Abstract

Hydrogen sulfide poses significant challenges to develop oil and gas fields and manage risks in terms of personal and process safety. There is a great need to accurately interpret a measured H2S concentration from a gas/oil/water sample for material selection and design during development stage and for sulfide scale/corrosion control after water breakthrough.

The objective of this work was to develop an integrated tool comprising ionic chemistry and hydrocarbon thermodynamics in a single and easy to use spreadsheet based tool that is also applicable at HPHT (High Pressure High Temperature) and high saline brine conditions. The tool must account for evaporation of water from brine phase and also have the capability to estimate CO2 and H2S partitioning and speciation, bubble points and density calculations, H2S fugacity and pH of the aqueous phase.

A simulation tool for ionic chemistry has been developed to model the H2S distribution with Pitzer ion interaction model to account for the speciation of CO2 and H2S, water activity, and the pH of the aqueous phase. The tool was evaluated by comparing the prediction to experimental data at relevant oilfield condition. The dependence of Pitzer's coefficients for ion activity coefficients on temperature and pressure was examined and incorporated into the model, which provides reliable predictions of solubilities and density that are consistent with over 5000 literature data at all conditions tested.

Hydrocarbon thermodynamics based on Peng Robinson equation of state with Peneloux specific volume shift correction has been used to model the partition of CO2, H2S and H2O in the gas/oil/water phase. In this paper we present the validation of the tool's capabilities against experimentally measured values over wide ranges of conditions, including single to multiple aliphatic/aromatic hydrocarbons, up to 6 molal NaCl salinities, and temperature up to 200 °C and pressure up to 10,000 psia with reasonable agreements.

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