Optimizing Sour Gas Qualification Testing – Modeling the Effects of Temperature and Total Pressure on H2S Fugacity, Activity, and Solubility Coefficients

Abstract

Traditionally, sour severity of high-pressure, high temperature (HPHT) oil and gas production wells were assessed by H₂S partial pressure (P_H2S): The mole fraction of H₂S in the gas (y_H2S) multiplied by the total pressure (P_T). However, P_H2S usually over-predicts the actual sour severity of HPHT systems, leading to sub-optimal material selection choices. To reflect recent advances in thermodynamic modeling and to avoid over conservatism, after careful deliberation, ANSI/NACE MR0175-2021/ISO 15156-2:2022 recently expanded the number of sour severity metrics to four: P_H2S, fugacity (f_H2S), chemical activity (a_H2S) and dissolved concentration (C_H2S) of the aqueous phase. The new metrics are often computationally derived and account for thermodynamic non-idealities, which are significant at HPHT conditions. Regardless of preferred metric, quantifying the sensitivity of each metric to a wide range of temperatures and total pressures is critical when conducting H₂S service assessments. In this article, the effect of increasing temperature and total pressure on the thermodynamically derived apparent H₂S solubility (K_H2S = C_H2S/P_H2S) was investigated. K_H2S is a critical parameter for quantifying changes in H₂S phase behavior/sour severity of HPHT systems. Apparent K_H2S values were calculated by two different thermodynamic models and benchmarked to two publicly available H₂S/H₂O datasets up to 120 °C and 10.3 MPa equilibrated in a brine containing 165,000 mg/L Cl⁻. The model that provided the best match to the experimental data was later used in a much broader thermodynamic sensitivity study of the H₂S/CH₄/H₂O/NaCl “oilfield” system. For this sensitivity analysis, changes in f_H2S, a_H2S, C_H2S and K_H2S were individually modeled between 4 to 204 °C, at total pressures up to 138 MPa, and in brines containing up to 25 wt % NaCl (180,000 mg/L Cl⁻). Lastly, a comparison of the predicted sour severity by pseudo-PH2S, fH2S, and CH2S metrics, over the same temperature and total pressure parameter space, is presented.