考虑支撑剂压实和嵌入双重作用的压裂裂隙渗透率模型研究

According to the report from Nature, the gas production rates from many shale gas plays in US gradually drops since 2010 and the decrease in hydraulic fracture permeability is one of the key influencing factors. Hydraulic fracture, filled with porous media (proppant pack), is a type of composite fractured media and there is still lack of proper permeability model for hydraulic fractures. Based on the pore compressibility theory and the contact theory, this project will establish a theoretical permeability model for hydraulic fractures, which emphasizes two influencing factors on hydraulic fracture permeability change: (1) The compaction of proppant pack due to the effective stress which gradually increases with the shale gas production; (2) The fracture closure due to the proppant embedment under contact stress. The model could be verified by existing experimental data: (a) The proppant pack permeability data under rigid confining plates are used to verify the permeability model only considering the compaction effect; (b) The permeability data under shale plates are used to verify the permeability model considering the combined effect of compaction and embedment. Moreover, this project will setup a triple porosity (matrix-natural fracture-hydraulic fracture) shale gas flow simulation model coupled with the permeability model developed. The influence of hydraulic fracture permeability change on long term shale gas productivity is revealed by simulation. The outcome of this work will extend existing fluid flow theory and provides theoretical basis for hydraulic fracture design and long term productivity analysis.

据《Nature》报道，美国多地页岩气产量自2010年起逐年下滑，压裂裂隙渗透率下降是重要的影响因素之一。压裂裂隙是充填着孔隙介质（支撑剂充填层）的复合型裂隙介质，尚缺乏合理的压裂裂隙渗透率模型。本课题将基于孔隙弹性和接触理论，建立压裂裂隙渗透率理论模型。模型重点考虑两种影响因素：（1）有效应力随页岩气开采增加导致的支撑剂充填层压实；（2）支撑剂嵌入裂隙面导致的有效渗流通道变窄。模型利用已有试验数据验证：（a）利用刚性夹板下支撑剂充填层渗透率应力敏感性数据验证只考虑压实作用的渗透率模型；（b）利用页岩夹板下支撑剂充填层渗透率数据验证考虑压实和嵌入双重作用下的渗透率模型。此外，将该渗透率模型耦合到“基质-天然裂隙-压裂裂隙”三孔页岩气渗流数值模型中，通过数值计算，揭示压裂裂隙渗透率变化对页岩气长期产能的影响。课题成果将拓展现有渗流理论，还将为压裂设计和长期产能分析提供理论依据。

页岩气是非常规天然气中重要的组成部分，也是一种新型的清洁能源。美国页岩气革命深刻地影响着世界，根据报道自2010年起美国页岩气产量逐年下滑，引起我们的深思。由于水力压裂是页岩气开采的重要增产手段，压裂裂隙渗透率下降是其产量下降的重要因素，而目前尚缺乏有关压裂裂隙渗透率模型研究的相关理论研究，本项目对支撑剂压实与嵌入双重作用下的压裂裂缝导流能力变化规律展开研究。研究内容主要有：压实性对支撑剂充填层渗透率的影响研究；考虑压实和嵌入双重作用的压裂裂隙渗透率模型研究；页岩多孔裂隙介质耦合数值模型建立和页岩气长期产能研究。基于孔隙压缩性理论与固体接触理论，建立了考虑压实与嵌入双重作用的压裂裂缝导流能力模型，利用建立的模型对前人研究得到的压裂裂缝导流能力实验数据进行了拟合，并对模型参数分布规律及其影响因素进行了讨论分析。研究结果表明：该模型可以较好地描述压实与嵌入双重影响下的压裂裂缝导流能力变化规律；支撑剂充填层初始孔隙体积压缩系数体现了支撑剂充填层孔隙的压缩性，其值越大，压裂裂缝导流能力变化越大；支撑剂充填层初始孔隙体积压缩系数变化率的绝对值越大，压缩系数变化量越大。压实与嵌入双重作用下压裂裂缝导流能力模型的建立，为预测压裂裂缝导流能力变化规律提供了理论依据，为我国页岩气增产开采提供理论参考依据和技术支撑。