Advanced Distributed Acoustic Sensing Vertical Seismic Profile Imaging of an Alaska North Slope Gas Hydrate Field

Abstract

Gas hydrates are found in significant quantities on the North Slope of Alaska in subpermafrost sand units and intermixed in lower portions of permafrost within the hydrate stability window. While conventional surface seismic data and established imaging methods can indicate the presence of gas hydrate reservoirs, producing high-resolution images of (seismically) thin layers remains challenging due to the preferential attenuation of the higher-frequency data components. An alternative strategy is to use distributed acoustic sensing (DAS) involving cementing optical fibers into boreholes to measure seismic wavefield energy closer to the strata of interest using vertical seismic profiling (VSP). DAS VSP imaging takes advantage of the shorter travel paths and reduced attenuation to generate higher-resolution near-well images. We illustrate these benefits on a DAS VSP data set acquired at the Hydrate-01 stratigraphic test well located in the Prudhoe Bay Unit of Alaska where significant gas hydrate deposits have been detected in two subpermafrost sand layers that are intended for long-duration production testing. Our DAS data preprocessing workflow effectively isolates the upgoing compressional-wave (P-wave) reflections required for subsurface acoustic imaging. After applying three-dimensional (3-D) tomography to improve the quality of the 3-D migration velocity model, we use 3-D reverse-time migration (RTM) to develop high-quality images of the two target sands and minor near-well faulting. We validate our RTM images through highly accurate well-ties with previously acquired petrophysical log data. This study demonstrates that combining 3-D RTM imaging with DAS VSP data provides significant value to gas hydrate and similar projects, and it suggests that more advanced inversion approaches such as (elastic) least-squares RTM could recover higher-resolution and more quantitative estimates of subsurface reflectivity, which would be valuable for refining the understanding of gas hydrate systems.

Planning and Operations of the Hydrate 01 Stratigraphic Test Well, Prudhoe Bay Unit, Alaska North Slope

“…In recent years, DAS technology has been shown to be an effective alternative to traditional VSP data acquisition in which the DAS fiber optic cable acts like a continuous series of geophones deployed along the entire depth of the wellbore. To develop a more complete understanding of the geologic conditions of the selected gas hydrate production test site, a DAS based 3D VSP data set was acquired in the Hydrate-01 STW in March-2019. ,,, VSP geophysical mapping of local and larger bounding faults and the interpretation of any major lateral changes in reservoir character in the area will inform the final selected location for subsequent wells in the planned testing program. In addition, the acquisition of the March-2019 DAS 3D VSP also provides a potential baseline geophysical survey that could be compared to a future postproduction test DAS 3D VSP to further evaluate the reservoir response to the planned depressurization production testing.…”

“…The March-2019 DAS 3D VSP included the acquisition of 1701 seismic-shot records in a spiral survey pattern centered around a midpoint between the Hydrate-01 wellhead and bottom hole location (Figure ) with the radius of the spiral shot pattern extended outward from the midpoint of the survey to a distance of ∼3200 ft (∼975 m). Postfield processing of the acquired DAS 3D VSP was conducted to produce high resolution 3D seismic images that would allow for improved characterization of the structural and stratigraphic setting of the of the selected production test site. ,,,, …”

Energy Transition and Climate Mitigation Require Increased Effort on Methane Hydrate Research

“…International methane hydrate production programs (offshore in Japan, , China, , India, , and South Korea , and onshore in U.S. Alaska North Slope ,− ) have revealed high methane hydrate concentrations in sediments with significant permeabilities required for gas production. , …”

Gas Hydrate Saturation Estimates, Gas Hydrate Occurrence, and Reservoir Characteristics Based on Well Log Data from the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

“…Planning for a gas hydrate production test and accurately interpreting the resulting data are dependent on thoroughly characterizing the reservoir itself as well as the nature of the gas hydrate within the reservoir. , More broadly, quantitatively characterizing natural gas hydrate deposits is essential for understanding the scale and occurrence patterns of gas hydrate deposits locally, regionally, and globally; such characterization informs studies of gas hydrate’s role in the carbon cycle, as a subsea hazard, and as an energy resource . Subsurface characterization to support the planned Alaska production test is provided by borehole log data , and sidewall core samples from the Hydrate-01 STW along with surface seismic and 3D distributed acoustic sensor (DAS) vertical seismic profile (VSP) data. ,, …”

“…16 Subsurface characterization to support the planned Alaska production test is provided by borehole log data 11,17 and sidewall core samples 18 from the Hydrate-01 STW along with surface seismic and 3D distributed acoustic sensor (DAS) vertical seismic profile (VSP) data. 17,19,20 Acquisition of borehole log data at gas hydrate study sites provides highly detailed information at the meter to centimeter scale by measuring a suite of physical properties from which reservoir and gas hydrate properties can be inferred. Logging-while-drilling (LWD) data are typically preferred over wireline data acquisition in gas hydrate studies due to borehole degradation and gas hydrate dissociation that can occur during the time between drilling and wireline logging.…”