[H-SC05] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC05-11] Observation of self-potential changes around a metal casing due to the injection of low pH solution in laboratory experiments

*Takuya Horikawa^1,2, Masao Sorai^1,2 (1.Geological Carbon Dioxide Storage Technology Research Association, 2.CO2 Geological Storage Research Group, GREEN, GSJ, AIST)

Keywords:self potential method, geobattery, redox potential, carbon dioxide capture and storage

In geological CO₂ storage and geothermal exploration, it is important to monitor the changes of subsurface fluid for a long time. Passive geophysical exploration techniques, e.g., self-potential (SP) method and gravity method, have advantages of cost and operation effort compared to the active techniques. The SP method is a technique for measuring the potential difference between two measurement points that spontaneously generated. The SP anomalies are caused by a variety of factors, including fluid flow (streaming potential) and changes in the redox condition (redox potential). It is well known that negative SP anomalies are observed near a conductor, e.g., a metal casing, when the conductor connects regions with different redox environments. This phenomenon is can be interpreted qualitatively by the “geobattery” model. Ishido et al. (2013) showed by a numerical experiment that injection and migration of CO₂ caused the change of the redox environment around the bottom of the metal casings and a potential variation of up to 80 mV was observed in injection wells, observation wells, and their surroundings. Therefore, the SP method is a promising and effective technique for sensing the approach of CO₂ to the well and the risk of leakage from the well. However, few experiments, regardless of in a laboratory or a field, have been performed to focus on the potential changes of the conductor coincided with injection or migration of CO₂.

A decrease in the pH of the groundwater due to the dissolution of CO₂ is thought to be one of the causes of the potential change in the conductor. In this study, we made the experimental sandbox using glass beads and a steel rod shown in Figure 1, and measured the potential of the rod while injecting solutions with different pH around the bottom of the rod. At the beginning of the experiment, the sandbox was saturated mostly with the solution of KCl 0.1 M, and its pH was almost 6.3. Next, the potential difference was measured between the rod and the reference electrode, placed on the opposite side of the rod, with a digital multimeter. Finally, the pH of the injected solution was adjusted to 2 ~ 6 by adding HCl into the KCl solution, and the potential of the rod was measured while injecting the solutions first from higher pH ones. Figure 2 is the experimental result, and shows that the potential dropped drastically by 170 mV or 500 mV coincided with the injection of the solution with pH 5 or 4.

This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and the Ministry of Economy, Trade and Industry (METI) of Japan.