Physical experiments of CBM coproduction: A case study in Laochang district, Yunnan province, China

Coalbed methane (CBM) coproduction in areas with multiple and thin coal seams has been an effective way to enhance the production of CBM wells and make the most of the CBM resources. However, the effects of CBM coproduction were not satisfying in early stages, and the gas production from multiple coal seams was even lower than that from a single coal seam. Previous research has employed various methods, including production data analysis and numerical simulation, to ascertain the controlling factors of CBM production in multilayer areas. But it is still unknown how they influence the gas production. To tackle these problems, a set of physical experiments simulating the process of CBM coproduction in multilayer areas is conducted. In these experiments, the permeability ratio, the reservoir pressure difference and the distance between two seams are set as independent variables, while the forward gas flow rate, the reversed gas flow rate in sublayers and the total gas flow rate in the tube well are the dependent variables, which are used to define sublayer interlayer interference index and analyze the mechanism of interlayer interference. The results are as follows. 1) The relationship between the production contribution of single layer and permeability ratio is exponential; that is, the production contribution of single layer is dominated by the permeability ratio. If the permeability ratio of the two seams is greater than 5 times, then an imbalance of production contribution between layers may occur. 2) The performance of the upper coal seam is dominated by the reservoir pressure difference; that is, the sublayer interlayer interference index of the upper coal seam increases with the reservoir pressure difference. 3) The performance of the lower coal seam is dominated by the distance between coal seams; that is, the sublayer interlayer interference index of the lower coal seam increases with the distance between coal seams. The sublayer interlayer interference index proposed in this paper provides a new quantitative standard for the magnitude of interlayer interference. The effects of permeability ratio, reservoir pressure difference and distance between coal seams on the gas production can be used as a reference for the optimization of reservoirs combination and drainage systems for the purpose of eliminating the interlayer interference and improving gas production.