Identification of pore-filling and fracture-filling hydrate by petrophysical simulation and acoustic experiment

Pore-filling and fracture-filling are two of the basic occurrences of natural gas hydrates in nature. To discriminate the type of gas hydrate is critically important for resource assessment, drilling safety and environment evaluation. In this paper, simulation experiment was carried out for the pore-filling and fracture-filling hydrate reservoirs in the South China Sea. The acoustic velocity and density of the two kinds of hydrate are obtained by petrophysical simulation and acoustic experiment simulation. The results suggest that the P wave velocity of the depositional mediums containing pore-filling and fracture-filling hydrate tends to increase with the volume fraction of hydrate, while the density decreases. Furthermore, we tested the impedance and the \rho \sqrt V _\rmp property of the two types of hydrate by combining velocity and density parameters together. The results also show that for the pore-filling hydrates, the properties of \rho \sqrt V _\rmp calculated by the petrophysical models and experimental \rho \sqrt V _\rmp both show positive slope, while The \rho \sqrt V _\rmp property of the fracture-filling hydrate shows negative slope. However, the differences between the model and experiment results of fracture-filling hydrate are obvious when the volume fraction of gas hydrate is less than 40%. It means that the petrophysical for fracture-filling hydrate needs to be further improved. In addition, pore-filling and fracture-filling hydrate in GMGS2-16 Site has been verified by the property of \rho \sqrt V _\rmp. The results show that the hydrate in the upper part of the well is mainly fracture-filling hydrate, as the bottom dominated by pore-filling hydrate. The verification has been confirmed by actual drilling results.