Research On Thermodynamic Behavior Of Marine Natural Gas-hydrate Drilling Sampling Progress

In the process of gas hydrates drilling sampling,in order to get the qualified sample successfully,the hydrate decomposition must be controlled well.This article research the thermodynamics behavior in sampling process of marine natural gas hydrate from several aspects such as: microscopic mechanism of hydrate decomposition,hydrate phase equilibrium condition,temperature field of coring bit rock breaking,borehole temperature field distribution,etc.The result is very meaningful for getting the qualified hydrate sample.First of all,the pressure change influence on SI methane hydrate stability was investigated with molecular dynamics simulation.The system conformation,radio distribution function,the potential energy and mean square displacement were analyzed,the dissociation mechanism of SI methane hydrate was announced from a microcosmic perspective.The result indicates that with the decrease of pleasure,the cage cavity structures composed by hydrones with hydrogen bond twist and disintegrate.The methane molecules inside escape from the cages and gather in space.Secondly,a finite elements model is established and the drilling process of coring bit is simulated.The temperature fields and changing by time of coring bit,hydrate bearing sediment and core sample are mainly studied.The results indicated that the highest temperature rising areas are distributed on the bottom of well in a point form.The friction generate heat when the points in the front face of cutters are primarily left in contact with sediment and the heat flowing from the points to inside lead to the temperature gradient.When the drilling speed and the coring bit rotate speed are faster,the temperature rise is higher.The friction heat generated in drilling process has little influence on the temperature rise of sample.It is necessary to use the pressure and temperature coring systemin the drill lifting process.Finally,the mathematic model used to predicte the temperature distribution of borehole is established referring to drilling fluid circulation in the sampling of rock stratum.On the basis of the simulation research on coring bit rock breaking temperature field and phase equilibrium calculation,the program used to predicate the thermodynamics behavior of natural gas hydrate sampling progress is composed with MATLAB programming language.Using the program could estimate whether the hydrate in borehole dicompose in specific condition.On the other hand,the program could obtain the technological parameters and their adjustable extend needed while the hydrate in borehole maintain stability.