Combined Scale Removal and Scale Inhibition Treatments

Abstract

Abstract Calcium carbonate scale impacts oil production in a large number of fields worldwide. This scale is generally managed by acid washing to removal the scale and/or by performing scale inhibition treatments. The choice between inhibition and regular stimulation is cost driven, with high cost operations generally selecting inhibition. However, even in wells where inhibition is planned, some scale can be deposited either prior to scale inhibitor deployment, or after the end of the scale inhibition treatment life. Consequently, stimulation treatments are required in many wells, in order to remove calcium carbonate scale. Combining scale inhibition with scale removal treatments offers several advantages. Firstly, in many operating areas, it would reduce the well intervention cost by making the operation a single intervention, offering significant economic benefits and a reduction in well intervention risk. Secondly, pumping a combined treatment not only reduces the risk of scale re-precipitation during the stimulation treatment, but it ensures that the zones that are stimulated are also inhibited. This directly protects value added by the scale removal treatment. This paper details the development of combined scale removal and inhibition treatments, from project initiation to readiness for field trials. The main challenges that need to be addressed in order to achieve an effective combined treatment are discussed. Data from a laboratory study, investigating the potential for combining scale inhibitors in hydrochloric acid, organic acid and scale dissolver systems are presented and the most effective combined systems are identified. Introduction Scale precipitation is a common cause of impaired well productivity, with calcium carbonate being the most common scale that is formed. Hydrochloric acid is frequently used for removal of carbonate scale,1–3 as it generally offers both the best performance and the lowest cost. Such acid treatments can be very effective in providing short term stimulation benefits to such wells, but the treatments are often short lived.3 For high temperature applications, organic acids have been used in preference to hydrochloric acid, due to corrosion concerns.4–5 The dissolution of calcium carbonate by chelating agents is also well known6–10. Treatments with chelating agents to remove calcium carbonate scale have been performed,6 but dissolution rates are generally lower than with acid and treatment economics tend to restrict their use. Calcium carbonate scale deposition can be effectively inhibited in most fields, with scale inhibitor squeeze treatments being widely used to prevent scale build-up11–13. In these squeeze treatments scale inhibitor is retained in the formation either via adsorption onto the rock surface14–16, or by precipitation (or phase separation) of the calcium salt of the inhibitor. Precipitation squeezes offer increased squeeze life17, but may have an associated risk of formation damage during treatment,18 especially in damage sensitive formations. Although scaling potential can be predicted19–21 and proactively treated, it is still common for scale to form in some wells, either before an inhibition treatment is pumped or after the end of the treatment life. This can occur if there is insufficient early warning of the onset of scale, or if limitations in well access or equipment availability delay a planned treatment. Consequently, even when inhibition is the scale management strategy, the need still arises for scale removal treatments. Conversely, when acid stimulation is the preferred scale management tool, there would be a benefit in reducing treatment frequency if the job life could be extended by simply adding an effective scale inhibitor to the acid system. Although the potential benefits of such combined treatments have been recognised, it has been reported that the post-acid treatment environment prevents scale inhibitors from performing effectively.22 The current approach to this problem, therefore, is to pump sequential scale removal and scale inhibition treatments.