Abstract

Well productivity is driven by establishing a clean connection through the near wellbore zone of drilling and completion induced permeability impairment commonly referred to as the "near wellbore damaged zone". This connection through the damaged zone is most often achieved by perforating with explosive shaped charges. The effectiveness of this connection is the result of perforator selection criteria and the well environment in which the perforating job is executed.

In the depleted oil field under study a typical completion is perforated using large Tubing Conveyed (TCP) guns in "shoot and pull" mode in a static underbalance environment. After shooting the well is killed to allow the TCP guns to be pulled safely and the completion run. Production from the newly perforated intervals declines quickly due to near wellbore damage caused by the kill cycle.

The challenge in this case was to identify the proper perforating gun system, conveyance method and perforating program to achieve the optimal productivity once the well is put on production. Modeling software was utilized to predict the productivity ratio (PR) for different perforating systems considering gun size, charge type, shot density, reservoir parameters and the well conditions at the time of shooting.

The perforating program was modified to perforate the well with gas lift on in a flowing condition maintaining under balance conditions after shooting to assist with cleanup. The under balance at the time of shooting was managed using a permanent down hole gauge installed in the completion string.

The first well completed using this process showed improved production of greater than 3 times what was expected when compared to similar wells in the field.

This paper will cover the job design criteria, the job execution requirements and evaluation of the results. In addition, a summary of the study leading to this work and the total cost reduction details for the completion operations is also included.

The result of this novel perforating job design has led to a new completion strategy for oil fields in Oman thus improving overall well performance.

Introduction

A key element of how productive a well will be is the effectiveness of the perforations. The perforation creates the path for the formation fluid to flow from reservoir to the wellbore. When the perforations are inadequate or plugged off inflow will suffer over the life of the well. The quality of the perforation job design and execution is a major concern in any completion design 1,2,3,4,5,6.

An effective perforating job is dependent on many factors including:

  • Perforating charge performance which is a function of reservoir rock strength, effective stress, fluid type and completion parameters

  • Perforating gun characteristics such as gun size, shot density, phasing and charge type

  • Well conditions at the time of perforating including underbalance and fluid type in the wellbore

  • What happens to the well post perforation e.g. a kill cycle which may damage the perforations and reduce productivity.

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