Using a real-time visualisation technique for the assessment of surface scale kinetics and mechanisms of inhibition

Improving the understanding of surface scale formation is important in implementing an effective scale management strategy in the oil and gas industry. Investigations into scale formation have largely been focused on precipitation in the bulk solution by assuming that surface scaling always results from pre-precipitated crystals in the bulk solution. However, recent studies have shown that scale inhibition efficiency and the effects on crystal morphology vary between surface and bulk processes The study is focused on the formation of BaSO4 and CaCO3 scale on stainless steel surface at constant saturation ratio (SR) using a visualization flow rig. The effects of SR, flow rates and temperature on surface scaling were studied. Also, the influence of SR and inhibitor concentration on surface inhibition efficiency of Polyphosphinocarboxylic acid (PPCA) and diethylenetriamine penta methylphosphonic acid (DETPMP) were studied. The range of SR considered is between 10 and 80 and with these values the experiments are conducted with no pre-precipitated crystals in the bulk solution. The flow rates used are between 10ml/min to 60ml/min. It is shown that the determination of surface crystallization mechanisms and kinetics allows for the correct type and dosage of inhibitor to be selected. Addition of inhibitors at the bulk minimum inhibition concentration (MIC) actually aggravates surface scaling. For a similar range of SR and test conditions, CaCO3 scale builds up on the surface shows the predominance of growth which results in bigger crystals and a smaller number of crystals while BaSO4 scaling results in a higher number of crystals nucleating from the surface asperities with slower growth rate and hence smaller crystals.