From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, October 5, 2007
Seismic Characterization of Multiple Fracture Sets:
Finite element (FE) modeling of the effective elasticity of the so-called digital rocks confirms that the non-interaction approximation (NIA), which simply adds the compliance contributions of cracks as if they were isolated and non-interacting, is sufficiently accurate in the entire range of the crack densities expected in practice. Moreover, the accuracy of the NIA does not deteriorate when the fractures form interconnected networks. The insensitivity of the effective elasticity to crack intersections and a strong dependence of the effective permeability on their presence imply that the crack-induced elasticity and permeability might be unrelated.
The NIA and FE modeling identify the quantities -- so-called principal crack densities and the fluid factors -- that govern the effective elasticity due to the presence of multiple, possibly intersecting fracture sets. The feasibility of estimation of these quantities and, thus, characterization of multiple fracture sets is demonstrated on seismic refection data acquired over the tight-gas Rulison Field in Colorado.