Underbalance Pressure Criteria for Perforating Carbonates

Abstract

Underbalance perforating has long been recognized as one of the best techniques for mitigating perforating damage. Earlier studies have attempted to establish the level of underbalance necessary to stimulate perforation clean-up in sandstone. These models are stated in terms of the underbalance pressure thought necessary to remove damage from perforations for a given matrix permeability (some models also give consideration to fluid properties and perforation geometry). The purpose of this study is to evaluate the extent to which these correlations for underbalance perforating may be applicable to carbonates. Single shot laboratory flow tests were performed with limestone and dolomite cores to determine the extent of perforation damage as a function of underbalance pressure. The calculated perforation skins from the carbonate perforating experiments are not well described by the earlier sandstone correlations. In particular, the primary reliance on rock permeability as a prognosticator of perforation clean-up does not work for carbonates. Rather than relying upon the initial underbalance pressure and matrix permeability as has been done previously for sandstones, perforation skin for carbonates is best related to peak underbalance pressure and rock strength. The underbalance pressure condition is not surprising. Progressive perforation clean-up with increasing underbalance is intuitive and has been well noted by previous investigators. This study extends the concept by making reference to a peak dynamic underbalance pressure occurring during perforating. Relating rock strength instead of rock permeability to perforation clean-up is a relatively new idea that may also have merit for sandstones. The results of this study suggest that the initial mechnism of perforation clean-up is primarily related to failure of the perforation tunnel wall under stress as opposed to fluid surge flow.