pH Model

pH is one of the critical parameters for predicting and assessing corrosivity in oil and gas production and transmission systems. The role of pH in corrosion and cracking has been well documented. Hence, accurate pH determination/prediction is essential for understanding system corrosivity and managing corrosion. This chapter provides a tool to determine in-situ pH in oil and gas environments.

Background

Understanding in-situ pH is essential for assessing and managing corrosion in oil and gas systems. In simple CO₂-H₂O systems, pH is mainly controlled by CO₂ partial pressure.1 - 3 A general relationship between pCO₂ and pH (at constant temperature) is presented in Figure 1.

Generic representation of pCO2-pH relation. after [1](#reference1)
Figure 1: Generic representation of pCO2-pH relation. after 1

As the corrosion reaction between iron and carbonate ions progresses, the pH of the fluid increases along with the concentrations of Fe²⁺ and CO₃²⁻ ions. When their solubility limit is exceeded, iron carbonate (FeCO₃) begins to precipitate, forming a protective layer on the steel surface, which can eventually reduce the corrosion reaction rate to some extent. The corrosion protection effectiveness of this layer depends on several factors, with pH, temperature, and ion concentrations being the most prominent.

The introduction of H₂S into the water-CO₂ system further complicates the situation due to the competitive formation of iron sulfide (FeₓSᵧ) deposits. These deposits can either slow down or sustain the corrosion rate, depending on environmental conditions (e.g., pH) and the properties of the formed sulfide layer

Tools

It is well known that open-cup pH measurements, taken after exposure to atmospheric conditions, often do not accurately represent the true in-situ pH of production environments. Misestimating pH can lead to significant underestimation or overestimation of system corrosivity. For effective system evaluation and material selection, a reliable method for determining in-situ pH is essential.

To address this need, an in-situ pH calculation tool has been developed based on the Brønsted concept.4 This tool enables accurate assessment of in-situ pH by incorporating key parameters such as temperature, pressure, CO₂ and H₂S partial pressures, fugacity coefficients, and the presence of ionic species—including weak organic acids like acetic and formic acid. The model outputs include in-situ pH, ionic strength, balancing ion concentrations, and saturation pH, which can help in assessing the scaling potential.

The in-situ pH generated by this tool can be applied to estimate corrosion rates in CO₂/H₂S environments, predict scale formation tendencies, support material selection in accordance with NACE MR0175 / ISO 15156 for H₂S-containing oil and gas production environments, and define appropriate laboratory testing conditions that reflect field realities.

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References

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