Application of Computer Models to Optimise Perforating Efficiency
Authors: Carnegie A. 
Published: December 2007

When optimising a perforating procedure, many questions need to be answered. For example,is it better to perforate overbalanced with casing guns or underbalanced with tubing conveyed guns?, orwhich through tubing gun system would be most cost effective?

The answers are influenced by many factors, which include phasing, shot density, penetration, level of underbalance, cost, reliability, temperature and pressure downhole, debris and positioning of the gun within the well bore. The questions must be efficiently and accurately answered, with due regard being paid to environmental factors, such as mechanical and flow properties of the formation being perforated. Often, decisions made by a company on perforating methodology are based upon just two considerations:its common sense notions about perforating for example, high shot density is needed in highly laminated formations, andits practical, but limited experience.

This paper describes a methodology in which this traditional approach can often be significantly improved by efficiently factoring in the results of extensive industry wide research and engineering programs. A key ingredient to the methodology is the application of an easy to use desktop computer model of the perforating process. The paper also presents the theoretical basis for this model, the guidelines for its use, and some practical examples demonstrating its application.

Introduction

Often, decisions made by a company on perforating methodology are based upon just two considerations:its common sense notions about perforating - for example, high shot density is needed in highly laminated formations, andits practical, but limited experience.

These approaches can often be enhanced by factoring in the results of extensive industry wide research and engineering programs. These have advanced the scientific understanding of perforating to the extent that now it is possible to reliably model the effects on the productivity of perforated completions, of changes in various key parameters which are under the control of the production engineer. For example, it is now possible to simultaneously calculate the effects on the productivity of a perforated completion caused in changes in underbalance, gun phasing, perforation tunnel depth and diameter, and completion interval length. Considerable effort has been expended to create a user-friendly desktop based program which enables these calculations to be performed economically and quickly, with minimal loss of accuracy. This program is called SPAN, short for Schlumberger Perforation Analysis Program. This program consists of two modules: penetration calculation and productivity calculation. In the penetration module, the length and diameter of the perforations are calculated under down hole conditions. These parameters are then used in the productivity module to evaluate anticipated productivity of the perforated completions.

This paper describes where in the scheme of completion design process SPAN usually lies, the theoretical basis underlying its algorithms, the methodology for its exploitation, the preparation of input data to the program, and finally some practical examples of its use. Extreme over balance (EOB) perforating is not considered in this paper. More information on how to decide whether it is better to apply EOB or to follow a conventional perforating approach can be found in [1]. The discussion in this paper relates mainly to Darcy inflow of liquid (oil or water) from conventionally perforated zones. Therefore, zones with gravel packs, and the case of gas inflow are not considered in this paper. This is despite the fact that SPAN handles both cases. More information on the productivity of gravel packs may be found in [2]. More information on the productivity of zones experiencing turbulent inflow of gas can be found in [3,4,5].

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