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This research determined that frequency attenuation can be used as a potential porosity/permeability detector within a carbonate reservoir in a Silurian pinnacle reef in the northern portion of the Michigan Basin. This reef, the Charlton 30/31 Field in Otsego County, Michigan, produced 2.6 million barrels of oil during its primary production phase and was selected for enhanced oil recovery operations in the early 2000s. As part of a U.S. Department of Energy study of this enhanced oil recovery project, a four-dimensional (4-D) P-wave seismic survey was acquired in an attempt to monitor the flow of CO2 through the reservoir.

A three-dimensional (3-D) P-wave seismic survey was acquired prior to the injection of the CO2, which served as the baseline for the 4-D survey. Various interpretation methods were performed on this initial 3-D data set in order to develop a reservoir characterization and a dynamic model for multiple reservoir simulations. During the initial phase of the project, a relationship was identified between the low instantaneous frequency derived from the full-azimuth seismic volume and zones of high porosity and permeability in the reef identified from well logs. This relationship was used for the reservoir characterization. Forward models developed to approximate the reservoir’s characteristics support the relationship. Multiple analyses and data sets acquired following the creation of this reservoir characterization confirmed the interpreted matrix porosity distribution developed using this relationship.

Following the injection of 29,000 tons of CO2 into the Charlton 30/31 reef, a second 3-D P-wave seismic survey was acquired that served as the monitor survey for the 4-D survey. Amplitude analysis of the monitor survey provided a good correlation to zones predicted to be filled with CO2 by the final, predictive reservoir simulation. However, one particularly strong amplitude anomaly immediately to the northeast of the CO2 injection point was observed that suggested this reef may contain fracture porosity as well as zones of high matrix porosity.

Although few individual, open fractures have been reported in these reefs, the available data sets are not optimum for the recognition of vertical fracture systems. In an attempt to identify and characterize any open fracture trends that may exist within this reef, four azimuthal seismic volumes were developed and analyzed. Isofrequency volumes were developed from these azimuthal volumes through spectral decomposition, and then they were compared in an attempt to identify frequency attenuation within the reservoir. In theory, zones of high matrix porosity and permeability affect all azimuthal volumes in an isotropic manner, producing zones of frequency attenuation in all azimuthal volumes to approximately the same degree. However, due to their often linear nature, fracture trends produce anisotropy that is potentially recognizable in an azimuthal volume.

Increased amplitude in some lower frequencies, specifically 15 Hz, in the 160° azimuthal volume was observed to the northeast and southwest of the CO2 injection point on the same time slice corresponding with the CO2 flow interpreted on the 4-D monitor survey. This suggests that some open, northeast-trending fractures exist near the injection point, resulting in increased directional system permeability at that location that channels more CO2 to the northeast than is explained by the matrix porosity and permeability alone.

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