Apparent Permeability for Gas Flow in Shale Reservoirs Coupling Effects of Gas Diffusion and Desorption
Apparent Permeability for Gas Flow in Shale Reservoirs Coupling Effects of Gas Diffusion and Desorption
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DOI：
10.15530/urtec-2014-1921039
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发表时间：
2014-08
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影响因子：
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通讯作者：
Keliu Wu;Xiangfang Li;Chenchen Wang;Wei Yu;Zhangxin Chen
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作者：
Keliu Wu;Xiangfang Li;Chenchen Wang;Wei Yu;Zhangxin Chen
页岩气藏的天然气运移机理和视渗透率与常规气藏有很大不同，这主要是由于页岩气藏的纳米尺度现象和有机质作为天然气储集和气源的介质所致。然而，气体流动特性对页岩储层中的井动态起着重要作用。因此，开发一种新的考虑气体运移机理的视渗透率模型是非常必要的。在这项工作中，我们提出了一个新的表观渗透率模型描述的气体在纳米孔隙中的纳米孔隙流动的页岩气藏集成体气体流动和气体从纳米孔壁解吸。虽然气体分子的平均自由程近似于纳米孔直径的数量级，但气体在纳米孔中的流动应该是联合收割机粘性流动和努森扩散的结合。总气体通量不应是粘流通量和努森扩散通量的简单相加，而应是基于它们不同贡献的加权总和。加权因子主要由粘性流和Knudsen扩散之间的相互作用控制，在新模型中，可以通过气体分子之间相互碰撞和与纳米孔壁碰撞的概率来量化。基于Langmuir等温方程和质量平衡方程建立了考虑气体解吸的视渗透率模型。该模型能分别从粘流、努森扩散和解吸三种情况准确计算出表观渗透率。此外，它可以提供关键的见解，了解大规模的气体流动和气体解吸从纳米孔壁的机制。该模型可用于开发新一代的页岩储层数值模拟器。
Gas transport mechanism and apparent permeability in shale reservoirs are significantly different from those in conventional gas reservoirs, which are mainly caused by the nanoscale phenomena and organic matter as being the media of gas storing and sourcing. However, gas flow behavior plays a significant role in well performance in shale reservoirs. Hence, development of a new apparent permeability model considering gas transport mechanism is critically desirable. In this work, we propose a new apparent permeability model describing gas flow in nanopores of shale gas reservoirs by integrating bulk gas flow in nanopores and gas desorption from nanopores wall. Although the mean free path of gas molecule is similar to the order of nanopore diameter, gas flow in nanopores should combine viscous flow and Knudsen diffusion together. The total gas flux should not be a simple summation of viscous flow flux and Knudsen diffusion flux, but a weighted summation based on their different contributions. The weighted factor is primarily controlled by the interaction between viscous flow and Knudsen diffusion, which can be quantified by probabilities between gas molecules colliding with each other and colliding with nanopores wall in the new model. The apparent permeability considering gas desorption is established based on Langmuir isotherm equation and mass balance equation. The new model can accurately calculate the apparent permeability from viscous flow, Knudsen diffusion, and desorption, respectively. Furthermore, it can provide critical insights into understanding the mechanisms of bulk gas flow and gas desorption from nanopores wall. Also, it can be used to develop the new generation reservoir simulator in shale reservoirs.