Experimental study on the mechanism of coupled dynamic–static fracturing on damage evolution and crack propagation in tight shale

Abstract

To reduce the energy release risk and optimise the crack network of tight shale reservoirs, this study experimentally investigated the coupled dynamic–static fracturing technique and its mechanism for the damage evolution and crack propagation in tight shale. First, an innovative coupled dynamic–static hydraulic fracturing test system with a fluid pressure loading strain rate of 10−4–100 s−1 was developed, and physical model experiments were conducted. Then, the responses of the fluid pressure, acoustic emission (AE) parameter, and crack propagation pattern were obtained. Further, the mechanism of dynamic pulsation parameters, such as the pulsation amplitude (PA) and the upper pressure limit (UPL), on the damage evolution of shale matrix and the crack propagation patterns were analysed. The results show that during coupled dynamic–static fracturing, the initiation pressure and the transient initiation energy were significantly reduced by 0.65%–25.58% and 5.36%–31.51%, respectively, whereas the cumulative AE energy under dispersed release of energy increased by an average of 29.29%. In addition, the pulsation parameters mainly affected the cumulative damage of the shale matrix by controlling the microcrack propagation scale and number. The cumulative damage under increasing PA and UPL showed a trend of first increasing and then decreasing, implying that there are optimal PA and UPL for the fracturing process. Moreover, the dynamic pulsation parameters have control effects on the crack-propagation direction. With the increase in PA and UPL, the propagation of microcracks around the borehole presents a trend of ‘preferential propagation in the axis-section direction, alternate propagation in the axis-section and cross-section directions, and preferential propagation in the cross-section direction’, which induces the subsequent static propagation of hydraulic macrocracks along the direction of dominant microcracks. Based on the experimental results, a reasonable fracturing strategy was proposed to realise the coalescence of crack networks in the tight shale reservoirs.