Robert A. Bauer

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.

Behaviour of abandoned room and pillar mines in Illinois

SummaryLittle comprehensive information has been reported on the behaviour of room-and-pillar mines. The objective of this paper is to present case data on mine failures in the Illinois basin for use in practice. Presented are results of an ongoing study and details on the site characteristics of cases where sags have developed on the surface. Site data are reported to show the geologic, mining, and sag conditions that existed. Sags mainly develop from pillar, floor, or pillar-floor failure. The character of the sags depends upon the type of mine failure as well as the overburden response.Preliminary results show that the statistical no-risk tributary pressure decreases over 300% as the mine age increases from about 2 to 100 years at a long-term value of approximately 300 psi (2070 kPa). As more information is collected and more analysis is done, the allowable tributary pressure can be determined for different site conditions.A plot is also reported that depicts the relationship of the maximum subsidence to site conditions. It was found that the modified subsidence factor was heavily dependent upon the overburden rock thickness.

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.

Using a Relational Data-Base Management System to Help Determine Causes of Damage to Structures in Illinois

Relational data-base management systems can be useful tools for engineering geologists investigating a wide variety of problems. Scientists at the Illinois State Geological Survey, a division of the Department of Energy and Natural Resources, and investigators for the Illinois Mine Subsidence Insurance Fund are using such a system to help differentiate causes of damage (mine subsidence, soil conditions, or other factors) to structures identified by insurance claims. From as many as ten files at the same time, researchers and investigators may compare nonconfidential data entered from reports contributed by both agencies. Files include data in 149 fields pertaining to damaged structures, soil conditions, underlying mines, and causes of damage. Using microcomputers linked by modem to the Department9s Prime 750 minicomputer, site investigators can consider damage to a single structure in the context of what is already known about damage to other investigated structures in the area. Researchers can use the data base to gain regional perspectives of problem areas by plotting key elements taken from site-investigation data onto base maps showing features such as surficial geology or mined-out areas. Researchers may then know where to collect supplementary data to increase understanding of problem areas. The new data will, in turn, be available to site investigators seeking information on specific areas. Although the data base was basically founded on insurance claims for alleged subsidence damage, less than 17 percent of the total claims actually result from mine subsidence; most of the damage is related to soil problems and to construction and maintenance practices inappropriate for local conditions. Damage resulting from these causes will also be of interest. The use of a relational system to study causes of damage is an example of how a relational system can facilitate investigations of many types of problems in engineering geology.