Abstract

Calcium carbonate scale impacts oil production in a large number of fields worldwide. This scale is generally managed by acid washing to remove the scale and/or by performing scale inhibition treatments. The methodology adopted is usually cost driven with high cost operations generally selecting scale prevention rather than removal.

Recently reported work1 showed the potential to integrate scale removal and scale inhibition treatments into a single package, offering clear economic and technical advantages. The combined treatment inherently reduces well intervention costs and well downtime, and protects the value added by the scale removal treatment - by assuring that all of the zones that are stimulated are also inhibited.

Combining acid stimulation chemicals and scale inhibitors is by no means a simple process. Compatibility between the acid, the acid additives and the scale inhibitor presents a significant issue in both live and spent acids. This paper will examine these technical challenges and describes the desired properties of such combined systems. Case histories of recent field trials of combined scale removal and inhibition treatments will be presented, including details of job design, job execution and post-job evaluation. Data demonstrating the scale inhibitor return profile in these treatments will be shown, and lessons learnt from the initial trials will be discussed. Comparative performance data for previous acid treatments will also be presented.

Introduction

Acid stimulation treatments are often used to improve well performance. Hydrochloric acid (HCl) is generally the acid of choice when calcium carbonate is the suspected damage mechanism, unless corrosion cannot be adequately controlled. For high temperature applications, organic acids have been used in preference to HCl, due to such corrosion concerns.2–3 The stimulation benefit of such an acid treatment is often only maintained if a scale inhibitor is subsequently deployed.

Recently reported work1 demonstrated that certain scale inhibitors are not only compatible with HCl but also that they retain their ability to adsorb onto reservoir rock under highly acidic conditions. Hence a scale inhibitor could be deployed directly in the acid system, negating the need for a separate scale inhibition treatment. Previously, it had been thought that scale inhibitors could not perform effectively in the post-acid treatment environment.4

Background

Two field trial candidate wells were identified, both of which had sand control completions installed. The wells were in different fields and one of the wells had been matrix acidised approximately one year earlier. The cause of decline in each candidate was inconclusive with both calcium carbonate scale deposition and/or fines migration being plausible options. Both of the wells had already been selected as acid stimulation candidates. Combining scale inhibition with the acid treatment offered the advantage that the treatment could be used not only as a stimulation treatment but also as a diagnostic treatment to assess the dominant damage mechanism.

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