The Influence of SC-CO₂ on the Nanopore Structures of Different Types of Shales Based on Small-Angle Neutron Scattering and Multifractal Methods

Abstract

Supercritical carbon dioxide (SC-CO 2 ), which has a high density, low viscosity, strong solubility, and high diffusivity, is considered an ideal nonaqueous fracturing fluid for shale gas and oil extraction. However, there is still a lack of effective methods for quantitatively evaluating the changes in the pore structure of reservoir shales at the nanoscale after SC-CO 2 treatment. Small-angle neutron scattering (SANS) can not only measure the pore structure parameters of connected pores but also obtain the structural parameters of unconnected pores. This is crucial for evaluating the influence of SC-CO 2 on shale nanopore structure. In this paper, felsic laminae-type shales and mixed laminae-type shales were subjected to SC-CO 2 treatment under water-heat coupling conditions for 24 h. Then, small-angle neutron scattering (SANS) experiments were conducted on these four samples. These shale samples were taken from the Cretaceous Qingshankou Formation in the Gulong Depression of the Songliao Basin, NE China. The key parameters, such as the pore size distribution, cumulative pore area, pore volume, and cumulative porosity, were obtained. In addition, the results obtained from the SANS experiments were analyzed by one-dimensional fractal and two-dimensional multifractal methods. The combination of fractal and SANS experiments is crucial for describing structural information on complex material. The multifractal method was used to analyze the anisotropy degree of the pore size distributions (PSDs). The experimental results showed that the changes in PSD among different shale types after SC-CO 2 treatment. Moreover, the change in the pore volume distribution influenced by SC-CO 2 in the felsic laminae-type shale was significantly different from that in the mixed laminae-type shale. The results of multiple fractal analyses showed that for felsic laminae-type shales, the PSD was altered in both 1−100 nm. In contrast, for mixed laminae-type shales, PSD changed mainly in the range of 1−10 nm.