가스하이드레이트 해리지역 모니터링을 위한 시간경과 주시역산 알고리듬의 개발

Abstract

Unconventional resources like as shale gas or gas hydrates have attracted great attention as alternative energy resources to take precautions against severe fluctuation of oil price by virtue of the progress of technology nowadays. In the production stage of gas hydrates, the success largely rests on securing the monitoring technology to detect the dissociation or breaking down of upper gas hydrate layer which plays a role as cover rock of methane hydrates. The dissociation or breaking down can be detected using seismic time-lapse monitoring by the decrease of velocity due to the phase change of gas hydrates. In order to perform this seismic time-lapse monitoring effectively, the design of optimal survey geometry is prerequisite. In this study, an efficient seismic time-lapse traveltime inversion algorithm was developed and the optimal monitoring survey geometry was investigated and suggested using this developed algorithm based on the rock physics model reflecting the gas hydrate bearing zone in Ulleung-Basin. In particular, the virtual source method and OBC (Ocean Bottom Cable) array was introduced in order to maintain the geometry consistency during monitoring period and cover the large horizontal area. Fresnel volume approach instead of conventional ray tracing method was also used in order to retain the maximum sensitivity between wells and reduce the high computational costs in traveltime inversion. In addition, various recent inversion techniques were employed in this algorithm in order to detect the low velocity anomaly successfully. To enhance the resolution and reduce the ill-posedness of inversion result, the minimum gradient support and reference model were implemented. Based on these techniques, integrated sensitivity analyses were conducted in accordance with seismic survey geometry in the vertical and horizontal well cases and the horizontal width of target area. The inversion tests were also conducted on the basis of the sensitivity analyses. From results of this study, the effective and stable gas hydrate production monitoring could be realized using virtual sources created from OBC array. It is expected that this developed time-lapse traveltime inversion algorithm can be also used for monitoring of various area where velocity variation as well as gas hydrate dissociated area. In addition, the traveltime inversion technique with eikonal solver and fresnel volume approach developed in this research might be able to be applied to the traveltime borehole tomography which is used to explore various buried objects.