Calcium carbonate (CaCO3) is one of the most common deleterious inorganic scale in oil and gas production and distribution restricting fluid flow, posing major operational challenges, and resulting in significant costs due to loss of production and remediation when not properly handled. The management of mineral scale is usually a complex process that requires an in-depth understanding and accurate estimation of prediction, inhibition, and other mitigation strategies. With new regulations in the oil and gas industry, there has been a shift to substitute conventional inhibitors with more environmentally friendly strategies particularly reliable scale prediction, and the use of anti-scaling surfaces and green inhibitors as potential remedies.
There are numerous studies on the formation of CaCO3, most have been geared towards understanding bulk precipitation processes, often in closed systems where saturation ratio (SR) decreases as a function of time and measurements are taken off-line. The focus on bulk precipitation has been driven by the assumption that surface scale is always a result of pre-precipitated crystals in the bulk solution migrating and settling on the surface. Thus, there has been little attention on surface deposition mechanisms which is the main challenge in proper scale prediction and mitigation in the oil and gas and desalination industries. Recent observations have established that bulk and surface scaling mechanisms and kinetics are different, yet a full mechanistic understanding of how scale layers build up on component surfaces is still limited. Focus is being directed towards surface deposition as fouling on surfaces often lead to operational problems and the rates cannot be predicted by consideration of bulk precipitation processes alone. To improve current prediction tools and inhibition methods, understanding the interactions between different factors that influence scale formation kinetics requires a suitable methodology that reflects real field scenarios. This is essential for the development of accurate kinetic prediction models.
In this study, a recently developed once-through capillary flow rig set-up that offers the ability to control thermodynamic and hydrodynamic parameters was modified and utilised to evaluate surface kinetics and mechanisms of CaCO3 scale formation under dynamic flowing conditions. An important attribute of this set-up is an “iso-supersaturated” condition that allows SR, regarded as the “thermodynamic driving force” for nucleation to remain constant in the capillary cell due to the short residence time of the flowing brines, and ensure that there is no bulk (pre-precipitated) crystals in the solution when it flows through the cell. The capillary flow rig allows assessment and variation of different scaling parameters, such that CaCO3 surface deposition can be investigated reliably. This permits the reliable kinetic examination of the scale growth in the capillaries and provides an improved mechanistic understanding of surface scaling.
This study investigates the kinetics and mechanisms of CaCO3 deposition on stainless steel surfaces under flow conditions over a wide range of SR, at 25⁰C and 70⁰C. Surface induction time (tind) and CaCO3 growth are evaluated from the differential pressure build-up when scale is deposited on the walls of the capillaries in real-time. Also, the influence of surface wettability by the application of silane and pre-scaled surfaces was studied. This was used to assess the interplay between surface feature/condition, and scale nucleation and growth kinetics, and to understand how surface condition/characteristics promotes or reduces the likelihood of scale formation in a flowing system.
The study shows that the deposition of CaCO3 on metallic surfaces can occur by heterogeneous crystallization and growth, and not necessarily by the adhesion of colloidal crystals from the bulk solution. The results also show that the nature of a surface is a factor that influences the induction time and growth rate; a low wettability/low-surface energy substrate delays the onset of nucleation and reduces the amount of surface scale deposit, this surface characteristic seems to improve scale performance. The pre-existing scale layer acts as active sites initiating nucleation and promoting further scale growth. The surface fouling mechanism is thought to be mass transport controlled during the early phase of scale formation and subsequently surface reaction controlled in the latter part.
Another novelty of this research is centred on the detailed investigation of the early nucleation period, results here show nucleation and crystal growth already taking place during what is normally referred to as the “induction period” in the tube blocking tests. Also, the de-supersaturation profile in static bulk test and the capillary rig was obtained via atomic adsorption spectroscopy, which enabled validation of predictions by MultiScaleTM software (based on thermodynamic factors). Bulk predictions were in close agreement with the experimental mass of scale precipitated, but there were extensive dissimilarities between scaling estimated by MultiScaleTM and actual deposit on the capillary surface.
The estimation of scale deposit thickness using the Hagen-Poiseuille equation is deemed unreliable due to the non-uniformity of the scale layer in the capillaries. A semi-empirical model that incorporates influential parameters particularly flow rate for the prediction of the induction time based on the classical nucleation theory, and scale deposition growth rate as a function of the deposition flux is proposed from the experimental work in this study.
