Perforating Requirements for Fracture Stimulations
Authors: Behrmann L.A. , Nolte K.G. 
Published: December 2007

Background

Perforating provides the means of communication between the well bore and the reservoir, and in a fracture- stimulated reservoir, the perforation is the fluid conduit between the fracture and the wellbore. Within this paper, fracturing implies using proppant. However, in general the presentation also applies to acid fracturing. The choice of perforating parameters for 1) size and type of gun, 2) type of charge, 3) shot density, 4) shot phasing, 5) interval length and 6) gun orientation can have a significant affect on the quality of the subsequent fracturing or matrix stimulated treatment (Nolte, 1982, 1988, Daneshy, 1973, Warpinski, 1983). For the combination of gravel packing and fracturing (frac and pack), perforating practices are governed by the gravel-packing considerations. These considerations are discussed in the "Frac and Pack and High-Rate Water Packs" section.

The objective of perforating for fracturing is to choose perforating parameters that minimize near-wellbore pressure drops during both the fracturing operation and production with limited-entry fracture placement being an exception. Some of these near-wellbore effects are perforation friction, microannulus pinch points from gun phasing misalignment, multiple competing fractures and fracture tortuosity caused by a curved fracture path (Romero, et al., 1995). Several of these near-wellbore effects from Romero, et al. are illustrated in Figs. 1 and 2. For any type of well treatment, there are two additional perforation-related parameters that may also affect the choice of the perforating system:the integrity of the cement/sandface hydraulic bond (microannulus) after perforating andresidual fractured-sand grains in the perforation cavity, particularly for a matrix treatment.

Effective matrix treatments require communication through most of the perforations. This can be achieved by either effective underbalance (Behrmann and McDonald, 1996), extreme overbalance (see "Extreme Overbalance Stimulation" section), or ballout. If a reservoir is perforated with insufficient underbalance to remove most of the perforation sand debris, then fluid injection may cause the comminuted sand to create an external filter cake on the perforation cavity during fluid injection. This was first observed on a water injector and later on extreme overbalanced tests (Behrmann and McDonald, 1996). Two unique characteristics were observed in these tests: productivity was not affected, and this "filter cake" was also an injection pressure barrier with an estimated pressure drop of more than 1000 psi. The existence of comminuted sand in the perforation cavity limits injectivity and increases the injection pressure. High pump rates and high fluid viscosity enhance these effects, which are more important for extreme overbalance stimulation.

A microannulus is usually present after perforating and/or immediately after pumping begins. Maintaining a good bond during the breakdown phase can be problematic because of a hydraulically propagated microannulus that is analogous to hydraulic fracturing, as discussed in Appendix A. Fracturing then proceeds as though from an open hole with some defects (perforations) that may be near the preferred hydraulic fracture plane (PEP). Most laboratory fracturing studies have taken extraordinary measures to avoid a microannulus by epoxying the casing to the rock, using 0-rings around the perforations, etc. Thus, the generality of these laboratory findings must be viewed with caution. The magnitude of the microannulus is dependent on the wellbore fluid and size and type of perforating gun (Table 1).

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