Dynamic Underbalance perforating is a completion technique which uses a perforating system engineered to create a rapid underbalance immediately upon formation perforation. 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).
Over the past 3 years, we have conducted dozens of perforate-and-flow laboratory experiments, carefully controlling and measuring wellbore transients, and measuring post-shot productivities. Our results indicate that the cleanup mechanisms are more involved than conventionally understood. We observed that a dominant mechanism of dynamic perforation cleanup is the increase in effective perforation length. We also confirmed previous findings that DUB perforating increases tunnel diameter, while reducing the thickness of the "crushed zone" of impaired permeability, which surrounds each tunnel. Although these processes have been mentioned in general terms previously, conventional models typically simplify things by reducing perforation cleanup to the enhancement of crushed zone permeability. Though this simplification has offered a convenient means of interpreting lab data, it can yield misleading results when applied downhole.
While increasing crushed zone permeability does indeed improve the productivity of real wells, the additional processes of enlarging tunnel diameter and reducing crushed zone thickness improve productivity further. Increasing the effective tunnel length provides yet another means of productivity gain, and under most circumstances is the dominant beneficial effect. We present productivity predictions of various downhole scenarios to quantify these newly-recognized effects.
These findings suggest the performance differential (between DUB and non-DUB techniques) at downhole conditions can be far greater than previously recognized.