Distribution patterns of various occurrence states of deep coalbed methane in the Baijiahai Uplift,Junggar Basin,Xinjiang

YANG Zhaobiao; LI Cunlei; GUO Qiaozhen; WANG Bin; LIU Changqing; QU Huailin; LIANG Yuhui; WANG Yuqiang

doi:10.13247/j.cnki.jcumt.20240080

Abstract：Deep coalbed methane(CBM)exhibits significant quantities of free gas,distinguishing it from shallow CBM and rendering it a crucial component of reservoir gas resources.The distribution patterns of different occurrence states of CBM vary notably between low and medium-high coal rank reservoirs.This study investigates the distribution law of various phase states of deep CBM in low coal rank reservoirs,focusing on the Xishanyao Formation of the Baijiahai Uplift in the Junggar Basin,uti-lizing porosity and permeability experiments under variable temperature and pressure conditions,a-longside isothermal adsorption tests under equilibrium water conditions.Incorporating mathematical modeling,the S-DR model considers changes in methane adsorption state density in temperature and pressure fields,establishing a prediction formula for adsorption state content in the Baijiahai uplift.Prediction charts of different gas content are developed based on the porosity prediction formula,inte-grated with temperature-pressure-dependent phase density of CBM.Results reveal three critical depths of gas content:adsorbed gas(about 900 m,11 m3/t),free gas(2 500～2 600 m,9 m3/t),and total gas(about 1 500 m,18 m3/t).The proportion of free gas increases with depth,reaching 50％at 2400 m and up to 80％at 5 000 m,influenced by adsorbed state density,free state density,and poros-ity changes under deep temperature and pressure.The Baijiahai uplift's CBM demonstrates enrichment characteristics of'one low and three high':Low coal rank,high gas content,high free gas propor-tion,and high saturation.Considering resource scale and composition,the optimal development depth range in this region is found to be 1 500～2 600 m,with high total gas content(15～18 m3/t)and free gas proportion(40％～55％),indicating significant recoverable potential.