Spectrophotometric Determination of pH Change of Formation Water Under High CO<sub>2</sub> Pressure Using a Mixed pH Indicator

SUGAI Yuichi, SUSANTO Very, SASAKI Kyuro, MORI Ryo

doi:10.2473/journalofmmij.123.518

In geologic CO₂ sequestration, the pH of formation water may be decreased due to CO₂ dissolution, which may cause the change of porosity and permeability of reservoir rock. The pH changes of formation water are widely varied depending on CO₂ pressure and the content of substances having pH buffering action, therefore, it is important to determine the wide range of pH change of various types of formation water under the various CO₂ pressure conditions. We considered a determination method of pH change of various types of formation water under the various CO₂ pressure conditions based on the spectrophotometry using a windowed high-pressure cell and a mixed pH indicator consisting of 4 single pH indicators. The well-defined absorption peaks were found at the wavelength of 614 nm or 444 nm when the pH of the solution was 5.6 or <5.6 respectively, therefore, two different calibration curves were derived from the absorbance of standard pH buffer solutions at each wavelength. The validity of this method was confirmed by an experimental result that the pH change of deionized water under 0.1 MPa CO₂ pressure had been determined accurately by this method. We carried out experiments on this method using the real formation water samples which contained bicarbonate ion having pH buffering action with different concentration under various CO₂ pressure. The results of the experiments demonstrated that this method is capable of determining the pH change of various types of formation water under various CO₂ pressure conditions.

Spectrophotometric Determination of pH Change of Formation Water Under High CO<sub>2</sub> Pressure Using a Mixed pH Indicator

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