Scale threat assessment modelling tools are based on thermodynamic solubility models with no account for surface deposition kinetics. Many operators use critical saturation ratios (SRs) and excess solute trigger values to describe conditions for deposition. This leaves a large area of uncertainty especially at low SRs, thus conservatism in design of barriers to manage scale.
This study utilizes two distinctly different techniques to understand the extent to which the presence of a stainless-steel surface impacts the kinetics of scale formation in comparison to only bulk precipitation in low (below 10) SR solutions. The two techniques are shown to be complimentary, providing insight into different aspects of crystallization such as homogenous and heterogeneous nucleation.
Experimental tests have been carried out at SR values of 10, 5 and 3 to follow the kinetics of bulk precipitation by using static tests to track the change in the calcium ion concentration in the solution with time. The static jar test was also used to provide information on the induction time for bulk precipitation which is important for identifying if the nucleation process for the surface deposition test in the bead pack is controlled by homogenous nucleation and growth in the bulk solution, heterogeneous nucleation, and growth on the surface, or both.
The surface deposition tests were conducted in a bead pack, a newly designed setup adapted from a sand pack technique. The bead pack was used to investigate whether the presence of a high surface area can provide sufficient deposition to obtain quantifiable data on the kinetics of scale formation in low SR solutions. The effect of temperature namely tests at 50°C and 90°C is also presented. The results show that significant variation exists in the rate of precipitation between CaCO3 bulk and surface scaling at different temperature and SRs.
The study describes a set of tests using the bead pack setup to provide quantitative surface precipitation rates at low SR in a controlled composition environment. This work provides a framework for the development of kinetic models targeted at reducing the conservatism in design of hydrocarbon production and carbon capture usage and storage (CCUS) facilities.