Microseismic monitoring, event occurrence, and the relationship to subsurface geology

The Illinois Basin - Decatur Project (IBDP) is one of the US Department of Energy National Energy Technology Laboratory-funded carbon dioxide (CO₂) sequestration projects that is approaching the goal of injecting one million tonnes of CO₂ within three years. Since mid-November 2011, IBDP has maintained the target injection rate of approximately 1,000 tonnes of CO₂ per day into the Mt. Simon Sandstone at a depth of approximately 7,000 ft (2134 m). Several measurement, monitoring, characterization, data integration, and modelling technologies have been implemented on this project, including real-time continuous microseismic monitoring which commenced several months prior to start of injection. Much of the extensive site characterization effort at IBDP has been motivated by the desire to understand the source mechanisms for observed microseismicity toward the ultimate goal of developing predictive capability. A rich dataset of microseismic observations has been acquired over nearly 4.5 years of monitoring to-date. These observations form semi-linear clusters in space, supporting the interpretation of a structural source mechanism. However, corresponding structural features are not observed in the existing 3D seismic data. This lack of direct observation of a structural source feature prompted a multi-disciplinary geoscience based approach to understanding the source mechanism for the observed microseismicity. Relationships between microseismic event occurrence and subsurface geology are observed in multiple domains and at multiple scales. Geomechanical characterization efforts show that the orientations of clusters are consistent with the in-situ tectonic stress regime. Microseismic event first motion analysis suggests focal mechanisms also consistent with the tectonic stresses. While no faults or other structural discontinuities may be unambiguously interpreted from the seismic data using conventional amplitude interpretation methods, investigation of specialized edge detection seismic attributes reveals a directional fabric in the rock mass that is consistent with the orientation of microseismic clusters. At a macro scale, some microseismic clusters appear to be associated with topographic features in the Precambrian basement interpreted from 3D seismic data. At the opposite end of the scale, the fractures observed in grains are suggestive of a tectonic stress regime consistent with both the geomechanical analysis and the microseismic cluster orientation. Through these multi-disciplinary, multi-scale studies, an understanding of the relationship between subsurface geology and observed microseismicity is evolving. While no single observation set supports unambiguous correlation, the consistency between multiple lines of investigation supports an interpretation of in-situ stresses, rock fabric anisotropy, grain scale failure, and relationships to basement structure, all consistent with realistic hypotheses for the microseismic source event mechanisms. One such source mechanism hypothesis has been tested using sophisticated numerical fluid flow and geomechanical modeling followed by seismological calculations, preliminary results of which are presented here.