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.