页岩储层多相流体耦合作用及动态地质效应

The interaction and transportation of micro- to nano-scale fluids in shale reservoir is one of the key scientific issues for exploitation of shale gas. The existing researches are mainly focused on the gas transportation in shale pores, but deficient to the interaction of multiple-phase fluids in shales, such as the surface-adsorbed-, capillary-bounded-, and bulk- water, as well as the bulk-, capillary-bounded-, and adsorbed- methane gases, and discuss the influences of multi-phase water on the adsorption/desorption and diffusion of the multi-phase methane. Based on series of specially designed low-field NMR simulation experiments, this project would analysis the occurrence rules of the multiple-phase fluids in shales, and discuss the dynamic relationship between the change of water saturation and the transportation mechanisms such as adsorption/desorption and diffusion, with respect to the changes of reservoir temperature and pressure. Then, this project would found a NMR-based quantified method for measurement of wettability and gas-water relative permeabiliy of shales. Using these methods, this project would explain the prerequisites and reasons for the change of gas-water wettability in in-situ shale reservoir, and discuss the change of gas-water relative permerbility with respect to different reservoir pressure and temperature. Finally, this project would comprehensively conclude the microscopic dynamic mechanisms of the adsorption/desorption, diffusion, wettability, and seepage of multiple-phase fluids in shale reservoirs. This project could provide certain theoretical supports for better understanding the transportation mechanism of shale gas, and for high-effective development of shale gas.

页岩储层微纳尺度内流体相互作用与流动机理是页岩气藏开发中亟需解决的关键科学问题之一。现有研究大多侧重于采用解析推导或数值模拟来研究微纳米孔隙内气体流动传输，而对多相态流体（表面吸附水、孔内毛管束缚水和自由水；吸附态甲烷、孔束缚态甲烷和自由态甲烷吸）相互作用的研究较少。本项目首先以前期形成的基于低场核磁共振的多相态流体分析技术为主要手段，通过针对性开展系列物理模拟实验，分析多相态流体在页岩中的赋存规律，阐明压降引起的含水变化与甲烷的吸附/解吸、扩散等作用的动态耦合机制。其次，形成基于核磁共振的页岩润湿性与气、水相对渗透率分析方法，模拟研究开发过程中气、水润湿性反转发生条件和内在机理，揭示不同温、压条件下页岩储层气、水相对渗透率变化规律；最后，综合阐明多相流体在页岩储层中的吸附/解吸、扩散、渗吸和渗流等微观动力学机理。研究将为深入理解原位条件下页岩气渗流机理和科学高效开发页岩气藏提供理论基础。

围绕“页岩储层多相流体耦合作用及动态地质效应”这一关键科学问题，本项目开展了技术装备研发、实验物理模拟及分子动力学模拟等研究工作，取得了主要创新成果有：(1)研制了基于核磁共振的高温高压渗吸动态监测装置和采用核磁共振技术研究孔隙间流体相变的实验装置；(2) 研发了一套基于核磁共振的页岩中多相流体（包括吸附态与游离态甲烷，吸附态、孔束缚态与自由态水）定量表征技术体系，提出了基于“双截止值法”的页岩全孔径表征及多态孔隙流体分类方法，提出了页岩中吸附气与游离气占比、吸附相甲烷密度、绝对吸附等温曲线、储层原位含气量模拟计算方法，为页岩储层原地气资源量计算与气体产出规律预测提供了新的技术手段；(3)阐明了富有机质页岩纳米孔隙成孔机制，揭示了页岩微纳米孔隙及分子结构中气、水的微观赋存机理；(4) 将Handy经典渗吸理论与核磁共振流体分析原理相结合，推导了页岩渗吸饱和孔隙面积计算方法，通过引入等效毛管力与渗吸渗透率两个概念，创造性地提出了一种全新的页岩自发渗吸数学模型，弥补了传统渗吸模型对复杂多孔介质渗吸动力学过程解析能力的不足；(5)揭示了龙马溪组页岩在不同渗吸不同边界条件下的自发渗吸与强制渗吸动力学机理及渗吸过程，为水力压裂过程中压裂流体的泵入、关井和压裂液返排设计提供了理论依据。