Study On The Feedback Of The Output Of CBM Vertical Wells To The Gas Content In Dafosi

The production process of coalbed methane(CBM)well is the process of drainage,pressure reduction and gas recovery,and the corresponding control mechanism is desorption,diffusion and seepage.The gas production of CBM wells were controlled by many factors,such as geological factors and engineering factors.However,the gas content of coal reservoir restricts the gas production of CBM wells.In order to find the chang rule of the gas content in the coal reservoir during the production process,a reasonable drainage system should be formulated to provide reference and basis for the optimization of the drainage system for the CBM vertical wells.It is of great significance to guide the development of CBM in China.In this paper,the characteristics of the pores and fissures of the coal reservoir were studied through collecting the geological data of the Dafosi field and the actual production data of CBM vertical wells.It was found that the fractured coal reservoirs were more developed,and the content of micropores and pores was relatively high,which was favorable for the adsorption and production of CBM.Based on the mechanism of CBM production,the characteristics of water production,gas production and bottom hole flowing pressure of CBM vertical wells were analyzed to divide reasonably the drainage stage.And the CBM drainage stage was divided into six stages according to the variation of gas production.Based on groundwater dynamics,coalbed gas geology and the Theis equation,the coal reservoir water conduction coefficient calculation model and formula of the radius of influence of straight wells were established.The single well drainage area was determine by the quasi steady stage characteristics.Then the relationship between the gas content of the coal reservoir and the time of drainage in the drainage area of coal seam gas well was calculated.The results showed that the gas content was linearly decaying in the whole production stage.At the stage of stable production,the attenuation rate was the largest,and the attenuation rate was the slowest in the stage of rising gas production,which was in accordance with the actua gas production.The daily water production,cumulative water production,daily production capacity and cumulative gas production data of three vertical wells in Dafosi were studied by mathematical method.It was found that there was a linear relationship between the daily/cumulative water production and cumulative gas/gas production.Therefore,the quantity of gas production could be controlled by controlling the water production during the late production process of coalbed gas wells.At the same time,the bottom hole flow pressure was ensure slowly and steadily to depress to reduce the matrix shrinkage of the coal reservoir.It made the coalbed methane well get high yield and stable production,so as to achieve the purpose of efficient and progressive development of CBM.