Posted by Cullen Young
Surface three-dimensional (3-D) seismic acquisition surveys and various targeted drilling programs have confirmed the regional extent of methane gas hydrate deposits on the North Slope of Alaska at Kuparuk. The hydrates have been found in significant quantities in sub-permafrost sand units within the gas hydrate stability zone. High-resolution seismic investigations are desired to better understand the nature of these hydrate-bearing reservoir units; therefore, a group collaboration among the Department of Energy-National Energy Technology Laboratory (DOE-NETL), Japan Organization for Metals and Energy Security (JOGMEC), and the U.S. Geological Survey (USGS) had fiber-optic cables installed within test wells, enabling the acquisition of distributed acoustic sensing (DAS) 3-D vertical seismic profiling (VSP) surveys.
A high-frequency (to 100 Hz) 3-D acoustic reverse-time migration (RTM) technique designed for the DAS 3-D VSP quality survey geometry was developed to produce high- interpretable subsurface imaging results. The produced image clearly resolved reflectivity observed throughout the model at locations expected from the source-receiver migration aperture, including the four main horizons of interest. Observations show that the reflections corresponding with the thin-bed gas hydrate sand units (Top D1 and Top B1) are laterally continuous over the area of data coverage as well as two horizons of interest including the base of significant ice-bearing permafrost (BSIBPF) and base of ice-bearing permafrost (BIBPF).
100 Hz 3-D acoustic RTM imaging results oriented in the azimuth of the deviated well extracted with the Vp well log shown in blue. The depth of four main interfaces inferred from log data are shown to the right of the image, including the tops of the D1 and B1 sands as well as the base of ice-bearing permafrost (BIBPF) and base of significant ice-bearing permafrost (BSIBPF).
Improvements to the final image volume are desired to provide information for both the reservoir tops and bottoms, as well as to obtain information on subsurface reflectivity. Research is ongoing to expand on the 3-D acoustic RTM-imaging procedure using: (1) acoustic least-squares RTM (LSRTM); (2) elastic RTM and LSRTM; and (3) elastic full-waveform inversion (FWI). For more information, please see Young et al., 2022.
That’s a great project. There has been much concern to investigate the presence of hydrate gas in sub-sea permafrost as they significantly produce gas when they warm up and thaw thereby causing the hydrates within or below them to destabilize and consequently produce gas. I would love to read much about it in due time.