Modeling and parameters analysis on a pulsating hydro-fracturing stress disturbance in a coal seam

• A numerical model which can simulate the stress response of coal rock during pulsating hydro-fracturing is proposed.
• The disturbance zone caused by pulsating hydro-fracturing is much larger than the zone caused by quasi-static fracturing.
• The pulsating hydro-fracturing effects could be improved by increasing the source frequency, but the limitations of the stress parameters result in a threshold value.
• The pulsating hydro-fracturing effects could be improved by increasing the source amplitude and mean stress, the larger the better.
• Excessively high source frequency is unnecessary during pulsating hydro-fracturing.

Pulsating hydro-fracturing (PHF) technology, based on hydraulic fracturing and pulsating injection, is extremely applicable for coal seams because of the remarkably low mechanical strength of coal rock. Consequently, PHF is currently under development, and new applications are being explored in China. Current research studies on PHF mainly focus on coal rock damage via experimental methods; however, to the best of our knowledge, none of these studies consider stress disturbances, which significantly impact the stimulation effect. This paper presents a stress-disturbance numerical model in an infinite elastic formation that uses a high-order, staggered grid finite-difference scheme and boundary conditions that combine a perfectly matched layer with a quasi-static confining pressure load. We define the effective stress disturbance zone as an area where the principal tensile stress exceeds the tensile strength of the coal rock and investigate the influences of the technical parameters (frequency, amplitude, and mean stress) on the PHF stress disturbance effect. Several unique behaviors were observed. First, the effective disturbance zone caused by PHF is much larger than the zone caused by quasi-static fracturing. Second, although the disturbance zone created by PHF can be improved by increasing the source frequency, limitations in the stress parameters likely result in a threshold frequency. Third, in contrast with frequency, the disturbance zone created by PHF is positively correlated with the amplitude and mean stress, which have no threshold values. Finally, the impacts of the pulsating stress parameters are greater than those of the time parameter. As a result, the amplitude and mean stress should be maximized, while an excessively high frequency is unnecessary. These research results will provide guidance for optimizing the technical parameters for using pulsating hydro-fracturing technology in a coal seam.