RESERVOIR CHARACTERIZATION USING SHALLOW WELL MICROSEISMIC MONITORING

The analysis of microseismicity is commonly used for detecting induced seismicity resulting from fluid injection. Projects in enhanced oil recovery have developed microseismicity tools for delineating fracture orientations in reservoirs. These tools have been applied to the Black Warrior Basin of Alabama to characterize saline reservoirs for potential carbon dioxide sequestration. The project is built around a 4915 foot exploratory well, two 5 mile long perpendicular 2-D seismic reflection profiles, and a zero-offset and three 2500 foot offset VSPs at azimuths of 18°, 145°, and 267° around the main well. Three 300 foot permanent microseismic monitoring wells were drilled around the exploratory well at distances of 565, 569, and 1251 feet. Each well contains 3 geophone pods at depths of 89, 189, and 289 feet and within each pod are 3 component seismometers. Each component of the seismometers comprises 2 geophones in series to increase the signal to noise ratio. The geophone pods have been grouted into place in uncased boreholes.

The monitoring wells were used to record five small volume water injections at the main exploratory well in addition to the vibroseis signals from the VSPs and naturally occurring local seismic events. Currently, most monitoring projects have used pre-existing well bores that may be too far away from the injection interval, or contain well casing that impedes acoustic signals. In the Black Warrior Basin project, the depth ratios of the main exploratory well to the monitoring wells are 16.5 to 1 and the wells are in close proximity to the injection interval. This configuration could present a less costly alternative for future monitoring projects.