Rock Physics Modelling for Estimation of Gas Hydrate Saturation Using NGHP-02 Well Data in the Krishna-Godavari Basin

The gas hydrate found in the eastern continental margin of India is a boon for this energy-deficient developing country. We used four wells of the National Gas Hydrate Program (NGHP) Expedition 02 from area B of the Krishna-Godavari (KG) basin to estimate gas hydrate saturation. The gas solid hydrate increases the resistivity and sonic velocity of sediment due to its presence. We first used Archie's method to estimate the saturation using deep resistivity log data, assuming that the sedimentary formation is saturated with brine water and gas hydrate only. Then we chose the modified Hashin-Shtrikman upper bound model from the rock physics analysis to estimate saturation from sonic log data using Gassmann's equation. The study of gas hydrate morphology suggests that the gas hydrates are pore-filling and grain-supporting in nature compared with the various rock physics models. Theoretical models were prepared for gas hydrate occurrence in coarser sediments with 25% shale and 75% quartz sediments with the critical porosity being 38%. The porosity above the critical porosity suggests that sediment-saturating fluids control the elastic properties of the formation. We also developed a rock physics template (RPT) for various porosity and gas hydrate saturation scenarios by assuming gas hydrates as a component of the rock matrix and the pore spaces of the sediments saturated by 100% water. The RPT was used to estimate saturations in the four wells. The estimated gas hydrate saturations obtained using the Archie method, rock physics model, and RPT method were compared for all wells, obtaining good agreement with the results.