High production indexes and the key factors in coalbed methane production: A case in the Hancheng block, southeastern Ordos Basin, China

Abstract

Abstract An understanding of gas well productivity in coal reservoirs is vital for the recovery of coalbed methane (CBM). The Hancheng Block has become one of the most productive CBM areas in China. Relatively few studies have investigated the productivity and key controls of CBM wells in the coal reservoir. In this work, with an analysis of the production characteristics of the CBM wells that have been producing for more than 2 yr in the Hancheng Block of the southeastern Ordos Basin, the correlations between various factors and numerous gas production data were analyzed by correlation scatter diagrams. The effect of six factors (the burial depth, the thickness of the coal seam, the ratio of the critical desorption pressure to the initial reservoir pressure, the gas content, the permeability and the effect of fracturing) on gas production was evaluated quantitatively based on the grey system theory, which is suitable for solving the complex interrelationships between multiple factors and variables. The results indicate that the Hancheng Block is in the early stage of development. The factors affecting gas production of the CBM wells are, in decreasing order, the ratio of the critical desorption pressure to the initial reservoir pressure, the volume of the fracturing fluid per meter, the gas content, the volume of the fracturing sand per meter, the thickness, the permeability and the depth. Among these factors, the ratio of the critical desorption pressure to the initial reservoir pressure determines frequently the degree of difficulty of the water drainage and decompression; the gas production potential (the thickness×the gas content) and the gas production deliverability (the gas production potential×the permeability) could characterize the comprehensive effect of the thickness of the coal seam, the gas content and the permeability on gas well productivity. Higher ratio of the critical desorption pressure to the initial reservoir pressure, higher gas production potential and deliverability are more favorable for gas productivity. Hydraulic fracturing has always been an indispensable part of the development of CBM fields. In combination with gas well production performance, the parameter indexes of high production CBM wells are presented as follows: the buried depth range should be 400–700 m; the thickness of the coal seam should not be lower than 5 m; the ratio of the critical desorption pressure to the initial reservoir pressure should be greater than 0.3; the gas content should be greater than 6 m 3 /t; the volume of the fracturing fluid per meter should not be less than 40 m 3 /m; the volume of fracturing sand per meter should not be less than 3 m 3 /m; and 1% KCl active water fracturing fluid should be used as the fracturing fluid.