Highly Overbalanced Perforating
Authors: Dees John M. 
Published: December 2007

This paper is SPE 30342. Technology Today Series articles provide useful summary information on both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significant concept, technique, or development within a specific area of technology. Journal of Petroleum Technology, May 1995.

Summary
This article briefly describes a new perforating technique that will result in clean, open perforations. The well is normally stimulated when highly overbalanced perforation techniques are used. This perforation process improves formation evaluation because the well flows better and is easier to test.

Introduction
Underbalanced perforating is generally accepted as the petroleum industry standard for perforating. However, a recent review of posperforation well performance indicated that inconsistent results are achieved with conventional perforating techniques. Pressure buildup analyses after perforating often indicate a range of skin factors from -1.0 to more than +50. An average perforation efficiency of 25% was observed in one perforation study.
On the basis of such results, the industry has been looking for a new method to perforate a well that could improve perforation efficiency. Possible stimulation techniques include propellants and explosives. Fig. 1 illustrate show the pressure vs. time relationship varies for four separate stimulation methods. Explosives or propellants apply a high pressure to the perforations in micro- or milliseconds, respectively. Hydraulic fracturing loads the perforations very slowly. Overbalanced perforating loads the perforations rapidly and maintains the pressure for a sufficient time to create clean fractures at the tips of each perforation (Fig. 2).

Mechanics
A perforating jet exerts a stress of 4 to 5 million psi at the tip. This high stress greatly exceeds all the principal rock stresses, and, consequently, a perforation hole and tunnel are formed. A reasonable estimate of the tunnel length can be made by considering such factors as charge specifications, in-situ rock properties, and wellbore conditions.
If a sufficient amount of overbalanced pressure is applied to the perforations after they are formed, two potential benefits can be gained. First, the prolonged application of high pressure will allow for some stabilization of the tunnel walls. Second, the action of the fluid used to apply the pressure can extend induced fractures created through each perforation.
Erosion of the fracture faces can occur if the velocity is high enough. Fluid velocity alone may be insufficient to etch the induced fracture faces properly in some cases. The addition of a reactive fluid, such as acid in a carbonate reservoir, or a scouring material, such as fracturing proppant, can improve the etching patterns on the fracture faces significantly. The addition of a scouring material may also be beneficial in eroding and enlarging the perforating-entry-hole diameter through the casing and cement sheath of the wellbore.
Highly overbalanced perforating can be done with any of the currently available perforating guns. The preferred gun type is a hollow steel carrier because the gun body will retain most of the charge debris. Tubing-conveyed guns will allow for the application of the highest bottomhole pressure (BHP).Fig. 3 shows a typical tubing-conveyed setup. Before perforating, pressure is applied to the wellbore that will result in a downhole pressure at least as large as the fracturing pressure of the formation. A typical applied pressure gradient is between 1.1 and 1.3 psi/ft. Ideally, the pressure gradient will exceed all the principal in-situ stresses. The rule of thumb for minimum applied pressure is the formation fracturing gradient plus 0.4 psi/ft.
The pressure is applied by use of all liquid, all gas, or a combination of the two. The preferred method will incorporate a liquid column directly across the perforation interval and up to a predetermined fluid level from the surface. A gas column will be used above this liquid to provide the additional pressure required to achieve the desired gradient. At the time of perforating, the expansion of the gas will translate directly into horsepower applied to the formation. Injection rates that are not attainable by practical means from the surface can be achieved downhole. In instances where reducing surface pressures is desirable, nitrogen has been used to achieve a predetermined surface pressure, followed by pumping of a weighted brine water to complete the initial pressurization. This has reduced surface pressures by several thousand pounds per square inch on some well treatments.
The high rate of fluid displacement will exceed the capacity of the perforations to accept fluid. This event will last from 1 to 15 seconds until the pressure falls below the fracture-extension pressure. The breakdown is greatly enhanced by performing additional pumping operations with additional gas and/or liquid volumes. In some instances, a small stimulation treatment maybe done together with the breakdown.
The final fluid pumped into the reservoir can be tailored to enhance the relative permeability to the reservoir produced fluid. In low-pressure gaswells, only small liquid volumes of liquid (1 to 5 bbl) are required for the breakdown. This small liquid volume can be dispersed easily by injecting gas, such as nitrogen or carbon dioxide. Surfactant blends that will leave the formation water-wet can be used in oil wells.