Iridium oxide-based chemical sensor for in situ pH measurement of oilfield-produced water under subsurface conditions

The objective of this study was to develop a pH sensor for monitoring hydrogen ion activity in situ in oilfield-produced water under subsurface conditions. An iridium oxide film was prepared, and the performance of the iridium oxide film was evaluated in produced water using a series of high salinities, temperatures, and pressures. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were employed to assess the morphology and oxidation state of the iridium oxide film. A series of open-circuit potential (OCP) measurements were performed to access the sensitivity and stability of the iridium oxide film electrode in produced water. Research results indicated that the pH sensor exhibits a very high sensitivity in the environment with varied interference ions, including Na⁺, Mg²⁺, Ca²⁺, Cl⁻, and SO₄ ²⁻, and the existence of sodium chloride in produced water made a slight positive shift of potential. The iridium oxide film-based pH sensor exhibited a near-Nernstian response at −57.9 mV/pH in produced water. An increase in temperature from 24 to 80 °C decreased the pH potential. Compared with the theoretical Nernst equation, the iridium oxide sensor showed fair Nernstian behavior at high temperatures. The iridium oxide pH sensor was accurately investigated under high pressure; the sensor seems independent of pressure, and the slope is the same at different potentials under a wide range of pressures.