What is pH?
pH is a scale used to measure how acidic or basic a solution is. Acidic solutions have lower pH values, while basic (or alkaline) solutions have higher ones. The scale is logarithmic and is based on the concentration of hydrogen ions in a solution. While this might sound like a concept confined to chemistry class, pH actually plays a critical role across many industries — from water treatment and agriculture to nuclear energy, CO₂ storage, and, of course, oil and gas.
Why does fluid pH matter in oil and gas?
Oil and gas production generates enormous volumes of water, known as produced water or brine. This water must be separated from the hydrocarbon stream before it can be sent to refineries. Water is also heavily used in enhanced oil recovery and throughout refinery operations — in boilers, distillation columns, and more.
The pH of these waters directly influences a wide range of critical decisions, including material selection, maintenance planning, and day-to-day operations. When pH is off, the consequences can be serious: scale buildup, formation damage, reduced effectiveness of chemical additives like corrosion inhibitors and surfactants, and poor filtration performance, to name just a few.
Perhaps most concerning for operators is the link between pH and corrosion. Produced waters often contain dissolved gases such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S), both of which can cause significant damage to pipelines, equipment, and metallic components — including uniform corrosion, localized attack, hydrogen embrittlement, and cracking. The severity of this damage is closely tied to the in-situ fluid pH. Getting pH right isn’t just good practice; it’s essential for protecting assets and maintaining operational integrity.
Figure 1. Produced water samples with different water to oil ratios.
Why predict pH instead of just measuring it?
Measuring pH in the field sounds straightforward, but in practice it’s anything but. There are three main challenges operators face:
First, contamination. Hydrocarbons, high salt concentrations, wax deposits, and microorganisms can all foul pH probes and produce unreliable readings.
Second, inaccessibility. Some locations — such as deep downhole environments, high-temperature zones, or the tops of pipelines where condensation produces tiny water droplets — make direct measurement difficult or impossible.
Third, sample alteration. When a fluid sample is brought to the surface and exposed to the atmosphere, dissolved gases begin to escape, changing the sample’s chemistry and making the measured pH different from what it actually was in-situ.
Accurate pH prediction fills these gaps, giving engineers reliable data even when direct measurement isn’t feasible.
Why is OLI software the right tool for pH prediction?
Many pH prediction tools exist, but most come with significant limitations — typically restricted to temperatures below 150°C, pressures below 50 bar, and moderately concentrated brines. Some rely on simple empirical equations (such as the Oddo-Tomson or Millero models), while more advanced approaches like the Pitzer model account for interactions between dissolved species but still fall short in extreme conditions, such high temperature and high pressure (HTHP).
OLI software takes a different approach, built on the Mixed-Solvent Electrolyte (MSE) model — a comprehensive thermodynamic framework capable of predicting phase and chemical equilibria as well as thermal and volumetric properties across an exceptionally wide range of conditions. This includes temperatures up to 300°C, pressures up to 4,000 bar, and fluid concentrations ranging from pure water all the way to highly concentrated salt solutions. It also accounts for interactions in both the liquid and gas phases, and covers a far greater number of chemical species than any other commercially available platform.
In short, the MSE model in OLI software is the most capable and accurate tool available for pH prediction in oil and gas environments.
To illustrate this, two real-world scenarios were modeled: CO₂- and H₂S-saturated aqueous solutions (Figures 2 and 3) — both commonly encountered in production. In each case, OLI’s predicted pH values closely matched experimental data across a range of conditions, demonstrating the reliability and practical value of the model.
Figure 2. pH vs. total pressure for the H2O rich phase in an H2O-CO2 open system.
Figure 3. pH vs. NaCl concentration in H2O-NaCl-CO2 and H2O-NaCl-H2S open systems at 25oC and 1 atm total pressure.
What OLI tools are available for pH prediction?
pH prediction is available through OLI’s thermodynamic property package, which uses both the MSE and MSE-SRK models. These capabilities are accessible across OLI Studio, OLI Flowsheet ESP, and OLI APIs — making them easy to integrate into your existing workflows, whether in the office or in the field.
To learn more about OLI’s pH prediction tools, visit olisystems.com/software/oli-studio or explore OLI’s corrosion management solutions at olisystems.com/solutions/aqueous-corrosion-management.
Ready to speak with an expert? Contact OLI at olisystems.com/contact-us or email [email protected].
References
- Springer R.D., Wang P., Anderko A., SPE Journal (2015) Paper No. 173902.
- Plennevaux C. et al., CORROSION Conference (2013) Paper No. 2843.
- Oddo J.E., Tomson M.B., J. Pet. Technol. 34 (1982) 1583–1590.
- Millero F.J., Geochim. Cosmochim. Acta. 59 (1995) 661–677.
- Millero F.J., Hershey J.P., American Chemical Society (1989) 282–313.
- Felmy A.R., Weare J.H., Geochim. Cosmochim. Acta. 50 (1986) 2771–2783.
- Pitzer K.S., Activity Coefficients in Electrolyte Solutions, 2nd ed., CRC Press, 2018.
- Pitzer K.S., J. Am. Chem. Soc. 102 (1980) 2902–2906.
- Hinds G. et al., CORROSION, 65 (2009) 635–638.