskissons coal interval. In addition, available desorption canister data, gas composition data, and conventional well logs are interpreted to investigate probable thermal effect on gas content.Our analyses of different datasets reveal that the thickness of intrusions and their positions with respect to the Hoskissons coal interval are variable in the studied wells. Permeability varies from 1091 mD down to zero owing to heterogeneous fracture and cleat systems. Interpreted natural fracture/cleat systems are well correlated with measured permeability from DST data analysis. This highlights the role of open natural fractures/cleats in fluid flow characteristics of the Hoskissons coal interval. Results indicate that the mineralizing effect of hydro thermal fluids derived either from magmatic intrusions or coal itself is not a controlling factor in fluid flow capacity of the Hoskissons coal interval in the studied wells. This is described by either the distance between coal section and major intrusions in some wells or perhaps emplacement of intrusive bodies prior to development of cleat and natural fracture networks. The destructive impact of intrusions on permeability is observed in one of the studied wells in which in-situ stress perturbation is large (due to presence of magmatic intrusions in sedimentary rocks). Variable in-situ stress orientation can decrease fracture connectivity and consequently fluid flow properties are affected. Gas content largely varies in heat affected coal intervals. This signature is the result of thermal effect fading with distance and is more pronounced when intrusions are in close proximity to coal intervals. Gas composition is variable in the studied wells. Gas composition data indicate that high quality desorbed gas with methane concentration higher than 90% could be found even in coal intervals which are heavily intruded. (C) 2016 Elsevier B.V. All rights reserved