Abstract

Calcium carbonate scale is commonly found deposited in process system configurations and upper tubulars of oil and gas producing wells. Remedial treatment of this scale can be costly and time consuming and requires acid dissolution. Hydrochloric acid is commonly used as a superficial low cost acid treatment but can often result in tubular damage. In an attempt to limit this problem, corrosion inhibitor packages are often incorporated into the acid treatment, however success has been limited. Alternatively organic acids can be used. They offer a practical alternative to mineral acids with lower corrosion implications, but to date have typically been deployed in fields where operating temperatures do not exceed 100°C.

Continued advancement in oilfield production technology has allowed the exploration of high temperature, high pressure fields, which in turn has led to the introduction of corrosion resistant alloys into production assemblies. The corrosivity of existing carbonate scale dissolving mineral acids for HP/HT application can be controlled, however existing corrosion inhibitor packages do not exhibit good environmental properties. As a result of this industry trend there is a requirement for scale removing chemicals with good environmental characteristics. These products must be capable of withstanding HP/HT conditions whilst exhibiting good performance and in particular be compatible with the high alloy steels which are currently utilised.

This current test work has investigated the possibility of an inhibited carbonate scale dissolver treatment that will combine a high carbonate dissolution rate with low corrosivity towards the high alloy steels commonly in use in HP/HT systems. Various static corrosion test methods were considered throughout the duration of the test work, and a method for high temperature testing was developed.

This has resulted in organic carbonate scale dissolvers which are suitable for use in HP/HT fields, that efficiently dissolve carbonate scale whilst providing adequate corrosion protection towards a range of novel corrosion resistant alloy steels.

The data contained in this paper details the development of these products and also discusses the test methodology and equipment design employed to evaluate these products.

Introduction

Calcium carbonate scale, which forms in well tubing and topside process equipment, is a commonly occuring problem in oil and gas production operations. Calcium carbonate scale may precipitate out of aqueous systems containing calcium and bicarbonate or carbonate ions. This deposition is most prevalent where CaCO3 solubility is low, for example in high pressure and temperature (HP/HT) conditions1.

Scale deposition can be very problematic as it can quickly plug the payzone, production tubing and topsides, leading to a reduction or even total loss in oil production. As an increasing number of high temperature, high pressure fields are developed in the North Sea the problems associated with calcium carbonate scale are changing. They are now much greater downhole rather than in topside equipment where they were more commonly encountered previously.

The removal of calcium carbonate scale in oil production systems has traditionally been dealt with by using mineral and organic acid solutions; typically hydrochloric and acetic acid. These acids will usually be applied with a corrosion inhibitor to minimise the corrosive interaction which occurs between these acids and the metallurgy in use. At elevated temperatures where the corrosion rate is much higher, control can be problematic. Many existing commercial acid inhibitor packages are unsuitable for use in high temperature systems due to instability and thus offer little protection against corrosion under these conditions.

To ensure that the remedial treatment of calcium carbonate scale can be effectively performed in high temperature fields, without adversely affecting the integrity of production assemblies, an effective product with low corrosivity is required.

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