Production Characteristics Of CBM Vertical Wells And Parameters Inversion In Dafosi

The production of coalbed methane(CBM)is a process of pressure reduction and drainage. With the constantly changing of the fluid pressure and effective stress of coal reservoir, the dynamic effect of pore fissure system is embodied in the change of permeability of coal seam, and the impact of coalbed methane well productivity effect to a great extent. The containing gas and reservoir comprehensive of coal storage layer influence of CBM gas production effect, and the effect of CBM type in the late selection and placement.After collecting CBM vertical wells production data and geologic data in Dafosi mine of Binchang mining area, the characteristics and variation of coal reservoir and its parameters were studied. using the testing and production data of typical vertical wells, based on the study of coal reservoir hole cracks and the characteristics of coalbed methane enrichment in the study area, the classification and productivity analysis of CBM vertical wells drainage was carried out, and the production curve was divided into five types. Based on fluid mechanics and coal reservoir geology, the Theis formula of variable flow formula was modified, change of reservoir parameter model and coal gas production model in the process of drainage and depressurization were established. All the production data analysis were achieved by programming in VBA which is Excel own development tool, the models results were in good agreement with the actual data. The conclusion of the paper is that: in the stage of drainage and depressurization, the permeability decrease sharply at first, then slowly decreases until smooth into the two-phase flow of gas and water, the permeability gradually increase at first, and then a sharp rise to the maximum which achieving about 100 mD.Coal seam of Dafosi has a nature of high permeability. Due to the elastic self-adjusting positive and negative effects, the production of vertical wells which in the period of stable stage basically have the ability of self-adjusting. It is suggest that maintaining the surface casing pressure in a stable value for gas production staying in stable level. With the steady growth of gas production in the later period, should prevent casing pressure drop too fast, for slowly extending of pressure funnel, driving water in the distance start to flow, realizing stable yield, for leading the exploitation of CBM achieve a high efficiency sustainable pace in conclusion.