|dc.description.abstract||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
• Mixed PbS and ZnS sulphide scale is found to be easier to inhibit than either scale