Deep Coalbed Methane Development in Eastern Ordos Basin, China

Exploitation of unconventional gas resources becomes conventional as with new technology and applications being used, and the marginal hydrocarbons attract strong interests from geologists and engineers, one of them is the deep coalbed methane (CBM). The deep CBM resource generally means the CBM buried deeper than 1500 m (Kuuskraa et al., 1993; Tonnsen and Miskimins, 2010). The CBM resources in China is assumed to as much as 36.8×10 m in coals buried shallow than 2000 m in 2009, with 32.4% (11.9×10 m) in coals of depth from 1500 – 2000 m, and 28.8% (10.6×10 m) in coals between 1000 – 1500 m. Even 1500 m is generally taken as the separation of shallow and deep, but it cannot be neglected that most of the CBM target reservoirs around world is shallower than 1000 m and even upper on 800 m. The CBM development in eastern Ordos basin also shows different turning points about the variation of ground stress, gas content, reservoir physical and mechanical properties as with different coal burial depth . * Industry has been reluctant to pursue these "deep coals" for generally four reasons: (1) limited downhole data; (2) higher drilling costs; (3) assumed unfavorable desorption characteristics; and, (4) concerns about low permeability (Kuuskraa, 1993). The CBM development is mostly affected by the following two parametes: (1) gas content, which determines the amount of resources; (2) permeability, which closely related to the gas production potential. The gas content of coal increases with depth firstly for the reason of increasing reservoir pressure, and then decreases as going into deeper strata for the disadvantage of increasing ground temperatures affected on gas desorption. The permeability of coal is strongly affected by the in-situ stress which directly acts on the underground reservoir stress condition. Injection/falloff methods have been widely used for the well test and data acquisition, and the detailed procedure for the discussion of in-situ stresses has been reported by Li et al. (2014) with showing the coal reservoir condition variation as with depth in Liulin CBM block, which located in the middle of eastern Ordos basin. Further, the strata temperature can also be detected during the well test, and gas/water production rates can be obtained from CBM well production records. Based on the in-situ stresses, strata temperatures and gas/water production rates recovered from 67 CBM wells in eastern Ordos basin, the variation characteristic of coal reservoir properties are discussed in this article, with a reflection to the critical point of deep CBM.