From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, March 23, 2012

Estimating subsurface fracture spacing using microscopic fractures

John Hooker
Bureau of Economic Geology, Jackson School of Geology

Constraining fracture spacing in the subsurface is a challenging problem for reservoir characterization. Steeply dipping fractures are difficult both to detect seismically and to sample using vertical cores. A current widely used core-based approach is to assume the core volume is representative of the reservoir volume, and so requires large amounts of core. A new method we developed takes advantage of our ability to detect microscopic fractures (microfractures)using SEM-based cathodoluminescence. We estimate fracture spacing by establishing a fracture frequency equation among microfractures and extrapolating to larger scales. We have observed that natural fractures commonly follow power-law size distributions with a narrow range of exponents. Assuming a fixed power-law exponent and varying only the coefficient to best-fit microfracture data substantially improves spacing predictions.