Determination and application of a reasonable drainage rate of coalbed methane in vertical wells in the single-phase flow stage

Abstract

The determination of a reasonable drainage rate of coalbed methane (CBM) in vertical wells in the single-phase flow stage can provide maximise the transmission of water pressure over distance. Based on the principle of effective stress and Darcy’s law, a mathematical model for dynamic changes of the permeability in the single-phase flow stage was established; on this basis, the relationship between permeability and threshold pressure gradient was experimentally attained; according to the linkage of changes of the transmission distance of water pressure, permeability, and pressure drop in the wellbore in the drainage process, a mathematical model for a reasonable reduction rate of the working fluid level in the single-phase flow stage taking the change of the permeability into account was established. The accuracy of the mathematical model was verified according to practical drainage data from CBM wells in Daning Block in Qinshui Basin, Shanxi Province, China. The results show that the rate of pressure drop decreases in a negative exponential manner with the increase of the drainage time. Different rates of pressure drop were required in coal reservoirs with different permeabilities; when keeping other conditions constant, the larger the permeability of coal reservoirs, the lower the threshold pressure gradient and the lower the rate of daily pressure drop. The research results provide a theoretical basis and reference for the reasonable drainage system in the single-phase flow stage.