A Study of Solubility of Strontium Sulfate

This paper describes the results of a solubility study of strontium sulfate in sodium chloride brine. A predictive equation for the solubility of strontium sulfate is presented, which is useful for calculating solubility in water containing 0 to 200 g/L (0 to 200 g/dm3) sodium chloride at temperatures from 100 to 300 degrees F (38 to 149 degrees C), with pressures from 100 to 3,000 psig (689 to 20 684 kPa), and total ionic strengths of 0 to 3.43. This paper also describes the experimental technique employed and the special equipment designed for this study. Introduction Strontium sulfate scale formation has become a growing concern in oil-production systems. Until recently, the appearance of strontium in oilfield scales has been primarily in the presence of barium sulfate scale. Almost pure SrSO4 scale now is observed in several production wells around the world. The scale formation is primarily a result of subsurface commingling of waters, which results in a water supersaturated in SrSO4. The literature does not readily provide solubility data that permit prediction of SrSO4 scaling under downhole conditions. Jacques et al. presented the most recent SrSO4 solubility data in 1979. Their work presented a comprehensive literature survey, new solubility data developed in synthetic brine systems from 77 to 212 degrees F (25 to 100 degrees C), and studies of effective SrSO4 scale inhibitors. The study showed that the solubility of SrSO4 increased with increasing ionic strength and decreased with increasing temperature once 104 degrees F (40 degrees C) had been exceeded. A method of predicting SrSO4 precipitation in field brines was suggested on the assumption that solubilities based on pure sodium chloride systems were the limiting thermodynamic case. This model did not provide for pressure effects and it was limited to 212 degrees F (100 degrees C). Some work available at the time indicated slight increases in SrSO4 solubility with pressure, but the study was limited to 95 degrees F (35 degrees C) maximum and low salinity. To predict SrSO4 precipitation and scaling under downhole conditions, solubility data are needed that bracket all possible temperatures, pressures. and salinities of oilfield waters. This study reports the results of new solubility determinations developed under broader test conditions. The results then are combined with the 1979 data 4 to provide a predictive solubility model. Theory To develop data that could be used generally to predict SrSO4 solubility in most oilfield brines, the solubility of SrSO4 was studied in pure synthetic NaCl brines. The salinity, temperature, and pressure ranges used in this study were selected to encompass most oilfield conditions. Minimum Maximum Salinity, g/L, NaCl 0 200Pressure, psig (kPa) 100(689) 3,000 (20 700)Temperature, degrees F 100 (38) 300 (149)( degrees C) To develop the most useful data possible over the ranges of the three variables listed demanding the fewest possible experiments, we used the Box-Behnken experimental design as taught by DuPont. This space-filling design was followed by a second design, a 2 3 factorial, inside the first design. SPEJ P. 292^