High Performance CaSO4 Scale Removers Functioning at Downhole and Surface Flow Lines in Oil and Gas Production

Abstract Calcium sulfate (CaSO4) is one of the most common scales formed in oil\gas wells and surface flow lines, contributing to serious operating problems and inhibiting production. The removal of this scale is costly, but in many cases is considered as inevitable and economically feasible to enhance productivity of wells and ensure oil/gas transportation in surface flow lines. Though many chemicals have been reported as effective for CaSO4 removal at high temperature for downhole applications, seldom is found effective for surface flow lines at lower temperatures. This work aims to investigate CaSO4 removing mechanisms and provide high performance CaSO4 scale removers that could work under both scenarios of downhole and surface flow lines. In this study, 15 chemicals as CaSO4 scale removal were studied by immersing CaSO4·2H2O crystals in solutions for a certain time. The temperatures were maintained at 50°C (representing onshore surface flow line conditions) and 100°C (presenting wellbore conditions) respectively. The efficiency was evaluated by visual observation and weight measurement on residual solids after treatment. Chemical loadings and scale dissolving in a time scale were also evaluated of the selected chemicals. The 15 chemicals can be categorized into three groups based on the removing mechanism: 1) converters that convert CaSO4 to CaCO3, 2) dissolvers that directly dissolve CaSO4 based on chelator-calcium reaction, and 3) dispersants that disperse the crystals into tiny particles to be moved more easily by fluid flow. Three chemicals, one converter (Chemical-A), one dissolver (chelator, Chemical-M), and one dispersant (Chemical-LX), showed the best performance in each group. The scale removal efficiency could reach up to 98 % when the scale/chemical ratio is as low as 1 g/10 mL and decreased to 70% when the scale/chemical ratio is 2/10 at 100 °C for Chemical-A. The efficiencies for Chemical-M were 83% and 42% at scale/chemical ratios of 1/10 and 2/10 respectively. When the temperature was decreased to 50 °C, the scale efficiency decreased to 50% and 25% for Chemical-A and Chemical-M respectively at scale/chemical ratio 1 g/10 mL. For Chemical-LX, the crystals were fully dispersed into slurries in an hour at both high temperature and low temperatures, which could be easily carried away by produced fluid flow when the scale/chemical ratio was 2/10. Therefore, Chemical-A and Chemical-M are suitable for wellbore applications and Chemical-LX is effective for surface flow lines considering the application scheme. By comparing the performance of three types of CaSO4 scale removing chemicals, (converters, dissolvers and dispersants), the work provides three high performance chemicals to deal with the CaSO4 caused flow problems both at high temperature wellbores and low temperature surface flow lines. Besides, this work gives a hint on to the selection of the appropriate scale removers based on mechanisms, and thus benefits future oilfield operations.