Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG 2003 Meeting, Salt Lake City, Utah, May 1114, 2003
Fracture Characterization in the Austin Chalk: a Horizontal Core from the Lower Austin Chalk, Pearsall field, Frio County, Texas
Julia F. W. Gale and Jon Holder
The Upper Cretaceous Austin Chalk is an important low-permeability fractured reservoir, commonly exploited through horizontal drilling. Successful wells optimize well-bore azimuth and length, target horizon, and storage volume potentially connected to the well bore through fractures. To achieve this end, fracture orientation, aperture, effective aperture, spacing, length, height, fill, and permeability must be determined. Two horizontal laterals and a vertical segment of an Austin Chalk core (Newsom No.1B from Pearsall field, Frio County, Texas) serve to (1) illustrate how each parameter affects well potential and (2) provide examples of both natural and drilling-induced fractures.
distributions in the horizontal sections of the core follow power laws.
Spacing-size distributions are negative-logarithmic or lognormal, and
fractures are clustered. The aperture size above which fractures are open
is 0.14 mm, meaning that many small fractures contribute to system connectivity.
Fracture height is governed by truncation effects at marl horizons and
pressure-solution seams, which in turn partly depend on fracture aperture
and thickness of the inhibiting layer.