Experimental study of the unblocking of coalbed with single-slit filling by pulse hydraulic fracturing

As an unconventional natural gas resource, coalbed methane has huge reserves in China and has good prospects for exploitation. During the coalbed methane extraction process, a large amount of coal powder is produced. Coal powder accumulates in fissures and blocks them, which is one of the important factors affecting the permeability of coalbed methane. A hydraulic fracturing system with adjustable frequency pulsation and single-slit filling prefabricated samples were developed for testing coalbed fissure unblocking. Experiments were carried out to study the evolution process of the unblocking pressure and the unblocking effect of pulse action under different frequency conditions. Results show that the evolution process of the unblocking pressure can be divided into three stages: pressure rise phase, decline phase, and pressure fluctuation stability phase. Compared to that, under a steady flow, the pressure threshold under the pulse action is lower, pressure drop duration is shorter, and pressure drop amplitude is smaller. The migration of pulverized coal under the action of steady flow is concentrated in the pressure drop stage. Under the action of pulsating pressure, pulverized coal migrates in the pressure drop stage and the pressure fluctuation stable stage. The pressure drop time is positively correlated with the total amount of pulverized coal transported, but the total transport volume of pulverized coal is equivalent to that under the action of steady flow. The fluid infiltration along the radial direction mainly occurs during the pressure rise to the peak pressure. The filtrate penetration radius is smaller due to the shorter unblocking time under the pulsating flow, and the reservoir damage is small. Comprehensively considering the unblocking pressure, coal migration, unblocking seepage path, and unblocking seepage depth, especially the unblocking pressure and unblocking seepage depth, as the main evaluation factors, the best unblocking effect is under the condition of 3 Hz, which has a lower solution.