Controlling factors of coalbed methane well productivity of multiple superposed coalbed methane systems: A case study on the Songhe mine field, Guizhou, China

The Songhe mine field located in western Guizhou Province of China is the most typical region of multiple coal seam development. However, the coalbed methane development practices of commingled drainage in recent years have been proved to be less ideal. In this work, on the basis of dividing gas system, the correlations between geological and engineering factors and gas production data were studied to determine the controlling factors of coalbed methane well productivity. The results indicated that the gas production had a positive correlation with cumulative thickness and gas content of fractured coal seams overall. However, the large cumulative thickness is generally resulted from the increase of coal seam number, and thus increases the reservoir heterogeneity and interlayer difference. The coalbed methane well productivity performance for this type region was a result of the strong interaction of cumulative thickness, burial depth, gas content, permeability, and reservoir pressure of fractured coal seam. However, the interlayer interference was the most direct factors restricting the productivity of commingle drainage by affecting the speed of dewatering and lowering of pressure and gas desorption time of gas-bearing systems. Additionally, the correlations between gas production and interlayer difference and interference were quantitatively analyzed, and the results showed that the wells with large interlayer difference and interference tend to have a poor productivity performance. As a result, the drainage method was particularly an important factor controlling the well productivity for multiple coalbed methane systems, because a proper combination of gas-bearing system and drainage and dewatering sequence are the keys to decrease the interlayer interference. Finally, the commingled drainage in this field was suggested to be conducted in steps according to the reservoir pressure, critical desorption pressure, and gas production pressure of each coalbed methane system.

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