Abstract: The micro-pores present different wettability in shale gas.The organic pores are oil-prone wettability and the inorganic pores are water-prone wettability.Based on the differences of wettability between different pore types of shale,Nuclear Magnetic Resonance (NMR) T₂ spectrum is used to determine the pore size distributions of both organic and inorganic pores.The specific procedures are as follows.Firstly,NMR experiments are performed separately on shale core samples saturated with brine and dodecane respectively to identify T₂ spectra of the organic pore and the inorganic pore.Secondly,joint mercury injection capillary pressure (MICP) and liquid nitrogen adsorption experiments are carried out to establish the quantitative relationship between T₂ and pore-diameter distribution(r_d=52T₂).Finally,this formula is used to determine the pore size distribution of organic pore and inorganic pores.These methods are applied to the evaluation of the pore size distribution of the shale gas reservoir in the Silurian Longmaxi Formation in Fuling area,Sichuan Basin.The diameters of organic pores are mainly distributed in the range of 2~50 nm with a peak at 10 nm,and some are in the range of 200~500 nm.The inorganic pores show a wider range of diameter,i.e.2.5 nm~500 nm,with a peak at 50 nm.The micro-cracks show the largest size scale and range from 4 μm to 10 μm in width,with a peak at 5 μm. The pore size distributions derived from NMR coincide with that derived from FIB-SEM quantitative analysis.NMR experiment can measure the whole core and thus can reflect the real in-situ pore size distributions of organic pores and inorganic pores of gas shale.Moreover,the cost of NMR measurement is low,thus demonstrates a good application prospect in gas shale evaluation.

Key words: NMR, organic pore, inorganic pore, shale gas, Longmaxi Formation, Fuling area, Sichuan Basin