Recharge Models for Devonian Shales

Abstract

SUMMARY Devonian shale gas reservoirs, which are believed to be highly fractured, require the use of dual porosity simulators to adequately describe the dynamic production processes. When using a dual porosity model, one has the option of using the pseudosteady- or the unsteady-state matrix recharge models. The recharge model determines how much gas can flow from the shale matrix, where most storage takes place, to the fracture system which provides the formation permeability. This study examines rigorously the differences between the two recharge models and how these differences impact specifically the performance calculations of Devonian shale wells. The paper presents the circumstances for which each of the models is appropriate. A parameter is given which roughly estimates when the unsteady or the pseudosteady models should be used. This parameter depends only on the shale matrix properties. The paper shows, both theoretically and by example, that even when the pseudosteady model is approximately valid for the calculation of cumulative productions, shut-in pressure calculations may still require the use of the unsteady model.