The Multitype Pore Structure Evolution Characteristics of Ziliujing Formation Shale from the Northeastern Sichuan Basin in China: Experimental Study Using Small Angle Neutron Scattering Technology

Abstract

The high complexity and heterogeneity of shale pore compositions and abundant nanopores complicate the multiscale characterization of a shale reservoir pore structure. As the degree of geological evolution changes, the pore structure and connectivity also undergo complex changes and the thermal simulation experiment is a practical method to reveal the evolution characteristics of shale within a relatively short time period using the time–temperature compensation principle. In this study, thermal simulation experiments were used to obtain shale samples of the Ziliujing Formation in Northeastern Sichuan Basin with different evolution degrees. The pore structure of the samples was quantitatively characterized by SANS combined with N2 adsorption and mercury injection capillary pressure (MICP) methods. The pore morphology was qualitatively characterized by the field emission scanning electron microscopy (FE-SEM) experiment. Finally, the influence of thermal evolution on the pore structure of shale samples was investigated. The results show that the pore size distribution of SANS is similar to that of MICP and N2 adsorption methods, and the pore volume is mainly concentrated in 5–100 nm. The porosity and pore specific surface area obtained by the three characterization methods showed the trend of SANS > MICP > N2 adsorption, and the difference in pore structure was mainly affected by fluid accessibility. With the increase in thermally simulated temperature, the total pore volume and pore specific surface area of shale samples show a trend of an increase first and then a decrease. The proportion of >10 nm pores increases gradually, and the fraction of closed pores shows a downward trend as a whole. Basically, with the increase in thermally simulated temperature, the pore connectivity is enhanced, and the sample heterogeneity is weakened, all of which are conducive to the migration of shale reservoir fluid. Overall, clarifying the evolution of closed pores in shale reservoirs is of great significance for estimating original oil and gas reserves more accurately and improving oil and gas recovery.