Inorganic scale formation in oilfield systems is greatly influenced by the fluid-flow regime, with shear rates and turbulence affecting nucleation, surface growth, morphology and adhesion. However, current scaling risk analysis relies upon thermodynamics-based predictions together with simple rules-of-thumb to address kinetics because the impact of flow regime is poorly understood. Similarly, scale inhibitor performance is typically evaluated under static or low-shear, laminar-flow conditions, which are highly unrepresentative of those in most production systems.
This paper examines scale deposition at metal surfaces and bulk-phase precipitation as a function of shear and turbulence, highlighting how rate and even scale composition can change in systems with mildly oversaturated sulphate tendencies. Conventional static and low-shear methods are compared with those using rotating cylinder and jet impingement to achieve high Reynolds numbers and shear stresses up to 400 Pa (and even 10,000 Pa via more sophisticated approaches). This builds upon earlier work (SPE 169761) and demonstrates the influence of fluid-flow dynamics on the critical supersaturation required to induce scaling, and by extension, on the nature of the deposit that may form where a system is mildly oversaturated for more than one scale. An example is where a sulphate scale at lower supersaturation precipitates under high shear conditions preferentially over another sulphate scale with higher supersaturation.
This work also highlights the significance of better matching shear and turbulence in the ranking of scale inhibitors, reporting differences in the relative performance of different generic inhibitors as a function of shear.
Finally, the paper describes the new test methods by which scaling propensity and inhibitor performance can be determined under conditions of shear and turbulence that are more representative of production conditions. A key aspect of this is that it facilitates testing of mildly supersaturated systems without having to adjust brine chemistry, leading to better understanding of the magnitude of uncontrolled scaling and of the Minimum Effective Dose of inhibitors to control it.