Effects of Variogram Characteristics of Coal Permeability on CBM Production: A Case Study in Southeast Qinshui Basin, China

Abstract

The coalbed methane (CBM) resources of China are located mainly in 9 basins, Ordos, Qinshui, Jungar, Diandongqianxi, Erlian, Tuha, Tarim, Tianshan and Hailaer. Qinshui Basin, one of the richest CBM basins in China, has boosted its annual CBM production to 3×109 m3 (106 Bcf). The coal seams in Qinshui Basin are significant with high gas content but strong heterogeneous permeabilities ranging from 0.1 to 10 mD. This paper investigates the effects of spatial distribution characteristics of coal permeability on CBM production. The study area is the South Shizhuang CBM district, Southeast of Qinshui Basin. The distributions of porosity, ash content, coal density and gas content of the coal seam are generated using sequential Gaussian simulation (SGS) with only one realisation because this paper only justifies the effects of coal permeability on CBM production. The permeabilities of 17 wells are determined by matching these wells' water and gas production with bottom-hole pressure as constraint. Then, the distributions of coal permeabilities are generated using SGS with a commercial simulator. The history matched permeabilities range from 1.5 to 12 mD with average of 2.9 mD of the 17 wells. Eight variogram models are used to build the distributions of permeability. The cumulative gas productions of two different well-spacing cases, 300 m and 2000 m, are compared. There are 20 realisations of permeability for each of the eight models. The results show that historical matching can be used to obtain the porosity multipliers and the permeabilities in wells. The major direction of variogram has less effect on the uncertainty of field CBM production than variogram range. The effects of variogram range on the uncertainty of CBM production are positive for the case with short well spacing and vice versa for the case with long well spacing.