页岩以纳米级孔隙为主,具有特殊的孔隙结构、复杂的岩石组成和超低的渗透性等特点,这给常规表征技术手段的应用带来了困难。基于对四川盆地龙马溪组海相页岩的低场核磁共振系统实验分析,提出了一套较为完善的针对页岩孔隙度、渗透率、孔隙类型、孔隙结构和甲烷吸附能力的精细定量表征技术,从理论和技术两个角度阐明了核磁共振弛豫谱技术在页岩储层物性与流体特征分析中的应用。结果表明:核磁共振技术可有效识别页岩的黏土束缚流体、毛管束缚流体和可动流体,并计算它们的孔隙度;提出的基于双T2截止值的页岩孔径划分方案可有效应用于评价页岩的储集和产出性能;提出的基于饱和流体和束缚流体双T2几何平均值的SDR渗透率计算模型在页岩渗透率预测方面具有较强的适应性;利用核磁共振技术可定量识别页岩中吸附态、孔束缚态甲烷和游离态,可获得样品的等温吸附曲线;利用核磁共振实验模拟页岩中注CO2后甲烷的相态变化过程,发现注CO2可有效提高页岩中吸附气的采收率。总体上,基于低场核磁共振的页岩储层物性和流体特征表征为页岩储层研究提供了一种全新的技术思路。

Gas shale is characterized by well-developed nano-pores,special pore structure,complex organic and inor- ganic compositions,and ultra-low permeability,which brings major challenge for the analyses by the conventional ana- lytical methods. On the basis of nuclear magnetic resonance ( NMR) experimental analyses of the marine Longmaxi formation shales from the Sichuan basin,this study provided a comprehensive methodology for measurement of the po- rosity,permeability,pore type,pore structure,and methane adsorption capacity of shale,and from theoretical and tech- nical perspectives,discussed the NMR applications in the characterization of reservoir physical properties and fluids transportation in shale. Results show that NMR technique can be used to quantitatively identify the clay-bonded fluid,capillary-bonded fluid,and movable fluid in shale,and to calculate their porosities;The dual-T2 cutoff based pore size distribution classification method is effect for evaluation of gas accumulation and gas transportation in shale;The SDR permeability evaluation model deduced from the double-exponential relationship with the T2 geometric mean in the full water-saturated state and in the bound water state,is applicable for shale permeability calculation;NMR technique can be used to quantitatively identify the adsorbed methane,porous-medium-confined methane,and bulk methane in shale, and to obtain isothermal adsorption curve of shale;A real-time NMR-based monitoring of the CO2 -CH4 displacement in shale indicated that additional 25% of adsorbed methane gas can be recovered by the injection of CO2 . It is concluded that the applications of NMR in analyses of the petrophysical properties and fluids transportation,provide a new solu- tion for shale gas reservoir characterization.