The world-first offshore production test of gas hydrate was just performed in the deepwater Nankai Trough along the Pacific coast of Japan in March, 2013. A week-long flow test successfully produced methane gas by depressurizing method from the subsea methane hydrate reservoir.

While methane gas was successfully extracted from hydrate, there is a strong urge for understanding the dissociation behavior and characterizing production process of hydrate through the depressurization method. This is a crucial knowledge for the future commercial production of gas hydrate.

The dissociation of methane hydrate is an endothermic reaction, and the drop of formation temperature is expected to occur as the dissociation progresses. By capturing the phenomenon of temperature decrease in situ, the data attributed to the dissociation of methane hydrate can be obtained during production.

Two monitoring wells, uniquely designed for minimizing thermal disturbance and better thermal coupling were drilled in the vicinity of the production well, and the two types of temperature sensors using DTS (Distributed Temperature Sensing) and array-type RTD (Resistance Temperature Detector) were deployed in the monitoring wells and recorded the data not only during the flow test period but also before and after the test. DTS covered the entire wellbore interval while the array-type RTD sensors were strategically placed across the gas hydrate reservoir with higher temperature resolution and accuracy.

In both monitoring wells, the temperature decreases were observed distinctively with both sensors as the flow test progressed over a week. The data quality check confirmed both measurements were conformable to the design specification and demonstrated the strong advantages of having both sensors in this monitoring system for further interpretation to investigate the dissociation behaviors.

The framework for temperature data analysis was defined to perform thermal characterization of gas hydrate reservoir during the production test stage. The preliminary analysis on temperature transients was performed, and the results that could explain the dissociation were obtained.