Lawrence A. Behrmann

New Predictive Model of Penetration Depth for Oilwell Perforating Shaped Charges

For cased and perforated natural completions, well productivity depends strongly on perforation parameters. These include system parameters (shot density and phasing), and the characteristics of the individual perforation tunnels (depth extending beyond drilling damage, the nature of the crushed zone surrounding the tunnels, and to a lesser extent the tunnel diameter). This paper describes recent research directed at improving the accuracy of penetration prediction at downhole conditions. Most conventional industry penetration models are based on laboratory experiments spanning the 1960’s through the 1990’s. It has been recently observed, perhaps not surprisingly, that these models are not always accurate predictors of the true downhole performance of modern shaped charge perforators. This situation is further complicated by the recent discovery that different industry models can give significantly different predictions of downhole penetration. The authors have addressed this situation by conducting an extensive laboratory experimental program, using the results as the basis for developing significantly improved penetration correlations. The new correlations are based solely on stressed rock performance, no longer dependant on unstressed concrete targets. This test represents a significant departure from previous approaches, but one which is necessary in order to focus our attention to targets of relevance to the downhole environment. Furthermore, this paves the way for optimization of charges for certain downhole environments, rather than for unstressed concrete. Several hundred modern charges of different sizes have been shot into multiple rock types under simulated downhole stress conditions up to 20,000psi. The new penetration correlation exhibits significantly improved accuracy in depicting the true influence of overburden stress, pore pressure, and formation strength on modern perforators. Consistent with historical work, we found that penetration depth in liquid saturated sandstones varies inversely with rock strength and applied stresses. Although the conventional models have been shown to be optimistic, this effort confirmed that the modern premium deep penetrator charges did outperform their predecessors at downhole conditions. Additional experiments with gas saturated sandstones indicated that penetration depth was shallower and less sensitive to stress level than in liquid saturated sandstones. Stronger rocks exhibit less fluid dependence. The improved correlations developed in this work are being implemented into the author’s penetration and productivity software. This will enable more accurate and reliable prediction of well performance. Introduction The primary objective of perforating a cased wellbore is to establish efficient flow communication with the reservoir. The key perforating parameters which influence reservoir deliverability are well known, and the relative influence of each has been quantified by a number of researchers [1, 2, 3, 4, 5, 6]. These key parameters include shot density, phasing, depth of penetration (DoP), tunnel diameter, and the nature of any permeability-impaired (“crushed”) zone which remains surrounding the perforation tunnels. Shot density and phasing are fixed system parameters, and therefore their values at downhole conditions are known. However, downhole values of perforation tunnel depth, diameter, and crushed zone characteristics cannot be known with certainty. These quantities must be estimated with predictive models. Therefore the accuracy of these predictive models is an essential ingredient in the accuracy of any productivity prediction. The authors are currently engaged in active research to increase our understanding of perforation tunnel depth, diameter, and crushed zone characteristics at downhole conditions. This research is leading to the development of models which more