Development of Ocean Bottom Multicomponent Seismic System for Methane Hydrate Dissociation Monitoring

Abstract

Abstract OBC (Ocean Bottom Cable) is a common tool used in the exploration of oil and natural gas fields. But its application to deep sea areas still has to be studied. In this paper we introduce the development of a new 4-conponents monitoring system called ‘DSS’ (Deep-sea Seismic System) using multi-component seismic sensors aim to delineate the methane hydrate dissociation zone for the offshore methane hydrate production trial scheduled in FY2012. Back in the year 2006 we developed RSCS (Real-time Seismic Cable System) which was composed of three component gimbaled geophones. The reflection seismic survey trials conducted in the Nankai Trough sea area confirmed the the high quality records that can be obtained with the system. Following the same design concept, we improved the RSCS receiver by reducing its size to 2/3, adopting accelerometers and a hydrophone. The new system was named as ‘DSS’. The receiver case has a protective metallic exterior and the cable is protected with steel-screened armoring, allowing for deployments up to 2,000m water depths and burial using ROV for sub-seabed deployment. This system realizes a unique survey style in which the receivers are left on the seabed between pre-test baseline survey and post-test repeated survey, this interval might be up to 6 months long. The fixed location of the receiver is very important for time-elapsed monitoring surveys. The newly developed DSS, comprising 36 receivers, was deployed and buried on the seabed off the Atsumi Penisnula in August 2012 to conduct a pre-test baseline survey at 1,000m water depths. The pre-test baseline survey was successfully completed and the obtained data was of very high quality. The post-test repeated surveys are planned forApril and August, 2013. Introduction MH21 (Research Consortium for Methane Hydrate Resources in Japan) is planning to carry out the offshore methane hydrate production trial during the FY2012. In this production trial, it is very important to know and understand how the methane hydrate dissociation progresses. The dissociation process progress and changes can be captured only by long-term reflection seismic monitoring which is conducted in intervals as a pre-test survey and a post-test survey, before and after the production trial. The RSCS, developed as a dedicated reflection seismic survey system, was available and its capabilities have been confirmed when we successfully obtained clear BSR (Bottom Simulating Reflector) records during survey trials conducted off Shima cost in 2006. But RSCS receiver size and weight make it very difficult to achieve the requirements for methane hydrate dissociation process monitoring, which requires a receiver interval of 25m at water depths of 1,000m. The estimated cable load was very high and its precise deployment to deep water was too difficult due to the hydrodynamics of the receiver stream. Based on the RSCS design concept, we have developed " DSS" with a more compact and lighter receiver. We have successfully carried out experimental performance trials off-shore in November 2011, and carried out pre-test survey in August 2012. This paper describes about DSS development and the result of pre-test survey.