Reservoir characteristics and coalbed methane resource evaluation of deep-buried coals: A case study of the No.13–1 coal seam from the Panji Deep Area in Huainan Coalfield, Southern North China

Abstract

The Panji Deep Area in Huainan Coalfield, Southern North China is a newly discovered deep coal-bearing block with abundant coalbed methane (CBM) resources. To date, systematic geological studies regarding CBM reservoir in this region are still deficient, thereby seriously hindering the further exploitation and utilization of CBM in Huainan Coalfield. Results indicate that No.13–1 coal seam in the Panji Deep Area has good material conditions for CBM accumulation and preservation. All the coal samples from the No.13–1 coal seam are medium ash and medium−high volatile bituminous coals (Ro,max, 0.72–0.82%), which have different proportions of macerals and minerals. Pore structure and morphology analyses indicate that No.13–1 coal samples have various pore types and complex pore structures. Among them, the ink-bottle shaped pores are mainly observed with a pore size diameter of 4 nm. The gas content of No.13–1 coal samples is 16.11–21.83 m3/t (avg. 18.16 m3/t), which is strongly affected by the effective thickness of coal reservoir, reservoir pressure and temperature. Gas content is significantly positively correlated with burial depth, reservoir pressure, and temperature below 700 m, 6 MPa, and 34 °C, respectively. However, the upward trend tends to be constant after these demarcation points. Numberous micropores hold the retention capacity of coal matrix to methane molecules, thus the residual gas content increases along with the volume and surface area of micropores. By contrast, the correlations between gas content and coal thickness and roof thickness are not obvious. The total CBM resource of No.13–1 coal seam is approximately reserve of 205.36 × 108 m3. Moreover, the total CBM resources in the northern and southern parts of the Panji Deep Area are 73.83 × 108 m3 and 131.53 × 108 m3, respectively. On the basis of multi-level fuzzy model and the evaluated factors, the most favorable areas for CBM development of the No.13–1 coal seam were predicted to be around Zhutuan and south Gugou town in the Panji Deep Area.