Predicting, monitoring and controlling geomechanical effects of CO2 injection

This chapter discusses the effects of pore-fluid pressure change on effective stresses in porous reservoir rock. While there is substantial information on pore pressure/stress coupling during pressure depletion in hydrocarbon fields, little is known about such effects during fluid injection. Thus, in cases where CO2 storage in severely depleted reservoirs is envisaged, geomechanical methods that predict depletion-induced faulting, such as by Streit and Hillis, can be useful for estimating whether fault-seal damage was induced. In cases where Biot's α can be determined for reservoir rocks in CO2-storage sites, velocity-effective stress relationships should be established to also measure effective-stress changes with seismic techniques in addition to direct fluid-pressure measurement in monitoring wells. To test for injection-related changes in total horizontal stresses in CO2 storage or demonstration projects, extended leak-off tests and hydraulic-fracturing tests can be conducted. In storage sites with an identifiable risk of injection-related fault reactivation to produce events of noticeable magnitude, geophones, and transducers should be installed for the monitoring of induced MS events. The monitoring can be used to detect accidental over-pressurization of the formation as it is likely to allow for real-time adjustment of injection pressures.