Hydromechanical impact of basement rock on injection-induced seismicity in Illinois Basin

The common explanation of observed injection-induced microseismicity is based on the measured stress state at the injection interval and the assumption that it remains the same in the vicinity. We argue here that representing the stress state in different geologic formations over the injection site with the single Mohr's circle is insufficient due to local stratigraphic features and contrast in compressibilities of the involved formations. The role of hydromechanical coupling in the microseismic response is also crucial for the proper assessment of the problem. Thoroughly monitored Illinois Basin Decatur Project revealed the majority of CO2 injection-associated microseismic events being originated in the crystalline basement. Even though basement faults can serve as the conduits for fluid flow-the predicted pressure increase seems to be insufficient to trigger seismicity. To address this issue, accurate laboratory measurements of rock properties from the involved formations are conducted. The pre-injection stress state and its evolution are evaluated with the hydromechanically coupled numerical model. It appears that the presence of an offset in a stiff competent layer affects the stress state in its vicinity. Therefore, both the pre-injection stress state and its evolution during the fluid injection should be addressed during the induced seismicity assessment.

Automated summary

This study discusses the explanation of observed injection-induced microseismicity, indicating that representing the stress state in different geologic formations over the injection site with a single Mohr's circle is insufficient. The role of hydromechanical coupling in the microseismic response is crucial. The study also reveals that CO2 injection-associated microseismic events in the Illinois Basin Decatur Project mainly originate in the crystalline basement.