Experimental investigation of the effects of long-period cyclic pulse loading of pulsating hydraulic fracturing on coal damage

Abstract

Coalbed methane recovery can largely ease the crisis of annually increasing energy consumption of China. Low permeability and porosity of deep coal reservoirs mainly cause the low gas production rate. Pulsating hydraulic fracturing (PHF) is an alternative tool to improve the reservoir permeability and stimulate the gas productivity. However, little has been done to directly observe the fatigue damage in coal created by the pulsating loading. Because the fatigue damage is primarily governed by the time-length of PHF period. Therefore, the effect of long period pulsating loading of PHF on coal damage has been investigated in this work. Results show that long-period pulsating loading of PHF can cause significant damage in coal. The low-field NMR data indicates coal porosity increases with the increase of maximum pulsating pressure. Pulsating pressure also plays an important role in the cracking pressure. A larger pulsating pressure leads to a lower cracking pressure. Comparison of PHF and HF damaged coal investigated by the X-ray CT indicates the PHF creates more fractures and higher porosity than conventional hydraulic fracturing. The increase ratios of the coal porosity by HF and PHF are 40 %–60 % and 80 %–100 %, respectively. This paper provides a better understanding the mechanism of the coal damage by the PHF under long-period pulsating loading which is of great importance for optimizing PHF technique and generating connected fracture networks.