近十年来在北美掀起的页岩革命, 释放了大量的深地资源, 改变了人们对油气资源稀缺性的认识, 受到了科学界和工业界的广泛关注. 页岩油气是我国重大战略性能源, 其高效开发和利用对于保障我国能源安全、缓解能源供需矛盾有极其重大的意义. 然而, 由于我国页岩储层埋藏极深、构造复杂、低孔低渗、吸附量大, 造成页岩气的采收率极低. 未来我国页岩油气的高效开发, 亟待突破采收率低的桎梏. 这在一方面需要结合工程实际, 精确描述页岩储层孔隙系统特征及其内部流体的赋存和相平衡规律; 另一方面需要从力学的基本原理和第一性的实验出发, 揭示岩石储层孔隙系统的力学–化学演化机理. 本文结合国家对页岩油气资源的重大需求和采收率极低的工程科学基本难题, 以实验研究为主线, 重点围绕页岩储层孔隙系统的多尺度特征、储集空间中流体的相平衡、纳米孔隙的力学–化学演化等三个方面内容开展了研究工作, 具体如下:  

页岩储层孔隙系统的多尺度特征. 页岩储层既是烃源岩又是储集岩, 其孔隙系统是经过漫长的地质历史演化形成的, 是页岩油气的主要赋存空间和运移通道. 明确页岩储层孔隙系统的孔隙结构及其动态演化等关键信息是页岩油气生成、储集、富集、成藏、解吸、驱替和运移等研究的基础和前提. 本文联合采用多种先进实验技术手段, 系统地厘清了页岩储层纳米孔隙的多尺度特征, 发现页岩储层中纳米孔隙主要为呈不规则圆形、狭缝形的有机介孔, 其孔径呈不对称双优势峰分布, 孔喉直径小、连续性低、连通性差, 整体呈孤立状.

储集空间中流体的赋存状态与相平衡. 首先, 揭示了页岩储层中气体随压力变化的赋存状态和相平衡规律: 页岩储集空间中吸附态、游离态甲烷均随气体压力增加而增加; 在较低气体压力下, 吸附态甲烷含量大于游离态的含量; 随着气体压力的上升、吸附位逐渐饱和, 游离态甲烷的增长速率大于吸附态, 最终其含量超过吸附态甲烷. 其次, 阐明了页岩储层的流体渗吸动态过程和主导效应: 在渗吸初期, 流体的运动主要由孔隙的毛细作用驱动, 为自发渗吸; 随着岩心的流体饱和度逐渐增加, 自发渗吸逐渐消失, 岩心端面的注入压力开始起主导作用, 流体运动逐渐达到平衡状态.

页岩储层纳米孔隙的力学–化学阻塞机理. 阐明了力载荷作用下页岩样品比表面积的发展和演化规律, 厘清了其孔容积的变化历程, 分析了页岩储层在力载荷作用下分形维数的演化趋势. 结合搭建的超临界流体实验平台上的原位吸附实验, 厘清了页岩储层孔隙系统在吸附超临界二氧化碳后比表面积与孔容积的变化历程. 结合原位表征实验, 阐明了该过程中的物理吸附、缔合化学吸附和解离吸附导致纳米孔隙阻塞的微观机理.

本文系统地研究了页岩储层孔隙系统的多尺度特征和力学–化学演化机理, 为解决页岩气开发中采收率极低的工程科学基本难题提供了实验基础和依据, 为加速推进形成页岩油气高效开发核心技术, 为我国页岩油气产业的发展起到支撑作用.

In the past decade, the shale revolution in North America has released a large amount of deep resources. It changed the public’s view of the scarcity of oil and gas resources, and has received extensive attention from the scientific fields to industy. Shale oil and gas are the major strategic energy resources in China. Their efficient development is of great significance for easing energy supply and ensuring energy security. However, due to the extremely depth of the shale reservoirs, the complex geological structure, low porosity, low permeability and large adsorption amount, the current recovery efficiency of shale oil and gas in China is extremely low. The development of shale oil and gas in China in the future is urgent to break through low recovery efficiency. On the one hand, it is necessary to combine the engineering practice to accurately reveal the characteristics of the pore-system and the occurrence state of fluids in the shale reservoirs. On the other hand, it is necessary to elucidate the mechanical-chemical evolution mechanisms of the pore-system based on the basic principles of mechanics and the experiments. This paper combines the country’s major demand for shale resources, and focuses on the low recovery efficiency. Three main issues are investigated: multiscale characteristics of the pore-system, the phase balance of fluids in the reservoir space, and the mechanical-chemical evolutions of nanopores. Specific as follows.

Multiscale characteristics of pore-system. The shale reservoirs are both the source rock and the reservoir rock. Their pore-system were formed by a long geological history and is the main channel for the storage and migration of shale oil and gas. The key information such as the pore structure and its dynamic evolution is the basis and premise for the generation, accumulation, enrichment, accumulation, desorption, displacement and migration of oil and gas. Combined with a variety of advanced experimental techniques, we systematically clarify the multiscale characteristics of nanopores in shale reservoirs. It is found that the nanopores in shale reservoirs are mainly irregularly round, slit-shaped organic mesopores. The pore size shows an asymmetric double-dominant peak distribution. The pore throats are small, and their continuity and connectivity are very poor with an isolated distribution.

The occurrence state and phase equilibrium of the fluids in the reservoir space. Firstly, the occurrence and phase equilibrium of gas in shale reservoirs are revealed. It is found that both adsorbed methane and free methane in the reservoir space increase with increasing gas pressure. At lower pressures, the content of adsorbed methane is greater than that of free state. As the gas pressure increases and the adsorption sites approach saturated, the growth rate of free methane is greater, and finally its content exceeds the adsorbed one. Secondly, the dynamic process and dominant effects of fluid imbibition in shale reservoirs are clarified. In the initial stage of imbibition, the movement of fluids is mainly driven by the capillary action of pores. As the fluid saturate the core gradually, the spontaneous imbibition disappears, and the injection pressure begins to dominate. At last, the fluid motion reaches equilibrium.

The mechanical-chemical blocking of nanopores in shale reservoirs. The development law of shale’s specific surface area and the variation history of pore volume are clarified. The evolution trend of fractal dimension of shale samples under force load is analyzed. Based on the in-situ adsorption experiments conducted on the supercritical fluid platform, we clarify the variation process of the specific surface area and pore volume of the shale reservoirs after surpercritical CO₂ adsorption. Combined with in-situ characterization experiments, the microscopic mechanism of nanopore blocking caused by physical adsorption, associative chemisorption and dissociative chemisorption is clarified.

This paper systematically explores the multiscale characteristics and mechanical-chemical evolution mechanism of the pore-system in shale reservoir. It provides experimental basis for solving the basic problems of low recovery efficiency in shale oil and gas development. It may accelerate the formation of core technologies for the efficient development of shale oil and gas and supports their future industrial exploition.