页岩气和致密砂岩气藏微裂缝气体传输特性

页岩气和致密砂岩气藏发育微裂缝,其开度多在纳米级和微米级尺度且变化大,因此微裂缝气体传输机理异常复杂.本文基于滑脱流动和努森扩散模型,分别以分子之间碰撞频率和分子与壁面碰撞频率占总碰撞频率的比值作为滑脱流动和努森扩散的权重系数,耦合这两种传输机理,建立了微裂缝气体传输模型. 该模型考虑微裂缝形状和尺度对气体传输的影响. 模型可靠性用分子模拟数据验证.结果表明:(1)模型能够合理描述微裂缝中所有气体传输机理,包括连续流动,滑脱流动和过渡流动;(2)模型能够描述不同开发阶段,微裂缝中各气体传输机理对传输贡献的逐渐变化过程;(3)微裂缝形状和尺度影响气体传输,相同开度且宽度越大的微裂缝,气体传输能力越强,且在高压和微裂缝大开度的情况下表现更明显. 

GAS TRANSPORT BEHAVIOR THROUGH MICRO FRACTURES OF SHALE AND TIGHT GAS RESERVOIRS

Gas transport mechanism is considerably complex in micro fractures of shale and tight gas reservoirs, which is caused by a fracture with nano-micro scales aperture. A unified model for gas transport in micro fractures of shale and tight gas reservoirs is proposed by coupling slip flow and Knudsen di usion, where, the ratios of the intermolecular collision frequency and the molecule-wall collision frequency to the total collision frequency are the weight coe cients of slip flow and Knudsen di usion, respectively. The present model considers the e ects of micro fracture shape and dimension on gas transport, and is validated with molecular simulation data published. The results show that: (1) the present model can describe all known gas transport mechanisms in micro fractures, including continuous flow, slip flow and transitional di usion; (2) the present model can reasonably describe the process of gradual mass transform for di erent gas transport mechanisms in micro fractures during di erent development stages of gas reservoirs; and (3) The micro fracture shape and dimension significantly impact gas transport, at the same micro fracture aperture, the greater the micro fracture width, the stronger the gas transport capacity, which is more obvious in the case with higher pressure and greater micro fracture aperture.