Postfracturing permeability prediction for CBM well with the analysis of fracturing pressure decline

Hydraulic fracturing is a widely used reservoir stimulation technique to exploit CBM in China. Compared with the original permeability of coal reservoir, the postfracturing permeability is more critical for the productivity evaluation of CBM wells. Based on hydraulic fracturing, microseismics monitoring, and well test data of the CBM wells in Zhengzhuang block of Qinshui Basin, this paper proposes a simple method to obtain the fitting pressure by the classic fracturing pressure decline analysis theory under the condition of corrected dynamic overall leakoff coefficient. In addition, the postfracturing permeability of coal reservoir is predicted by the injection well testing theory. The morphology of hydraulic fractures in the study area is mainly determined by in-situ stress, and most fractures are vertical ones propagating along the direction of the maximum principal stress. The fracturing curves of the study area can be classified into stable curves, stably fluctuating curves, descending curves, ascending curves, and fluctuating curves, where the stable and descending fracturing curves represent better fracturing effect. The length of hydraulic fractures is much greater than the height. The PKN model has been selected as the hydraulic fracture propagation model to analyze the fracturing pressure decline. The slope value of liner segment of G(delta,0) curve is proposed as the criterion for the fitting pressure calculation. The postfracturing permeabilities of coal reservoir range from 1.55 to 8.83 x 10(-3) mu m(2), all of which are remarkably higher than that of the original reservoir.