Porosity, permeability, and grain size of sediment cores from gas-hydrate-bearing sites and their implication for overpressure in shallow formations: Results from NGHP-02, Krishna-Godavari Basin, India

The existence of overpressure in shallow argillaceous sediments, which is constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements were conducted on core samples from gas-hydrate-bearing regions offshore from Krishna-Godavari Basin, eastern India (the National Gas Hydrate Program Expedition 02). Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 0.65 at 0 MPa to 0.4 at 10 MPa. Permeability and the corresponding hydraulic diffusivity decreases from 10^-17 to less than 10^-18 m² and from 10^-7 to 10^-8 m²/s at 0.5 and 5 MPa, respectively. Grain sizes were larger and sand fraction more scattered in channel-filled sediment sites when compared to slope sediment sites. The preconsolidation stress evaluated from consolidation curves indicates the absence of overpressure at shallow depths. In contrast, a comparison of on board measurements and proposed normal consolidation curves, suggested that porosity was enhanced at greater depths. These enhanced porosities are interpreted as a sign of overpressure, and thus lower effective stress than would be predicted under hydrostatic pressure. The predicted overpressure, calculated from the porosity gaps between measured data and normal consolidation state, gradually increased with depth, and reached to near lithostatic stress. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, when high fluid influx or lower permeability than that derived from laboratory data were assumed. The relatively small overpressure generation in channel-filled sites compared with slope sites can likely be explained by the higher permeability due to coarser grain size and larger sand fraction in this environment.