Characterizing Physical Properties and Controls on Gas Hydrate Saturation Using Borehole Log and Core Data at the Alaska North Slope Gas Hydrate Production Test Site

Abstract

Abstract The JOGMEC-DOE-USGS Collaborative Gas Hydrate R&D Project in Alaska has acquired a wealth of geophysical data at the Kuparuk State 7-11-12 site on the Alaska North Slope to facilitate site characterization and interpretation of the 2023–2024 extended duration reservoir response gas production test data. We integrate borehole logging and core data to characterize the reservoirs and bounding layers, and to investigate geologic factors controlling gas hydrate saturation. Using borehole density logs and core-derived grain density information, we calculate formation porosity for the four gas hydrate wells at the site. In parallel, we develop a higher-resolution porosity model based on density data acquired on pressurized cores. We derive three independent gas hydrate saturation (Sh) estimates based on electrical resistivity and P-wave and S-wave sonic velocity (VP and VS, respectively) log data. These log-based estimates are consistent with one another, with direct measurements of Sh in pressure cores, and with estimates from nuclear magnetic resonance (NMR) logs. Linking our P-wave-derived Sh estimate with various parameters associated with reservoir quality, we observe that clay content (indicated by X-ray diffraction analysis of core samples and clay fraction estimated from gamma-ray logs) shows the strongest correlation with Sh, with higher Sh found in strata with the lowest clay fraction. Data from across the two target reservoirs indicate that clay content increasing from 6% to 14% corresponds with Sh decreasing from 85% to 30%. This same clay content range also corresponds with variation in NMR-log-derived intrinsic permeability from 1500 down to 6 mD. We observe that larger grain sizes are correlated with higher Sh, but overall, we observe less clear relationship between grain size and Sh.