页岩气绿色高效开发中关键理论问题的研究

The shale gas has been taken as the most important non-conventional hydrocarbon resource and the next generation of strategic energy source, because of the abundant reserves of natural gas which is currently recognized as the clean energy source with high efficiency. Though the significance of the efficient recovery of shale gas is very remarkable, but the production of shale gas is very difficult. The natural gas is trapped in the extremely dense shale and is hard to be recovered, specific theoretical knowledge and techniques are in desire for the production of shale gas but are still lacked. This project plans to study the micro molecular adsorption behavior of methane in the nano pore of shale, investigate the effect of temperature, pressure, characteristics of nano pore and additives on the the micro phase equilibrium in the nano pore of the dense shale by combining theoretical molecular simulation methods and experimental approaches, to build up the thermodynamic and kinetic rules about the variation of the molecular state of methane in micro view, which would be the key theoretical foundation of the recovery techniques of shale gas. The project plans to study the effect of surfactants on the molecular behavior of methane in shale, find out the particular preferred surfactant to enhance the desorption of the methane from the surface of shale and induce the formation of nano bubbles using desorpted free methane as the gas agent. By studying the rule of the formation, coalescence and transportation of the nano bubbles in the fine porous media, and the effect of polymer on the rheology of the foam fluid, the optimum low density foam fracture fluid with cutting carrying capacity, width generation capability and supporting performance at the same time could be designed. The theoretical and technical innovations of the project are both very distinct, and the environmental significance is very remarkable, too. The environmental risk of the in situ low density foam fracture technique could be controlled very well by using green surfactant and natural polymer as additives, and the leakage of methane to the reservoir could be avoid .

页岩气作为新一代战略能源，其勘探开发的重要性毋庸置疑，但页岩气圈捕在仅为纳米尺度的孔隙中，开采难度大，高效开发需要新的有针对性的理论研究和技术突破。本项目以分子模拟和实验技术相结合，研究页岩纳米尺度孔隙中甲烷的吸附状态和分子行为，探索温度、压力、孔吼特性以及添加剂对甲烷分子行为和微观相态的影响，研究储层条件下甲烷纳米气泡的形成、稳定、聚并以及在纳米尺寸孔道和微米宽度裂缝中的运移规律和微观流体动力学；在以上研究的基础上，优选表面活性剂和活性聚合物提高甲烷在页岩孔隙中的解吸附效率并调控流体的流变性质，设计具有支撑、携岩、造缝多种效能的微细泡沫低密度压裂液提高页岩气开采效率。本项目对致密页岩纳米孔隙中的微观相平衡以及微观流体动力学的研究填补理论空白，理论意义非常重大；采用绿色添加剂构成多效低密度压裂液提高页岩气开采效率并确保对环境无污染，经济、技术、环境意义均十分重大。

国内外关于页岩气藏的研究大都停留在宏观层面，对于气体在页岩纳米孔隙中的微观行为规律的认识较为缺乏，并且由于页岩组成的复杂性以及页岩气开采过程中复杂的作业环境，很难从宏观或表观层面从实验研究中得到准确的理论认识，因此严重制约了对页岩气开采技术的创新和有效应用。. 本项目运用分子动力学（molecular dynamics, MD）、巨正则蒙特卡洛（grand canonical Monte Carlo, GCMC）等分子模拟方法，结合密度泛函理论（density functional theory, DFT），深入探究了CH4与CO2气体在干酪根、石英、蒙脱石、方解石等典型页岩组分组成的单一或混合纳米孔隙中的微观状态及分子行为规律，通过系统考察孔隙结构、尺寸及孔隙表面亲疏水性变化等对孔隙中气体的微观行为和吸附特性的影响，发现和证实了CO2/CH4竞争吸附的普遍存在性，为提出运用CO2提高页岩CH4采收率的技术创新路线提供了确凿的理论依据。通过系统考察CO2对多种页岩纳米孔隙中残留CH4的驱替效率、微观机制及影响因素，证实CO2-EGR技术不仅能够实现页岩气的增产，同时还能实现对CO2的埋存，实现经济效益和环境效益双赢。. 根据已有的认识，压裂造缝是页岩气藏的有效开采方式，其中水力压裂是目前应用最广的技术，虽然取得了较好的效果，但带来水资源浪费、环境污染等诸多问题，且无法很好地应用于水敏性地层。基于项目研究取得的理论认识，本项目提出了采用二氧化碳泡沫压裂提高页岩气采收率的创新技术路线。探究了不同气体形成的泡沫的稳定机制，深入研究了表面活性剂、聚合物及复合体系的泡沫性能以及加和增效机制，设计出了具有优异稳定性的二氧化碳泡沫体系，探索了纳米气泡的形成、稳定和运移机制，并选取SiO2纳米颗粒及陶粒作为支撑剂代表物考察了泡沫体系的携砂能力。研究表明设计的CO2泡沫体系性能优异，在页岩气藏的压裂增产方面具有良好应用前景，为CO2-EGR技术的应用打下了坚实的基础。. 本项目研究充分揭示了二氧化碳强化页岩气开采的重要意义和应用前景，此外还探究了采用沥青、方解石等天然矿物构建纳米孔隙捕集烟道气中CO2的机制和效果，为自烟道气捕集二氧化碳技术的发展提供一定的理论指导，对本项目采用二氧化碳提高页岩气开采技术的应用提供必要的保障，因此也具有重要的意义。