Impacts of mineralogy and pore structure on spontaneous imbibition in tight glutenite reservoirs

Spontaneous imbibition plays a critical role in the two-phase flow in hydraulic fractures of tight reservoirs. Although massive contributions have been devoted to addressing this issue, very few studies focus on tight glutenite reservoirs, which is an important member of tight reservoirs. As a result, the imbibition mechanism and its influencing factors are still unclear, several key factors fail to be captured, and in-depth analyses are lacking in spontaneous imbibition for tight glutenite reservoirs. In light of the current knowledge gaps, this study conducted a series of water imbibition experiments, combined with X-ray diffraction tests, thin section and scanning electron microscopy observations, high-pressure mercury intrusion experiments, and corrected contact angle experiments to reveal the impacts of rock mineralogy and pore structure on spontaneous imbibition, and significance levels of main influential factors were determined by grey relation analysis. Results show that: 1) the hydrophilic mineral that exceeds 94% provides a strong driving force for water imbibition. Intergranular pore and gravel boost imbibition recovery through enhancing imbibition and drainage ability, respectively. However, intergranular dissolved pore is unfavorable for water imbibition due to extremely rough surface and small radius. Medium-size sand also has a negative impact because of incremental flow tortuosity and pore coordination number reduction; 2) A large percentage of micropore and macropore can significantly enhance imbibition rate and recovery. Three fractal characteristics in micropore, identified for the first time, complicates water imbibition process in tight glutenites. A critical total fractal dimension of micropore is observed, above which the negative influence magnitude of complex pore structure on spontaneous imbibition gradually decreases; 3) Water imbibition sequence and diversities of imbibition phenomena are significantly affected by the combination of multi-fractal dimensions and heterogeneous distributions of pore volume in different fractal regions. The superposition of smallest fractal dimension and the largest pore volume leads to the best imbibition recovery in the transition stage of imbibition, which is an extremely time-consuming process; 4) Upon grey relation analysis, oil drainage ability is the most critical factor for water imbibition in tight glutenites. These observations deepen our understanding of the distinctive imbibition mechanisms in tight glutenite reservoirs.