Numerical simulation of hydraulic fracturing and associated seismicity in lab-scale coal samples: A new insight into the stress and aperture evolution

Hydraulic fracturing and its associated seismic behaviour are increasingly important in the resource sector. In this paper, hydraulic fracturing and induced seismicity by both the constant flow rate and constant pressure injection was successfully simulated in lab-scale coal samples based on the discrete element method (DEM). Using the self-developed code in particle flow code (PFC), numerical simulations of fluid injection, fluid transport and seismic response are achieved simultaneously. The code has been applied to simulate water injection into an intact coal sample as well as coal samples with one pre-existing fracture. Model results follow observations from field practices and laboratory experiments, which demonstrate the reliability of the self-developed code. Induced seismicity by two injection methods shows that hydraulic fractures on the side of the sample with a pre-existing fracture present different cracking propagation patterns compared to the other side with no pre-existing fracture. Meanwhile, more peaks of extension-to-compression ratios calculated from the moment tensors of AE events are observed, which can be used to imply differential stress as well as reveal the impact of the pre-existing fracture on the aperture evolution of hydraulic fractures.