Prediction model for free gas content in deep coal seams based on effective porosity

SHI Rui; BIAN Liheng; ZHANG Wei; MA Zhuang; JIAO Jiahui; SHEN Jian

doi:10.13247/j.cnki.jcumt.20240320

Abstract：The content of free gas is a crucial indicator in the exploration and development of deep coalbed methane,offering significant theoretical and engineering value for evaluating the development potential of such resources.This study addresses the limitations of existing free gas calculation models by analyzing coal pore compression characteristics under deep reservoir conditions and developing a prediction model for free gas content in deep coal seams.The re-sults indicate that the free gas content of coal seams is primarily influenced by two factors:those related to the gas state(reservoir temperature,reservoir pressure,and gas compression factor)and effective porosity.The gas state,controlled by reservoir temperature,pressure,and the gas compression factor,affects gas content,while effective porosity(which accounts for pore compression and the pore volume occupied by water)determines the size of the free gas storage space.With increasing burial depth,free gas content in the Danning-jixian block initially increases before gradually stabilizing.Within the same coal seam thickness range,po-rosity is the predominant factor affecting free gas content,and pore compression characteristics and water content must be considered when calculating free gas content.As burial depth in-creases,free gas content across different coal grades rises gradually,with the increasing rate diminishing with depth.This study achieved the goal of accurately predicting free gas content in deep coal seams by considering pore compression characteristics and the pore volume occu-pied by water,thereby providing more precise reservoir physical parameters for deep coalbed methane exploration and development.