Geological Controls Of Coalbed Methane AccumulationEvolution In The Weibei Coalbed Methane Pilot Field

The basic parameters(e.g. gas content, saturation and permeability) are highly heterogeneous in CBM reservoirs in Weibei field. To better understand this phenomenon, the CBM accumulation evolution process was simulated. The heterogeneous distribution characteristics of reservoir physical properties were proposed, and the geological control mechanism of the heterogeneity in these parameters is also discussed. Results achieved as follows.(1) The coal reservoir evolution process in Weibei CBM field was investigated. The deposition–burial, geothermal, and organic maturation evolution histories of the Weibei CBM field were investigated. The tectonic evolution characteristics of structural section and the whole research area were analyzed based on numerical simulation studies(e.g. numerical simulation on tectonic deposition–burial, geothermal and organic maturation evolutions). The geothermal evolution history which is mainly affected by magmatic heat was also discussed. The hydrocarbon evolution history mainly contain two periods of gas generation and expulsion process.(2) The characteristic of energy field(e.g. groundwater dynamic field, in situ stress field and reservoir thermal field) in coal reservoir was investigated. Based on stable isotope analyses and quality tests results for the groundwater dynamic field distribution feature was simulated by using software Aquachem. The research area is subdivided into five hydraulic structural domains. The relationships between the fluid flow pathways, CBM accumulation and three thrust faults were analyzed. A calculation model of in situ stress based on geophysical logging data was established to reveal the in situ stress field distribution characteristics in this area. The reservoir thermal field distribution characteristic was studied, and the variable differences between each tectonic domain were discussed.(3) The CBM accumulation evolution mechanism was revealed in this area. Based on numerical simulation and experimental research, the generation, retention, migration, accumulation, and dissipation process of gas during the six evolution stages were portrayed. The effects of critical tectonic events on current gas content, gas saturation and reservoir physical parameters were dissected. The first critical tectonic event mainly controls the primary CBM generation. The second critical tectonic event dominates the secondary gas generation and dissipation. The third and fourth critical tectonic events are the main cause of variations in the gas distribution in this area.(4) Three gas accumulation models were proposed in this area. The groundwater dynamic field characteristics was combined with groundwater and methane stable isotope analyses results to dissect the control mechanism of hydrogeology characteristic in variable distribution of gas content and gas saturation property. The proposed three gas accumulation models were: first, thermogenic-migration-fault sealing; second, hydrodynamic pressure and lithology trapping of deep structures; third, hydrodynamic flushing for gas losing.(5) A calculation model using well logging data was proposed for evaluation of reservoir permeability of coal seam. The deep lateral resistivity logging and microsphere focusing logging data was used to investigate the distribution characteristic of reservoir permeability. Then, the calculated results was utilized to discuss the control mechanism of geological setting in reservoir permeability heterogeneity combing with in situ stress field and tectonic evolution characteristics.(6) A comprehensive evaluation system was used to evaluate the CBM resources in this research. A forecasting method of favorable prospective targets for CBM production was also proposed.