The world-first offshore production test of gas hydrate was just performed in the deep water 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 natural 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 the temperature transients was performed, and the results that could explain the dissociation behaviors were obtained.

The world's first offshore gas hydrate production was successfully carried out in the deepwater Japan at Nankai Troughin Q1 2013. In this project, one production well and two sandface monitoring wells were drilled and installed with a combination of distributed temperature sensing (DTS) and array-type RTD (Resistance Temperature Detector) sensors. The objective of the sandface monitoring system was to capture the hydrate dissociation front dynamically changing during the production test and to monitor long-term reservoir stability with the selected temperature sensors. An ability to continuously monitor the response of these temperature data during production test would facilitate tracking of the dissociation front and yield valuable information for engineering design and verification of numerical reservoir simulators. The temperature sensors are cemented behind the casing and also strategically installed to cover the hydrate zone of interest and the entire wellbore. Due to operational constraints, the monitoring system was designed to be autonomous self-operated system by the subsea battery without cable connection from the sea surface for a period of 18 months from the day of installation of the monitoring system. The deployment of this monitoring system in this shallow unconsolidated hydrate reservoir was an unprecedented and challenging operation. In this paper, we will show the details of the key system components of the sandface monitoring system and the deployment process.

The world's first offshore production test of the methane hydrate (MH) project was successfully carried out in the deepwater Nankai Troughin 2013. In February and March 2012, a production well and two monitoring wells were drilled and cemented to deploy temperature monitoring systems for which a combination of distributed temperature sensing (DTS) and array-type RTD (Resistance Temperature Detector) was employed.For cement hydration, the chemical reactions are exothermic in nature so that a measureable temperature increase will be observed once the heat is released. With a typical cement system in wellbore configuration, this temperature increase could result in MH dissociation prior to the production that may have some impacts on the extraction of actual methane gas. Additionally, potential problems of thermally induced cracking are another concern.To evaluate the thermal effect from the cement heat to MH, a 1D numerical model of cement hydration is developed based on the radial heterogeneity of thermal properties. The temperature data acquired during cementing stage is used as the input data for the model, and the parameters such as activation energies for the cement hydration heat correlation in the model are determined. It was shown that the variation of simulated temperature from different MH saturation within the upper MH zone is so small that a reliable estimation of MH saturation becomes difficult while the estimated MH saturations from temperature data inversion in the lower MH zone are relatively high. A sensitivity study is also performed to understand which parameters in the developed model most significantly influence the temperature during the different stages of the cement hydration, respectively.

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