Experimental And Numerical Simulation Studies Of Natural Gas Hydrate Production By Depressurizing

Natural gas hydrate (NGH) is regarded as the development focus of energy for its large reserve. With the severe situation of oil and gas resources being less and less, the energy crisis in current society can be alleviated by the rational development of NGH reservoir. The governments all around the world pay high attention to the fundamental research of NGH exploitation. Depressurization is an important method of developing NGH reservoir. The forward topics on the NGH development include: the impact of the formation and decomposition of NGH in porous media during depressurizing development on the reservoir parameters (such as saturation, permeability, porosity, acoustic velocity, rock resistivity); the measurement and quantitative interpretation of some important development parameters (such as the NGH saturation); the impact of the distribution of salt concentration of reservoir water and the NGH saturation on the development performance, and the study on mathematical model and numerical simulation. So far, there is little research on the subject at home and abroad.These problems are studied through laboratory experiment, theoretical research and numerical simulation in this paper. There are two main line: one is experimental research, including the development of NGH experimental equipment, the measurement of NGH reservoir physical properties and the experiment of NGH development, the other is numerical simulation study, including the construction of mathematical and numerical model, the programming of corresponding software and the numerical simulation. On the basic of full investigation two sets of devices for hydrate experiment are designed: one for measuring the physical properties and developing the NGH reservoir, the other for measuring the one-dimension resistivity of NGH. These devices are specially used to study the variations of reservoir parameters and measure the resistivity of hydrate reservoir. A three-phase (gas, water and hydrate), eight-component mathematical model for NGH reservoir of the salt-containing, water-rich phase is presented, which can describe multiphase, non-isothermal seepage.The conclusions are as follows:(1) The law of reservoir permeability and porosity with hydrate saturation is testified. Exponential-type formula, a new formula for describing the relationship between the permeability and saturation is presented. The left shift of methane hydrate phase-equilibrium curve caused by salt component in the solution is testified, a new empirical formula between salt concentration and super-cooling degree is regressed.(2) The modified formula of Wyllie equation, which describes the relationship between acoustic velocity and hydrate saturation in the reservoir, can interpret the hydrate saturation.(3) Saturation resistivity interpretation model suitable for NGH system of salt-containing, water-rich phase is proposed, which is suitable for resistivity log interpretation of saturation in the hydrate reservoir. One-dimensional measurement experiment of resistivity is carried out, and resistivity explanation model is used to obtain the distribution of hydrate saturation.(4) The kinetic hypothesis of NGH's thermal decomposition is presented, and the integrated process of the depressurization and thermal decomposition is analyzed. Therefore, the mathematical model can be applied for the depressurizing and heating process. The mathematical model mentioned above includes mass conservation equation of the components, energy conservation equation, transport equation of gas components, salt and inhibitor, and decomposition kinetic equation of NGH. The factors, such as temperature, components, phase equilibrium, are fully considered in the multiphase seepage model, so as to well describe the development mechanism of salt-containing, water rich phase NGH reservoir.(5) The mathematical model is discretized by finite difference method and solved by sequential solution method, which is to successively solve the pressure, saturation, temperature, mass fraction of gas phase, the distribution of salt or inhibitor in a time step. It is proved that this method not only meets the demand of numerical calculation, but also simplifies the processing procedure.(6) The self-made program is used in the one-dimensional depressurization numerical simulation of methane hydrate and the influencing factors of depressurization are analyzed. The results suggest that the factors, such as distribution of hydrate saturation and salt concentration of reservoir solution, have a great effect on depressurized development performance of the hydrate.