Major types and quantitative characterization of self-sealing during the accumulation of unconventional hydrocarbons in the whole petroleum system

The mechanisms underlying the self-sealing accumulation of unconventional hydrocarbons form the core of the whole petroleum system (WPS) theory. Forces driving the self-sealing process originate from intermolecular interactions, and their manifestations and mechanisms vary with reservoir media and geological conditions. In this study, a series of results and insights are obtained through systematic investigation. Essentially, the self-sealing accumulation of unconventional hydrocarbons is a non-buoyancy process dominated by intermolecular interactions. For the first time, three major mechanisms behind the self-sealing accumulation are systematically identified: interfacial effect, confinement effect, and steric hindrance effect. The principles and scaling effects of these mechanisms are accordingly defined. Factors influencing the forces that drive the self-sealing of various unconventional hydrocarbon resources are determined. Specifically, the self-sealing of tight hydrocarbons and free shale hydrocarbons is predominantly driven by capillary pressure at megapascal (MPa) level, governed by the pore size, interfacial tension, and wettability of reservoirs. In contrast, the self-sealing of adsorbed shale hydrocarbons and coalbed methane (CBM) is primarily driven by molecular adsorption forces under the confinement effect, with the adsorption energy jointly influenced by mineral surface properties, pore structure, temperature and pressure conditions, and fluid characteristics. Models describing self-sealing governed by the relative sizes of pores and hydrocarbon molecules are established. These models reveal that under ultra-tight pores, where pore sizes are comparable to molecular sizes, the steric hinderance effect predominates, causing mechanical obstruction for large molecules. When pore sizes are less than 38 times molecular sizes, differential adsorption of hydrocarbons becomes significant, leading to the gradual emergence of the confinement effect. Conversely, when pore sizes are far larger than molecular sizes, self-sealing is primarily driven by the interfacial effect. This study presents a systematic elucidation of various types of intermolecular interactions involved in the self-sealing of unconventional hydrocarbons, along with methods for their quantitative characterization. The results of this study deepen the understanding of the mechanisms governing the self-sealing accumulation and offer a theoretical guide for research into the distribution patterns of unconventional hydrocarbon reservoirs.