In 1993, Shell began a cooperative evaluation with the Gas Research Institute to study factors controlling gas production from the naturally fractured Devonian shale Antrim formation in Montmorency Co., Michigan. The presence of multiple intersecting sets of natural fractures is the primary control on well deliverability and the Antrim must be fracture stimulated to be economical. However, due to the shallow depths (500- 2000 ft) and naturally fractured nature of the Antrim, fracture geometry is complex, and the determination of the optimal fracture treatment or completion methodology is not straightforward.
To evaluate the effectiveness of fracture treatments in the Antrim, Shell conducted pressure transient modeling, microseismic tomography surveys for imaging created fracture dimensions (length, height, and dip), and the cutting of multiple coreholes to core the created hydraulic fractures. The integrated results suggest that fracture growth in the Antrim is very complex. A series of sub-vertical fractures were created that were parallel to the maximum principle stress and defined a zone of fracturing at least 50 feet wide. The fractures grew asymmetrically in height above the top perforation and appeared to follow a tortuous path along joint-set directions. Two coreholes drilled at 35-degree angles from vertical encountered 8 propped hydraulic fractures. Several of the cored propped fractures had varying azimuths suggesting that pre-existing natural fractures, which trend primarily NE/SW and NW/SE, were propped during the fracture treatment.