Monitoring variations in subsurface properties using direct-wave arrivals recorded by downhole fiber-optic sensors

Abstract

Placing seismic sensors in or in an immediate vicinity of the of the zone of change allows precision monitoring of subsurface properties through measurements of direct wave arrivals. Use of permanent receivers such as distributed acoustic sensors (DAS) allows monitoring to be done continuously or on de mand using either controlled or natural seismic sources. Indeed, changes of formation stiffness (or receiver coupling) cause changes of dynamic strain amplitude measured by DAS, while corresponding changes of interval velocity cause changes in traveltimes measured below the zone of change. Application of this technology to monitoring of a small CO2 injection using permanent seismic sources and downhole DAS receivers shows substantial reduction of strain amplitude and traveltime delays immediately after the start of the injection, consistent with full-wave modeling. Passive seismic recording using a DAS array installed in an abandoned well shows variations in DAS amplitudes caused by variations of cementation/casing.