Abstract

Precipitation of mineral scales causes many problems in oil and gas production operations: formation damage, production losses, increased workovers in both producers and injectors, poor injection water quality and equipment failures due to under-deposit corrosion. The most common mineral scales encountered with downhole and topside processes are sulphate and carbonate-based minerals. The development over the past few years of fields where high temperature and high salinity brines are being produced with associated hydrocarbon has presented a more challenging environment for scale management. In such fields hydrogen sulphide gas is quite a common component of the produced fluids. To remove the gas from the process stream hydrogen sulphide scavengers are applied. The introduction of these chemicals can result in a secondary problem arising from their relatively high pH. Changes in localized pH associated with the introduction of such production chemicals has been observed to induce carbonate scale precipitation and in the case of one high salinity brine system unexpected sulphide scales of lead and zinc.

This paper will focus on the mechanism of occurrence of such scale types within a North Sea field and how an inhibitor package was developed that controls both carbonate and sulphide scale formation while it also effectively controls associated hydrogen sulphide. It will outline the field and laboratory testing methods utilized for assessment of the scale challenge, the chemical screening methods used and assess the currently available prediction software so that evaluation of the potential problem within fields during appraisal can be determined prior to production commencing.

Introduction
Oilfield Scales

The most common scales encountered in oil field operations are sulphates such as calcium sulphate (anhydrite, gypsum) barium sulphate (barite), strontium sulphate (celestite) and carbonates (calcite). Numerous studies on scale inhibition have been published over the past few years with regard to controlling such scale within the reservoir and in production equipment (downhole and topside).1–8 Other less common scales have also been reported such as iron oxides, iron sulphides and iron carbonate. These scale types are most commonly associated with iron generation from corrosion products although iron carbonate scale has been reported to form from produced water drawn from formations where iron containing authigenic minerals are present within the formation.9–11 As with the sulphate and carbonate scale types described above even iron carbonate scale can be controlled by inhibitor molecules.12

Lead and zinc sulphide scale has recently become a concern in a number of North Sea oil and gas fields. These deposits have occurred within the production tubing and within the topside process facilities. Investigation of the literature led to a number of references where such scale had been observed13–18 but little information was available on their inhibition by chemical means.18,19

Hydrogen Sulphide Scavenger Technology

During the production of oilfield fluids it is not uncommon for hydrogen sulphide (H2S) to be produced in brine solutions. It is a weak acid ionising to generate HS and S2- ions. Owing to its toxic nature and the resultant elevated corrosion risk that can result in stress cracking in metal pipe work the removal of hydrogen sulphide from water oil and gas is essential. There are various techniques for the removal of hydrogen sulphide from produced fluids. These include zinc or iron oxide beds, scrubbing the gas with caustic liquid systems and application of amine-aldehyde condensates. The latter methods are generally found to be the most effective. The amine-aldehyde reaction relies on the reaction of the "triazine" group with the hydrogen sulphide to form organic polysulphide, trithianes and other reaction products, Figures 1 and 2.

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