煤系页岩瓦斯吸附解吸渗流与变形耦合作用多尺度联动机制研究

Indefinite mechanism of gas-shale coupling effect is the main reason of low recovery, in fact shale exploit is multiscale linkage process of gas microscopic desorption cumulation to macroscopic seepage presentation caused by mining, and the mechanism and law has not still been clear. Taken coal shale as subject, technique of nuclear magnetic reasonance is introduced, and experimental inversion method is used. Based on macroscopic experiment with parameters of engineering scale, macroscopic parameters characteristic of coupling effect on gas adsorption,desorption,seepage and deformation under different load combination will be explored. Non-magnetic triaxial permeameter for variable confining pressure, axial pressure and pore pressure will be produced. Nuclear magnetic experiment of same parameters will be made. Law of microscopic nuclear magnetic indexs such as gas adsorptive capacity, free capacity and porthole radius will be obtained. Corresponding with parameters obtained by macroscopic experiment such as critical desorption pressure, desorption speed ratio and seepage rate, law of engineering scale will be inversed, and multiscale linkage mechanism of gas-shale coupling effect will be revealed. Using the correspondence with T2 distribution spectrum of lateral relaxation time and parameters of macroscopic seepage rate and microscopic porthole deformation, macroscopic and microscopic linkage mechanism of gas-shale coupling effect will be linked, the gas-solid coupling mechanical model of shale transportation and output will be established and solved numerically, thus output prediction criterion offering reference to shale exploit will be presented.

瓦斯-页岩耦合作用机理不清是造成页岩气采收率低的根本原因，实际上页岩气开采是人为采动致瓦斯微观解吸累积到宏观渗流显现的多尺度联动过程，目前其研究远未达到机理清晰、规律明确程度。拟引入核磁共振技术，采用实验反演方法，以煤系页岩为研究对象，基于工程尺度参数的宏观实验，探究不同荷载组合瓦斯解吸渗流与页岩变形耦合作用宏观参数变化特征，研制变围压、轴压、孔隙压非磁性三轴渗透仪，进行相同参数核磁实验，得到瓦斯吸附量、游离量、孔道半径等核磁微观指标变化规律，与宏观实验得到的临界解吸压力、解吸速率、渗透率等参数对应，反演工程尺度产出规律，阐明瓦斯-页岩耦合作用多尺度联动机制。利用横向弛豫时间T2分布谱与宏观渗透率和微观孔结构变形参数对应关系，将瓦斯-页岩耦合作用宏微观联动机制联系起来，继而基于气固耦合理论建立页岩气运移产出多尺度联动力学模型，并加以数值求解，提出产能预测判据，为页岩气开采提供参考。

瓦斯-页岩耦合作用机理不清是造成页岩气采收率低的根本原因，实际上页岩气开采是人为采动致瓦斯微观解吸累积到宏观渗流显现的多尺度联动过程，目前其研究远未达到机理清晰、规律明确程度。引入核磁共振技术，采用实验反演方法，以煤系页岩为研究对象，基于工程尺度参数的宏观实验，探究了不同荷载组合瓦斯解吸渗流与页岩变形耦合作用宏观参数变化特征，研制了变围压、轴压、孔隙压非磁性三轴渗透仪，进行了相同参数核磁实验，得到了自由态、游离态、吸附态瓦斯吸附量、解吸量、横向弛豫时间T2等核磁微观指标变化规律，与宏观实验得到的有效应力、瓦斯压力、瓦斯吸附增量、瓦斯解吸增量等参数对应，从宏微观结合角度发现了一些新现象，总结了新规律，获得了新认识，建立了新指标。横向弛豫时间T2反映了瓦斯在页岩孔裂隙中不同赋存状态，可作为吸附态瓦斯、游离态瓦斯和自由态瓦斯划分指标。依据核磁共振信号幅值变化可以实现自由态瓦斯、游离态瓦斯和吸附态瓦斯含量变化的定量化表征。试验发现宏观吸附态瓦斯含量与微观吸附态瓦斯增量间呈对数函数关系，微观吸附态瓦斯解吸量与瓦斯压力、有效应力间呈指数函数关系。考虑煤系页岩气吸附膨胀作用，建立了含吸附气煤系页岩有效应力公式。基于气固耦合理论、各向异性材料力学，考虑压裂液滤失附加应力，建立了煤系页岩横观各向同性水力压裂水平井井壁应力力学模型，给出了水力压裂起裂判据。分析了水力压裂压强、气藏埋深、侧压比、水力喷射布置方式、喷射角度对煤系页岩气藏应力分布特征，反演了工程尺度产出规律，阐明了瓦斯-页岩耦合作用多尺度联动机制，为煤系页岩气开采提供了科学参考。