Mechanistic evaluation of sulphide scale formation in the oilfield
Okocha, Cyril Emeka
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Scale deposits, such as more conventional sulphate and carbonate scales, impair oil and gas production and lead to problems such as production loss, equipment failure and additional expenditure. However, more recently the occurrence of sulphide scales (e.g. FeS, ZnS and PbS) is increasingly being reported. The mechanisms of formation and inhibition of sulphide scales in oilfield systems are not yet fully understood. This thesis presents a modelling, methodological and mechanistic study of sulphide scale (FeS, ZnS, PbS) formation and inhibition. A number of specific aspects relating to sulphide scaling are studied in this thesis, as follows: (i) Sulphide scale formation and the related mechanisms of inhibition by chemical scale inhibitors; (ii) the effects of commercial scale inhibitors (e.g. PPCA, DETPMP and other blends) on sulphide and mixed scale formation; (iii) the effect of THPS as a sulphide scale dissolver and the additional effect of brine composition (Ca2+ and Mg2+ ) on its performance; (iv) the effect of scale inhibitors on the morphology of both BaSO4 and CaCO3 when co-precipitated with sulphide scales; (v) the effect of scale inhibitors on mixed sulphide scaling (e.g. ZnS/PbS etc.); (vi) a sulphide prediction model was also developed which gives a description of the sulphide precipitation interactions and this was tested against experiment. These various experimental studies were carried out using an integrated combination of techniques such as a modified static bottle tests (performed in an anaerobic apparatus for some cases to avoid Fe2+ oxidation), dynamic static tests, particle size analysis, inductively coupled plasma (ICP), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). These studies have enabled us to develop some new insights into the mechanisms of sulphide scale formation and inhibition in this work. For example, some of the specific contributions to the understanding of sulphide scale formation an d inhibition mechanisms, from this work are listed as follows: • A number of predictions were made using a detailed sulphide model of experimental quantities such as final solution pH, saturation ratios (SR), mass of FeS etc. These predictions were compared directly with the experimentally measured quantities and excellent agreement has been found. • A number of novel types of FeS inhibition behaviour have been observed and describes when treated with scale inhibitors e.g. some FeS solutions are found to become clear from black 24hrs after the scale inhibitor treatment. • There is significant barite crystal distortion when co-precipitated with FeS, but no distortion is observed when barium sulphate co-precipitates with PbS and ZnS. • Mg2+ has a profound effect on the ability of THPS to inhibit FeS and this work demonstrates that THPS is significantly enhanced by the presence of Mg2+ which has more effect than any other ions. • CaCO3 crystals deposit in bulk solution, rather than on the metal surface when coprecipitated with ZnS. The polymorphs of CaCO3 form on the metal surface when coprecipitated with PbS. • Mixed PbS and ZnS sulphide scale is found to be easier to inhibit than either scale individually.