Permeability anisotropy and relative permeability in sediments from the National Gas Hydrate Program Expedition 02, offshore India

Gas and water permeability through hydrate-bearing sediments essentially governs the economic feasibility of gas production from gas hydrate deposits. Characterizing a reservoir's permeability can be difficult because even collocated permeability measurements can vary by 4-5 orders of magnitude, due partly to differences between how various testing methods inherently measure permeability in different directions and at different scales. This study uses a customized flow anisotropy cell to investigate geomechanical and hydrological properties of hydrate-bearing sediments focusing on permeability anisotropy (i.e., horizontal, k(h), to vertical, k(v), permeability 'ratio) and relative permeability. Two cores recovered during India's National Gas Hydrate Program Expedition 02 (NGHP-02) are tested in this study. Near in situ effective vertical stress, similar to 2 MPa, the permeability anisotropy is approximately k(h)/k(v) = 1.86 for the seal core (from a fine-grained non-reservoir overburden sedimentary section) and k(h)/k(v) = 4.24 for the gas hydrate reservoir core with tetrahydrofuran (THF) hydrate saturation Sh = 0.8. Permeability anisotropy increases exponentially with effective vertical stress, as described by k(h)/k(v), = alpha(sigma(v)/MPa)(beta) , with alpha = 1.6, beta = 0.22 for seal sediment and alpha = 3, beta = 0.5 for THF hydrate-bearing sediment. Results imply the measured permeability from permeameter tests with vertical flow may underestimate the reservoir's flow performance, which is mainly horizontal (radial) toward a vertical well. Hydrate in sediment increases the gas-entry pressure and residual water saturation, but decreases the water retention curves shape factor (m), resulting in a steeper curve. Distributions of available pore space sizes for flow in sediment with and without THF hydrate (S-h = 0.8) follow a log-normal distribution. Hydrate formation decreases the apparent mean pore size from similar to 10 mu m to similar to 2 mu m, without evidently changing the pore size distribution's standard deviation. Gas hydrate dissociation increases effective permeability and relative permeability to gas.