Abstract:

The bilayer or multilayer co-drainage is an important approach to improve the single well production. In order to improve the drainage efficiency of bilayer drainage wells in Zhengzhuang Block of southern Qinshui Basin, the drainage data and co-drainage trial data from the southwest of Zhengzhuang Block are analyzed and the method is put forward to judge the gas production capacity of No.15 coal seam in real time by the change trend of coal production after the hydrodynamic level drops below No.15 coal seam. Two methods predicting the gas production capacity of each layer by key parameters of both geology and engineering or by early desorption drainage parameters are proposed. A new drainage method is put forth and the trial is carried out in the southeast of Qinshui Basin, that is the “variable rate drainage, gas released at certain casing pressure, high-production-rate and certain stable production kept at high pressure”. The results show that if the production continues to rise after the fluid level drops below the No.15 coal seam, the gas production capacity of No.15 coal seam is high. On the contrary, it indicates that gas production of No.15 coal seam is low. When the resistivity of No.15 coal seam is less than 1 000 Ω·m or the construction pressure is low, the gas production capacity is relatively low. The data of bottom-hole flowing pressure and casing pressure of No.15 coal seam after casing pressure come out in coalbed methane well can be used effectively to predict the gas production capacity of each layer, with high prediction accuracy. The application of the new drainage method, proposed by this paper, results in 22.2 % shorter of the production-increasing cycle decreasing from 180 d to 140 d and more than 20 % increase of the average single-layer production from 1 000 m³ to 1 200 m³, compared with the old drainage method. The great difference in the development performance among the 20 wells is affected mainly by the gas production capacity of each well. The wells with higher gas production capacity have higher stable gas production and longer stable gas production time. The productivity prediction method and the co-drainage method of bilayer vertical coalbed methane wells have great reference significance to the productivity releasing of co-drainage bilayer vertical coalbed methane wells.

Key words: bilayer drainage, vertical well, drainage method, gas-production capacity of each layer, southwest of Zhengzhuang Block, Qinshui Basin