An experimental study of the optimal drainage working system in the single-phase gas flowing stage of coalbed methane wells

Accurately determining the optimal drainage system in the single-phase gas flow stage of coalbed methane (CBM) wells is important when trying to guarantee maximization of gas production. By using a self-made physical simulation device for stress-permeability-cumulative gas production, tests on continuously exponential and wave trough-type cumulative gas production under four drainage systems and different permeabilities were performed. The four working systems involve a stepped wave trough, stepped wave crest, stepped decline, and a stepped increase. On this basis, the optimal drainage system in the single-phase gas flow stage at different permeabilities of coal seams was determined and the associated permeability-drainage rate-drainage time chart was established. The results show that there is high cumulative gas production under the drainage system with the drainage rate that first decreases, then increases. The initial pressure difference, acceleration/deceleration coefficient of pressure difference and rate of change of effective stress jointly influence the cumulative gas production. For low-permeability coal samples, decreasing the acceleration/deceleration coefficient of pressure difference weakens the influence of the negative effect of effective stress and thereby leads to high cumulative gas production. For high-permeability coal samples, increasing the initial pressure difference and acceleration/deceleration coefficient of pressure difference shortens the time of action of the negative effect of effective stress, thereby resulting in high cumulative gas production. The research results provide a method of optimizing the drainage system in the single-phase gas-flow stage in CBM wells at different permeabilities.