Correlation between Injection Rates, Pressure Data and Induced Microseismicity

In this paper we investigate the relation between injection rate, fluid pressures and the activity rate of microseismicity. Two case studies are presented, the Longyearbyen CO2LAB on Svalbard and the Krechba, In Salah CO2 injection project in Algeria. The CO2LAB project has so far only injected for short testing periods (maximum of 5 days) and injection fluids were water and brine. One significant microseismic event occurred about 17 hours after shut of the 5-day water injection, followed by a series of 7 aftershocks that could only be retrieved with master-event cross-correlation methods. Following injection tests with smaller injection volumes did not induce any measurable microseismicity, although clear pressure drops accompanied by an increase in injection rate were observed. This increase in permeability is likely due to aseismic deformation. In the second case study at In Salah, CO2 is continuously injected at about 1.9 km depth in three horizontal wells. Close to one of the injection wells, a microseismic network (at times only consisted of one 3-component geophone) recorded significant amounts of microseismic activity with over 5000 microseismic events during about two years of observation. The microseismic events mainly occur in clusters and are clearly related to the CO2 injection process.