The Effect of Perforating Conditions on Well Performance
Authors: McLeod Harry O. 
Published: December 2007

The productivity of a perforated gas well is affected strongly by nondarcy or turbulent flow through the compacted zone around each perforation. The turbulence coefficient depends on the permeability of this compacted zone. This permeability, a function of perforation condition, can be used with perforation dimensions to predict gas well performance. predict gas well performance.

Introduction

Recent work by Jones et al. and Mach et al., describes pressure drop in turbulent flow through gravel-packed pressure drop in turbulent flow through gravel-packed perforations. No method has been presented yet to perforations. No method has been presented yet to describe similarly turbulent flow in perforated wells that are not gravel packed. This paper presents an approach to this problem. It can be used to analyze producing gas wells, or it can be combined with flowing well analysis to calculate the perforations needed to complete a gas well in a consolidated or competent formation.

The General Radial Gas Flow Equation

Gas flow into a perforated well can be described by the well-known equation

(1)

The skin factor, s, accounts for viscous flow through the damaged zone around the wellbore. including the effects of perforations. The term Dq accounts for the extra pressure drop as a result of turbulent gas flow around the pressure drop as a result of turbulent gas flow around the wellbore. Other terms are defined in the Nomenclature. These general terms, s and Dq, are evaluated by transient pressure testing, or they can be determined by multirate flow tests. They provide a measure of total additional pressure drop caused by wellbore damage and turbulent flow. In evaluating well completions or proposing a certain way of perforating, we need a more proposing a certain way of perforating, we need a more specific relationship to wellbore geometry and condition. By analyzing the effect of perforations on well flow from experimental parameters from laboratory perforation tests, one can show the dominating influence of perforation tests, one can show the dominating influence of real perforations on wellbore pressure drops in a high-permeability formation. These same procedures also can be used for low-permeability formations; however, the perforation effect is not as striking as in a perforation effect is not as striking as in a high-permeability formation. Fig. 1 shows a simple schematic of a perforation connected to the wellbore. Around each perforation made in rock there exists a compacted zone with a thickness of about 0.5 in. (1.25 cm). The permeability of this compacted zone will vary from 10 to 25% of the permeability of the rock just before perforating. The permeability of the rock just before perforating. The compaction takes place when the hole is created by the impact of the disintegrated shaped charge metal liner. The permeability can be reduced further by the presence of dirty perforating, fluids or drilling mud, particularly when pressure forces fluid into the perforation, For a perforated well, the factor D is defined as follows. perforated well, the factor D is defined as follows.

(2)

This equation is developed in Appendix A along with the equation defining beta dp,

(3)

JPT

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