Evaluation of Microscopic Pore Structure Characteristics and Flow Mechanism of Tight Oil Reservoirs in the Ordos Basin

Abstract

To elucidate the impact of the pore structure in tight sandstone reservoirs on the seepage dynamics during water flooding, an exhaustive characterization of the micro-pore architecture of the Yanchang Formation’s Chang 6 tight sandstone reservoir was undertaken. This involved employing a suite of advanced techniques, encompassing Field Emission Scanning Electron Microscopy (FESEM), Nuclear Magnetic Resonance (NMR) spectroscopy, and High-Pressure Mercury Intrusion (HPMI) experiments. The study delved into how these intricate pore structures influence oil-water seepage characteristics, further analyzing the mechanisms governing water displacement of oil within the micro-pores of the tight sandstone. The results indicate that the primary pore types in the Yanchang Formation Chang 6 tight sandstone reservoir of the Wuqi area are residual inter-granular pores, dissolution pores, inter-granular pores, and a limited number of micro-fractures. The pore size distribution (PSD) curves for these reservoir types display both unimodal and bimodal morphologies, with a corresponding decline in reservoir performance and permeability, as well as an increase in heterogeneity. When the permeability is less than 0.15mD, submicron pores (0.1-1.0 µm) are the primary contributors to permeability, averaging at 59.1%. Conversely, when permeability exceeds 0.15mD, micron pores (>1.0µm) become the main contributors, with an average of 52.5%. The oil displacement efficiency in submicron pores is significantly higher (average 13.1%) compared to that in nanopores and micropores. Notably, when permeability is greater than 0.15mD, the oil displacement efficiency in micropores increases markedly and becomes dominant. This research achievement offers theoretical support for the effective utilization and enhanced oil recovery in tight oil reservoirs.