页岩气藏普遍发育有微裂缝，降压生产时应力敏感引起的微裂缝宽度变化将会显著影响页岩气的质量传输。综合考虑页岩气体在微裂缝中传输时的滑脱效应、克努森扩散和表面扩散等影响因素，运用弹塑性力学、解吸附理论，建立了考虑页岩微裂缝缝宽动态变化的气体质量传输模型，并应用分子模拟验证了模型的可靠性。在此基础上，研究了页岩储层微裂缝中气体质量传输规律和不同传输机理对气体质量传输的贡献。结果表明：①该模型能够很好地反映微裂缝中共存的连续流动、滑脱流动以及克努森流动等多种流态。②地层压力大于3.4MPa时，缝宽变化导致气体传输能力下降，下降后的传输能力最小仅为未考虑缝宽变化时的0.45倍；地层压力小于3.4MPa时，缝宽变化使气体传输能力增大，增大后的传输能力最大为未考虑缝宽变化时的4.5倍。③气体质量传输能力与裂缝压缩性呈负相关，而与岩石杨氏模量、泊松比呈正相关；地层压力小于4MPa时，气体质量传输能力与气体解吸附性呈正相关；地层压力大于4MPa时，不同解吸附性对质量传输的影响几乎一致。④表面扩散只有在纳米级别的微裂缝和低地层压力下才会对气体的质量传输产生较大影响。当表面扩散作用对气体传输贡献较小时，滑脱流动和克努森流动对质量传输的贡献呈现“此消彼长”的趋势；当表面扩散所占比重较大时，随着其比重的降低，滑脱流动和克努森流动对质量传输的贡献将先同时增加，然后“此消彼长”。
Shale gas reservoirs generally develop micro-fractures.During pressure-relief production,the change of micro-fracture width caused by stress-sensitivity is an important factor affecting shale gas transport.Based on the cubic grid model,the slippage flow model,the Knudsen diffusion model and the surface diffusion model,a gas mass transporting model considering the dynamic change of shale micro-fracture width is established by using elastic-plastic mechanics and desorption theory of adsorption gas.Meanwhile,the molecular simulation results verify the reliability of the model.On this basis,considering the dynamic change of micro-fracture width,the law of shale gas mass transport is studied,and the contribution of different transporting mechanisms to the total gas transport is discussed.The results show that:(1)Considering the change of micro-fracture width caused by stress-sensitivity,the model established in this paper can well reflect these coexisting flows including continuous flow,slippage flow,Knudsen flow and surface diffusion flow.(2)Compared with original transporting capacity without considering the change of micro-fracture width,when the formation pressure is higher than 3.4MPa,the change of micro-fracturewidth decreases the gas transporting capacity,and the minimum transporting capacity is only 0.45 times of the original transporting capacity,while the formation pressure is less than 3.4MPa,the change of micro-fracture width increases the gas transporting capacity,the maximum transporting capacity is 4.5 times of the original transporting capacity.(3)The gas mass transport is negatively correlated with the compressibility of micro-fracture and positive correlation with the Young’s modulus and Poisson’s ratio of the rock.When the formation pressure is less than 4MPa,the gas mass transport is positively correlated with the gas desorption.When the formation pressure is greater than 4MPa,the effect of different gas adsorption on gas mass transport is almost the same.(4)Considering the change of the micro-fracture width,only when the micro-fracture is nano-scale and the formation pressure is relatively low,the surface diffusion can exert a great influence on the gas transport.When the contribution of surface diffusion to total gas transport is small,the contributions of slippage and Knudsen flow respectively to total gas transport show a tendency of “shifting from one another”.When the proportion of surface diffusion is larger,with the decrease of contribution of surface diffusion,the contributions of slippage and Knudsen flow respectively to total gas transport will increase together in the first stage and then “shifting from one another” in the second stage.
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