The fluid produced by the geothermal wells in the Nigorikawa field which is located in southern Hokkaido, Japan's northern island, is rich in carbon dioxide(C02). The dissolved C02 causes a scale deposition problem of calcium carbonate(CaCOs). Japan Metals & Chemicals Co., Ltd.(JMC) has already solved the CaC0, scale deposition problem by injecting a scale inhibiting chemical fluid directly and continuously through a injection tube extending into the production we1 1. The CaCO, scale deposition occurs at the flash point. I t is important, therefore, to estimate the flash depth correctly so that the depth of the chemical injection point can be determined. However, the boiling point curve of the geothermal fluid with dissolved CO, is different from that of pure water(H20). So, a wellbore flow model in the presence of CO, gas has been devel oped. This model was tested by comparing the temperature and pressure values calculated by the computer to depth profiles drawn the Nigorikawa field data. As a result of the cmparison, a satisfactory fitting has been obtained. A brief explanation of this model and a few examples of analysis by this model are presented.

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