System Monitors Sandface for Deepwater Offshore Gas-Hydrate Production
Authors: Wilson Adam 
Published: February 2015

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25328, ’A Deepwater Sandface-Monitoring System for Offshore Gas- Hydrate Production,’ by S. Chee, SPE, T. Leokprasirtkul, T. Kanno, O. Osawa, Y. Sudo, M. Takekoshi, and H. Yu, SPE, Schlumberger, and K. Yamamoto, Japan Oil, Gas, and Metals National Corporation, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.

The world’s first offshore gas-hydrate production was carried out successfully in deepwater Japan at Nankai trough in 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 resistance-temperature- detector (RTD) 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.

Introduction

Interest is growing worldwide in methane hydrate as a unique and challenging natural-gas resource. In order to produce and use these resources, it is necessary to establish the production technology and understand how methane-hydrate dissociation behaves. Japan has studied methane-hydrate exploration during the past decades and planned to conduct the world’s first offshore methane-hydrate production test in 2013. In such a production test, the dissociation progress and the stability of the methane-hydrate reservoir can be captured by long-term continuous temperature monitoring, which is conducted in real time and autonomously before, during, and after the production test.

One of the more challenging requirements is that the deepwater sandface-monitoring system must be self-operated by subsea battery without cable connection from the surface for a long-term monitoring period of 18 months. Thus, the reliability and redundancy of the monitoring system were key design challenges above data measurement quality and offshore deployment tasks.

Outline of the Sandface-Monitoring Wells

One production well and two monitoring wells were drilled. The methane-hydrate reservoir in the Nankai-trough area has methane-hydrate-bearing sand-rich intervals in turbidite fan deposits. For the production test, approximately 50 m of methane-hydrate zone was selected that is located in shallow unconsolidated reservoir approximately 300 m below the seafloor in a water depth of 1000 m.

Deepwater sandface-monitoring systems were deployed in the two monitoring wells in the first quarter of 2012, approximately a year before the methane-hydrate production test started in the first quarter of 2013. The production test continued for a week, and then the recovery of the sandface-monitoring systems was performed in the third quarter of 2013. Therefore, the in-situ temperature monitoring started 1 year before the production of methane hydrate and continued for approximately half a year after the production test in order to monitor the long-term stability of the methane-hydrate- reservoir behavior. After the monitoring system was recovered from the seafloor, temperature data were retrieved from data storage for analysis.