井下原位燃烧加热与降压联合开采冻土区天然气水合物基础研究

A fundamental research on the exploitation of the natural gas hydrates (NGH) in permafrost by means of down-hole in situ combustion and depressurization will be carried out based on the reservoir characteristics of the NGH in permafrost of Tibetan Plateau of China. Firstly, the NGH phase equilibrium conditions, the dissociation kinetics parameters, and the optimum combustion conditions of the down-hole in situ combustion are measured by experiment. Theirs influence mechanisms and the down-hole in situ combustion and explosion characteristics are studied and discussed. The mechanisms of the NGH decomposition kinetics and the replacement of NGH with CO2 hydrate are studied by molecular dynamics simulation. The prediction models of the NGH phase equilibrium and dissociation kinetics are established based on these studies. Then, the NGH exploitation experiments of three-dimensional simulation are carried out to study the gas/water production rates, the gas-liquid seepage and the heat and mass transfer, so as to clarify theirs influence mechanisms. The three-dimensional NGH exploitation theoretical model is thus established base on the NGH decomposition, gas-liquid seepage, and heat and mass transfer. Finally, the three-dimensional numerical simulation is used to further optimize the operating parameters of the NGH exploitation process, and to study the "ice blockage", the "secondary hydrate formation", the CO2 hydrate formation and the NGH replacement mechanism by CO2. The influence mechanisms, and the controlling and regulation principle are also studied by this three-dimensional numerical simulation. The key issues occurred in the process of the permafrost NGH exploitation by down-hole in situ combustion and depressurization stimulation, such as the down-hole in situ combustion method, NGH dissociation kinetics, gas-liquid seepage, heat and mass transfer, "ice blockage", "secondary hydrate formation", and the CO2 hydrate formation, will be solved through the implementation of this project. The research will provide a new exploitation method for the permafrost NGH of our country, and will also provide the basic technical data and theoretical supports for the application of this new exploitation method.

针对我国青藏高原冻土区典型天然气水合物（NGH）藏特征，开展井下原位燃烧加热与降压联合开采冻土区NGH基础研究。首先通过实验研究冻土区NGH相平衡条件与分解动力学特性，井下原位燃烧与爆炸特性及最佳燃烧条件，阐明其影响机制，采用分子动力学模拟研究NGH分解动力学机理和CO2置换开采NGH机理，建立相平衡与分解动力学预测模型；然后采用三维实验模拟研究开采过程产气、产水规律，气液渗流、传热传质规律及影响机制，建立基于NGH分解、气液渗流、传热传质三维NGH开采理论模型；最后通过三维建模，采用数值模拟进一步优化开采过程操作参数，阐明开采过程水合物"冰堵塞"、"二次水合物"、CO2水合物形成与置换开采的影响机制与调控原理。解决开采过程井下原位燃烧，NGH分解、气液渗流、传热传质， "冰堵塞"、"二次水合物"及CO2水合物形成等关键问题。研究将为我国冻土区NGH开采提供一种新方法及基础数据和理论支持。

针对井下原位燃烧加热与降压联合开采青藏高原冻土区典型天然气水合物（NGH）的基础科学问题，重点研究了冰点附近多孔介质中水合物相平衡、形成分解动力学，水合物燃烧与安全控制，开采传热、气液渗流，“二次水合物”形成、CO2置换开采等。测定了NGH相平衡、形成分解动力学基础数据，阐明了水合物“自保护效应”和“冰堵塞”机理及其影响机制，水合物分解速度在冰点上下分别受反应与扩散过程控制，建立了水合物相平衡及分形缩核分解动力学理论模型，确定了祁连山冻土区水合物开采操作条件及稳定区域。阐明了井下水合物燃烧特性、机理、影响机制及安全燃烧控制方法，分解水层是影响水合物燃烧、火焰熄灭的关键，分解水增加，燃烧火焰会自行熄灭；建立了具有自主知识产权的井下反扩散火焰高速燃烧器原位燃烧加热与降压联合开采冻土区NGH的工艺技术方案，获得了优化的反扩散火焰高速燃烧器结构及操作条件，证实了其可行性。阐明了燃烧烟气CO2对 “二次水合物”形成、CO2水合物置换开采与封存的影响，多孔介质中CH4及CO2水合物形成特性及影响机制等，测定了CH4+CO2水合物相平衡数据，沉积物表面水合作用加速了水合物成核和生长；多孔介质有利于提高CO2水合物形成速率，水的可获得性和孔隙效应是控制CO2水合物生成的关键；CO2置换开采速率主要受流动相中CH4逸度的影响，置换过程在压力超过热力学推荐区域时仍然可进行；N2更易在水合物相中扩散，与水形成氢键，提高了CH4置换率和置换速率。阐明了开采过程水合物分解、传热与气液渗流控制机理，建立了含水合物多孔介质渗透率模型和基于水合物分解、传热及气液流动的NGH开采理论模型，数值模拟阐明了双水平井降压+定热流注热开采典型冻土区NGH产气产水规律、开采能量效率，温度、压力、水、气、水合物饱和度空间分布变化规律及影响机制等。研究成果为我国NGH开采提供新方法和基础数据及理论与技术支撑。