基于REV尺度格子Boltzmann方法的页岩气藏渗流规律

页岩储层一般天然微裂缝发育，基质孔隙结构复杂，使得页岩气渗流过程呈现出多尺度多场耦合的特征。为了研究页岩气藏复杂的渗流规律，重构了天然微裂缝发育的页岩储层多孔介质模型，并围绕页岩气多重运移机制对广义格子Boltzmann模型进行了修正，建立了适用于模拟页岩气渗流特征的表征单元体（REV）尺度格子Boltzmann模型（LB模型），并基于天然微裂缝物性特征以及气体滑脱、吸附/解吸、表面扩散效应等渗流特征对该模型进行了敏感性参数分析。结果表明：当页岩储层天然微裂缝较发育时，微裂缝为气体在基质中流动的主要通道；其中裂缝密度是影响储层表观渗透率的主要参数，裂缝密度增大3～4倍，储层表观渗透率可增大10倍以上，而裂缝长度以及裂缝开度的影响程度均次之；努森数（Kn）是影响页岩气渗流的主要参数，随着Kn增大，克氏效应愈显著，特别当Kn > 0.1时，多孔介质表观渗透率增幅显著增大；页岩储层多孔介质表观渗透率会随着吸附气量的增大而减小，特别是当储层压力较低时，该现象更为显著；气体表面扩散效应对页岩气渗流过程的影响程度大，同等条件下考虑气体表面扩散效应的储层表观渗透率较忽略该效应可提升2～5倍，但提升作用受制于储层吸附气量的多少。该研究成果为页岩气微观渗流理论研究提供了新思路，为页岩气藏高效勘探开发提供了技术支撑。

Gas seepage flow law of shale gas reservoirs based on REV-scale lattice Boltzmann method

Shale gas reservoirs are characterized by multi-scale and multi-field transport behaviors owing to the various natural micro-fractures and complex matrix pore structures. In order to study the complex seepage law of shale gas reservoir,a micro-fractured porous medium was reconstructed,the generalized lattice Boltzmann model was modified,and representative elementary volume(REV)scale lattice Boltzmann model(LB model)suitable for simulating the seepage characteristics of shale gas was established. Based on the physical properties of natural micro-fractures and seepage flow characteristics of gas slippage,gas adsorption-desorption and surface diffusion, the sensitivity parameters of the model were analyzed. The results show that the micro-fractures become the main channel for gas flow in the matrix when the natural micro-fractures of shale reservoir are relatively developed, the fracture density is the main parameter affecting the apparent permeability of the reservoir. When the fracture density was increased by 3-4 times,the apparent permeability of the reservoir can be increased by more than 10 times,and the fracture aperture and fracture length have less effect. Knudsen number(Kn) is a primary parameter which has great influence on the shale gas flow behaviors. The Klinkenberg effect become more remarkable due to the increasing Knudsen number,especially when Kn > 0.1,the increase rate of the apparent permeability becomes more significant. Moreover,the increasing adsorbed gas concentration is accounting for the decreasing apparent permeability,particularly in case of the lower pressure. The surface diffusion has great impacts on the gas flow behaviors,the apparent permeability could be 2 to 5 times larger than its counterparts in case the effect of surface diffusion has been ignored. However,the strength of this effect is subject to adsorbed gas concentration. This study could provide some instructive insights into the theoretical research of the shale gas seepage mechanism and some technological support for the exploration and exploitation of shale gas reservoirs.