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) (T2) 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 T2 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 T2 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.