Controlling factors of free gas enrichment in deep coal seams and their implications for exploration and development

XI Zhaodong; TANG Shuheng; DUAN Lijiang; ZHANG Songhang

doi:10.13247/j.cnki.jcumt.20250202

Abstract：In recent years,significant breakthroughs have been made in the exploration of deep coal-bed methane(CBM)in North China,revealing substantial resource potential.However,the un-clear mechanism governing the differential enrichment of free gas has hindered"sweet spot"predic-tion and development planning.Focuses on the typical deep CBM blocks in North China,a ternary coupling mechanism of"burial depth-preservation-reservoir capacity"which controlls the enrich-ment of free gas in deep coal seams was elucidated systematically by geological analysis,experimen-tal testing,and numerical simulation.The results shows that burial depth exceeding the critical ad-sorption depth is a crucial prerequisite for free gas formation,favorable preservation conditions are the key to free gas retention and accumulation,and sufficient reservoir space determines the scale of free gas enrichment.These three factors form an inseparable organic whole,and the absence of any one condition leads to differential enrichment of free gas in deep coal seams.Typical blocks in the southern Ningwu Basin,the Yushe-Wuxiang Block of the Qinshui Basin,and the northern eastern margin of the Ordos Basin were selected for analysis.The results shows that in the southern Ningwu Basin,free gas enrichment is absent because the coal seam burial depth does not reach the critical depth for adsorption capacity.In the Yushe-Wuxiang Block of the Qinshui Basin,intense tectonic modification in later stages caused large-scale gas escape.In the Linxing-Shenfu Block on the northern eastern margin of the Ordos Basin,differences in reservoir capacity primarily governs the differential enrichment of free gas.The results of numerical simulations further indicate that the degree of free gas enrichment is a key controlling factor for the production of deep CBM wells.A higher degree of free gas enrichment is conducive to achieving higher productivity.The efficient ex-ploration and development of deep CBM require defining the optimal"sweet spot"depth window.Within this window,coal seams should possess both high gas content and a significant degree of free gas enrichment to ensure favorable resource composition and scale.Furthermore,it is essential to comprehensively evaluate the stress-strain characteristics and fracability of the coal seams to guaran-tee effective hydraulic fracturing and economic recoverability.This research provides a crucial theo-retical basis for selecting exploration areas and optimizing development strategies for deep CBM.It holds positive significance for promoting the scaled development of China's CBM industry.