Fracture Interpretation in the Barnett Shale, Using Macro and Microseismic Data
The Barnett Shale was deposited in the Fort Worth Basin during the Mississippian in a marine setting and unconformably overlies Ordovician carbonates. In the area of this study, the underlying Viola limestone has been eroded, justxtoposing the Barnett Shale against the porous and water-bearing Ellenberger Limestone. We explore the challenge of Barnett Shale development: to maximize stimulated reservoir volume within the shale, without tapping the adjacent Ellenberger water reservoir.
The Barnett Shale ranges in thickness from about 50 feet in the south to nearly 1000 feet to the northwest. From the southwest boundary of the Llano arch, the Barnett Shale dives from a depth of 2500 feet down to 8000 feet near the Muenster Arch, the northern field boundary. The top of the Barnett in the study area is located at 6400 feet and has a thickness of 500 feet. Barnett shale porosity ranges from 0.5 to 6% with permeabilities as low as 70 nanodarcies. Low clay content (20-30%) makes the Barnett more "fracture friendly" than typical shales, which is verified by analyzing microseismic magnitudes and density.
Investigation of stress information and our measurement of published microseismic data all indicate a current stress orientation along a 50-60/230-240 degree azimuth. Common geologic models of the Muenster Arch indicate that the primary modern-day stress is to the southwest, supporting the 230-240 degree estimate. Time-lapse analysis of microseismic from this project also indicates preferred induced fracturing along this same orientation. Of particular interest is the high variability of microseismic activity on adjacent fracturing stages, which correlates with "macroseismic" indicators of natural fracturing.
Circular "collapse chimneys" of fractured rock pockmark much of the study area and range from beneath the Ellenberger and can extend up thousands of feet to the Pennsylvanian Caddo (Atoka) Limestone. While opinions vary on the relative influence of karsting and basement faulting upon the creation of these fractured columns, it is essential to avoid these features both during well planning and completions. This study indicates how "macroseismic" attributes are used to interpret fracture volumes for well planning purposes. Further, calibration of microseismic measurements with extracted macroseismic attributes provides predictive capabilities for estimating fracturing intensity and orientation, for optimal completions design.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009