Experimental study of the effective stress coefficient for coal permeability with different water saturations

The effect of effective stress on coal reservoir permeability is inevitable during coalbed methane (CBM) production. Studying the effective stress coefficient for permeability (ESCK) is crucial because it determines the effective stress and stress sensitivity of a coal reservoir. Considering the actual drainage process of a CBM well, a series of stress sensitivity experiments with different water saturations were designed and implemented. The experimental results show that the gas measured permeability of the coal sample decreased with increasing water saturation. The change in permeability with different water saturations had a significant power relationship with increasing net pressure. Using the response surface method (RSM), ESCK values with different water saturations were obtained. The ESCK value, regardless of the stress conditions, is a constant at a certain water saturation, and it shows an increasing trend with increasing water saturation. The ESCK values with different water saturations indicate that the effective stress decreases with increasing water saturation. The stress sensitivity and dynamic change in coal reservoir permeability were also analyzed using ESCK values with different water saturations. The stress sensitivity coefficient decreased with increased water saturation, and the reservoir permeability decreased more obviously with an ESCK value of 0.7675 than that with a value of 1.0. During CBM well production, water saturation decreases, and the stress sensitivity of coal reservoir permeability increases.