HYDRAULIC PULSE TESTING OF SINGLE FRACTURES IN POROUS AND DEFORMABLE HARD ROCKS

Single borehole pulse injection tests were carried out on single natur al fractures in granitic rocks at a depth of 80 to 417 metres to deter mine the hydraulic aperture. Pressure pulses were created by injecting a small volume of water into the fracture using a compressed-air driv en downhole valve. The pulses were of short duration with a fast press ure decay and apertures as small as a few microns could be measured. T he field tests were modelled with a finite element program which simul ates coupled fluid flow and mechanical deformation in both the fractur e and the ambient rock matrix. The results of modelling show that if f racture flow dominates, then the storativity is dominated by the mecha nical deformation of the fracture. For the short duration pulse tests used in this study, the deformation of the fracture and thus the stora tivity is restricted by the stiffness of the ambient rock. The results of the modelling indicated that the fracture storativity is dose to 3 .5 x 10(8). Field tests in hard rocks can therefore be evaluated with conventional curve matching methods using an assumed storativity to de termine flow dimension and transmissivity. The known storativity makes the determination of hydraulic aperture of the fracture less subjecti ve.