| Natural gas hydrate is a new type of high-quality energy source with broad prospects.Dissociation of gas hydrate by depressurization is a relatively common hydrate extraction technology.The dissociation of gas hydrate by depressurization involves changes in multiple physical fields such as phase transformation,multiphase seepage,and temperature.The dissociation of hydrate will affect the mechanical properties of the reservoir,causing deformation of the reservoir even the submarine geological hazard.The establishment of a multi-physics coupling numerical model which can correctly describe the dissociation behavior of hydrate,and study the gas and water production laws and the influence mechanism on reservoir deformation along with the hydrate dissociation play important reference significance and engineering application value on the natural gas production.In this research,the method of combining theoretical analysis and numerical simulation is used to study the multi-physics coupling behavior in the process of the dissociation of hydrate by depressurization.The specific work is as follows:(1)Based on the multi-physics coupling theory of hydrate dissociation,a multi-physics coupling numerical model that can describe the chemical changes,gas-liquid seepage,heat transfer and mechanical deformation in the process of the dissociation of gas hydrate by depressurization is established.The model is combined with COMSOL Multiphysics?software to establish a finite element calculation program which is capable of simulating the dissociation behavior of gas hydrate by depressurization.(2)The results for the dissociation test for the hydrate in Berea sandstone core samples and the triaxial shear test for the hydrate-bearing sediments are used to verify this multi-physics coupling numerical model,which show the good applicability of the numerical model.The influence of hydrate saturation,effective confining pressure and temperature on the mechanical properties of hydrate-bearing sediments are analyzed.(3)This research simulates the hydrate dissociation test by depressurization,and deeply analyzes the dissociation,seepage,temperature,and mechanical properties of hydrate-bearing sediments during the hydrate dissociation process.It shows that the hydrate dissociation and the stress-strain relationship of the hydrate-bearing sediments predicted by this model fits well with the experimental results.(4)Based on the multi-physics coupling numerical model,this thesis studies the dissociation properties and gas production law of gas hydrate under various parameters.The sensitivity analysis of multiple parameters of is carried out in this thesis,which provides a certain reference for the follow-up hydrate dissociation by depressurization simulation research. |
