Hydraulic fracturing of rock cannot be seen insitu, and the end result must mostly be judged on basis of roundabout evidence. As there is no question about the desirability of visual observations and direct measurements, the only way out of the dilemma seems to lie in experimental models. This part of the work carried out at Northwestern University is therefore directed toward the development of suitable experimental techniques and transparent laboratory models that could be used to simulate the various problems associated with drilling into hard impermeable rock and fracturing it for energy extraction. The material selected for the transparent models is epoxy resin (Epon 828 resin with phthalic anhydrate hardener). This material is commonly used for three-dimensional photoelastic experiments which employ the stress freezing technique. The principal reason for using this material, at least for the present, in studies of hydraulic fracture is that it can be cast into large blocks with relative ease in the laboratory. The only equipment needed forth is purpose is a curing oven with accurate temperature control.

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