Experimental study of pulsating water pressure propagation in CBM reservoirs during pulse hydraulic fracturing

• An experiment was conducted to study PWP propagation in coal fissures in this work.
• The peak pressure first decreases and then increases in a non-plugging fissure.
• The plugging rate has a great effect on PWP propagation in a plugging fissure.
• This work gives a basis for predicting and assessing of initial fracture in coal.

Pulse hydraulic fracturing (PHF) technology can effectively improve reservoir permeability for coal bed methane (CBM) extraction. In this work, laboratory experiments were conducted to study the laws regarding pulsating water pressure (PWP) propagation during PHF in a CBM reservoir at different pulse frequencies and plugging rates. The results demonstrate that the average PWP is stable over the length of the reservoir fissure, and the PWP peak primarily depends on the PWP amplitude. The experimental results show that for a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases with the maximum pressure occurring at the fissure tip. When the fissure is blocked, a portion of the pressure waves are reflected in front of the blockage, forming reflection waves, which interact with the incident waves and lead to an increase in the PWP peak pressure. When the plugging rate is 95%, there is a significant difference in the PWP peak pressure in front of and behind the blockage because of the pressure wave reflections and interactions. As the plugging rate decreases, the blockage effects are reduced, and the wave proportion that is reflected decreases such that the transmission and creeping wave components increase. When the plugging rate is 65%, the PWP peak pressure behind the blockage is larger than that prior to the blockage. The findings build a basis to predict and control fissure extensions to improve the effect of PHF by optimizing the technical parameters.