Tittle: Structure characteristics and evolution mechanism of nanopore in transitional coal-bearing shale
Abstract: Nanopore plays a crucial role in shale gas adsorption, and its characteristics and evolution are significant for the evaluation of shale reservoirs with strong heterogeneity. In this study, transitional coal-bearing shale samples from the south margin of North China were selected for investigation of pore structure characteristics and evolution mechanism. Results indicated that nanopores of the studied shales are fractal with strong heterogeneity. The higher fractal dimension leads to larger surface area and micropore volume of shale samples. The abundant terrigenous material and limited hydrocarbon generation and expulsion are favorable for the development of intraP pores in clay, so that the Organic matter (OM) pores are poorly developed and are partially filled with clay minerals and pyrite framboids. Shale samples having higher levels of clay content and lower levels of quartz content tend to develop more heterogeneous micropores, producing a more complex pore structure in shale. The evolution of nanopore in studied shales is simultaneously determined by the coupling relationship between clay content and the thermal maturation of OM. In the early oil and low clay content stage, the low degree of the thermal evolution of OM leads to generation of less acidic fluid during the decarboxylation of kerogen, resulting in a weak physical combination and chemical corrosion between OM and clay minerals, with the macropores taking the main place in the pore structure. In the main to late oil and medium clay content stage, hydrocarbon generation of OM leads to the development of pores in kerogen and the gradual generation of more acidic fluids that dissolve clay minerals, so that the mesopores are dominant. In the wet gas and high clay content stage, increases in hydrocarbon generation intensity and clay mineral content cause development of more complexes of OM and clay, with a large number of micropores produced in shale reservoirs.
Citation: Yu K., Ju Y., Shao C., (2020). Structure characteristics and evolution mechanism of nanopore in transitional coal-bearing shale. Journal of Petroleum Science and Engineering, 184, 106545.
