ABSTRACT

Laboratory flow tests have been performed to assess the effects of normal and shear stresses on the permeability of radial fractures around borehole. The rock specimens are prepared from Phu Kradung sandstone to obtain hollow cylinders having outside and inside diameters of 18.6 and 3.3 cm with a length of 15 cm. The rock is uniform and effectively impermeable. A radial fracture is artificially made by tension inducing method. It cuts through the borehole axis and along the specimen diameter. After applying a constant diametrical loading, the water is injectedunder constant head into the center hole. The fracture permeability is determined for various fracture orientations with respect to the vertical loading direction with 15° apart. The flow tests arerepeated 3 times under each vertical load to assess the permanent closure of the fracture under loading. The diametrical loads are progressively increased from 0.63MPa to 1.85MPa. Finite difference analyses have been performed to calculate the normal and shear stress distributions on the fracture under various orientations. The results indicate that the increases of the normal stresses rapidly decrease the fracture permeability.When the normal of fracture is deviated from the loading direction, the shear stress can increase the fracture permeability. A permanent closure of the fracture is observed as evidenced by the permanent reduction of the fracture permeability measured from the second and third cycles. The changes of aperture, water flow rate, and applied are used to calculate the changes of the fracture permeability. The fracture permeability is in the range between 1 × 10–18 m2and 1.5× 10–15m2.

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