页岩气储层压裂液滞留机理与模拟研究

A general observation from the completion of shale gas wells is that a large fraction of the injected water remains in the formation after flowback, and that the fluid loss from any single well can exceed 60% of the original injected volume. This may affect the post-fracturing productivity of the well. However, there is lack of direct experimental and visual evidence of the extent of fluid retention, evolution of the resulting invaded fluid front, and how they relate to potential productivity hindrance.. In this study, laboratory experiments have been carefully designed to represent the migration and retention of the invaded fluid in the neighborhood of hydraulic fractures. Laboratory experiments are carried out on several shale samples in order to mimic the fluid injection, flowback and shut-in periods of the well.Then, aided by laboratory invaded fluid saturation, relative-permeability, capillary pressure data and their dependency to stress in a shale gas reservoir, the flow-simulation model are able to reproduce the suspected fluid migration and retention behavior. Results from this investigation are expected to provide fundamental insight to improve the understanding of the physical mechanisms that control fluid transport and subsequent gas recovery.

页岩气储层在水力压裂过程中需要泵注大量的水，现场统计超过60%的压裂液滞留在地下不能够返排，这种现象可能影响页岩气井的压后产能。然而，目前缺乏直接可视的实验数据证明滞留液的分布范围、前缘运移动态以及这种现象是如何影响页岩气产能的。. 本项目详细设计了室内实验来重现侵入压裂液在水力裂缝及其周围复杂基质孔隙间的运移和滞留过程。物理模拟了压裂液泵注、返排和关井等几个重要施工步骤。辅以实验获得的侵入液饱和度、毛细管力、相对渗透率、应力敏感等参数，建立了流动模拟数值模型来重现侵入压裂液的运移和滞留过程。项目研究结果将有助于深入理解清水压裂液在页岩气储层中的运移和滞留控制机理，同时提高对页岩气采收机理的进一步认识。

大规模清水压裂技术是目前公认的页岩气开发的有效手段，现场施工结果显示清水压裂改造后的页岩气井返排率极低。然而，压裂返排过程中侵入液在水力裂缝周围复杂基质孔隙间运移和滞留的控制机理尚不明确，滞留液在页岩气生产过程中的影响也亟待解释量化。. 本项目利用高温高压驱替-核磁共振扫描一体化仪器，通过对压后关井过程的自发渗吸和返排过程进行物理模拟，建立了针对页岩清水压裂液的返排物理模拟实验方法，形成了纳米级孔喉内流体运移识别与评价新技术。在此基础上，建立了考虑毛管力、化学渗透、裂缝闭合、热对流与传导、气体解吸附以及气体高速非达西流动的气水盐三孔双渗页岩气井压裂返排数学模型，编制模拟器开展了压后返排因素的敏感性分析。最后，通过结合实际页岩气压裂井的压后返排数值模拟和施工数据拟合,明确了压裂液的滞留机理和滞留关键因素，为后续开展压裂返排制度优化、页岩气井压后产能预测工作奠定了基础。. 本项目的研究结果对页岩气清水压裂增产改造技术提出了更深刻的认识和见解，为页岩气水力压裂技术的施工方案设计和增产效果评价提供了理论依据，同时对我国非常规天然气的压裂增产改造也具有一定的参考和借鉴意义。