Spatial distribution and variation of coalbed methane reservoir characteristics and its significance for CBM development in Western Guizhou

Western Guizhou has the most abundant coalbed methane resource in South China with complicated CBM occurrence conditions and development disadvantage. In this study,based on the test data of CBM wells,the scheme for dividing the coal reservoir permeability in the research area is determined. The spatial distribution and variation char- acteristics of in-situ stress,together with permeability,pressure system and gas reservoir types under its control are ana- lyzed. The reservoir control effect of in-situ stress and its significance for CBM development is discussed. Regionally, Liupanshui coalfield has significantly higher in-situ stress than Zhina coalfield,which leads to a medium-low and low permeability and abnormal over-pressure reservoir,and is characterized by “in-situ stress sealing”. Zhina coalfield de- velops a medium and medium-low permeability and under-pressure coal reservoir,and is characterized by “ pressure- driven”,the pressure drawdown can spread freely. Vertically,the critical conversion depth of in-situ stress is defined as 300 m and 600 m,which are also the critical depth controlling CBM accumulation characteristics. In different depth ranges of above 300 m,300-600 m and below 600 m,it is represented by horizontal stress dominant,horizontal stress and vertical stress jointly dominant,vertical stress dominant,respectively. Overpressure reservoirs develop within the depths of 300-600 m. Unified pressure system tends to develop above 600 m whereas multi-layer superposed pressure systems tend to develop below 600 m,and the reservoir energy type is transformed from “pressure-driven” to “in-situ stress sealing”. Based on the stepped change characteristics of reservoir parameters in vertical and multi-level fuzzy synthetic evaluation,the depths of 300 -600 m is speculated as the golden zones for CBM exploration and develop- ment,the producing layers of high production well in Zhina coalfield are all within this depth range.