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The importance of gas hydrate as an alternative energy resource is ascending because of its worldwide distribution along the continental margins, in the permafrost regions and deep-water lakes, its richness, and high fuel productivity. Seismic survey is used predominantly to identify gas hydrate deposits at the regional scale, whereas drilling and coring are used for ground-truthing at the local scale. However, reliable quantification is the utmost priority to evaluate its resource potential commercially and understand its effect on the environment. Pressure core analysis is a direct way to know the morphology and amount of gas hydrate in a reservoir, while rock physics modelling is used widely to estimate gas hydrate saturation from remotely sensed geophysical data. Elevated velocity and resistivity are two parameters mostly used in rock physics theory to estimate the gas hydrate concentration in sediments, whereas, pore-water salinity is one of the important proxy methods used to estimate gas hydrate saturation. Several rock physics models based on the empirical, semi-empirical, and laws of physics exist in the literature that relate the observed anomalous physical properties like velocity and resistivity to reservoir properties like porosity, permeability, saturation etc. However, both direct and indirect methods have certain limitations, which are important for accurately quantifying gas hydrate. We review in detail the existing rock physics theories with possible uncertainties to estimate gas hydrate saturation using velocity, which provides reliable estimates with less uncertainty than that from resistivity. [ABSTRACT FROM AUTHOR]

Causes and effects of global warming have been highly debated in recent years. Nonetheless, injection and storage of CO (CO sequestration) in the subsurface is becoming increasingly accepted as a viable tool to reduce the amount of CO from the atmosphere, which is a primary contributor to global warming. Monitoring of CO movement with time is essential to ascertain that sequestration is not hazardous. A method is proposed here to appraise CO saturation from seismic attributes using differential effective medium theory modified for pressure (PDEM). The PDEM theory accounts pressure-induced fluid flow between cavities, which is a very important investigation in the CO-sequestered regime of heterogeneous microstructure. The study area is the lower Tuscaloosa formation at Cranfield in Mississippi, USA, which is one of the active enhanced oil recovery (EOR), and CO capture and storage (CCS) fields. Injection well (F1) and two observation wells (F2 and F3) are present close (within 112 m) to the detailed area of study for this region. Since the three wells are closely situated, two wells, namely injection well F1 and the furthest observation well F3, have been focused on to monitor CO movement. Time-lapse (pre- and post-injection) log, core and surface seismic data are used in the quantitative assessment of CO saturation from the PDEM theory. It has been found that after approximately 9 months of injection, average CO saturations in F1 and F3 are estimated as 50% in a zone of thickness ~ 25 m at a depth of ~ 3 km. [ABSTRACT FROM AUTHOR]

India's National Gas Hydrate Program (NGHP) Expedition-02 was conducted in 2015 to investigate the prospective locations of sand-rich gas hydrate reservoirs along the eastern offshore, India. We select Area-B among four areas A, B, C and E of expedition NGHP-02 as it was completely unexplored during NGHP-01. Logging while Drilling (LWD) data were collected at twelve sites in area B in Krishna Godavari (KG) offshore basin, where a huge amount of gas hydrate (up to 90% of the pore volume) is present as pore-fill, fracture-fill and both pore- and fracture-filled (mixed) morphology. The resistivity log generally overestimates gas hydrate saturation because of the increase of resistivity of fresh water near gas hydrate accumulation and also in the presence of fractured-filled gas hydrate. Though gas hydrate saturation obtained from pressure cores are assumed to be ground truth but they are discrete. Hence, the application of an advanced rock physics model is required to obtain continuous quantification of gas hydrate throughout the depth considering the gas hydrate morphology and microstructure of sediment. We apply a combination of self-consistent approximation (SCA) and differential effective medium (DEM) theories to model biconnected marine gas hydrate reservoir that can deal with inherent anisotropy due to the presence of clay platelets and pore-filling, fracture-filling or mixture of both morphologies of gas hydrate. First, we determine the distribution pattern of clay platelets from the observed background velocity (water-saturated sediment) and then estimate the gas hydrate saturation at four holes of area B, where pressure core measurements are available. We predict maximum gas hydrate saturation as 50–90% of the pore space above the BSR in this area. Comparison with other rock physics model shows that prediction from the combined SCA-DEM theory matches better with the pressure core measurements. Image 1 • Rock physics modeling of clay and sand-rich reservoir incorporating anisotropy of sediment and morphology of gas hydrate. • Application of Self Consistent Approximation and Differential Effective Medium (SCA-DEM) rock physics theory. • Assessments of gas hydrate existing in pore spaces and in fractures separately from rock physics modeling. • Application of variable orientation distribution pattern of clay minerals in sediment with depth. [ABSTRACT FROM AUTHOR]

Hikurangi margin, New Zealand, the shallowest subduction zone on Earth is the unique place to study the slow slip creep-like deformation and seafloor failure, and their probable link to the presence of gas hydrate, cold seeps and fluid migration. International Ocean Discovery Program expedition 372 discovered gas hydrate within thin intervals (ranging from ~5 to 25 m) of the deformed sediments in the Tuaheni landslide complex area of the northern Hikurangi margin. Seismic profiles show typical BSRs (bottom simulating reflectors) cutting across the dipping strata throughout the sections. To characterize the reservoir, here, we invert both acoustic impedance (product of velocity and density) and porosity along two perpendicular seismic profiles crossing the well using a model-based post-stack seismic inversion technique. Results show the layer structure of alternate high-impedance with low-porosity and low-impedance with high-porosity sediments. Interestingly, each layer is consisting of thin and relatively low impedance intra-layers, which clearly indicates the possible pathways for fluid and gas migration, and one of the reasons for seafloor collapse in this region. Very low impedance observed below the BSR is due to the gas-charged sediments at the dipping strata, and is the probable source of free-gas migrating upward. Next, we apply rock physics theory to know the distribution of gas hydrate at Site U1517 as well as along two perpendicular seismic profiles. Rock physics modeling using sonic velocity at well location shows that gas hydrate is distributed mainly within the depth intervals of 5–50 m, 90–115 m and 130–155 m with an average saturation of about 10–15% and the maximum concentration of about 35% of the pore space at 100 m depth. Prediction of gas hydrate saturation from the resistivity method using neutron porosity and NMR (nuclear magnetic resonance) logs in Archie's empirical formula shows less saturation compared to that from the sonic log. Whereas, estimated gas hydrate saturation from the chloride concentrations ranges from 0 to 45% of the pore space. The spatial distributions of gas hydrate along seismic profiles are predicted by rock physics theory using the inverted impedance and porosity. Saturation of gas hydrate along the seismic profiles varies from 0 to 40% with an average of ~10% of the pore space. The correlation between inverted impedance and porosity indicates that the high impedance layers are due to low porosity consolidated sediment without significant gas hydrate concentration. Gas hydrate is distributed in relatively high porosity and low impedance layers along the seismic profiles. Our result shows that the amount of gas hydrate concentration is low in this region and sediments are highly deformed/disturbed in the presence of many fluid/gas migration pathways. Image 1 • Integration of rock physics modelling with well log and seismic data to characterize a gas hydrate reservoir. • Estimation of acoustic (seismic) impedance and porosity along seismic lines from model-based post stack seismic inversion. • Various features related to the deformation of the sediments and slope failure. • Their possible links to the presence of gas hydrate distribution/amount in the northern Hikurangi margin, New Zealand. [ABSTRACT FROM AUTHOR]