Pore size distribution is of extreme important in the shale gas reservoir evaluation and exploration. However, shale is kind of quit complicated material, and traditional well logs (i.e. acoustic, density and neutron) only measure total porosity, while it cannot quantity the different pore size. Also, the laboratory NMR test is not suitable for filed scale well evaluation. Thus, we used NMR well logs to analyze different pore size distribution, we found that relaxation time <4.5ms is the bound fluid volume, which occupied the most part of the total NMR porosity. Furthermore, we also found that another new cutoff (except T2 cutoff) presence in the T2 distribution figure. We conclude that such new cutoff is for routine free fluid volume and micro fractures volume. Finally, we found that bound fluid volume is the main part for shale gas storage, and the micro fractures did not contribute a lot.
Fully understanding of the shale's pore size distribution and bound water T2 cutoff are key important for shale gas reservoir (Cao Minh et al. 2012, Odusina and Sigal 2011), especially for the reservoir capacity estimation and hydraulic fracturing (Yu et al. 2016a, Yu et al. 2016b). Shale gas reservoirs are of complex pore types which presented the multi pore system: nano-pores in the organic matter, micro pores between the hard minerals and clay and micro-fractures along the bedding (Sondergeld et al. 2010). However, such pore characteristic especially bound fluid volume and micro-fractures, which directly influence the gas storage and hydraulic fracturing method. Loucks (2012) reported that nanometer- to micrometer-size pores and the natural fractures along with them which is the flow path during gas production (Loucks et al. 2012). Saltt also claimed that microscale and nanoscale pores within organic matter in shales come from SEM is importance to storage (Slatt and O'Brien 2011). The former studies have fully investigated the pore types in the shale, but limited literature to demonstrate the method to quantity them. These different types of pore will show bound fluid volume and free fluid volume from the NMR data Nadia (2016) has reported that the T2 cutoff for shale is 0.24ms−0.26ms, this study data is from NMR experiment (Testamanti and Rezaee 2017), however, it is not suitable for NMR logs (Georgi et al. 1999). Therefore, no significant attention has been to given to evaluate the NMR well logs for pore size distribution in the shale gas reservoir. Thus in this paper, we used NMR log data to get different pore size distribution, and find the T2 cutoff for bound fluid volume.