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Abstract

Shell U.K. Exploration and Production (Shell Expro) operates in the U.K. sector of the North Sea on behalf of Shell/Esso and other joint venturers.

This paper discusses operational problems experienced to date, in Shell Expro's fields, due to the formation of barium and strontium sulphate scales downhole in producing wells. Particular reference is made to the Shell/Esso Brent and Dunlin Fields.

Introduction

The formation of heavy deposits of barium and strontium sulphate scale has become a major operational problem in Shell Expro operated fields as the breakthrough of injected seawater at producing wells has become widespread over the last two years. Barium and strontium ions present in the formation water combine with sulphate ions in the seawater to produce scales which are, unlike carbonate based scales, resistant to removal with acid.

In the case of wells producing on stratified reservoirs, as in the Brent sequence, seawater breakthrough times may differ between sand bodies, such that mixing of the two waters occurs mainly in the wellbore. The scales formed are of such low solubility that deposition occurs in the liner and lower section of the tubing tailpipe. If not treated this scale can rapidly lead to severe productivity impairment.

Following initial attempts in Shell Expro to remove this scale, which had only limited success, programmes of regular scale inhibition squeeze treatments were implemented during 1985.

This paper discusses experiences to date, including progress in optimising treatment designs and in identifying wells where treatments are no longer required.

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