Sulphate scale can be predicted from thermodynamic models and over recent years better kinetics data has improved the prediction for field conditions. However, these models have not been able to predict the observed deposits where flow disruptions occur such as chokes, gas lift and safety valves. In recent years it has been recognised that the turbulence found at these locations increases the likelihood of scale formation and experiments have been able to demonstrate that with increased turbulence there is an increase in the mass of scale observed and an increased concentration of scale inhibitor is required to prevent its formation. In this paper a field case is investigated where strontium sulphate was observed in a location downstream of a gas lift valve. Laboratory tests were conducted to confirm whether the expected scaling was observed in a low shear flow loop and also to investigate whether the location of the scale changed when additional turbulence (gas injection) was introduced to the system. The flowrate was chosen so that the shear stress generated on the test piece was approximately 1-2 Pa, similar to the value expected in typical field pipe flow. At the end of the test, the scale adhered to each of the five sections of the test piece pipe work was analysed separately to give data on both the mass and location of scale. A second test was also carried out to investigate the effect shear and turbulence induced by gas lift had on scale formation by modifying the test piece to introduce a flow of gas into the system. The test method was then used to evaluate a scale inhibitor and assess whether its performance was affected by the different flow regimes. The introduction of the ‘gas lift’ had a significant effect on the location of scale. Instead of being spread evenly throughout the test piece, the majority of the scale deposited upstream of the gas injection point. This is likely due to the induced turbulence and expansion in the tubing diameter at the T-piece increasing the residence time and thereby enhancing scale growth. A significant difference in scale location was also observed when the inhibitor dose was too low to prevent deposition and a higher dose was required to achieve complete inhibition in the ‘gas lift’ system. The findings from this study have significant impact on the design of test methods of evaluating scale risk in low saturation ratio brines and the screening methods for scale inhibitor for field application that should be utilised to develop suitable chemicals that perform better under higher shear conditions.

A pilot-scale test system has been developed to evaluate scale deposition and inhibition under flow conditions that are more representative of the field than conventional laboratory methods. This system has been used to investigate an example of strontium sulphate scaling and the effect of hydrodynamics on the location and the quantity of scale deposit formed. The approach used in this study involved flowing a large quantity of heated brine through a 1" diameter steel test piece at 5 L/min for several hours in a single-pass system. The test piece consisted of short sections allowing the configuration to be altered and allowing the scale deposited in each section to be quantified. Tests were run with configurations simulating a straight pipe, an ICD nozzle, and a gas lift system to investigate the effect of turbulence and shear stress on scale location and mass. The effect that injection of a chemical inhibitor had on preventing scale deposition in this system was also tested. The results obtained demonstrated that the flow regime in the test piece had a significant effect on the mass and location of strontium sulphate scale deposits. A much greater quantity of scale was observed when the fluid experienced the increased shear stress and turbulence generated by passing through a narrow restriction, or by the injection of gas into the flow, compared with straight pipe flow. Successful chemical inhibition of surface deposits was demonstrated in this system. The use of smaller scale benchtop tests to demonstrate the effect of saturation ratio on obtaining sufficient deposition in single-pass tests compared with a recirculating approach are also discussed. Design and implementation of large-scale tests is an important industry tool for laboratory evaluation of scaling in circumstances where conventional test methods do not give sufficient confidence to develop a scale management strategy and a more field-representative method is required. This paper describes an approach for successfully carrying out such tests and demonstrates the breadth of results that can be obtained from them. It also adds to industry knowledge of the effect that regions of high turbulence and shear have on promoting greater scale formation.