Natural fractures in shales: Origins, characteristics and relevance for hydraulic fracture treatments

Authors: J. F. Gale and J. Holder

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 oral session chaired by S. C. Ruppel and R. G. Loucks (http://www.aapg.org)

Abstract: The subcritical crack index of different shale facies was measured. 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 occurred 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.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, “Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the United States.” The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological-, engineering-, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)