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.
For cased hole completions communication between the wellbore and producing formation is re-established through the perforations. It is accepted that underbalance perforating can facilitate perforation clean-up and thus permit more effective flow. Other researchers have investigated the level of pressure underbalance necessary to clean-up perforations in sandstone. The extent to which perforating underbalance correlations and their underlying assumptions may be applied to carbonates was unknown.
This report describes nine single shot laboratory flow tests performed at the Schlumberger Productivity Enhancement Research Facility (PERF) in Rosharon, Texas to evaluate underbalance pressure perforating strategies in carbonates. The perforating experiments were carried out under simulated downhole conditions. The perforation damage and subsequent clean-up were evaluated as functions of rock and simulated wellbore conditions. The perforating experiments were conducted with Bedford Limestone and Silurian Dolomite cores and 15 gram and 21.7 gram deep penetrating charges. The 15 gram and 21.7 gram charges are hereafter refered to as charge A and B respectively. The perforation tests were conducted at initial wellbore underbalance pressures ranging from 500 to 4000 psi. Five of the nine perforation tests were shot into limestone core targets using both charge A and B. Four of the nine perforation tests were shot into dolomite core targets with charge A.
The results of the perforating experiments with carbonate target cores are not well described by published 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 starting 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.
Several authors have proposed correlations to estimate the level of underbalance pressure necessary for perforation clean-up. This earlier work focused on sandstones and is not necessarily applicable to carbonates.
King, Anderson, and Bingham1 compared the level of total pressure underbalance and formation rock permeability to the success of acid treatment for 90 wells. Successful acidization was defined as improving the production rate by 10% or more.