Abstract: Scaling Characteristics of Natural Fracture Systems

W. C. Belfield

Fracture aperture is an important parameter to the fluid-flow properties of naturally fractured reservoirs. Information from micro-imaging logs collected in several horizontal wells provide an extensive database for evaluating how the distribution of this attribute varies with scale and how this variation can impact the fracture description.

Descriptions of fracture systems using simple box-counting techniques or variograms are deficient in the sense that they treat all fractures as being equal. Clearly, large aperture fractures will be significantly more permeable than very narrow ones. Both the aperture distribution of intersected fractures and their scaling characteristics can be incorporated using a multifractal description. This formalism basically describes how the cumulative "mass" of fracture apertures is distributed along the wellbore length. It results in a scale-independent description in the sense that any mapping of the aperture distribution to a longer or shorter well length will yield the same multifractal spectrum.

To illustrate how scaling characteristics can affect a description we consider average fracture spacing. This statistic is commonly used to describe a similarly oriented set of fractures. Of interest is how the average spacing changes if we can observe smaller and smaller fractures. Average spacing is found to follow a power law decrease when incorporating smaller fractures. This observation renders the meaning of average fracture spacing irrelevant unless the size or number of fractures is explicitly considered.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994