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We estimate the rate of upward pore fluid flow and chlorinity (Cl−) concentrations at depth through a joint analysis of Cl− concentration and temperature versus depth profiles obtained from gas hydrate related test sites drilled and cored in the Krishna-Godavari Basin off the eastern coast of India. Cl−, measured on conventional core samples, decreases with depth at all sites but some of the Cl−profiles show a prominent convex shape, whereas in situ temperature profiles, obtained by the Advanced Piston Coring Temperature probe, are mostly linear at all sites established during the National Gas Hydrate Program 02 Expedition (NGHP-02). Assuming a one-dimensional, time-dependent model for the advection of pore fluid including the sedimentation effect and depth-dependent diffusivity, we estimate darcy velocity and the Cl− concentrations at depth. The best-fit darcy velocity of 1.2–1.6 × 10−11 m/s was estimated for the sites along the crest of the regional anticlinal structure in the NGHP-02 Area B, which was significantly faster than those on the flanks of the anticline. Because the thermal diffusion coefficient is much larger than the chloride ion diffusion coefficient, estimated darcy velocities are not great enough to generate nonlinear temperature profiles with depth, which is consistent with observed linear thermal profiles. • We estimate pore fluid flow rate through analysis of Cl− & T profiles in hydrate-bearing sediments of Krishna-Godavari Basin. • Some Cl− profiles show convex shape, whereas temperatures are mostly linear, constraining pore fluid flow rate. • Maximum flow rate is 1.2~1.6 × 10−11 m/s on anticlines in Area B, significantly faster than in flank or in other areas. [ABSTRACT FROM AUTHOR]

The existence of overpressure in shallow sediments, which is often constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements in laboratory experiments were conducted on core samples from gas-hydrate-bearing regions offshore from the Krishna-Godavari Basin, eastern India. Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 65 % at 0 MPa to 40 % at 10 MPa. Permeability and the corresponding hydraulic diffusivity decrease 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 the sand fraction more scattered in channel-filled sediment sites compared to slope sediment sites. The preconsolidation stresses evaluated from consolidation curves indicate the absence of overpressure at shallow depths. In contrast, a comparison of ship-board measurements and standard compaction curves suggested that measured porosity was higher than the predicted porosity at greater depths. These porosity anomalies are interpreted as a sign of overpressure that approaches near lithostatic values at greater depths in slope sediment sites. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, under the assumption of large basial fluid influx or lower permeability than that derived from laboratory data. The modeling results suggest that near-hydrostatic pressure at shallow depths and significant overpressure at greater depths proposed by the porosity gap method is explained by the non-linearity of transport properties. The relatively small overpressure generation in channel-filled sites compared with slope sites can be explained by the higher permeability due to coarser grain size and larger sand fraction and by the smaller basal influx. On the contrary, considerably large basal influx associated with clay mineral dehydration and methane gas supply from deep sediments was expected to promote overpressure at slope sites, which is confirmed by Cl− concentration-depth profiles. • Porosity-depth curves deviate from normal compaction curves at greater depths. • The maximum past stress of mud suggests hydrostatic pressure at shallow depths. • The mud permeability is very low, leading to overpressures driven by sediment load. • Large grain size and sand fraction result in low overpressure at channel filled site. • Slope basin sites generate high overpressure due to large basal influx. [ABSTRACT FROM AUTHOR]

Physical properties like porosity and density are important parameters for size estimations of fossil fuel reservoir in host sediments and rocks. X-ray CT based imaging and subsequent analysis of the images by using various techniques for estimations of these properties of host rocks and sediments has been attempted. However, most of those techniques tend to be biased and subjective due to complications in image processing. Here we describe a new procedure for estimating porosity and density without processing the XCT images, but using the raw data that are used for producing the images. In this study, large-size cylindrical core samples (up to 150 cm long and 6.6 cm diameter) were imaged in a matter of minutes by using a medical XCT scanner soon after their recovery from seafloor onboard drillship in order to record as realistic as possible condition of hydrate-bearing sediments. Despite some uncertainty in relative volumes of solid, liquid and gas phases at voxel level, the results obtained by this rapid and non-destructive procedure are in good agreement with those obtained by time- and sample-consuming conventional methods of physical property measurements. • XCT scanning of hydrate-bearing sediments soon after their recovery from seafloor. • New method for porosity estimation of sediments from raw XCT data (CT number). • Much faster than conventional method of porosity estimation. [ABSTRACT FROM AUTHOR]