Block scale investigation on gas content of coalbed methane reservoirs in southern Qinshui basin with statistical model and visual map

Abstract

This study performs a block scale investigation on gas content of a coal reservoir in Zhengzhuang Block of the southern Qinshui basin in China. The gas content of Coal Seam No. 3 in this coal reservoir was measured in field and laboratory in conjunction with tests on coal properties such as adsorption isotherm, maximum vitrinite reflectance, coal composition and maceral component etc. Total 36 coal cores collected from 3 adjacent coalmines in the southern Qinshui basin were investigated, including analysis of logging data from the drilling wells. The investigations provided experimental data for block scale modeling and visualization analyses on the correlation between gas content and the key factors such as coal properties and geological conditions of the coal reservoir. Data obtained by field and lab tests were analyzed by statistical models in order to correlate gas content and individual type of coal properties and geological variables. The statistical model was then used to map the gas content of the target coal seam in the studied area, resulting in a flood map of gas content at a 1:50000 scale. The flood map was further visualized with other variables in terms of the properties of coal and coal reservoir and its geological conditions. These visualized maps provide useful geological interpretation for block scale investigation of the comprehensive relationships between the gas content and the coal properties and regional structure in the given coal reservoir. The results show that gas content has little correlation with coal rank, maceral composition, coal thickness, cap and bottom lithology, while it is highly related to the structural properties such as burial depth and effective cover thickness. A stagnant hydrodynamic condition is favorable to the higher gas content on the whole but does not contribute to gas lateral and local variation. Canonical correlation and principal component analysis on the statistical model reveal the key factors that control the gas content are burial depth, effective thickness of overlying strata, groundwater level and moisture content in coal seam.