Observations of a potential size effect in experimental determination of the hydraulic properties of fractures

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Abstract
In several recent investigations, experimental studies on the effect of normal stress on the hydraulic conductivity of a single fracture were made by using three rock specimens ranging in cross‐sectional area from 0.02 m2 to over 1.0 m2. At the maximum stress levels that could be attained (10–20 MPa), minimum values of the fracture hydraulic conductivity were not the same for each rock specimen. These minimum values increased with specimen size, an indication that the determination of fracture conductivity may be significantly influenced by a size effect. The implications of these results are important. Cores collected in the field are normally not larger than 0.15 m in diameter. However, the results of this work suggest that when a core of this size is used for laboratory investigations, the results may be nonconservative in that fracture permeabilities will be significantly lower than those that will be found in the field. If the results with an ultralarge Core (0.95 m in diameter) are more indicative of field conductivities for a fracture under stress, then further work is needed to determine optimum specimen size so that reliable results on flow in fractures under stress will be available.

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