Pore accessibility by wettable fluids in overmature marine shales of China: Investigations from contrast-matching small-angle neutron scattering (CM-SANS)

The characterization of pore accessibility is essential for shale oil and gas reservoir evaluation. However, owing to the extremely low permeability and high heterogeneity of shale components, accurate nanoscale evaluation of pore accessibility remains challenging. In this study, a systematic investigation of nanoscale pore accessibility characteristics and their controlling factors was undertaken via contrast-matching small-angle neutron scattering (CM-SANS) on four overmature marine shale samples from China. In addition, a series of experiments integrating the air-liquid contact angle, spontaneous imbibition, mercury intrusion capillary pressure (MICP), and field-emission scanning electron microscopy, were conducted to determine the shale wettability and pore connectivity characteristics. A novel accessibility index was developed and utilized to analyze the accessibility of fluids with different wetting phases in nanoscale pore networks and their changes over time. The results from the CM-SANS test of Wufeng-Longmaxi shale samples indicate that higher pore volumes (pores >7 nm) were filled with toluene due to the development of organic pores and the connection between organic pores and inorganic pore systems. Meanwhile, more water-accessible pores were observed within a pore size of less than 7 nm. This result indicates that the pore volume is perhaps composed of hydrophilic pores related to clay minerals or created by the swelling of clays. Furthermore, the combination of CM-SANS on samples (before and after combustion) and MICP showed that pore accessibility to water and toluene was controlled by pore surface wettability and pore connectivity. Thus, CM-SANS is an effective means of evaluating the nanoscale pore accessibility of shale, revealing the relationship between wettability and pore connectivity, which can help better understand the migration of hydrocarbon fluids and the imbibition of fracturing fluid.