Enhancement of Coalbed Methane via Nitrogen Injection in a Coal Mining Area: A Laboratory and Field Study

Gas flooding is a key technique for increasing coalbed methane (CBM) production of declining wells in coal mining areas. Nitrogen is clean, pollution-free, low-cost, and relatively safe compared with carbon dioxide. In this study, nitrogen drive experiments were conducted under different conditions using a coal-gas-liquid relative permeability device and gas chromatography. The effects of the temperature, intermittent time, nitrogen purity, and displacement pressure on the output characteristics were investigated, and then, field tests were conducted. The results show that with increasing displacement time, the produced gas volume and flow rate are dominated by methane at the beginning and then quickly transform into nitrogen. The volumes of both methane and nitrogen are positively correlated with the displacement time via a power function. Additionally, increasing the nitrogen temperature, intermittent time, nitrogen purity, and injection pressure can effectively increase methane production. In the field test conducted in the Fanzhuang block in the Qinshui Basin, China, the methane production of 10 monitoring wells was 11,700 m(3)/day, which was 2,700 m(3)/day higher than that before nitrogen injection. This paper provides an optimized scheme for the stimulation of CBM via nitrogen displacement in coal mining areas, which has a good application prospect.