页岩气多尺度复杂流动机理与模型研究

Study on the multi-scale nonlinear flow mechanism and model of shale gas

页岩气储层纳微米孔隙、裂缝结构复杂,存在多尺度流动,气体的流动规律不同于常规气藏.本文对多孔介质内气体流动进行了研究,利用努森数划分不同尺度下气体流态,阐明了不同区域的流动机理和流动特征;综合考虑达西渗流、滑移扩散效应、井筒附近高速非达西效应等多重非线性效应,建立了页岩气储层多尺度统一流动模型.引入页岩气储层基质-压裂缝耦合两区模型,建立了页岩气储层压裂井定压条件下的两区压力分布和产能预测方程,并结合生产实例进行了参数敏感性分析.结果表明:随着滑移扩散系数、分形系数、压裂半径的增大,页岩气井产能增加,且增加幅度减小;考虑高速非达西效应较不考虑高速非达西效应时,页岩储层产能偏低,且高速非达西效应的影响小于滑移扩散对产能的影响.该模型为体积压裂页岩气产能预测及开发指标优化提供了理论依据.

Shale gas reservoirs have extremely complex structures with nano-pores and fractures. And the multiscale flowing states are also existed in it. The gas flowing characteristic is apparently different with normal gas reservoirs. In this paper, gas flow in porous media is was researched studied under different gas flowing state by the Knudsen number and the flowing mechanism, and the characteristics are explained. Then we build a unique general flowing shale gas model with multi-scale, by comprehensively considering Darcy flow, slip flow, diffusive flow and non-Darcy flow with high speed at the bottom hole, which are all the non-linear effects. A new shale gas model with combining original shale and fracturing areas is introduced to calculate the gas productivity by considering settled pressure conditions. The simulative results are calculated by the gas productivity functions and analyzed the sensitivity of parameters with the real productivity. The results show that shale gas productivity would increase by the increasing on the diffusion coefficient, parting coefficient and the radius of fractures, but the increasing rate would have the opposite effect. The productivity with considering non-Darcy effect was a little lower than the result without considering this effect. And the high speed non-Darcy had less effect on gas productivity when comparing with the slippage-diffusion. This model has offered a series of theories for forecasting and optimizing the productivity of shale gas with SRV.