Microscopic pore throat structures and water flooding in heterogeneous low-permeability sandstone reservoirs: A case study of the Jurassic Yan'an Formation in the Huanjiang area, Ordos Basin, Northern China

Abstract

• The pore throat structure is classified based on inflection points of fractal curves. • The heterogeneity is caused by the varying proportions of micro-structure types. • The water flooding is mainly affected by the heterogeneity of pore throat structure. The complexity of the pore throat structures of heterogeneous low-permeability reservoirs causes uneven water flooding, resulting in the retention of a considerable amount of residual oil. Therefore, it is necessary to characterize pore throat structures and the effects of these structures on water flooding. Such structures and effects were investigated using scanning electron microscopy (SEM), cast thin sections (CTS), high-pressure mercury intrusion (HPMI) and microvisualization seepage experimentations. In the fractal dimension curves, there were two relatively close inflection points, corresponding to pore throat radii of approximately 3 µm and 0.02 µm. On this basis, the pore throat structures of all the samples were categorized into macrothroats, mesothroats and microthroats. All the samples contained these three types of pore throat structures. The heterogeneity of the pore throat structure was caused by the varying proportions of the different pore throat structure types. This was the first analysis of the effects of these varying proportions of pore throat structure types in one sample on water flooding. The influences of the proportions and pore throat radii of the different pore throat structures on water flooding were notable. Through this study, the effects of the pore throat structures on water flooding were found to be controlled by the proportions of the different pore throat structure types. This finding is of great significance to the efficient development of heterogeneous low-permeability reservoirs.