Numerical Analysis of Condensate-Flow Impairment in Retrograde-Gas Naturally Fractured Reservoirs and Its Interplay With the Estimation of Diffusion Coefficients

Abstract

Summary Previous studies dedicated to the analysis of retrograde condensation in naturally fractured gas/condensate reservoirs have concluded that the presence of a multimechanistic-flow environment in tight naturally fractured reservoirs hosting retrograde gases can have a great effect on the depletion behavior of the reservoirs (Ayala et al. 2004; Ayala et al. 2006; Ayala et al. 2007). In these systems, condensate dropout below dewpoint conditions is typically immobile and greatly impairs the flow of the other phases, adversely affecting reservoir productivity and ultimate recovery. Whenever this flow impairment becomes severe, diffusion can take over as the main recovery mechanism. In such cases, fluid recovery is driven by both molecular-concentration fields (i.e., diffusive transport governed by Fick's law of diffusion) and the pressure field (i.e., bulk transport governed by Darcy's law). Previous multimechanistic studies relied upon the assumption that the diffusive flow component could be represented as a constant value throughout the reservoir's productive life. However, diffusion coefficients in hydrocarbon systems can be shown to vary significantly during the reservoir's producing life as a function of prevailing pressure, temperature, and composition conditions. The present study captures the influence that varying composition and fluid properties, such as density and viscosity, have on the value of the diffusion coefficient at different stages of reservoir depletion. It is concluded that the assumption of constant diffusive coefficients can yield good predictions for cases where fracture-depletion rates are small in particular. Diffusion coefficients and diffusive effects are also shown to exhibit a strong dependency on fracture-depletion rates. The present work reassesses the effect of multimechanistic flow on the isothermal depletion of tight, naturally fractured retrograde-gas reservoirs while capturing the full dependencies of the diffusion coefficient on reservoir fluid properties and their interplay with the formation of the condensate bank around matrix edges.