Experimental investigation of the influence of pulsating hydraulic fracturing on pre-existing fractures propagation in coal

Abstract

This study aims to investigate the propagation behavior of pre-existing fractures in a coalbed methane (CBM) reservoir during pulsating hydraulic fracturing (PHF). An innovative test system capable of true triaxial loading and multi-mode PHF was developed to conduct PHF on pre-existing fracture specimens of synthetic coal. The fluid pressure, strain around the pre-existing fracture, and propagation morphology of the pre-existing fractures were measured to analyze the influences of the coupling action between the pulsating parameters and the confining stress on pre-existing fracture propagation and fatigue damage of the coal. The results show that the evolution of fluid pressure under PHF can be divided into four periods: slow growth, rapid rise, steady drop, and decline. In the steady drop period, the intermittent fluid injection causes alternating pressure pulsation in the pre-existing fractures and cyclical strain fluctuation around the pre-existing fractures, leading to an abrupt propagation. With the increase in confining stress, the propagation morphology changes from single to cross direction exhibiting a “T” shape. Under flow-control injection mode, the frequency and amplitude of the pulsating pressure significantly influence the fatigue damage of coal, stimulating fracture propagation. With the increase in pulsating frequency, the pulsating amplitude decreases gradually, and the pre-existing fracture propagation rate and the pressure drop rate both increase first and then decrease; thus, an optimal pulsating frequency can be chosen to accelerate fracture propagation. Furthermore, the pulsating pressure has weakening effects on the coal around the pre-existing fractures, thus enriching the seepage channels around the primary fracture. Based on this work, a PHF method integrated with multi-mode injection is proposed to achieve the control of hydraulic fractures and perform efficient fracturing of CBM reservoirs.