Improved Methods for the Modeling and Conduct of Vertical Interference Tests Across a Tight Zone

Abstract

ABSTRACT Vertical interference testing to determine the effective vertical permeability of tight zones is an important part of reservoir characterisation. The analytical model of Bremer has been extended to include the effect of wellbore storage and skin using a real time convolution formula. This appraoch has been validated by comparison with the results of the dual permeability model in Laplace space proposed by Economides. Practical considerations relating to the effective conduct of such tests are given. In particular procedures for pressure testing of packers and eliding sleeve doors are recommended along with good cement bond logs. Reciprocal testing of both layers as active and observation layers gives much improved results. Several field examples have been analysed with success. The depth of investigation of such vertical interference tests is related to the gauge resolution. The effect of wellbore storage on observation layer response can be predicted by analysing the active layer response in the conventional manner. This reduces uncertainty in the determination of vertical permeability. Large negative skin factors due to acid stimulation lead to problems with the Laplace space method. Analytical results have been complemented by numerical simulation to characterise for example the effect of tight zone capacity and extension. Numerical simulation was also used to show up the effect of leaks behind casing on the observation layer response. A distinct change in the character of its signal occurs with a casing leak.