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Previous HitPredictingNext Hit Macrofracture Permeability from Microfractures

MARRETT, RANDALL, ORLANDO ORTEGA, ROBERT REED, and STEPHEN LAUBACH, University of Texas at Austin, Austin, TX

Macroscopically visible fractures have a much more significant impact on Previous HitreservoirNext Hit permeability than do microfractures, however sampling statistically significant numbers of macrofractures in the subsurface is typically difficult and expensive. We are developing rapid and inexpensive techniques for using the abundant microfractures in fractured Previous HitsandstoneNext Hit reservoirs as quantitative proxies for macrofractures. The fundamental assumption of our approach is that fractures across a wide range of scales are intimately related in terms of mechanics, diagenetic history, and fluid flow. One objective of the work is to predict Previous HitreservoirNext Hit-scale fracture permeability, including the effects of macrofractures, from microfracture observations.

In a subsurface sample, which may be as small as a side-wall plug, we commonly find large numbers of microfractures. The microfractures are often cryptic or invisible petrographically due to diagenetic quartz-fill that is in optical continuity with fractured sand grains and intergranular cement. However, these fractures are readily imaged by their cathodoluminescence under high-magnification SEM, and fracture attributes such as length and width may be systematically quantified and analyzed. Preliminary data are consistent with the hypothesis that macro- and microfractures follow length and width scaling relations from the scale of individual sand grains to the scale of lithologic layering. Unlike purely geometric scaling relations, attribute scaling relations allow direct quantification of aggregate Previous HitreservoirNext Hit properties such as pemmeability, Previous HitporosityTop, and elastic shear-wave anisotropy. Future work will test the accuracy of such predictions.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.