Abstract
Permeability of coal reservoirs influence the extraction of coal gas from coal seams. Twelve coal samples were collected at an anticline and a syncline of the No. 3 coal seam in the Sihe coal mine. Porosity, permeability, pore size, vitrinite reflectance, and liquid nitrogen adsorption of the samples were evaluated. Structural curvatures at the sample locations, and the distance between the sampling locations and the nearest faults were calculated based on seismic data. The influences of the evaluated parameters on permeability were analyzed. Major factors that influence permeability of the No. 3 coal seam were extracted using principal component analysis (PCA). Based on the porosity–permeability model derived from the Archie formula and classic Kozeny–Carman equation, we deduced that the permeability of coal increased with an increase in porosity. With an increase in average vitrinite reflectance, permeability decreases first and then increases. PCA results showed that coal permeability was regulated by three key components representing three modes. The first component included pore size, depth, and pore complexity accounting for 52.59% of the variability indicating that it was the most important in controlling permeability. The second component included specific surface area, structural curvature, and porosity, and the third component comprised of specific surface area, porosity, and average vitrinite reflectance. Overall, pore diameter and complexity had significant effects on coal permeability. The results show that researchers and stakeholders must consider the interactions among multiple factors rather than single factors to understand the influences on permeability to facilitate efficient utilization of coalbed methane resources.
Highlights
Permeability of coal reservoirs reflect the flow performance of fluids such as coal gas in coal seams and it is important for the simulation and identification of fluid systems in coal seams (Harpalani and Schraufnagel 1990).It is important to understand the factors that influence coal permeability for the successful exploration of coalbed methane
Our study aims to analyze the relationships between geological factors and coal permeability in the Sihe coal mine
This study examined the physical properties of 12 coal samples obtained from the No 3 coal seam in the Sihe coal mine
Summary
Permeability of coal reservoirs reflect the flow performance of fluids such as coal gas in coal seams and it is important for the simulation and identification of fluid systems in coal seams (Harpalani and Schraufnagel 1990). It is important to understand the factors that influence coal permeability for the successful exploration of coalbed methane. Many studies have been conducted on factors that control coal permeability such as fissures, coal rank, fracture aperture, geological structures, and stress on strata Cracks in coal are classified as exogenous and endogenous (Gamson et al 1993; Yao and Liu 2009). Endogenous cracks occur due to the shrinkage in coal volume and internal tensions in coal caused by the influence of temperature and pressure during coal formation. Exogenous cracks are formed by tectonic structures such as faults and folds (Yao et al 2014). Sorption-induced strain displays a non-linear behavior with continuously sorbing gas injections, and both types of strains are competing processes in causing the volumetric change in coal (Fan and Liu 2018)
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