Laboratory evaluation of induced seismicity reduction and permeability enhancement effects of cyclic hydraulic fracturing

摘要

A new concept of cyclic hydraulic fracturing with cyclic injection was suggested to replace the conventional hydraulic fracturing with continuous injection to help reduce induced seismicity during fracturing. Proof of the concept was done in laboratory. Test results showed that 80% of the average breakdown pressure in continuous injection is able to induce hydraulic breakdown by cyclic injection, and the total number of cycles generally decreased with increasing maximum injection pressure. Induced seismicity reduction was confirmed based on acoustic emission monitoring results. Apertures of induced fractures in granite specimen were estimated through computed tomography (CT) images and they were relatively smaller in case of cyclic fracturing compared with conventional hydraulic fracturing. Moreover, injectivity index was calculated based on re-injection tests. The results of fracture aperture and injectivity index indicate that permeability enhancement effect for cyclic hydraulic fracturing is not as effective as conventional hydraulic fracturing considering isostatic loading condition. Hydraulic fracturing efficiency is expected to be improved by both modifying the cyclic injection style and considering hydro-shear by applying confining pressure and differential stresses to the samples.