关键词: 沁水盆地, 煤层气, 渗流特征, 扩散, 临界压力

Abstract:

Coalbed methane(CBM) exists in pores of coal matrix mainly in adsorption state under an artesian condition, it can not be exploited until draining water and dropping pressure. Compared with low-permeability natural gas reservoirs, coalbed methane reservoirs usually have a higher level of the initial water saturation. During gas exploitation, two phases of gas and water always co-exist even at the third stage of exploitation, in which the aqueous saturation still maintains a higher level though the water production rate is very low. The presence of formation water usually makes porous channels narrow, resulting in the capillary pressure of low-permeability water-bearing gas reservoirs universally high, which can significantly affect gas permeability. The present paper investigated seepage characteristics of some coal rocks sampled from the Qinshui Basin and categorized curves of the water drainage and gas exploitation into three representative types, namely upward-protruding, downward-concaving and composite types. It was found that the higher the aqueous saturation, the greater the gradient of the start-up pressure, which is of great significance not only in making a proper procedure of the water drainage and gas exploitation but also in improving effects of water drainage and gas production.

Key words: Qinshui Basin, coalbed methane, seepage characteristics, diffusion, critical pressure