Calcium Carbonate Crystallization Kinetics in Relation to Surface Scale Formation in Oil and Gas Pipelines

Calcium carbonate (CaCO3) mineral scale is one of the major problems faced by the oil and gas industry; as its deleterious crystallisation process causes serious flow assurance problems in oilfield facilities. In this thesis, a suitable methodology based on experimental and modelling approaches was developed for assessing the crystallisation mechanisms and kinetics of CaCO3 precipitating spontaneously in a bulk solution and at metallic surfaces. Several parameters have been assessed during the simultaneous crystallization at bulk and surface: supersaturation, temperature, flow rate, nature of metal substrate and time. Bulk nucleation kinetics revealed interfacial tension values in the range 0.92 – 25.46 mJ/m2, suggesting homogeneous nucleation for higher supersaturation (11 ≤ σ ≤ 55), and heterogeneous nucleation for the lower supersaturation brines (1.52 ≤ σ ≤ 4.77). These interfacial tension values calculated for 25°C, 40°C and 80°C are in good agreement with literature for calcite and vaterite. Bulk growth rate varied from 5*10-13 m/s to 1.25*10-15 m/s over a range of supersaturation and temperatures. The mechanisms obtained for bulk crystallisation suggested spiral growth is predominant at lower saturation ratio and 2D nucleation at high saturation ratio at 25°C. A de-saturation profile was obtained via direct measurements of solution speciation and validated using MultiScale for scale predictions. These results supported the crystal evolution approach proposed based on experiments. Surface crystallization results have shown that in presence of corrosion, a localized supersaturation at the metal of steel enhances surface kinetics and there is a competition between CaCO3 and ferrous corrosion products to occupy the surface nucleation sites. At higher temperature FeCO3 was more likely to co-precipitate with CaCO3, whilst at lower temperature mostly CaCO3 polymorphs and Fe2O3 were formed. The bulk crystallization kinetics and mechanisms observed in absence of corrosion (i.e., stainless steel) showed to be identical to the spontaneous crystallization process. This thesis delivers an appraisal of the engineering science around the crystal formation (scaling) in oil and gas relevant conditions. The mechanisms and rates of formation of CaCO3 in the solution and at the surface of a corroding substrate are discussed