Rock-Physics Modeling And Physical Properties Inversion Of Gas Hydrate

Gas hydrate(hereinafter referred to as hydrate)is a new type of clean energy with huge reserves.The gas hydrate has more broad prospects for exploration and development with the conventional oil and gas resource which is gradually more difficult and expensive to explore.Nowadays,using seismic data to identify the presence of hydrate is the most common method.The BSR reflection characteristics of seismic in the hydrate formations can be used to delineate the existence of hydrate.However,the application of seismic data to identify hydrate has some drawbacks such as multiple solutions and uncertainties.Therefore,it is extremely necessary to carry out further research on hydrate seismic identification.Rock physics modeling is an important bridge between hydrate elastic parameters and physical parameters.The microscopic occurrence of hydrate in the ground is a significant factor on rock physics modeling.According to the hydrate occurrence status,it can be divided into pore-filling mode,load-bearing mode,cementation mode and crack filling mode based on the difference of isotropy and anisotropy assumption.In this paper,according to the seismic rock physics theory and modeling methodology,the equivalent medium modeling methods of hydrate formation with pore-filling mode and load-bearing mode are proposed.While the modes of pore-filling and load-bearing are ideal assumptions for the occurrence of hydrate and they can be easily affected by hydrate saturation.Especially,this effect cannot be quantitatively determined at present.To overcome the above shortcomings,we propose an innovative method of petrophysical model assuming that hydrate simultaneously exists in the matrix and pores.This model is more general and it further can simulate the actual occurrence of hydrates.The effectivity and reliability of the proposed model are verified by numerical simulation and measured data.Reservoir physical properties are important to describing and evaluating reservoir by seismic data,and the accurate prediction of reservoir physical parameters also plays a key role in the discriminate of hydrate.Especially,physical parameters are of great significance to the exploration and development of hydrate and the characterization of reserves.In this paper,the pre-stack seismic inversion method is first studied for the prediction of gas hydrate reservoir sensitive parameters.We select sensitive parameters of gas hydrate reservoirs through petrophysical rendezvous analysis.Subsequently,the spatial distribution is further determined by optimizing sensitive parameters of hydrate reservoirs.Two pre-stack inversion methods for reservoir physical parameters of hydrate reservoirs are developed.One is the inversion method of hydrate reservoir physical parameters based on empirical petrophysical relationships,under the framework of Bayesian inversion theory,this method uses empirical petrophysical relationships to realize the inversion of the physical parameters of the reservoir through prestack elastic impedance,in which the logging data is used as prior information.In especial,the inversion result has a good agreement with the logging data.In the above-mentioned inversion method,it is the key to construct the petrophysical relationship between reservoir elastic parameters and physical parameters.However,the petrophysical relationship obtained from few logging data cannot represent the entire work area.For this reason,a probabilistic inversion method for hydrate reservoir physical parameters based on petrophysical models is developed.This method derives the reflection coefficient equations of hydrate physical parameters,which is based on seismic rock physics theoretical models.Besides,Monte Carlo stochastic inversion algorithm is also been added in this method to achieve direct pre-stack seismic inversion of the physical parameters in hydrate reservoirs.The rationality and reliability of the method by model testing and field data are verified.