An Efficient Chelator - MGDA to Remove Calcium Sulfate Scales in Oil Production Systems

Scales can accumulate and obstruct flow pathways in oil production systems, causing production rates reduction, energy consumption increasing and operational disturbances. The mitigation of scale deposition is costly but pivotal for maintaining high-performing production processes. Calcium sulfate, an acid-insoluble scale, requires efficient chelator to be removed. Herein, this paper demonstrates the effectiveness of trisodium dicarboxymethyl alaninate (MGDA) to dissolve CaSO4 at various temperatures, presenting the scenarios of downhole and flow lines in oil production system. CaSO4 scale removal tests were conducted by immersing CaSO4·2H2O crystals (10 mm * 2 mm) in solutions for a certain time at 50°C (representing surface flow line conditions) and 100ºC (presenting wellbore conditions) respectively. The original and residual weight of crystals were measured to calculate the removal efficiency. Factors like MGDA concentration, scale/solution ratio and dissolving rate in a time scale were also evaluated to present the chelator performance and dissolving mechanism. Besides, whether the addition of K2CO3 has positive effects on CaSO4 removal is investigated. MGDA is selected to be investigated since it presented a higher removal efficiency comparing with Glutamic acid diacetate-Na4 (GLDA) and penta-sodium diethylene-triaminepentaacetic acid (DTPA) in the lab screening. Results showed the scale removal efficiency using 8% MGDA at 100 ºC were 83% and 42% at scale/solution ratios of 1/10 and 2/10 respectively after 24h treatment. At 50 ºC, the scale efficiency decreased to 30% at scale/chemical ratio 1 /10 ratio, but the value is over 60% at 0.5/10 ratio. It is noted that increasing MGDA concentration will not obviously improve the crystal dissolving efficiency. Thus, it is recommended a higher solution volume at low temperature conditions. Besides, the time scale results presented the dissolving efficiency quickly increased in the first hour and then slowly increased with time. Furthermore, addition of potassium carbonate and subsequent acid washing of the residual solids will promote the conversion of CaSO4 to CaCO3 for acid removal, which slightly improved the removal efficiency at low addition concentrations. High concentration addition of potassium carbonate will deteriorate the scale dissolving by MGDA. This work demonstrates MGDA as an effective chelator in CaSO4 removing at various temperatures, which is a potential selection for future oilfield operations.