Amount of gas hydrate estimated from rock physics analysis based on morphology and intrinsic anisotropy in area B, Krishna Godavari offshore basin, expedition NGHP-02

摘要

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 poreand 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.