Bettina P. Goertz-Allmann

Characterization of induced seismicity patterns derived from internal structure in event clusters

Seismicity induced by CO2 injection at Decatur, Illinois, occurs in distinct clusters and shows no obvious correlation with the proceeding pressure front. We analyse some of these clusters in more depth by using a waveform cross-correlation approach. With this approach we can associate about 1400 events from two clusters, with moment magnitudes between 1.1 and -1.7, with specific formations of much smaller vertical dimensions (10th of meters) than the depth resolution of traveltime-based event locations. The differentiation of reservoir and basement events, and the definition of sub-clusters by waveform correlation, rather than by location, helps to better analyse the spatio-temporal evolution of the events within a cluster. In the Decatur case, this is characterized by event migration from the reservoir into the adjacent basement. The spatial variation of Brune stress drop and Gutenberg b-value exhibit signs of a fluid-driven triggering mechanism at the cluster level, revealing a punctual hydraulic connection between reservoir and basement, most likely associated with basement faults cutting into the reservoir. The observed clustering of seismicity can thus be explained by the lateral heterogeneity of permeability and crustal strength, and is overall consistent with a pressure-induced triggering mechanism. Hence, proper long-term risk mitigation for large-scale fluid injection close to the basement requires prior mapping of small sub-seismic basement-connected faults.

Integrating Active with Passive Seismic Data to Best Constrain CO2 Injection Monitoring

The Illinois Basin - Decatur Project (IBDP) is to date one of the largest CO2 sequestration projects in the United States. So far, 1 Mio tonnes of CO2 have been injected over 3 years into the Mt. Simon sandstone formation at about 2 km depth. A suite of various active and passive seismic monitoring techniques have been applied at the site, providing a rich monitoring dataset. Time-lapse 3D surface seismic and VSP measurements were carried out to delineate the progression of the CO2 front. In addition, passive seismic monitoring revealed over 10’000 microseismic events. As a novel method, we attempt to combine the active and passive seismic data for seismic tomographic inversion for the 4D velocity- and attenuation structure in the reservoir. The combined aperture and higher resolution focuses on the reservoir and may allow a more precise mapping of the injected fluid over time. To investigate 4D changes of velocities and attenuation a similar source and receiver distribution is required. This is a particular challenge for microseismic events. High microseismic event location accuracy is essential, which we intend to improve by near surface material characterization, both from downhole petrophysical logging and seismic velocity logging within newly drilled shallow wells.

Interpretation of Microseismicity from Hydraulic Fracture Modelling

We have presented a finite-element based model for simulation of hydraulic fracturing. Each element is assigned strength in terms of a strain limit, and each element may have its individual strength. The element fractures when the strength limit is exceeded. One or more elements may break in one event. We show a 2D example of hydraulic fracturing of a low-permeable rock, where the fracture geometry is obtained. The bottom hole pressure is computed, which shows the pressure drops that follows each event. Finally, the magnitude of the fracture events is plotted in both space and time, which can be compared with the data from micro-seismic monitoring. A calibration of the model may provide effective parameter values for the rock. The presented formalism can also be coupled to reservoir simulators and calibrated against rupture propagation theories.