| As a green and clean energy source,coalbed methane(CBM)plays an important role in current and future energy development and people’s livelihood utilization.However,at present,the understanding of transfer law between the matrix and fracture in process of CBM exploitation is still unclear,and needs further clarification and research.Through literature research,it was found that there are many studies on the transfer law of slight-compressible or incompressible fluids such as oil and water,and less for compressible fluids such as gas,especially CBM.The transfer law of single phase is concentrated,while the understanding of multiphase transfer is not clear enough,especially CBM-water two-phase transfer.In the theoretical aspect,paper derived the transfer functions of CBM single-phase and CBM-water two-phase considering the adsorption-desorption,diffusion,stress sensitivity and non-Darcy effects of CBM.These transfer functions are respectively based on Warren-Root model,Lim-Aziz model and Zimmerman model.In the experimental aspect,experiments law of transfer was obtained.The investigation results indicate that:(1)The transfer functions of CBM single-phase were obtained based on Warren-Root model,Lim-Aziz model and Zimmerman model,which considered the CBM adsorption-desorption,diffusion,Klinkenberg effect,non-Darcy effect and stress sensitivity of matrix.Besides,considering CBM-water capillary force,the transfer functions of CBM-water two-phase were also obtained.(2)CT scanning and high pressure mercury intrusion indicated that there are micro-nano pores and fractures in coal samples,which proves that study of dual-pore media for coal samples is correct.(3)Transfer experiment results showed that when boundary fracture pressure is instantaneously reduced from initial pressure to a certain pressure and maintain it constant or sealed,the transfer of CBM from matrix to fracture will occur.(4)Transfer flow lasts for several days;the relationship between fracture pressure,cumulative transfer volume and time can be expressed by a logarithmic function.Besides,the relationship between cumulative transfer rate and time is power function.(5)The measured instantaneous transfer rate increases rapidly at beginning and then decreases,and slow intermittent transfer phenomenon occures by desorption effect in the middle and late stages and the curve presents two-stage feature.(6)The instantaneous transfer rate obtained by fitting cumulative transfer volume is inversely proportional to time.(7)As depressurization amplitude increases,the transfer time and cumulative transfer volume all increase.The constant in instantaneous transfer rate formula obtained by fitting is also increasing.(8)In addition,as long as there is water,the cumulative transfer volume and transfer rate of CBM change immediately,which is most obvious in the initial stage of transfer.(9)Under the same pressure difference,as water saturation increases,the end time gradually decreases and some parameters such as transfer rates are also significantly reduced,and final transfer degree is also gradually reduced,which are most pronounced when pressure difference is small. |
