Stress relief coalbed methane drainage by surface vertical wells in China

This paper summarizes the status of engineering practice, technology and research related to stress relief coalbed methane (CBM) drainage using surface wells in China during the past 10 years. Comments are provided on the theory and technical progress of this method. In high gas mining areas, such as the Huainan, Huaibei and Tiefa mining areas, characterized by heavily sheared coals with relatively low permeability, stress relief CBM surface well drainage has been successfully implemented and has broad acceptance as a CBM exploitation technology. The fundamental theories underpinning stress relief CBM surface well drainage were established in China, and include elements relating to: (1) rock layer deformation theory, vertical zoning and horizontal partitioning, and the change in the stress condition in mining stopes; (2) a theory regarding an Abscission Circle in the development of mining horizontal abscission fracture and vertical broken fracture in overlaying rocks; and (3) the theory of stress relief inducing permeability increase in protected coal seams during mining; and the gas migration–accumulation theory of stress relief CBM surface well drainage. Three kinds of technical drainage modes are described: protected seam surface well drainage in areas influenced by mining represented mainly in the Huainan mining area; gob surface well drainage most prominently in the Tiefa mining areas; and the multipurpose surface well drainage used in the Huaibei mining areas. Serial well allocation in which wells are used for different purposes as extraction and mining proceeds, and well bore configurations and designs suitable for this purpose, are described. The development and utilization of CBM is significant to energy-saving, emission-reduction and to ensure safety in coal production.