Estimation of S-wave Velocity for Gas Hydrate Reservoir in the Shenhu Area, North South China Sea

Estimation of S-wave velocity using logging data has mainly been performed for sandstone, mudstone and oil and gas strata, while its application to hydrate reservoirs has been largely overlooked. In this paper we present petrophysical methods to estimate the S-wave velocity of hydrate reservoirs with the P-wave velocity and the density as constraints. The three models used in this paper are an equivalent model (MBGL), a three-phase model (TPBE), and a thermo-elasticity model (TEM). The MBGL model can effectively describe the internal relationship among the components of the rock, and the estimated P-wave velocities are in good agreement with the measured data (2.8% error). However, in the TPBE model, the solid, liquid and gas phases are considered to be independent of each other, and the estimation results are relatively low (46.6% error). The TEM model is based on the sensitivity of the gas hydrate to temperature and pressure, and the accuracy of the estimation results is also high (3.6% error). Before the estimation, the occurrence patterns of hydrates in the Shenhu area were examined, and occurrence state one (the hydrate is in solid form in the reservoir) was selected for analysis. By using the known P-wave velocity and density as constraints, a reasonable S-wave velocity value (ranging from 400 to 1100 m s−1 and for a hydrate layer of 1100 m s−1) can be obtained through multiple iterations. These methods and results provide new data and technical support for further research on hydrates and other geological features in the Shenhu area.