Evaluation of laboratory techniques for assessing scale inhibition efficiency

Injecting chemical inhibitors is the most common method to mitigate mineral scaling in the oil industry. As such, the effectiveness of the techniques employed to evaluate performance of chemical scale inhibitors and apply the appropriate dosage is a very important aspect to be considered during the design of a scale prevention treatment. In this paper, the kinetics of scale formation and its inhibition are studied using a conventional bottle test, a dynamic tube blocking rig and a recently developed in-situ flow visualization rig. Calcium carbonate scaling brine was prepared at two saturation indices (SI) of 2.1 and 2.8 at 50 °C and run through the rigs at flow rate of 20 ml/min. The conventional polphosphinocarboxylic acid (PPCA) inhibitor was used for the inhibition study at concentration ranging between 0.5 and 10 ppm. The MICbulk determined from bottle test and supported with the in-situ turbidity MICbulk for SI of 2.1 and 2.8 are 1 ppm and 8 ppm respectively. For the same SI values, a considerably lower concentration of PPCA, 0.5 ppm and 4 ppm for the surface inhibition test using the capillary rig were obtained compared to MICsurface of 4 ppm and 8 ppm from the in-situ visualization technique. The surface visualization technique enables the range of concentration of inhibitors at which both bulk and surface scaling are completely controlled to be determined. The different techniques are shown to give complementary information for different stages of crystallization process and inhibition.