Production characteristics and drainage optimization of coalbed methane wells: A case study from low-permeability anthracite hosted reservoirs in southern Qinshui Basin, China

Monitoring the production from 94 coalbed methane (CBM) wells in the southern part of the Qinshui Basin of China this study demonstrates production characteristics of CBM wells, and how the incorrect production system, including improper water drainage rates and wellhead pressures, can lead to diminished gas production. Using data from these wells our results suggest that high-production rate wells, medium-production rate wells, low-production rate wells, and drainage wells, are controlled by drainage conditions in addition to the well location and structural geology. The analysis of drainage parameters shows that the maximum wellhead pressure should be maintained around 1.5 MPa before stable production, and between 0.10 MPa and 0.30 MPa after stable production. The most efficient average water production rate is approximately 4 m3/day before gas production and should be maintained near 1 m3/day during gas production. Initial daily average water production rate should be maintained around 1.5 m3/day. Maximum water production rate should be regulated between 4 and 17 m3/day. The rate of water level reduction should be within 4 m/d and drainage time should be maintained for 50-200 days prior to gas production. Implementation of these optimal drainage parameters will promote and sustain peak gas production for several years. In addition, reservoirs with adequate permeability, > 0.1 mD, are ideal for electric submersible pump systems while sucker-rod pumps are better suited for reservoirs with poor permeability. The combination of these operating conditions and the appropriate pumps optimizes the extraction efficiency and recovery of coalbed methane from the anthracitic coals in the Qinshui Basin.