Lawrence Berkeley National Laboratory

Abstract Analytical solutions with infinite exponential series are available to calculate the rate of diffusive transfer between low‐permeability blocks and high‐permeability zones in the subsurface. Truncation of these series is often employed by neglecting the early‐time regime. In this paper, we present unified‐form approximate solutions in which the early‐time and the late‐time solutions are continuous at a switchover time. The early‐time solutions are based on three‐term polynomial functions in terms of square root of dimensionless time, with the first coefficient dependent only on the dimensionless area‐to‐volume ratio. The last two coefficients are either determined analytically for isotropic blocks (e.g., spheres and slabs) or obtained by fitting the exact solutions, and they solely depend on the aspect ratios for rectangular columns and parallelepipeds. For the late‐time solutions, only the leading exponential term is needed for isotropic blocks, while a few additional exponential terms are needed for highly anisotropic rectangular blocks. The optimal switchover time is between 0.157 and 0.229, with highest relative approximation error less than 0.2%. The solutions are used to demonstrate the storage of dissolved CO 2 in fractured reservoirs with low‐permeability matrix blocks of single and multiple shapes and sizes. These approximate solutions are building blocks for development of analytical and numerical tools for hydraulic, solute, and thermal diffusion processes in low‐permeability matrix blocks. Key Points We develop unified‐form approximate solutions for diffusive fracture‐matrix transfer for isotropic and anisotropic matrix blocks We determine the solution coefficients that depend only on area‐to‐volume ratio or aspect ratios for anisotropic blocks We apply the developed solutions to block and reservoir‐scale diffusion of dissolved CO 2 for solubility trapping