Fundamental Studies On Natural Gas Hydrate Reservoir Development Program By Depressurizing

As an alternative energy resource with huge potential, natural gas hydrates (NGH), commercial exploitation of NGH has been paid much attention to in recent years. The fundamental studies on NGH reservoir development is the premise and foundation to commercial exploration of NGH reservoir and is a very advancing step in NGH development, involving in many challenging issues such as NGH quality evaluation, production performance analysis for NGH reservoir of different quality as well as project economic evaluation for NGH reservoir of different quality. At present, there is no relevant research. Therefore, the study of NGH reservoir development is of great theoretical and practical value.Focusing on the challenges mentioned above, the NGH reservoir depressurization development as an important way for NGH exploration is studied through theoretical research, laboratory experiment and numerical simulation in this paper. The following research is conducted:The quality evaluation model of NGH reservoir is built, which analyzed comprehensively the gas-bearing NGH reservoir. Based on numerical simulation, the sensitivities of all parameters are analyzed, and 12 parameters with great impact are determined as the factor set of NGH reservoir quality. With reference to oil field evaluation criteria, the comment sets of NGH reservoir quality evaluation are ascertained, and interval division of each factor is defined for corresponding comment sets. With the multi-layered division of main controlling factors set that influence development effect, the weights for the factors are determined with the the analytic hierarchy process analysis. The two-stage quality evaluation model of NGH reservoir is finally established, and quality grades of 4 typical NGH reservoirs are obtained using this model.Guided by similarity theory, a set of quasi-3-D scale physical simulation model for NGH is designed, and the NGH reservoir development experiments of different qualities were conducted. By means of dimensional analysis and check analysis, 39 similarity criteria of NGH reservoir development are derived based on the NGH reservoir mathematical model, therefore providing design principle for the model similarity. Based on the similarity transformed data between the model and the prime type, the dynamic laws of depressurization development for different quality NGH reservoir are obtained. The experiments results showed that the NGH reservoir depressurization development could be divided into three stages: the free NGH gas production, dissolved NGH gas production and free & dissolved NGH gas production, the gas production decreased step by step. The dissolved NGH gas production occupied a higher proportion of the total gas production as the quality of NGH reservoir increased. The accumulative gas production and the production time decreased as the quality of NGH reservoir decreased.The mathematical model for NGH reservoir development is built, and the corresponding simulation calculation program is compiled. By using the program, depressurization development characteristics of different quality NGH were analyzed. The effects of bottom hole production pressure on NGH reservoir of different quality depressurization development parameters were analyzed. The numerical simulation results showed that the water production and cumulative water production have stronger sensitivity relationship with flowing bottom hole pressure and increase as the flowing bottom hole pressure decreases. Off-take rate and cumulative velocity appear less sensitivity with flowing bottom hole pressure.A complete set of economic evaluation methods for NGH reservoir development is proposed. By the method of internal rate of return, production limit of NGH is calculated based on numerical simulation. The economic evaluation for NGH reservoir of different quality is carried out. Economic evaluation results showed that the depressurization development of the first, second and third class NGH reservoir have commercial exploitation value. The depressurization development of the forth class NGH reservoir is not economically feasible.