Recovery of Retrograde Condensate from Naturally Fractured Gas-Condensate Reservoirs

This paper considers the flow behaviour of retrograde condensate in naturally fractured gas condensate reservoirs, and the possibility of recovering part of the condensate by gravity drainage. A geologically realistic model for strongly folded and faulted carbonate structures is assumed: a stratified reservoir with permeable layers between impermeable barriers, cut by a system of vertical open fractures. Two analytical flow models are presented that describe retrograde condensation and subsequent condensate drainage within the fractures and within the matrix rock, respectively. In both models, capillary phenomena at the fracture walls are taken into account. The models are applied to calculate the potential for retrograde condensate recovery in the Waterton reservoir (Alberta, Canada). In this gas-condensate reservoir, condensate may have accumulated by gravity drainage, as indicated by some production tests. The calculated results are in agreement with the observations: for fracture density, matrix permeability and reservoir-layer thickness typical of the Waterton reservoir, a small part of the retrograde liquid will indeed accumulate within a practical time span. Condensate accumulation and recovery will significantly increase if the reservoir pressure is restored e.g. by lean gas or nitrogen injection.