Occurrence characteristics and influential factors of movable fluids in pores with different structures of Chang 63 reservoir, Huaqing Oilfield, Ordos Basin, China

Exploration and development of Chang 6(3) reservoir in the Huaqing Oilfield, Ordos Basin is challenging because of the complex geological features, including strong heterogeneity, complicated microscopic pore structures, low saturation of movable fluids, and distinct fluid distribution characteristics. Confronted with this challenge, this study used the nuclear magnetic resonance (NMR) technique to quantitatively evaluate the movable fluids in pores with different structures. Moreover, microscopic experiments were conducted to analyze the occurrence characteristics and influential factors of movable fluids in pores with different structures, including conventional physical property analysis, image porosity measurement, casting thin section observation, SEM analysis, high pressure mercury intrusion testing, and constant pressure mercury intrusion testing. The results showed that gray and grayish brown lithic feldspar sandstones were the dominant types of sandstone. Residual intergranular pores and feldspar dissolved pores were the main pore types, with unevenly distributed pore throat radii, among which small pores and micropores were primary. Four types of pore structures could be divided according to comprehensive analysis of capillary pressure curves, namely Types I, II, III, and IV, and they were featured by movable fluids with distinct occurrence characteristics in them. Reservoir permeability was well correlated with movable fluid saturation. Multiple factors were demonstrated to influence the movable fluid saturation in pores with different structures, including pore throat radius, pore-throat radius ratio, sorting coefficient, effective pore and throat volume, maximum mercury saturation in pores and throats, and interstitial clay minerals. Generally, good physical properties led to larger variation of movable fluid saturation. Among them, effective pore throat radius and pore-throat radius ratio were the dominant factors, effective pore and throat volume as well as sorting coefficient had some degree of influence, and maximum mercury saturation and physical properties had little effect on the movable fluid saturation.