From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, December 13, 2013
Link to streaming video: available 12.13.2013 at 8:55am
AAPG Distinguished Lecturer
Bureau of Economic Geology
Natural Fractures in Shale Hydrocarbon Reservoirs
The diversity of shale-hosted fracture systems is illustrated using examples from shale reservoirs worldwide. Their relevance for hydrocarbon production is considered. Both open and sealed fractures can interact with and modify hydraulic fracture size and shape. Interaction may serve to increase the effectiveness of the hydraulic fracture network, or could work against it. Open fractures can enhance reservoir permeability but may conduct treatment fluids great distances, in some instances possibly aseismically.
We have addressed the challenge of incomplete sampling of subsurface fractures through extensive fracture data collection in cores and image logs and careful selection of outcrops, coupled with an understanding of how fractures and their attributes scale. We also use tested mechanistic models of how fractures grow in tight sandstones and carbonates to interpret fractures in shale. In order to predict fracture patterns and attributes it is helpful to understand their mechanism of formation and timing in the context of the burial and tectonic histories of the basin in which they are forming. A key variable is the depth of burial, and thereby the temperature, pore-fluid pressure and effective stress at the time of fracture development. For the most part the origin of fractures cannot be determined from their orientation or commonly-measured attributes such as width, height and length. The mineral fill in sealed fractures does provide an opportunity, however, and we use fluid-inclusion studies of fracture cements tied to burial history to unravel their origin.
Julia Gale has a background in structural-metamorphic history and shear zones in the Archean Block of Greenland and the Dalradian of northeast Scotland. "These sparked my interest in the often-conspicuous, late-stage vein systems that are present in these regions. Recognizing that vein geometries are commonly similar looking, independent of the scale of observation, I was also aware of fractal phenomena increasingly being reported in the Earth Science literature, and I began some quantitative studies of vein systems. Brittle fractures also display fractal relationships, and the prospect of using microstructures to predict properties of macrofractures forms the central concept of my work. Applications of this work are particularly relevant in the characterization of fractured hydrocarbon reservoirs."