Assessment of CaCO3 surface deposition mechanisms and kinetics in low supersaturation environments

Mineral scale formation is the deposition of unwanted materials on solid surfaces to the detriment of their function. Scale deposition can have a significant impact on operations. Scale restrictions can reduce flow capacity, hinder the correct operation of valves and other safety critical equipment. Removing this scale, which in some forms can be radioactive, and performing preventative treatments can run into tens of millions of dollars per well per event especially in subsea deep-water production. Calcium carbonate (CaCO3) scale is one of the most common mineral scaling types found in the oil and gas industry. CaCO3 formation at low saturation ratio (SR) is particularly challenging because its deposition progresses slowly until triggering catastrophic event. A reliable scale prediction tool is crucial to reduce the uncertainties associated with scale prediction. This requires a suitable methodology for generating reproducible data on the kinetics of CaCO3 deposition. The common techniques used for studying CaCO3 deposition such as the dynamic tube blocking rig and static jar setup are not suitable for generating measurable data at low SR within an acceptable time frame in the laboratory. This research focuses on understanding the mechanisms of bulk precipitation and surface deposition at low SRs (2 -10) and mid-range temperatures (50oC - 90oC). Static jar tests were carried out to follow the kinetics of bulk precipitation. An in-situ visualisation flow set-up was also used to follow the build-up of scale on surfaces as a function of SR and flowrate. The results showed evidence of nucleation and crystal growth. Finally, a newly developed beadpack technique, which has shown huge potential to accurately quantify scaling kinetics at low SR. The sensitivity of the beadpack to parameters such as area-to-volume ratio, surface roughness and flowrate was also evaluated. In contrast to previous studies, this study has demonstrated that bulk precipitation can occur at very low SR over a sufficient time. The beadpack design was effective for quantifying the kinetics of CaCO3 deposition on the surface at low SR due to the presence of a high area-to-volume (A/V) ratio in the pack. The results from the in-situ visualisation cell demonstrated that an increase in SR and flowrate led to an increase in the growthrate of CaCO3 on the surface at low SR. In addition, the growth-rate of CaCO3 was 10 times faster at SR 10 on the surface compared to the bulk and 5 times faster for SR 5. This result indicates the role of surface deposition should be seriously considered in the development of a reliable scale prediction model. The three techniques employed have shown to be complementary and provide insight into different aspects of the crystallisation process. The understanding and results generated in this study can serve as the building blocks for the development of a reliable kinetic model for predicting scale deposition at low SR in the oil and gas industry.