Insights into the wetting mechanisms in low-permeability sandstone reservoirs and its evolution processes: The shahejie formation in the Dongying Depression, Bohai Bay basin

Abstract

During the accumulation and development processes of low-permeability oil, wettability plays a crucial role in the percolation capacity of fluids. In particular, the evolution of low-permeability sandstones involves complex changes in formation fluid properties and mineral types, leading to a deficient understanding of wetting mechanisms in low-permeability sandstones. This gap significantly impedes research on the accumulation mechanisms of low-permeability oil. Focusing on the low-permeability sandstones in Shahejie formation of Dongying Depression, Bohai Bay Basin, tests like Casting thin section, scanning electron microscope, contact angle measurement, molecular dynamics simulation and the Amott method based on nuclear magnetic resonance technology were performed to comprehensively investigate sandstone wettability and its evolutionary process. The results indicate that the adsorption capacity of hydroxyl groups or monovalent cations (K⁺and Na⁺) for water molecules causes residual intergranular pores, feldspar dissolution pores, quartz dissolution pores, clay mineral intercrystalline pores and micro-cracks to exhibit hydrophilicity; while the adsorption capacity of divalent cations (Ca²⁺and Mg²⁺) for oil molecules causes the surface of dissolution pores of carbonate minerals to exhibit neutrality or lipophilicity. Additionally, changes in formation conditions (temperature, pressure, ion types, and ion concentration) significantly control the wettability of pore surfaces, further determining the overall wettability of low-permeability sandstone reservoirs. Throughout the evolutionary process of low-permeability sandstones, the Amott-Harvey index of low permeability sandstone is greater than zero, and the wettability exhibited hydrophilic or neutral. From A-stage eodiagenesis to B-stage mesodiagenesis, the Amott-Harvey index is 0.82, 0.12, 0.37, 0.03, and 0.49, and the wettability of reservoirs transitions through phases of strong hydrophilicity, weak hydrophilicity, hydrophilicity, neutrality, and hydrophilicity, respectively. Finally, an evolutionary model of the wettability of low-permeability sandstone reservoirs was established, which is of great significance for predicting the quality of low-permeability sandstone reservoirs.