海域天然气水合物连续注气驱替-置换开发机理及多相渗流产出规律研究

Injection of CO2-containing gas mixture has become one of the main research directions on hydrate exploitation due to its triple advantages of high gas recovery, CO2 long-term storage and geo-mechanical stability, and has tremendous potential for future industrialization application. It is expected to significantly suppress the water/sand migration and markedly enhance the gas-to-water ratio by the method of continuous gas injection and replacement, especially for the hydrate exploitation from marine hydrate deposits. Therefore, this study intends to establish an integration simulation system for hydrate formation, gas recovery, multiphase fluid flow and long-term evolution of the reservoir in marine sediments. Firstly, key parameters such as injection rate, gas composition and production pressure can be optimized to enhance the efficiency of injected gas to displace pore fluid. Based on the optimal results, the kinetic mechanism of hydrate phase transition interacting with gas-liquid-solid multiphase mass transfer, as well as the law of producing gas/water/sand can be revealed by describing the fields of temperature, pressure and concentration during the continuous gas injection and replacement. Then, the synergistic effects between CO2 and small-molecule gas in hydrate decomposition and regeneration can be further understood by macroscopic physical simulation, microscopic morphological observation and composition measurement. Further, the kinetic mechanism of the continuous gas injection and replacement can be established. Finally, the applicability of this technology will be evaluated in the marine hydrate development based on the above work.

注CO2混合气开采天然气水合物兼具高效产气，CO2原位封存和储层稳定的三重优势，已成为水合物开发技术的主要研究方向之一，极具应用潜力。尤其在海域天然气水合物开发过程中，通过连续注气驱替-置换开采的方式，有望显著抑制水/砂迁移，大幅提高产出气水比，降低出砂风险。为此，本项目拟建立海域水合物成藏-注采-输送-长期演化一体的考察模式，优化注气速率、注气组成和注采压力等关键参数以强化注入气驱替孔隙流体的效率，刻画连续驱替-置换过程中温度场、压力场和浓度场的时空演变规律，揭示水合物相变与气/液/固多相传递交互作用的动力学机制及气/水/砂产出规律。基于宏观物理模拟计算、显微观测和水合物分子成分测定，揭示CO2和小分子气体在水合物分解和再生成方面的协同作用，确立连续驱替-置换开发动力学机制。为注CO2混合气驱替-置换我国海域天然气水合物提供适应性评价。

注CO2混合气开采天然气水合物兼具高效产气，CO2原位封存和储层稳定的三重优势，已成为水合物开发技术的主要研究方向之一，极具应用潜力。目前的关键问题是探明注入气驱替孔隙流体及激发水合物分解和再生成的动力学规律和关键影响因素。本项目重点针对海域天然气水合物储层通常具有高含水、无封闭盖层的特征，在三维模拟装置中建立了敞开式的富水的甲烷水合物样品并完成了一系列连续注气驱替-置换开采甲烷水合物实验模拟研究。厘清了孔隙流体组成和甲烷水合物分解行为的关系，揭示了连续注气驱替沉积物中孔隙流体产出和上覆水下渗的规律，建立了储层气相空间、气体波及范围与注气速率的关联式，提出并评价了短注气强化甲烷水合物开采的新方法。建立了弱降压结合注气的联合开采方法，有效强化了注入气体波及范围，提高了开采效率，同时显著抑制外围水渗入并大幅提高产出气水比；在敞开式富水沉积层中实现了水合物1:1置换开采过程，其对强化储层稳定性和降低出砂风险具有重要意义。基于Raman光谱和显微成像联合观测技术，初步揭示了小分子气体和CO2在水合物分解和再生成方面的协同作用机制；针对项目执行期间发现的甲烷产出和CO2封存相互制约的问题，进一步提出并测试评价了交替注CO2/N2模式和变组成注气模式两种调控注入气体的方法，在保障高甲烷产出的基础上实现了高CO2封存率。本项目研究丰富了天然气水合物开采相关的基础数据和规律性认识，为安全、绿色高效开采天然气水合物和碳封存提供了新的技术思路。本项目资助发表SCI收录学术论文7篇，申请并授权中国发明专利1件。