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Abstract

This work will describe the management of scale in a United Arab Emirates offshore carbonate reservoir. Field pressure is sustained by water injection, which contributes to the formation of scale in downhole tubulars. This scaling can cause both safety and production problems, for example, by blocking a subsurface safety valve or chocking flow at downhole nipples.

ZADCO has initiated a successful strategy to manage the sulfate-scaling tendency in the completion tubing. Software is used to identify wells early on as potential scale candidates. This software uses real field measurements and analysis of injected and produced water information. The results are used to plan well monitoring and/or treatment. The identified wells are monitored using both slickline gauge measurements to measure actual scale growth and produced water surface analysis to update the software model.

Early corrective action is performed using a specialized sulfate scale-dissolving fluid pumped from the surface. More aggressive treatment for larger scale accumulations is performed using a barge, coiled tubing unit, and specialized downhole jetting system.

Results have shown that early diagnosis of the scale allows earlier, easier, and far less expensive scale removal treatment. The effect of scale on production decline is also reduced.

Introduction

ZADCO owns more than 300 wells in Upper Zakum field, offshore Abu Dhabi. To ensure that wells operate as per initial specifications for all the designed life and to minimize the risks associated, it is important to ensure individual well integrity.

ZADCO has currently in place a Well Integrity Management System (WIMS)[1] that helps to identify, prevent, and solve all the problems related to lack of integrity in each well. Well integrity is based on the establishment and maintenance of confinement barriers.

In the broader approach of managing well integrity in the Upper Zakum field, scale management constitutes an important part as scale deposition across the production string has various effects on the integrity of the wells, as described below.

a) Technical integrity is affected when the cross sectional flow area of a string (ID) is reduced or completely plugged off with scaling, which ultimately results in partial or total production loss. In addition, it restricts achieving the reservoir-monitoring program.

b) Safety barrier integrity is affected by eliminating the functioning of downhole safety valve (DHSV) and the check valve installed at the surface flow lines when scale deposits across them.

Types of Scale

Since 1991, two types of scale deposition have been experienced in the tubing strings of Upper Zakum producing wells. The main cause of such scale deposition in the producing wells is the incompatibility between the formation water (enriched with minerals) and injected seawater (enriched with sulfates/carbonates). Scaling phenomena is caused by the reaction between the minerals and sulfates/carbonates when the injected seawater breaks through and mixes with formation water accompanied with change in thermodynamic conditions. (Refer to Fig. 1 for schematic of injection pattern.)