Effects of brittle/ductile deformation of coal measure shale on pore structure in Juhugeng Mining Area, Qinghai

The effect of brittle/ductile deformation on coal measure shales is one of the key factors in the evaluation of coal measure shale reservoirs in complex structural areas. This paper takes the brittle/ductile deformation coal measure shales in the Juhugeng Mining Area of Qinghai province as the research objects, and selects the shale samples of ductile deformation and brittle deformation with nearly the same mineral compositions, and based on scanning electron microscopy (SEM), low temperature liquid nitrogen adsorption/desorption, and nuclear magnetic resonance (NMR) experiments, the macroscopic and microscopic characteristics of the brittle/ductile deformation of shales and their influence on the pore structure were summarized. The results show that the brittle deformation mainly causes a large number of tensile or shear fractures in coal measure shales, but does not change the arrangement of the mineral particles; while in the ductile deformation shales, there are a large number of wrinkles, kinking zones, and irregular erection of minerals changing the tight arrangement of mineral particles, which not only increases the pore volume and specific surface area, but also causes the original pores to be compressed and deformed. At the same time, the effects of brittle deformation and ductile deformation on the shale pore types did not show any obvious differences, but the total pore volume, specific surface area, porosity, and the effective porosity of the ductile deformation samples were significantly higher than that of the brittle deformation samples: ductile deformation samples werecharacterized by total pore volume of 11.970×10-3-13.820×10-3 ml/g, specific surface area of 5.779-8.287 m2/g,porosity of 4.39%-7.34%, and effective porosity of 1.34%～2.52%, while brittle deformation samples were characterized by total pore volume of 5.546×10-3-7.720×10-3 mL/g, specific surface area of 4.343-4.545 m2/g, porosity of 1.24%-2.76%, and effective porosity of 0.43%-0.72%,respectively, and the increase of pore volume and specific surface area caused by ductile deformation mainly occur within the range of pores smaller than 100 nm. In addition, as the pore size increases, the proportion of non-connected pores in ductile deformation samples gradually decreases, while that in brittle deformation samples does not change. So it can be inferred that the ductile deformation reduces the connectivity of pores with smaller diameters, but increases the connectivity of pores with larger diameters, while the brittle deformation affects the pore connectivity in different pore sizes without any difference.