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46 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 CHEMICAL TREATMENT acid gases in the aqueous phase and the effects of multiple precipitating scales that are frequently found in many heavy oilfield produced brines. Figure 1 illustrates the size of the error that can occur when assuming ideal chem- istry. The partial pressure of H 2 S at 50 °C as a function of total pressure is shown by the thin dashed line labeled P(H 2 S) to the left of the vertical line that divides the vapor- liquid and the liquid-liquid regions. The fugacity is shown by the thick solid line labeled f(H 2 S). The solubility of H 2 S in an aqueous phase with and without 3 m NaCl is also shown by the thin solid lines along with the activity of H 2 S in solution. The cal- culated H 2 S solubilities 5 (thin solid lines) are compared with the experimental data of Koschel, et al. 6 Although the P(H 2 S) and the f(H 2 S) appear to be similar on the left side of the graph, use of P(H 2 S) at ~35 atm would overestimate the amount of H 2 S in the vapor phase by a factor of ~16%. In the liquid- liquid region, Henr y 's law 7 is not directly applicable and it is no longer prac- tical to use partial pressure to characterize the vapor phase. Brines The nonideality of single-component brines is highly dependent upon the salt, its concentration, and the temperature. As shown in Figure 2, calculated using the model of Gruszkiewicz, et al. 8 and Wang, et al., 9 a NaCl brine at 25 °C has a mean activ- ity coefficient, γ ± , that hits a minimum of ~0.65 at ~5 wt% and then approaches 1.0 again as the concentration approaches sat- uration. However, the activity coefficient of a sodium sulfate (Na 2 SO 4 ) solution drops from 1.0 at infinite dilution to a minimum value of ~0.1 at saturation. The activity coefficient of potassium chloride (KCl) hits a minimum of ~0.58 at ~15 wt% and then stays near that value as the concentration increases to saturation. The activity coeffi- cient of magnesium chloride (MgCl 2 ) fol- lows the trend of Na 2 SO 4 at low concentra- tions and then increases to as much as ~20. If laboratory test protocols are based upon single-component brines with just NaCl and the results are supposed to simu- late corrosion in a multicomponent brine containing other ions, some of the assump- tions based upon the NaCl brine results may not be valid, even if scale and corro- sion product precipitation are not factors. Sulfide Stress Cracking Testing In 2005, Nelson and Reddy 10 discussed the importance of considering both the nonideality of sour gas at system pressures in excess of 35 MPa and the necessity of considering non ideal solution parameters. They found that "neglecting these nonideal ef fects causes large over-conser vatism" when performing fit-for-service stress cor- rosion cracking testing. Procedures have long existed to esti- mat e c ompre ssi bi lity for th e se mi xed chemistry gases. 11 Modern thermodynamic models have greatly simplified the process of determining compressibility as well as refined the accuracy. Complicating mat- ters, gases that contain more than a com- bined total of ~3% CO 2 and/or H 2 S require additional steps to determine the com- pressibility factor, z, for the gas. For exam- ple, a 20% CO 2 -10% H 2 S-70% methane (CH 4 ) gas at 10 MPa and 80 °C would have a com- pressibility factor of 0.8210 and the fugacity coef ficient of H 2 S w ould b e 0.71. Thi s means that instead of the partial pressure determination of 1 MPa of H 2 S, the actual H 2 S fugacity under these conditions would be 0.71 MPa. Including the consideration of heavier hydrocarbons in the calculation of the compressibility factor can place the envi- ronmental conditions near the critical boundaries for the mixed gas. These condi- tions can lead to fugacity coefficients as low as 0.25 or even lower, which in this case would translate into an error of 4 in the determination of the fugacity of the gas. If CH 4 was replaced in the earlier-mentioned gas mixture with a conceptual worst-case mixed hydrocarbon gas, instead of a fugac- ity of 0.71 MPa, the fugacity of H 2 S in the gas could be closer to 0.18 MPa. Further, if the reduced fugacity of H 2 S, the activity coefficient effects of a mixed- FIGURE 2 Mean activity coefficients of common brine components as a function of concentration at 25 °C. TABLE 1. CALCULATED H 2 S DISSOLVED IN AQUEOUS PHASE IN THE PRESENCE OF 20% CO 2 + 10% H 2 S + BAL. CH 4 AT 80 °C Total Pressure (MPa) Aqueous Solution Ideal Chemistry (molality H 2 S) Nonideal Chemistry (molality H 2 S) 10 Pure H 2 O 3% NaCl 3X seawater 0.29 0.29 0.30 0.21 0.20 0.17 17 Pure H 2 O 3% NaCl 3X seawater 0.43 0.44 0.45 0.26 0.25 0.21 70 Pure H 2 O 3% NaCl 3X seawater 0.88 0.90 0.93 0.34 0.32 0.28