Abstract

This paper reviews Mobil Oil Indonesia's experience in the completion of high temperature, high rate gas wells, gained during ten years as operator for the Arun Field.

Over 50 production and injection wells have been completed in the Arun Field, and some of these wells have been worked over once or even twice since their initial completion. problems which have been encountered during the operation or workover of the wells have led to changes in completion practices and equipment design. These problems are described in detail as are the subsequent completion changes and the results of these changes.

Introduction

The Arun Field is a giant natural gas condensate field located in Aceh Province at the northern tip of the island of Sumatra (figure 1). It was discovered in 1971. Mobil Oil Indonesia (MOI) is the operator for the field, acting as a production sharing contractor to the Indonesian national oil company, Pertamina. The field is a major source of LNG for Japan and Korea.

Abnormally high producing temperatures and pressures, the corrosive constituents of the well stream fluid, and high field production rates, have combined to present many challenging engineering and operating problems. Individual Arun wells are capable of production rates in excess of 150 MMSCF/D with wellhead temperatures above 300F. The initial reservoir pressure was 7100 psia and wellhead producing pressures were 5500 psig or greater.

As a consequence of these extreme conditions, the original Arun completions developed wellhead seal and tubing connection leaks. A comprehensive design review, including laboratory testing, identified the mechanism by which leakage occurred. An alternative completion design was then developed based on metal to metal wellhead seals on the final cemented casing, and premium shouldered connection tubings with multiple seals. New Arun wells were completed in this way and a workover program was undertaken to recomplete all wells which had the original completion.

The Arun Field workover program indicated that the weighted invert emulsion packer fluid resulted in solids settlement and emulsion breakdown. Extensive investigation eventually led to the specification of an unweighted refined oil as a suitable replacement packer fluid.

The workover program further showed that the seal assembly in every polished bore receptacle (PBR)/seal assembly type of completion was firmly stuck in the PBR. No seal assembly was pulled in any of the 29 workovers. A testing program was initiated to determine if a packer type completion with a rigid tubing to packer connection would be suitable for Arun usage. Subsequently an hydraulic set packer was specified.

II. THE EARLY ARUN COMPLETION DESIGN

The first Arun well to be drilled and completed as a development well was CIII-2 (A-15), in January 1977.

The well was completed as shown in figure 2, and included many of the features typical of the early Arun completion design. Features which should be specifically noted are as follows:

  • Resilient elastomer seals were used on all wellhead casing packoff sealing systems with only the tubing; hanger neck seal incorporating a metal to metal seal.

  • A premium thread tubing connection with limited back-out resistance was specified.

  • The production tubing downhole isolation assembly was the PBR/seal assembly type with the PBR being run as part of the 9 5/8 inch cemented casing.

  • A weighted invert diesel water emulsion was used as the packer fluid in the 7 inch by 9 5/8 inch annulus.

It became evident after some operating experience had been gained that this type of completion was not suitable for Arun producing conditions. The problems encountered will be discussed in the following sections.

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