Utilizing Moment Tensors to Characterize Out-of-Zone Steam Chamber Growth

*Summary Microseismic monitoring has long been used to monitor steam injection programs related to enhanced heavy oil recovery. It has been well established that microseismicity can be used to examine steam chamber dimensions, cap rock integrity or steam containment and fault activation associated with injections. More recently, source parameter investigations have identified the stress transfer mechanism associated with steam chamber development and the redistribution of stress to allow for out-of-zone growth. The underlying process associated with out-of-zone growth, however, has remained elusive. Here, we consider seismicity associated with a steam injection into an oil bearing reservoir in North Oman. Utilizing a multi-well multi-array configuration, moment tensor inversion was carried out for 824 microseismic events (571 general solutions, 253 double couple solutions) occurring both in the reservoir formation as well as in the adjoining cap rock. Our analysis showed a progressive change in principal stress-strain axes with decreasing depth, suggesting a flip in magnitude occurs between the reservoir and the cap rock thereby allowing for the activation of large structures and out-of-zone growth. Events within the reservoir appear to be dominated by volumetric failures whereas within the cap rock, shear-tensile failures with opening and closure events predominate. Based on these observations, we suggest that moment tensor analysis provides an approach for identifying the underlying processes associated with steam injection and moreover, that local variations in the stressstrain field due to steam injection can result in optimal orientations to allow for the activation of pre-existing faults, in this case located in the cap rock.