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Yasuhiro Yamada, Kyoto University (Japan)
Makoto Kunieda, Kyoto University (Japan)
Akira Ueda, Kyoto University (Japan)
Toshifumi Matsuoka, Kyoto University (Japan)
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Geological sequestration is one of the most accountable methods to reduce emission of green house gases to atmosphere. The key thing of this technique is to prevent leakage of injected gas from the underground reservoir. Potential leakage paths may be either breakage of natural seals or along the wellbore that drilled for the injection. The later includes leakages through casing cement due to its alteration, through the interface of reservoir and cement, and through the interface of cement and casing pipe. Since there are numerous uncertainties on this topic, we believe experiments are vital at the current stage to investigate all of these and extract factors that influence the alteration of materials and leakage of injected gas.
In this study, we used super critical CO2 because the pressure and temperature condition at the injection point may fulfill the condition of super critical phase of CO2. The specimens include pure mudstone, pure G-class casing cement, and combined material of cement and sandstone. These specimens are prepared in the form of cylinder and contained in pressure chambers with some water solution. The chambers are then filled with CO2 and the temperature and pressure are maintained at 60 deg C and 10MPa (i.e. supercritical CO2) during the experimental period (3 or 9 weeks). Then, the specimens are examined with stereoscopic microscope, polarizing microscope and scanning electron microscope (SEM and EDS). X-ray analysis on the specimens and atomic absorption analysis on the water left in the chambers are also performed.
Experimental results show that natural mudstone is not affected within a few months, but in all other specimens mineral precipitation (aragonite and calcite) was observed in the pore space of sandstone and casing cement. This result implies that carbonate mineralization in the pores of cement and sandstone might disturb flowing of CO2 and water in the wellbore materials, and that the cement degradation may be constrained. There are still some possibilities that carbonate minerals precipitated in the pores are dissolved due to continuous interaction with acidic brine for a longer time period.
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