Effects of Microheterogeneous Wettability and Pore-Throat Structure on Asphaltene Precipitation in Tight Sandstone: Results from Nuclear Magnetic Resonance

The pore throat structure and microheterogeneous wettability of tight sandstone reservoirs are complex, which leads to varying asphaltene precipitation locations, contents, and distributions in different pores during CO₂ flooding. Clarifying the heterogeneous wettability of different pore throat structures and their effects on asphaltene precipitation and adsorption is crucial for improving CO₂ displacement efficiency. A series of experiments were conducted in this study, including X-ray diffraction (XRD), cast thin section (CTS), field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion (HPMI), environmental scanning electron microscopy (E-SEM), nuclear magnetic resonance (NMR), and CO₂ flooding experiments, to investigate the pore structure complexity of tight sandstone reservoirs of the Yanchang Formation in the Ordos Basin, China. Furthermore, we investigated the variations in microheterogeneous wettability across diverse pore-throat structures and elucidated the impact of heterogeneous wettability on asphaltene precipitation during CO₂ flooding. The findings indicate that the type and configuration of pore throats are crucial factors influencing microheterogeneous wettability. The intergranular pores are dominated by mixed wetting, and most of the dissolution pores exhibit oil wetting. The surface of Illite shows drop-like water under E-SEM, which is mainly oil wetting, whereas the surface of chlorite shows film-like water, which is water wetting. The configuration of chlorite intercrystalline pores and intergranular pores shows water wetting, whereas the configuration of Illite intergranular pores and dissolution pores shows oil wetting. During the CO₂ flooding process, asphaltene tends to be adsorbed in the intercrystalline Illite with dissolution pores, reducing the dissolution pore volume and blocking small pores, and the displacement efficiency becomes low. In addition, asphaltene precipitation also occurs in the pore configuration of chlorite intercrystalline and intergranular pores, causing a wetting reversal on hydrophilic mineral surfaces. This reversal increases the pore throat structure complexity but has less of an impact on the flooding efficiency. A high Illite content is more likely to lead to asphaltene precipitation, significantly influencing small pore structure and the oil displacement efficiency.