Anisotropic analysis and saturation estimation of gas hydrate filled in fractures: a case of site NGHP01-10D, offshore eastern India

Gas hydrate filled in fractures was the important occurrence morphology in fine-grained sediments from deep basin of India. Gas hydrate existed as solid nodules and subhorizontal veins filled in high angle fractures. Gas hydrate was formed along the tectonic principal stress. Gas hydrate filled in fractured reservoir shows anisotropic properties. Gas hydrate saturation estimated from NGHP01-10D was about 40% of the pore space using isotropic velocity model. However, gas hydrate saturation estimated from pressure cores was about 20% of the pore space. To reconcile the difference, an anisotropic model based on laminated media was used to estimate gas hydrate saturation in fractured reservoir. In vertical borehole, the incident angles of wave propagation have a relation with fracture dip angles. Gas hydrate saturations and fracture dip angles can be estimated from both P-wave and S-wave velocities obtained from the logging data. Gas hydrate saturations estimated from NGHP01-10D vary from 15% to 25% of the pore space using anisotropic velocities for laminated media. The fractures are high angle fractures and the dip angles vary from 60 to 90 degrees. The saturations estimated using the new method fit the results estimated from pressure core gassing well.