[H-SC04] 地球温暖化防⽌と地学（CO2地中貯留・有効利⽤、地球⼯学）

[HSC04-P02] CO₂ detection experiment based on the corrosion potential changes of a metal casing

*堀川 卓哉^1,2、徂徠 正夫^1,2 (1.二酸化炭素地中貯留技術研究組合、2.地質調査総合センター 地圏資源環境研究部門 CO2地中貯留研究グループ)

キーワード：二酸化炭素地中貯留、腐食電位、自然電位、地球物理学的モニタリング

In geological CO₂ sequestration, one of the high-risk leakage pathways is the wells, especially around abandoned wells (Osada & Azuma, 2016). Therefore, monitoring the subsurface CO₂ in the vicinity of the wellbore for a long time is important for risk assessment of the leakage. The electric potential of a conductive body, often called corrosion potential, depends on the electrochemical reaction between the conductor surface and the neighboring aquifer. The negative potential anomaly is usually observed around the well because of the corrosion reaction of iron. CO₂ injection causes variations of redox condition around the bottom of the well following to dissolution of CO₂ into the aquifer, and then the corrosion potential changes on and around the wellhead are induced. Actually, previous field tests showed that a potential increase of about 50 mV was observed around the wellhead in connection with CO₂ injection (Nishi and Ishido, 2022). Monitoring the electric potential of a well casing or its neighbor from ground far from the wellhead has a possibility to detect the approach of CO₂ plume to the well and the risk of leakage from the well. However, few experiments have been performed to focus on the potential changes of the well induced by the approach of CO₂.
In order to understand the changes in the corrosion potential caused by touching CO₂ in the CCS field, we made a sandbox apparatus, which was composed of three horizontal layers corresponding to the reservoir, the caprock and the upper sand layer. In our experiments, an iron rod was stuck through the layers vertically, and the electric potentials on and around the rod were measured while electrochemical reactions between the rod and solutions in the layers progressed. First, we attempted to measure the potentials for almost 50 days after substituting the solution in the reservoir from brine to carbonated water., and the increase of the rod potential of ~40 mV was observed 12 days after the substitution (AGU, 2022). The appearance of the rod after the experiment suggested that corrosion products covered the rod surface and the following passivation increased the potential. Next, to evaluate the influence of the surface area of the rod exposed to the carbonated water on the potential increase, we changed the thickness of the reservoir and performed a measurement similar to the first experiment. In our presentation, the relationship between the exposed area of the rod and the potential changes will be presented.
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.