The Characteristics Of Pore-Fracturesin Middle-High Rank Coals In Shanxi And Its Influences On Permeability

Based on 23 coal sample,a series of experiments and analysis,such as nuclear magnetic resonance(NMR),mercury intrusion porosity(MIP)and N2 adsorption/desorption,were carried out to study the characteristics of pore-fractures and its influence on the permeability.The complexity of coal reservoir makes the characteristics of pore-fractures difficultly to define.Therefore,this article combines NMR,MIP and N2 adsorption/desorption with pore-fractures fluid to study the characteristics of pore-fractures and its influence on the permeability by using a clear and concise fractal method frommicro perspective.Firstly,the study area is located in the central Shanxi Province,and the coal bearing stratas are Shanxi formation and Taiyuan formation,the micro fractures are highly developed and its number is from 593 to 4540.Thestructure for micro fractures is dendritic and reticulation type,has a good connectivity,which indicates that Shanxi block is the structure coal developed area.Secondly,the heterogeneity of pore-fractures system in Shanxi is the fractal heterogeneity: NMR fractal dimension(DNMR)< N2 fractal dimension(DN)< MIP fractal dimension(DMIP).By the influence of bulk relaxation(T2B)and diffusion in internal field gradients(T2D),NMR method characterized the micropore has some limitations.The percentages of different pore-fractures have a huge jump at Ro,m =1.8-2.0%,which indicates a third coalification jump.Thirdly,the heterogeneity of pore-fractures fluid in Shanxi is that moveable fluid fractal(DM)> irreducible fluid fractal(Dir)>saturated fluid fractal(DW).The fluid space has four typical types.The shapes “C”,“Z”,“S”,and “T” correspond to four structure types A,B,C,and D.The influences of coalificationon the percentage of different pore-fractures aredifferent from the influences on the porosity.Finally,permeabilityindicates the influences of pore-fractures.The connectivity of pores becomes good and the weakening pore anisotropy gives fractal dimension DNMR aspecific value,and permeability is over 0.025 mD(millidarcy);permeability also presents a positive relation with fractal dimension DNMR;the surface morphology of pore-fractures produces thependular ring affecting the seepage ability and reducingthe permeability;permeability is affected by the irreducible fluid space(BVI)and the movable fluid space(BVM),indicates a positive exponential function with BVM/BVI for seepage pores,and a negativeexponential function with BVM/BVI for adsorbed pores and transition pores.The NMR,MIPand N2(77K)method combining fractal analysis withcoalification enables us to characterize pore-fractures and the effects of coalification on porefractures quantitatively.The relationship between permeability,pore-fracture heterogeneity and BVM/BVI is beneficial forpredicting the favorable areas for CBM exploitation.