柴达木盆地东部石炭系页岩气储层渗流特征研究

Percolation characteristics of Carboniferous shale gas in Eastern Qaidam Basin

页岩气是一种潜在资源量巨大的非常规天然气资源,页岩气藏具有多尺度的孔渗结构及多种渗流形态,研究页岩气藏储渗特征能够为页岩气的勘探开发提供理论支撑。该研究选取了柴达木盆地东部石炭系克鲁克组与怀头他拉组的页岩钻井岩心共5个样品,应用氩离子抛光-扫描电镜实验与孔径分布测试(包括压汞法、氮气吸附法、二氧化碳吸附法)对页岩的孔隙结构特征进行定性定量测试。基于页岩气质量流量渗流模型,给出页岩表观渗透率与平均压力的关系。通过甲烷渗流模拟实验测定页岩的表观渗透率。从表观渗透率随平均压力的变化特征出发,分析页岩储层中的气体渗流规律。研究结果表明:页岩气的渗流形态包括滑脱流、扩散流及达西流。在低压情况下,渗透率低的页岩中以扩散流为主,其次为滑脱流。随渗透率的增大,渗流主要形式转变为滑脱流。当压力大于2MPa时渗流形态以达西流为主,滑脱与扩散行为不明显。研究区内页岩微孔与中孔发育较多,达西渗透率与大孔的孔隙体积相关性较大,扩散流对表观渗透率的贡献与50nm以下的孔隙孔体积比例有一定的相关性。低压下扩散流对表观渗透率的贡献较大,扩散流是一种非常重要的页岩气运移形式。

Shale gas is one kind of unconventional gas resources with rich potential resources, and it has multi- scale porosity structures and a variety of seepage patterns. The study of the porosity structures and seepage law of shale gas can be used as the theoretical support for the exploration and development of shale gas. In this study, pore-structure tests and methane seepage experiments were conducted on shales from the Carboniferous Hurleg and Huitoutala Formations in Eastern Qaidam Basin. Based on the mass flow model of shale gas considering gas slippage and diffusion behavior, the relationship between the apparent permeability and the average pressure was introduced. The apparent permeability of shale was measured by methane seepage experiment, and the shale gas flow characteristics affected hy the slippage and the diffusion were analyzed. The results show that the shale gas seepage states include slip flow, diffusion flow and Darcy flow. The slip and diffusion behavior in the whole seepage weakens with the increase of the average pressure. Under the condition of low pressure, the lower the shale permeability, the greater the diffusion flow contribution. With the increase of permeability, the main seepage mechanism becomes slip flow. When the pressure is greater than 2 MPa, Darcy flow mainly influences the seepage, and the slip and diffusion behavior are not obvious. Darey permeability and the macropore volume have a positive correlation, and diffusion flow contributions to the apparent permeability have certain relation with pores whose diameter smaller than 50 nm. Diffusion flow is a very important gas transport process in shales.