Gas hydrate reservoir identification based on rock physics modelling and sensitive elastic parameters

Abstract

Abstract Seismic bottom simulating reflections (BSR) analysis and seismic inversion are commonly used for gas hydrate reservoir interpretation. The relationship between gas hydrate saturation and elastic parameters can be influenced by gas hydrate occurrence state (e.g. pore-filling type gas hydrate or load-bearing type gas hydrate), and this may cause inaccurate interpretation. We first used the simplified three-phase Biot equation (STPBE) to model a formation containing two types of gas hydrate at the same time. Then the effects of occurrence state and authigenic minerals on the relationship between saturation and varied elastic parameters are analysed. Results show that bulk modulus (K), shear modulus ($\mu $), P-wave velocity (${V}_p$), S-wave velocity (${V}_s$), velocity ratio (${V}_p/{V}_s$), Poisson's ratio (v) and $\mu \rho $ increase at different rates with gas hydrate saturation, ${V}_p/{V}_s$ and v show relative higher sensitivity to occurrence state. Ratios of elastic parameter increments are further used to highlight the anomalies caused by gas hydrate. Four attributes ($\Delta K/\Delta \mu $, $\Delta {V}_p/\Delta {V}_s$, $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $) show good sensitivity to both gas hydrate saturation and occurrence state. $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ can be used to distinguish gas hydrate with varied occurrence states from authigenic minerals (limestone, opal, pyrite and others). Two selected sensitive attributes $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ are applied to well logs, four gas hydrate-bearing intervals in well 2L-38 from Mallik permafrost area in Canada and one gas hydrate-bearing interval in well A from Shenhu area in South China Sea are identified. These results are consistent with the interpreted result from the resistivity log using Archie's formula. This investigation may provide effective tools for the seismic interpretation of gas hydrate reservoirs.