A Field Study of Underbalance Pressures Necessary To Obtain Clean Perforations Using Tubing-Conveyed Perforating
Authors: King G.E. , Anderson A. , Bingham M. 
Published: December 2007

Abstract
A study of 90 wells perforated with the tubing conveyed perforating system has found a correlation between underbalance pressure and formation permeability that can be used to achieve clean perforations. The data are from gas and oil producers in clean sandstones. Data for the report are from wells which were perforated, tested, acidized, and retested. There is a clear minimum underbalance line separating the data sets of wells that had clean perforations (unassisted by acidizing) from those wells that showed a significant productivity increase after acidizing.
The study includes data from oil and gas wells in the Gulf of Mexico, Louisiana (Tuscaloosa), New Mexico (Morrow), Rocky Mountain Overthrust, and Alberta, Canada.

Introduction
Underbalance perforating or perforating the pressure in the wellbore lower than the pressure in the formation is generally acknowledged to be one of the best methods for creating open, undamaged perforations. During the few microseconds that it perforations. During the few microseconds that it takes a shaped charge perforator to create a perforation, a focused pressure wave punches a hole through the casing and into the formation. The material in the path of the pressure wave is thrust aside and the path of the pressure wave is thrust aside and the part of the formation next to the perforation may part of the formation next to the perforation may be compacted. The resultant crushing of the formation next to the perforation can reduce the initial permeability by 70% or more. Several authors have permeability by 70% or more. Several authors have noted the presence of the crush zone surrounding the perforation and have recognized that it accounts for a large part of the damage that may inhibit production. Historically, acid breakdowns were commonly used to remove this permeability damage or reduce its effect. In underbalance perforating, the pressure differential from the formation to the wellbore helps remove this crushed formation from the perforation more successfully than perforation washing or surging.
The pressure differentials necessary to remove damage from a perforation is affected by pressure and flow rate. The pressure differentials necessary for perforation cleanup usually range from approximately 500 psi to over 4000 psi and have been established by trial and error in each field.
This study uses information from 90 wells that were underbalance perforated, tested, acidized, and retested. The intent of the research was to determine the minimum underbalance pressure necessary to achieve undamaged perforations. It is important to optimize the amount of underbalance pressure since excessive underbalance pressure, particularly where the cement or the formation is weak, can cause the casing to collapse or the formation to disaggregate.

Discussion

Tubing Conveyed System
One of the most popular systems for inward differential pressure perforating is the tubing conveyed system that was first described in 1975. The system involves running a perforating gun on tubing with a packer above the gun. Underbalance pressure can be achieved by swabbing or jetting out the completion fluid in the tubing to any desired height. After the packer has been set, the gun is fired either by dropping a bar, a battery pack, or by pressure firing. A perforated nipple below the pressure firing. A perforated nipple below the packer allows the formation fluids to flow into the packer allows the formation fluids to flow into the tubing after perforating.
Figure 1 is a record of bottomhole pressure during underbalance perforating. The data were collected with a bottomhole pressure recorder positioned immediately above the packer. The device positioned immediately above the packer. The device communicated with the tubing through a small port.