In this paper semi-analytical solutions for interference testing in a two-layer reservoir are presented. The two layers may or may not be separated by a semi-permeable shale barrier. The solutions are derived assuming that (i) pressure equilibrium existing in the more permeable layer, due to its high vertical permeability, allows only radial flow in this layer, and (ii) the radial flow in the tighter layer is negligible due to the permeability contrast between the two layers. The upper reservoir boundary condition allows the modeling of either a no-flow or a constant pressure upper reservoir boundary which can be used to represent a gas cap. Wellbore storage and skin effects are considered at both active and observation wells. The dimensionless pressure and pressure derivative solutions are obtained for both layers at the observation well and for the producing layer at the active well.
The results obtained indicate that the interference pressure behavior in a stratified reservoir with vertical crossflow taking place from the supporting layer to the producing layer is affected significantly by the transmissibility ratio of the two layers, the properties of the shale barrier separating the two layers, and the skin and wellbore storage at both active and observation wells. It is shown that depending on the magnitude of the wellbore storage at the active and/or observation well, the early time interference pressure behavior may be masked making proper interpretations of early-time pressure transient test data difficult.