Abstract:

Shale gas is one kind of unconventional gas resources with rich potential resources, and it has multi scale porosity structures and a variety of seepage patterns. The study of the porosity structures and seepage law of shale gas can be used as the theoretical support for the exploration and development of shale gas. In this study, porestructure tests and methane seepage experiments were conducted on shales from the Carboniferous Hurleg and Huitoutala Formations in Eastern Qaidam Basin. Based on the mass flow model of shale gas considering gas slippage and diffusion behavior, the relationship between the apparent permeability and the average pressure was introduced. The apparent permeability of shale was measured by methane seepage experiment, and the shale gas flow characteristics affected by the slippage and the diffusion were analyzed. The results show that the shale gas seepage states include slip flow, diffusion flow and Darcy flow. The slip and diffusion behavior in the whole seepage weakens with the increase of the average pressure. Under the condition of low pressure, the lower the shale permeability, the greater the diffusion flow contribution. With the increase of permeability, the main seepage mechanism becomes slip flow. When the pressure is greater than 2 MPa, Darcy flow mainly influences the seepage, and the slip and diffusion behavior are not obvious. Darcy permeability and the macropore volume have a positive correlation, and diffusion flow contributions to the apparent permeability have certain relation with pores whose diameter smaller than 50 nm. Diffusion flow is a very important gas transport process in shales.

Key words: shale gas, Eastern Qaidam Basin, apparent permeability, gas slippage, diffusion behavior