Quantitative Characterization and Pore Size Distribution Analysis of Tight Reservoir Wettability Using Integrated Alternating Spontaneous Imbibition and Nuclear Magnetic Resonance Techniques

Tight reservoir wettability directly influences the flow and storage characteristics of fluids within pores, which is crucial for evaluating the mobility of oil and water during tight reservoir oil reservoir development, as well as analyzing the storage capacity of carbon dioxide and hydrogen in tight reservoir reservoirs. However, the low porosity and permeability of tight reservoir, coupled with its complex pore network structure, significantly increase the difficulty of wettability assessment. Traditional methods struggle to achieve accurate quantification of the volumes of pores with different wettability types in tight reservoir, relying more on qualitative or indirect evaluations. This study proposes a novel quantitative method for characterizing tight reservoir wettability types based on alternating spontaneous imbibition combined with nuclear magnetic resonance (NMR) fluid quantification. Through a series of alternating imbibition experiments—oil imbibition (SI-O), water imbibition (SI-W), secondary oil imbibition (SI-2O), and secondary water imbibition (SI-2W)—coupled with dynamic T2 and T1-T2 NMR monitoring, the changes in fluid content and distribution within tight reservoir pores during imbibition were elucidated. The results indicate that tight reservoir pores can be categorized into oil-wet, water-wet, and mixed-wet types, each exhibiting distinct pore size distribution characteristics. The SI-O and SI-W stages represent the fluid filling phase, during which tight reservoir pores are rapidly saturated with oil and water, respectively. In contrast, the SI-2O and SI-2W stages represent the fluid equilibrium replacement phase, where the total fluid content in the pores remains unchanged, and only the fluids in mixed-wet pores are replaced according to the imbibition fluid type. Based on the differences in tight reservoir pore wettability, an alternating imbibition model was developed and validated through T2 NMR analysis and fluid content changes. Using this model, the volumes of the three wettability types of pores were quantified, and their pore size distribution characteristics were further clarified through T2 projection spectrum analysis. Compared to traditional methods, this approach addresses the gap in quantifying and analyzing pore size distributions of different wettability types in tight reservoir, significantly improving the accuracy and reliability of tight reservoir wettability assessment. It provides a new perspective for wettability analysis and quantification in tight reservoir.