Solvent injection has been considered as an efficient method for enhancing oil recovery from fractured reservoirs. The success of this method therefore depends on the degree of enhancement of the mass exchange rate between the solvent residing in the fracture and the oil residing in the matrix. If the mass transfer Would be solely based on diffusion, oil recovery would be unacceptably slow. A series of soak experiments have been concluded to investigate the mass transfer rate between the fracture and the matrix. In a soak experiment, a porous medium containing oil is immersed in an open space containing the solvent to simulate the matrix and the fracture, respectively. We use a CT scanner to visualize the process. The experimental data are compared with a simulation model that takes diffusive and gravitational forces into account. We find that the initial stage of all experiments can be described by a diffusion based model with an enhanced "effective diffusion coefficient". In the second stage enhancement of the transfer rate occurs due to the natural convection of solvent in the fracture. The experiments are quantitatively modeled by numerical simulations. We find that transfer rates depend on the properties of the rock permeability, the viscosity and the density of solvent and oil. The gravity enhanced transfer is quantified by comparison of experimental and simulated results.