Investigation into the mechanisms of formation and prevention of barium sulphate oilfield scale
Shaw, Scott Stewart
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The performance of barium sulphate oilfield scale inhibitors (SIs) is affected by a number of factors, including temperature, pH and brine composition. This thesis focuses mainly on the effect of varying brine composition – in particular, Ca2+ and Mg2+ divalent cations on SI inhibition efficiency (IE) and minimum inhibitor concentration (MIC) levels. The molar ratio of Ca2+/Mg2+ in field formation waters is known to vary widely and is typically between 1 and 10. Since Ca2+ tends to improve the performance of phosphonate scale inhibitors and Mg2+ “poisons” them, then the effect of Ca2+/Mg2+ ratio is of great practical importance in SI applications. This occurs since Ca2+ has the ability to be incorporated into the growing barium sulphate lattice whereas Mg2+ cannot. The effect of divalent ions on polymeric SIs is rather less and different SIs respond in different ways, as reported in detail here. In this work, the possible mechanisms of scale inhibition are discussed with regard to different generic SI types, e.g. sulphonated polymers, phosphonates, etc. A range of 9 phosphonate and 9 polymeric SIs are tested. The SIs tested are categorised into Type 1 and Type 2 scale inhibitors, with regard to their sensitivity to Ca2+ and Mg2+ cations. Furthermore, they are all sub-categorised into further sub-types – Type A and Type B – depending on their compatibility at higher levels of calcium, [Ca2+] = ~1000–2000ppm. At the end of this work, all SIs are given categorisation codes, e.g. Type 1A, Type 2B etc., depending on this classification. In series of additional experiments, the effect of varying pH on IE/MIC is examined; the degree of SI depletion from solution is monitored during static IE experiments (these are referred to as SI consumption experiments); and ESEM images and EDAX analyses of scale deposits are obtained. The relation between IE and SI chemical molecular structure is also explained. Of the SIs tested, only three are classed as Type 1 because MIC is primarily affected by BaSO4 Saturation Ratio, not molar ratio Ca2+/Mg2+. Conversely, the MIC of all other SIs tested is primarily affected by molar ratio Ca2+/Mg2+; these are classed as Type 2. There are notable differences between the SI consumption profiles ([SI] remaining vs. time) of Type 1 and Type 2 SIs. Generally Type 1 SIs are not consumed significantly and maintain good IE and a high % of SI in solution over long periods, e.g. 96 hours; whereas Type 2 species are consumed rapidly, sometimes to ~ 0% in solution and IE also declines rapidly. There are two exceptions to this general observation – HEDP and HPAA. Non-ICP analytical methods for SI assay, including C18/Hyamine and Pinacyanol techniques can be applied for the assay of non-ICP detectable SIs such as MAT during static IE/consumption experiments. The IE of all SIs depends on their chemical structure. Chemical structures of SI-metal complexes presented in this thesis illustrate that SI molecules containing multiple amino methylene phosphonate functional groups have the greatest tendency to be Type 1 (e.g. OMTHP, DETPMP, and PMPA). This relies upon the inclusion of nitrogen atoms within the main carbon chain of SI molecules.