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页岩储层束缚水影响下的气相渗透率模型
李靖1,2,李相方1,陈掌星2*,王香增3,吴克柳1,2,孙政1,曲世元
Permeability model for gas transport through shale nanopores with irreducible water saturation
LI Jing1,2, LI Xiangfang1, CHEN Zhangxin2, WANG Xiangzeng3, WU Keliu1,2, SUN Zheng1, QU

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摘要  在纳米孔隙单相气体传质理论的基础上,考虑实际样品孔隙-裂缝形貌特征,利用权重系数叠加滑脱流及分子自由流,建立了圆管孔与狭缝孔内的气体传输模型(无机质孔隙-裂缝多表现为狭缝形;有机质孔隙多表现为圆形)。进一步考虑含水饱和度在无机质与有机质孔隙的分布差异性,结合实际样品的孔隙分布特征,量化研究了含水饱和度对气体流动的影响。结果表明:束缚水对纳米尺度孔缝内气体流动能力的影响主要受控于流动条件,即气体努森数Kn。随Kn增大,微尺度效应(滑脱及扩散)影响开始显著,束缚水对气体流动能力的影响逐渐减弱。以狭缝孔(无机质孔隙)为例,在束缚水饱和度30%条件下,当Kn<0.001 时(微尺度效应不明显),气相渗流能力降低约51%;而当Kn>1.0 时(微尺度效应显著),气相渗流能力降低33%。因此,伴随页岩气藏开发,储层压力降低,气体努森数Kn增大,束缚水对气体流动的影响在一定程度上将被减弱,但该影响仍然不容忽视。本研究为合理评价及预测储层含水条件下页岩气井产能奠定了理论基础。
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关键词 : 页岩气;束缚水;纳米孔隙;微裂缝;气相渗透率
Abstract

Permeability models for single-phase gas transport though nanotubes and nanoslits were established by a weighted
superposition of slip flow and molecular diffusion (the inorganic pores were regarded as the nanoslits; the organic pores were
regarded as the nanotubes). Besides, the influence of water saturation on the gas transport was quantified by considering its distribution
characteristic inside the inorganic pores and the organic pores of actual shale formations. The results show that the effect
of water saturation on the gas flow capacity at a nanoscale is mainly controlled by a Knudsen number (Kn); as Kn increases,
the impact of nano-scale effect (slip and diffusion) begins to grow, and the decrease in gas flow capacity caused by the bound
water weakens. For slit-shaped pores (e.g. inorganic pores), when Kn < 0.001 (the nano-scale effect is not obvious), the gas-phase
permeability decreases by as high as 51% with an irreducible water saturation of 30%; instead, when Kn > 1.0 (the nano-scale
effect is significant), the gas-phase permeability reduces by about 33% in the same water saturation condition. Therefore, with the development of shale gas reservoirs, the reservoir pressure gradually reduces and the Kn gradually increases, leading to a
weakening effect of the bound water on gas flow; however, this effect still cannot be ignored. This paper provides a theoretical
basis for reasonable evaluations and predictions of gas production from actual shale formations with initial water saturation.

Key words: shale gas; irreducible water; nanopores; microfractures; gas-phase permeability
收稿日期: 2018-06-29     
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