Tracer testing using alcohols were conducted in the Matsukawa vapor-dominated reservoir since 2000 (Fukuda et al., 2005). In addition to the alcohols, two unexpected gases were detected by gas chromatography during the test conducted on well M-14 prior to the completion of the drilling. We concluded that the gases were derived from lost-circulation materials (LCMs) used for the drilling of well M-14, which was confirmed by heating experiments of the LCMs in the laboratory. The LCMs were products of cottonseed hull and pulp.

The LCM-derived gases were found in all the monitoring wells in the Matsukawa vapor-dominated reservoir, although the alcohol tracer was detected in the only two production wells. The return curves and arrival times were obtained by the monitoring of the LCM-derived gases and they showed that the fluid flowed from the southwest to the northeast within the system; this flow direction is consistent with the previous study of the pressure distribution (Hanano and Matsuo, 1990).

With the results obtained during the monitoring, we concluded that the LCM-derived gases can be used as a tracer to detect the breakthrough of the drilling fluid in the vapor-dominated reservoir.

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