Pore size distribution and methane equilibrium conditions at Walker Ridge Block 313, northern Gulf of Mexico

In-situ pressure, temperature, salinity and pore size may allow coexistence of three methane phases: liquid (L), gas (G), hydrate (H) in marine gas hydrate systems. A discrete zone of three-phase equilibrium may occur near the base of the gas hydrate stability zone (GHSZ) in sediments with salinity close to seawater due to capillary effects. The existence of a three-phase zone affects the location of the bottom-simulating reflection (BSR) and also has repercussions for methane fluxes at the base of the GHSZ. This project studied the hydrate stability conditions in two wells, WR313-G and WR313-H, at Walker Ridge Block 313 in the northern Gulf of Mexico. The pore size distributions were determined by constructing a synthetic nuclear magnetic resonance (NMR) relaxation time distribution. Correlations were obtained by non-linear regression on NMR, gamma ray, and bulk density logs from well KC-151 at Keathley Canyon. The correlations enabled construction of relaxation time distributions for WR313-G and WR313-H, which were used to predict pore size distribution through comparison with mercury injection capillary pressure measurements. With the computed pore size distribution, L+H and L+G methane solubility was determined from in-situ pressure and temperature. The intersection of the L+G and L+H curves for various pore sizes allowed calculation of the depth range of the three-phase equilibrium zone. In previous studies at Blake Ridge and Hydrate Ridge, the top of the three-phase zone moves upwards with increasing water depth and overlies the bulk three-phase equilibrium depth but this was not observed at Walker Ridge. In clays at Walker Ridge, the predicted thickness of the three-phase zone is approximately 5 m, but in coarse sands it is only a few centimeters due to the difference in absolute pore sizes and the width of the pore size distribution. The thick three-phase zone in the clays may explain in part why the BSR is only observed in the sand layers at Walker Ridge, although other factors may influence the presence or absence of a BSR.