Investigations of fracture behavior and pore structure change in pulse fracturing for cement block

Abstract

Pulse fracturing can reduce fracture initiation pressure and increase stimulation effectiveness. To investigate the effect of frequency and amplitude on fracture propagation behavior in pulse fracturing, tri-axial experiments with acoustic emission (AE) monitoring are applied. The results demonstrate that pulse fracturing can generate mixed mode fractures, and the distinct deviation of the fracture propagation near-wellbore can be seen for pulse fracturing. Both pulse frequency and amplitude are critical parameters for the propagation path, measured AE accounts, accumulative energy and pore connectivity. The increase in amplitude is beneficial for large fracturing energy, but the increase in frequency is not necessarily helpful for fracture effectiveness. The harmony between the rock and pump frequency can yield the largest acoustic events and pore connectivity. The pore structure and microfracture change of rock before and after pulse fracturing at different distances are measured with nuclear magnetic resonance (NMR) and CT scans, indicating that pulse fracturing has a strong capability to create a larger stimulated area than the monotonic fracturing model. In addition, obvious microfracture generation and pore space expansion can be seen near the injection port. However, the pore structure comparison indicates that the pulse fracturing mode mainly improves the scale of macropores, which is primarily because of the relatively small pulse fracturing parameters used in these experiments. Thus, this study provides an understanding of pulse frequency and amplitude effects for stimulation effectiveness related to pulse fracturing of tight reservoirs.