Natural Fractures in Shales: Origins, Characteristics and Relevance for Hydraulic Fracture Treatments

Julia F. Gale¹ and Jon Holder^2
1Bureau of Economic Geology, University of Texas at Austin, Austin, TX
²Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX

Most shales contain natural fractures. We review common fracture types and their characteristics based on core and outcrop studies from several different shales, including Devonian Woodford shales from the Permian basin and the Mississippian Barnett Shale of the Fort Worth basin.

We measured the subcritical crack index of different shale facies. Geomechanical modeling using the index as an input parameter allows prediction of fracture clustering. To do this rigorously, however, requires an understanding of the diagenetic history as it relates to evolving mechanical rock properties, and the timing of fracturing. Thus it is necessary to integrate fracture work with other fundamental geologic knowledge. For example, there can be many different causes of fracturing over the lifetime of a shale. Some fracture sets in Woodford Shale cores are seen to have been deformed by compaction, whereas some others are later. The mechanical properties of the pre-compaction rock at the time of early fracturing are likely to be very different from those prevailing at a later, post-compaction stage. The resulting fracture patterns and sealing characteristics for the different fracture sets are likely to be different also.

The relevance of natural fractures in these shale gas plays is that they are weak planes that reactivate during hydraulic fracture treatments. We have observed fracture planes only half as strong as the host rock during tensile testing. The first step towards understanding whether hydraulic fractures will be affected in a given zone is to predict the natural fracture patterns and measure the in situ stress.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas