Investigations of water saturation and fractal characteristics in tight sandstone gas reservoirs using centrifugation and NMR experiments

In this paper, the petrophysical properties were analysed by X-ray diffraction (XRD), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) experiments and the centrifugation and NMR experiments were conducted to investigate the water saturation and fractal characteristic in tight sandstone gas reservoirs. Experimental results showed that connate water saturation increases with decrease in permeability. Average connate water saturation, connate water saturation at the paraxial end and NMR connate water saturation are 58.4%~80.6%, 37.3%~60.7% and 64.6%~84.6%, respectively. The optimum centrifugal force is 1.70 MPa~2.40 MPa. A formula for calculating average water saturation was derived for different permeability cores. In addition, three models for calculating connate water saturation were presented. Finally, the fractal characteristics of pore throat using the improved NMR method were analysed. Results showed that the structure and surface of pore throat are irregular and heterogeneous, and the distribution of connate water becomes more complex and irregular after the centrifugation, compared with the saturated state. [Received: February 1, 2019; Accepted: January 20, 2020]