Enhanced mass transfer between matrix and filled fracture in dual-porosity media during spontaneous imbibition based on low-field nuclear magnetic resonance

Spontaneous imbibition (SI) of wetting fluids is an important process for many hydrological and geological applications. In this study, we investigated experimentally and theoretically the SI process in a dual-porosity medium consisting of the matrix and the filled fracture. Low-field nuclear magnetic resonance (LF NMR) technology was used to dynamically monitor the distribution of the imbibed water in the dual-porosity media during the SI experiments. Based on the LF NMR theory, a cut-off method was proposed to quantitatively distinguish the imbibed water from the matrix and the filled fracture. The results showed that the rate of matrix imbibition was greater in the dual-porosity media than in single-porosity media. This inconsistent rate of matrix imbibition between the single-porosity media and the dual-porosity media was caused by the enhanced mass transfer between the matrix and the filled fracture. An analytical model was then proposed to characterize this enhanced mass transfer between the matrix and the filled fracture. It was found that the rate of matrix imbibition was not only affected by the presence of the filled fractures but also depended on the size of the particles filled in the fracture. A clear non-monotonic relationship was found between the rate of matrix imbibition and the size of the particles filled in the fracture. The rate of matrix imbibition initially increased and then decreased as the size of the particles filled in the fracture increased. The proposed analytical model not only highlighted the mechanism of enhanced mass transfer between the matrix and the filled fracture, but was also able to determine the optimum equivalent capillary diameter of the filled particles in the fracture for enhanced rate of matrix imbibition in dualporosity media.