Natural fracture characterization of the New Albany Shale, Illinois Basin, United States
The New Albany Shale is an Upper Devonian organic-rich gas shale located in the Illinois Basin. A factor influencing gas production from the shale is the natural fracture system. I test the hypothesis that a combination of outcrop and core observations, rock property tests, and geomechanical modeling can yield an accurate representation of essential natural fracture attributes that cannot be obtained from any of the methods alone.
Field study shows that New Albany Shale outcrops contain barren (free of cement) joints, commonly oriented in orthogonal sets. The dominant set strikes NE-SW, with a secondary set oriented NNW-SSE. I conclude that the joints were likely created by near-surface processes, and thus are unreliable for use as analogs for fractures in the reservoir. However, the height, spacing, and abundance of the joints may still be useful as guides to the fracture stratigraphy of the New Albany Shale at depth. The Clegg Creek and Blocher members contain the highest fracture abundance. Fractures observed in four New Albany Shale cores are narrow, steeply-dipping, commonly completely sealed with calcite and are oriented ENE-WSW. The Clegg Creek and Blocher members contain the highest fracture abundance, which is consistent with outcrop observations. Fractures commonly split apart along the wall rock-cement interface, indicating they may be weak planes in the rock mass, making them susceptible to reactivation during hydraulic fracturing. Geomechanical testing of six core samples was performed to provide values of Young’s modulus, subcritical index, and fracture toughness as input parameters for a fracture growth simulator. Of these inputs, subcritical index is shown to be the most influential on the spatial organization of fractures. The models predict the Camp Run and Blocher members to have the most clustered fractures, the Selmier to have more evenly-spaced fractures, and the Morgan Trail and Clegg Creek to have a mixture of even spacing and clustering. The multi-faceted approach of field study, core work, and geomechanical modeling I used to address the problem of fracture characterization in the New Albany Shale was effective. Field study in the New Albany presents an opportunity to gather a large amount of data on the characteristics and spatial organization of fractures quickly and at relatively low cost, but with questionable reliability. Core study allows accurate observation of fracture attributes, but has limited coverage. Geomechanical modeling is a good tool for analysis of fracture patterns over a larger area than core, but results are difficult to corroborate and require input from outcrop and core studies.