تجزیه و تحلیل حساسیت مقیاس گذاری ضوابط در شبیه سازی فیزیکی مخزن هیدرات گاز

A numerical approach of sensitivity analysis of dimensionless parameters in gas hydrate reservoir physical simulation is proposed by analyzing the sensitivity factor defined as the relative variation ration of a target function with respect to the relative variation of dimensionless parameter. With this approach, the dominance degree of all the dimensionless parameters can be quantitatively evaluated and the dominant ones can be singled out conveniently. Taking a 1-D experiment of gas production from hydrate reservoir by depressurization as an example, we find that the order of sensitivity factors ranges from 10−5 to 100. The most dominant dimensionless parameter is the dimensionless initial temperature of hydrate reservoir and the dimensionless phase equilibrium pressure of gas hydrate, which just reflects that the consumed energy for hydrate dissociation comes from the energy contained in formation and the driving force for hydrate dissociation is very important in hydrate dissociation by depressurization.

Gas hydrate is regarded as a kind of future energy due to its tremendous reserve [1] and widespread deposition [2]. With the continuous researching development on gas hydrate, the commercial production of gas hydrate is probably becoming a reality. Some theoretical and experimental works were carried out on how to exploit nature gas from hydrate reservoir by the safe, commercial and efficient methods. A lot of laboratory experiments have been carried out to study gas hydrate dissociation [3], [4], [5], [6], [7] and [8]. However few literatures have reported experiment based on the principle of similarity. The reasons may be that the hydrate dissociation process involves some complex mechanism which now cannot be clearly explained. Therefore, most experiments intended to reveal those kinds of unknown mechanisms. On the other hand, few institutes can obtain the nature samples of gas hydrate. Moreover, the measurement values for the same parameter may be with the difference of several orders which implies the lack of common view. Physical simulation obeying the principle of similarity is an important approach to reveal mechanism of gas hydrate dissociation which is helpful for filed design and optimization of development programs. Compared with field tests, physical simulation seems to be cheaper, time-saving and easier to implement. There are so many dimensionless parameters in physical simulation of hydrate reservoir exploitation because gas hydrate dissociation associates with such complicated factors as multiphase fluid flow in porous media, kinetic and endothermic process of hydrate dissociation, heat convection and conduction. According to the similarity of principle [9], each field prototype should theoretically exist a fully similar model which satisfies all the dimensionless parameters. However, it is very difficult or sometimes even impossible to keep all the dimensionless parameters identical in the laboratory experiment. To tackle this kind of problems, an efficient and practical way out is to single out the dominant parameters and to relax the secondary ones in laboratory experiments. Theoretical and experimental arranging all the dimensionless parameters in the orders of importance is not easy or even impossible to realize. In our work, a set of scaling criteria of gas production from hydrate reservoir by depressurization is firstly derived from the mathematical models. Then a numerical approach of sensitivity analysis of dimensionless parameters is proposed based on the scaling criteria of gas hydrates exploitation. With this approach, the dominance degree of the dimensionless parameters can be quantitatively evaluated and the dominant ones can be singled out conveniently.

There are too many dimensionless parameters for hydrate exploitation experiment to keep all dimensionless parameters identical in one experiment. Therefore, we must know which one is dominant that should be satisfied with and which one is the secondary that can be relaxed. In our research work, we derive a set of scaling criteria for hydrate exploitation physical simulation. A numerical approach of sensitivity analysis of dimensionless parameters is proposed based on the analysis of the sensitivity factor defined as the relative variation ration of a target function with respect to the relative variation of dimensionless parameters. We find that the order of sensitivity factors ranges from 10−5 to 100. With this approach, the dominance degree of the dimensionless parameters can be quantitatively evaluated and the dominant ones can be singled out conveniently. The most dominant dimensionless parameter is the dimensionless initial temperature of the hydrate reservoir and the dimensionless phase equilibrium pressure of gas hydrate, which just reflects that the consumed energy for hydrate dissociation comes from the energy contained in formation and the driving force for hydrate dissociation is very important in hydrate dissociation by depressurization.