Vertical heterogeneity of permeability and gas content of ultra-high-thickness coalbed methane reservoirs in the southern margin of the Junggar Basin and its influence on gas production

Abstract

Ultra-high-thickness (>15 m) is one of the unique features of coalbed methane (CBM) reservoirs in Junggar basin, Xinjiang, China. However, there are few studies on the vertical permeability and gas content heterogeneity of ultra-high-thickness reservoirs. We use the Fukang west block on the southern margin of the Junggar Basin as a characteristic-example. Logging data describe the vertical distributions of the permeability and gas content of ultra-high-thickness CBM reservoirs. Flow modeling is used to explore the influence of those vertical heterogeneity on gas production. The results show that there is clear heterogeneity of these two parameters across the reservoir - from top to base - with different functional variabilities at shallow, intermediate and deep burial depths. At shallow depth (800–1100 m) the heterogeneity is controlled mainly by the increasing geo-stress and coalification. Permeability gradually decrease from the reservoir top to the base due to depth-increasing stress while gas content gradually increases due to permeability and depth-increasing degree of coalification. At intermediate depths (1100–1500 m) these two parameters show no particular trend. At the deep depths (1500–2000m) the reverse trend is observed as permeability increase from the reservoir top to the base while gas content gradually decreases. This heterogeneity is of great significance in the evaluation of the reserve, the recoverable resource and in the optimization of gas production. Neglecting the vertical heterogeneity of gas content across the seam results in a mis-estimation of ~4.4% of the recoverable resource with a 40% chance of exceeding this mis-estimation if only a single sample within the seam is used. The vertical heterogeneity exerts a significant influence on gas production, and the influence will be greater when the reservoirs with larger gas content. Production optimization favors perforation of separated rather than adjacent stages.