Microscopic pore-throat structure variability in low-permeability sandstone reservoirs and its impact on water-flooding efficacy: Insights from the Chang 8 reservoir in the Maling Oilfield, Ordos Basin, China

Abstract

The Chang 8 reservoir of the Maling Oilfield in the Ordos Basin, China is facing a series of challenges in hydrocarbon resource development, including rapidly decreasing production rates, declining dynamic fluid levels, and elevated water cuts in oil wells, along with heterogeneity in microscopic pore-throat structures and notable interstratal inconsistencies. To systematically address these issues, this study selected representative samples from the reservoir and conducted rigorous microscopic percolation experiments on them. A comprehensive evaluation of the heterogeneity in microscopic pore structures was conducted using an integrative methodological approach, involving physical property quantification, petrographic thin-section analysis, scanning electron microscopy, constant-rate mercury intrusion, and nuclear magnetic resonance techniques. The primary objective of this investigation is to elucidate the underlying formation mechanisms, states of occurrence, and spatial distributions of residual oil. Understanding of these issues will facilitate the establishment of empirical correlations between diverse microscopic pore structures and water-flooding efficiencies, and aid in the identification of key variables governing the distribution of residual oil. Analytical outcomes reveal substantial variations in seepage characteristics contingent upon the nature of microscopic seepage conduits. Specifically, the Chang 8 reservoir manifests four discernible categories of microscopic seepage pathways: solely intergranular pores, a confluence of dissolution and intergranular pores, exclusively dissolution pores, and micropores. A correlative decline in oil displacement efficiency is observed across these conduit types. Critical variables such as throat radius and its distribution patterns emerge as pivotal determinants influencing oil displacement efficiency, eclipsing the impact of conventional physical properties and mobile fluid saturation levels. Remarkably, samples characterized by a composite of dissolution and intergranular pores demonstrate superior displacement efficiency. Distinct types of pore structures correspond to noticeably different water-flooding oil pathways and oil displacement efficiencies. During the water-flooding process, fingering network displacement is dominant, and it exerts a significant control on oil displacement efficiency. Key factors affecting this efficiency include the injected water volume multiples and displacement pressure, values of which should be optimized during the actual water-flooding process.