Non-uniform Distributions of Gas Pressure and Coal Permeability in Coalbed Methane Reservoirs Induced by the Loess Plateau Geomorphology: A Case Study in Ordos Basin, China

Coal mine methane (CMM) accumulation and permeability distribution characterization are the bases for a successful CMM drainage design. Before extending degassing activities, the prediction of the gas pressure and permeability distributions of the pre-drainage area is recommended. The effects of CMM geology on gas occurrence and coal permeability have drawn much attention and contributed to numerous research findings. However, the loco-regional distribution characteristics of the gas occurrence and coal permeability induced by landform characteristics have barely been investigated. This paper presents a quantitative case study on the non-uniform distributions of the gas pressure and coal permeability induced by the Loess Plateau geomorphology. Based on the collection and analysis of a large amount of data, the non-uniform distribution of the burial depth was first obtained. The relationship between the burial depth and gas pressure was analyzed by using the safety line method. A series of efforts were made to perform experimental tests for determining the competing influences of the effective stresses and sorption-induced strain on the coal permeability, and the relationship between the burial depth and coal permeability was analyzed by using the modified PM model. Finally, a quantitative evaluation of the non-uniform distributions of the gas pressure and coal permeability induced by the Loess Plateau geomorphology was achieved. The results showed that the statistical burial depth, gas pressure, and permeability of the No. 3 coal seam were irregular with large loco-regional changes, and the range of the burial depth was from 349 to 479 m. Further, the predicted pressure was from 0.81 to 1.66 MPa, and the predicted permeability was from 0.0155 to 0.01968 mD. When using both the gas pressure and coal permeability as the evaluation indices of the difficulty of gas drainage, the study area could be divided into six parts and classified into two groups. The division of a coal seam influenced by the Loess Plateau geomorphology will provide essential information formulation and economical plan to degas it.