Geological Reservoir Characterization of a Gas Hydrate Prospect Associated with the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

Abstract

Geological reservoir characterization is essential for accurate evaluation of gas production performance from gas hydrate reservoirs. Particularly, the understanding of reservoir architecture and heterogeneity is of great importance since these are considered as major controls on fluid hydrodynamic and thermodynamic conditions. This study deals with well log and three-dimensional (3-D) vertical seismic profile (VSP) data acquired from the Hydrate-01 Stratigraphic Test Well within the 7-11-12 prospect, Prudhoe Bay Unit, Alaska North Slope and reports on the results of geological/geophysical evaluation related to the geological structure and reservoir properties of the 7-11-12 prospect. The structural trends of the target reservoirs, based on well correlations, are mostly consistent with the predrill prediction using the surface seismic data, and infer the existence of subseismic faults cutting through the Hydrate-01 well. The 3-D VSP data confirm a down-to-the-east normal fault that offsets the reservoir units across the Hydrate-01 well, which is concordant with the well identification of the same fault, and indicate a northeast-dipping relay structure associated with the overstepping normal faults. The edge enhancement attribute associated with discontinuity generated from the 3-D VSP data shows small faults/fractures, possibly as part of a complex fault network within the imaged normal fault system. These results reveal that the 3-D VSP data provide detailed structural information that is not present from the surface seismic data. The Hydrate-01 well log data confirm the occurrence of gas hydrate at high saturation in the two targeted sand units (B1 and D1 sands), and the comparison to a nearby preexisting well (7-11-12 well) shows the same general trend in gas hydrate saturation as a map of seismic impedance generated from surface seismic data. The well log data also suggest that the base of gas hydrate occurrence in the Hydrate-01 and 7-11-12 wells is almost aligned at the same depth in both of the targeted B1 and D1 sand reservoirs. Especially for the D1 sand in the Hydrate-01 well, the resistivity logs show a sharp transition from high gas hydrate saturation to fully water-saturated within the D1 sand, suggesting a common gas hydrate/water contact. The results of this study will be used to construct the geological models needed for reservoir simulation studies and they can provide important insights into the geological factors that control the occurrence of gas hydrate on the Alaska North Slope.

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

“…Evaluation of LWD NMR data provided a second interpretation of in situ effective permeabilities within the gas hydrate reservoirs. Standard methods of NMR analyses suggest low values (on the order of 0.1 mD) are associated with high bound-water fractions . However, reevaluation of the NMR log data indicates that higher permeability values consistent with those obtained from pressure cores are possible for the gas hydrate-bearing reservoir sections logged and cored in the Hydrate-01 STW. ,, …”

“…The pad lies at the intersection of the main PBU Spine Road and the road to the PBU Z-Pad to the south. As part of the well-planning effort, a portion of an industry-acquired 3D seismic data volume was made available to the project partners through agreements with PBU WIOs, which allowed for more detailed mapping of the potential hydrate reservoir sections in the area of the gravel pad from which the Kuparuk 7-11-12 exploration well was drilled …”

“…Within the 7-11-12 well, each potential gas hydrate-bearing unit was assessed to be ∼30 to ∼50 ft (∼9 to ∼15 m) thick, with gas hydrate saturations of 70% and higher (Figures –). The remaining pore fill was expected to include bound and free water only (i.e., no free gas). , In order to refine the final location of the drilling targets for the stratigraphic test well, the PBU WIOs enabled project scientists to access seismic data from the vicinity of the Kuparuk 7-11-12 pad to build on the seismic studies completed by the SOA-DNR.…”

“…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. ,,,, …”