Pore-fracture Structure And Mechanical Damage Mechanism Of High Volatile Bituminous Coal Reservoir During CBM Production In The Southern Junggar Basin

The Jurassic terrestrial high volatile bituminous coals(HVBC)are deposited in the southern Junggar Basin(SJB),which is currently regarded as one of the hottest spots for coalbed methane(CBM)development in China.Based on the sophisticated characterization of pore-fracture structure of HVBC in the southern Junggar Basin,researches on reservoir mechanical damage,including stress sensitive damage,water blocking damage,and gas trapping damage,were conducted based on experimental simulation and numerical simulation methods.Pore scale effect of reservoir mechanical damages and its relationship with pore-fracture structural complexities were studied by means of applying multiple low-field nuclear magnetic resonance techniques.Forming mechanisms of reservoir mechanical damages were revealed.The results showed that:(1)volumetric proportion of micro-pores and transition pores of HVBC deposited in the SJB was the greatest,followed by that of meso-pores,and that of macro-pores was the lowest.Morphologies of fractures and adsorption pores,pore-throat distribution,fractal characters,and pore-fracture connectivities were significant in heterogeneities;(2)pore scale effect existed in stress sensitive damage of HVBC,namely stress sensitive damaged degrees were different in different scaled pores,and macro-pores and fractures were the key apertures influencing stress sensitivities;(3)stress sensitivities and their variation of the up-and down-direction parts of steeply inclined reservoir were heterogeneous during CBM production;(4)water blocking damages of HVBC were closely related to retained water content.Water saturation increased logarithmically,as water flooding time increased.Invaded water content and retained water content after nitrogen displacing were closely related to the complexities of pore-fracture structure;(5)gas trapping damage degree was closely correlated to the morphologies of fractures and adsorption pores.Induced by gas trapping damage,water seepage channels were occupied by those trapped gas,leading to the reduction of relative permeability of water,and eventually resulting in the hindered groundwater depressurization process and decreased CBM production.Results of this article have theoretical significance in deepening the understanding of mechanical damages of HVBC during CBM production.This study can also give instructions on actual CBM production practices.