The kinetics of barium sulphate scale formation and inhibition in the bulk solution and on surfaces

The deposition and subsequent growth of inorganic scale on completion equipment is a major problem in the oil and gas industry. Several studies have been conducted on the kinetics of both bulk precipitation and surface deposition of barium sulphate. These studies were often conducted in a closed system and measurements were taken off-line and in this study, a flow cell was designed to study both kinetic processes in-situ and in an open system. The set-up allows real-time analysis of a metallic sample by following various scaling parameters such as surface coverage, number and size of crystals formed on the scaling surface. The experimental results were fitted to a diffusion-controlled model to study the mechanism of the surface crystallisation process.
The kinetics and mechanisms of barium sulphate bulk precipitation and surface deposition with the absence and presence of scale inhibitors (diethylene triamine penta methylene phosphonic Acid (DETMP), VinylSulphonate Acrylic acid co-polymer (VS-Co) and poly-phosphino carboxylic acid (PPCA)) were studied. The influence of saturation ratio, flow rate, pre-scaled surface and interval injection on the surface inhibition performance of PPCA scale inhibitor was studied. In addition, the formation of barium sulphate in a multiphase environment was investigated.
The results showed that the deposition of barium sulphate on a metallic steel could occur simply by heterogeneous nucleation and grow and not always by the adhesion of pre-precipitated crystals from the bulk solution. It also revealed the strong effect of saturation ratio and temperature on the nucleation mechanism of barium sulphate deposition on surfaces.
In terms of preventing surface growth by the application of scale inhibitor, the study showed that scale inhibitor could act as a promoter of the crystallization process rather than hindering the crystal growth. Furthermore, the study highlights the importance of starting the chemical treatment as early as possible to limit the pre-existing layer of scale which can considerably disrupt the efficiency of scale inhibitors.
It was found from the study that pre-existing layers of crystals can act as active sites for nucleation and further growth of crystals. The results also show the significance of taking into account the injection of scale inhibitor from the control valve into a process line; inappropriate injection could affect the prevention of scale during continuous injection.
The study demonstrated that the presence of oil droplets can influence the deposition of barium sulphate on surfaces. This suggests in choosing an anti-scaling surface to prevent scale formation, tests should be conducted to evaluate the performance of the surfaces both in an aqueous environment and multiphase environment.