Deep CBM extraction numerical simulation based on hydraulic-mechanical- thermal coupled model

In order to improve the accuracy of productivity prediction in deep reservoir and reduce the design error of CBM well,the authors analyzed the relationship between the characteristic parameters of coal reservoir and its seam depth. A series of numerical simulation were carried out based on an established hydraulic-mechanical-thermal coupled model (HMT) for deep CBM extraction. The law of deep extraction with different characteristic parameters and the in- fluence of seam depth on gas production were analyzed. Results show that the evolution of permeability ratio is the competitive result of the matrix shrinkage effect caused by geostress increase,temperature reduction and methane de- sorption,and the matrix swelling effect caused by reservoir pressure decrease. The reservoir temperature reduction and pressure decrease during CBM extraction will lead to an increase of permeability. The geostress plays a leading role in the variation of permeability ratio in deep coal reservoir,and the initial permeability of coal reservoir in stress-free state plays a controlling role in gas production. When seam depth is less than a critical depth,gas production will increase with seam depth. While exceeding the critical depth,gas production will decrease with seam depth. Hence,the effective measures for creating permeability is the key to achieve high yield for CBM extraction in the coal reservoirs buried deeper than 1 000 m.