Modified effective medium model for gas hydrate bearing, clay-dominated sediments in the Krishna-Godavari Basin

During NGHP-Expedition-01, well logs were obtained for gas hydrate exploration in Krishna-Godavari (KG) offshore basin. These logs coupled with a suitable rock physics model can be used to understand the interaction between the sediment grains of unconsolidated marine sediments as well as with hydrate. In this paper, we study the friction-dependent effective medium model (EMM) to understand these grain interactions. The compressional (P) and shear (S) wave velocities of fluid saturated sediments are estimated using different friction parameters at Site NGHP-01-03, which represent the background fluid-saturated marine sediment, and are compared with the observed velocities derived from sonic logs. Our analysis shows that the shear velocity is overestimated for the Hertz-Mindlin contact theory [noslip across the grain contact], but can be accurately estimated for the Walton's smooth contact model [zero friction across the grain contact]. It suggests that the background shear wave velocity need to be modeled without friction at the grain contact for unconsolidated marine sediments. Further, the friction-dependent EMM theory is tested at Site NGHP-01-07 which represents the load-bearing gas hydrate deposits in KG basin. The comparison between the gas hydrate saturations estimated from sonic and resistivity logs shows that saturations estimated from P-wave velocity match well with those estimated from resistivity and chloride anomaly and is largely independent of the frictional parameter. However, gas hydrate saturations estimated from shear wave velocity is overestimated in the absence of friction but agrees with the other estimates if an arbitrary small friction is included in the EMM. We further extended the friction-dependent EMM for multi-grain contact (clay + quartz + hydrate) in which the effective modulus of sediment matrix is estimated by accounting for all possible contact combinations among the grains like quartz-quartz (QQ), clay-clay (CC), clay-quartz (QC), quartz-hydrate (QH), clay-hydrate (CH), and hydrate-hydrate (HH). The gas hydrate saturations estimated from shear velocity assuming the same non-zero friction term are underestimated as compared to those estimated from P-wave velocity. Interestingly, the saturations estimated assuming zero-friction from both P- and S-wave velocities are comparable to each other and show a good match with those estimated from resistivity logs and chloride anomalies. The proposed EMM with zero friction and mixed grain contact is able to predict the velocities of fluid-saturated sediments as well as gas hydrate bearing sediments in KG offshore basin. (C) 2014 Elsevier Ltd. All rights reserved.