Summary Optimizing the soaking stage is essential for the success of hydrocarbon gas (HG) huff ’n’ puff (HnP) in shale reservoirs. However, the role of fractures in controlling solvent transport during the soaking stage is still not well understood. Here, we develop a fracture-matrix formulation that couples advection-diffusion in the fracture with diffusion in the matrix. The analytical solution quantifies fracture-matrix mass exchange under varying fracture velocities, effective diffusion coefficients in the matrix (Dm), and matrix porosities, thereby evaluating the sensitivity of the model to key transport parameters. We apply the developed fracture-matrix model on the available literature data on Eagle Ford shale samples and observe that (i) velocity strongly controls solvent concentration along the fracture; (ii) increasing Dm reduces the solvent concentration in the fracture by enhancing its penetration into the matrix; and (iii) the diffusive transport is negligible within the fracture. We conduct high-pressure and high-temperature visualization experiments on two Vaca Muerta (VM) shale cores, one fractured (VM-F) and one intact (VM-I), to validate the proposed model and evaluate HG transport during the soaking stage. We fit the analytical fracture-matrix model to the measured pressure-decline data of the soaking stage to estimate the effective diffusion coefficients in the matrix. We observe that VM-F exhibits a faster and larger pressure decline than VM-I, with the cumulative amount of HG transferred into the core being 1.7 times higher. Dm remains essentially comparable between the two cores (6.92×10−11 m2/s in VM-F vs. 7.36×10−11 m2/s in VM-I), indicating that the observed difference in pressure decline is not controlled by Dm. It is instead caused by the presence of fractures, which increase the available solvent-to-oil contact area and promote mass transfer into the matrix. We observe that HG transport in VM-F is advection-dominated in the fracture domain (NPe = 1,057) during the soaking stage, providing quantitative evidence that fractures enhance solvent supply in the matrix.
Zheng et al. (Wed,) studied this question.