International Journal of Coal Geology, Vol.202, 95-108, 2019
Current status and geological conditions for the applicability of CBM drilling technologies in China: A review
Coalbed methane (CBM) drilling technology is critical in achieving efficient CBM development. The geological conditions for CBM development in China are complex, including special terrain, high-rank coal areas, deep coal seams, multiple superposed coal seams and coal measure gas symbiosis areas, steeply inclined coal seam areas, and tectonic coal seam areas. Therefore, it is critical to apply the optimum drilling technology in each different area. This paper first reviews the current CBM drilling technologies and gas production behaviors in China. It is found that although vertical wells and cluster well groups are common well types for CBM development in China, U-, V- and L- shaped horizontal wells and multilateral horizontal wells have been favored for CBM development in recent years. The paper then discusses the applicability of different well types to geological conditions and proposes a modification method for well design in various geologic conditions. This method uses the coal structure, R-o, in situ stress, and ratio of critical desorption pressure to the reservoir pressure as the main inputs.
Keywords:Drilling technology;Geological conditions;Deep coal seam;High-rank coal;Multiple coal seams;Well type optimization