Perforation Cleanup by Means of Dynamic Underbalance: New Understanding

Abstract

Summary Dynamic-underbalance (DUB) perforating is a completion technique that uses a perforating system engineered to create a rapid underbalance immediately upon formation perforation (within tens of milliseconds or faster). This technique—properly applied—improves well deliverability by effectively cleaning the newly created perforation tunnels, regardless of initial static pressure conditions (overbalanced, underbalanced, or balanced). The authors are engaged in a multiyear program of perforate-and-flow laboratory experiments [along the lines of American Petroleum Institute Recommended Practice (API RP) 19B Section 4], carefully controlling and measuring wellbore transients and measuring post-shot productivities. Experiments thus far have been limited to static balanced conditions to ensure that any cleanup observed would be attributable solely to wellbore dynamics rather than to preshot static pressure conditions. Our results provide new insight into perforation-damage and -cleanup mechanisms. We observed that a dominant source of perforation damage can be the reduction in effective flowing perforation length, and a primary mechanism of DUB cleanup is to increase this effective length. Although increasing the permeability of the crushed zone that may surround the tunnel (a conventional simplified treatment of perforation damage and cleanup) does indeed improve the productivity of real wells, the additional processes of enlarging tunnel diameter and reducing crushed-zone thickness further improve productivity. Increasing the effective tunnel length provides yet another means of productivity gain and, under many circumstances, is the dominant beneficial effect. We present productivity predictions of various downhole scenarios to demonstrate and quantify these effects. These findings indicate that the performance differential (between DUB and non-DUB techniques) at downhole conditions can be more significant than previously recognized. Future work will explore varying initial static pressure conditions (underbalance, overbalance) in conjunction with shot-time wellbore pressure transients. Work is also ongoing to probe the limits of the findings reported herein, particularly the influences of formation properties (permeability, strength, and lithology) and drilling damage.

Comparison of Crushed-Zone Skin Factor for Cased and Perforated Wells Calculated with and without including a Tip-Crushed Zone Effect

“…(3) The comparison of the simulations reveals that there is a significant difference between each of the two tipcrushed zone scenarios (4) The numerical model gave almost identical results as for Karakas and Tariq's model, if the length of the perforations was assumed to be shorter than their real length by a thickness of crushed zone (5) The differences between the two simulations' results show that the currently available model [14] for estimating the skin factor for vertical perforated completions needs to be improved, based on which of the two cases is closer to reality (6) The study presented two novel correlations that give more than one option to calculate the crushed skin factor (7) The outcomes of this study underscore the need to include the crushed zone anisotropy effect through the improvement of available models for determining the crushed skin factor…”

“…One highly efficient way to remove the debris is through underbalanced perforating, although the precise underbalance degree required for generating effective perforations is still a topic of debate. The underbalance is usually calculated by considering the matrix permeability, along with tunnel parameters and fluid, as described by Tariq [4], Walton [5], and Grove et al [6]. More recent research has looked into various aspects of cleanup and productivity mechanisms in relation to perforation tunnels.…”