4:27 PM - 4:45 PM
[SVC30-08] The chemical compositions of chlorites from wells discharging acid fluid in the Hatchobaru geothermal field, Oita, Japan
Keywords:chlorite, acid fluid, alteration mineral, EPMA, geothermal
The Hatchobaru geothermal field is located in Oita Prefecture, Japan. In the Hatchobaru field, geothermal fluid flows along the NW-SE trending faults and NE-SW trending faults (Momita et al, 2000). While most production wells discharge neutral fluid, some wells targeting the Hatchobaru fault discharge acid fluid (pH=3~5, Shimada et al, 1985). The acid fluid is rich in Fe and Mg, compared to the neutral fluid (NEDO, 2000). According to X-ray diffraction analysis (XRD) of cutting samples collected from wells targeting the Hatchobaru fault, chlorite and illite were detected at the depth of feed points (Tsutsumi et al., 2020). It is reported that Al-chlorite occurs under the influence of acid fluid (Kawano and Tomita, 1991). In this study, we consider the relationship between the chemical composition of hydrothermal chlorite and pH of fluid in the Hatchobaru field.
The drill cuttings collected at the depth of feed points in six production wells in the Hatchobaru geothermal power plant were selected for analysis. With the total number of six samples, the chemical compositions of chlorite were analyzed using Electron Probe Micro Analyzer analysis (EPMA) at Kurume research park.
Based on the chemical composition, the chlorites were classified as Fe-Mg chlorite. Some chlorites from wells producing acid fluid are higher octahedral Al content compared to chlorite from wells producing neutral fluid. Pyrophyllite is identified at 50m above the feed point depth of acid fluid in the well targeting the Hatchobaru fault, and the chlorite of this well shows higher Al content. The chlorite was probably formed under the influence of pyrophyllite formation. On the other hand, chlorite and illite are present, but calcite is also identified at the feed point depth in one of the wells producing neutral fluid. Therefore, the chemical composition of chlorite is related to pH of fluid and the distribution of pyrophyllite. This result implies that the presence of relatively high Al chlorite at the Hatchobaru fault is probably applicable to the prediction of acid fluid existence even during well drilling. It needs to be taken into account that some chlorites from the wells targeting the other faults were relevant in Al contents with chlorites from the wells producing neutral fluid, so it is necessary to collect more data to compare to the cases in the other faults or the other fields.
We are grateful to JOGMEC and Kyuden Electric Power Co., Inc. (KEPCO) for their permission to publish the well data. This study was conducted as a part of the project financed by JOGMEC.
The drill cuttings collected at the depth of feed points in six production wells in the Hatchobaru geothermal power plant were selected for analysis. With the total number of six samples, the chemical compositions of chlorite were analyzed using Electron Probe Micro Analyzer analysis (EPMA) at Kurume research park.
Based on the chemical composition, the chlorites were classified as Fe-Mg chlorite. Some chlorites from wells producing acid fluid are higher octahedral Al content compared to chlorite from wells producing neutral fluid. Pyrophyllite is identified at 50m above the feed point depth of acid fluid in the well targeting the Hatchobaru fault, and the chlorite of this well shows higher Al content. The chlorite was probably formed under the influence of pyrophyllite formation. On the other hand, chlorite and illite are present, but calcite is also identified at the feed point depth in one of the wells producing neutral fluid. Therefore, the chemical composition of chlorite is related to pH of fluid and the distribution of pyrophyllite. This result implies that the presence of relatively high Al chlorite at the Hatchobaru fault is probably applicable to the prediction of acid fluid existence even during well drilling. It needs to be taken into account that some chlorites from the wells targeting the other faults were relevant in Al contents with chlorites from the wells producing neutral fluid, so it is necessary to collect more data to compare to the cases in the other faults or the other fields.
We are grateful to JOGMEC and Kyuden Electric Power Co., Inc. (KEPCO) for their permission to publish the well data. This study was conducted as a part of the project financed by JOGMEC.