Permeability Evaluation of Hydrate Reservoirs Based on NMR T ₂ Relaxation Time from Both Log and Laboratory Data, Alaska North Slope HYDRATE 02 Geo Data Well

This study investigates the permeability of gas hydrate-bearing sediments based on nuclear magnetic resonance (NMR) transverse (or spin–spin) (T 2 ) relaxation measurements obtained from both pressure core samples and downhole logging in the Alaska North Slope. Seven core samples from the B1 sand (unit B)─the reservoir interval of a recent extended-duration gas production test under the JOGMEC-DOE-USGS Collaborative Gas Hydrate R&D Project in Alaska─were analyzed to measure T 2 distributions, grain size, and both effective and intrinsic permeabilities under in situ stress conditions. These data were used to evaluate the predictive accuracy of several NMR-based permeability models, including the Timur–Coates (TC) and Schlumberger-Doll Research (SDR) models, as well as the hydraulic radius model. Among these, the hydraulic radius model using laboratory based NMR signals exhibited the best agreement with laboratory-measured permeabilities in sand-rich hydrate-bearing sediments, highlighting its robustness and practical applicability without fitting parameters. The modified TC model performed well even with a fixed parameter (α = 0.6) using NMR signals of hydrate-bearing sediments, uniquely allows for intrinsic permeability prediction for simulating hydrate dissociation. In contrast, the TC and SDR models showed greater deviations. Moreover, since T 2 distributions after hydrate dissociation cannot be obtained in wireline logging, intrinsic permeability cannot be predicted in practice using the TC and SDR models. In fine-grained, hydrate-free seal layer samples, the SDR model outperformed the hydraulic radius model, which tended to overestimate laboratory-derived permeability by about an order of magnitude. These findings emphasize the importance of selecting appropriate models based on sediment type and reservoir conditions.