The permeability of whole and jointed Barre Granite

Abstract

The permeability of whole and jointed Barre granite was measured at pressures up to 2 kbars. Jointed samples were actually split cylinders joined by surfaces with controlled surface roughness. Samples with induced tension fractures were also measured. The permeability of the whole rock ranged from about 10 −6 to 10 −7 darcies. The permeability of the jointed rock ranged from about 8 × 10 −5 darcies at low pressure down to that of the whole rock at high pressures. Permeability was not a simple function of the difference between external confining pressure (P c) and internal fluid pressure (P f). Changes in permeability were found to be proportional to (b dP f − a dP c) where b/a < 1 for the jointed rock and b/a ≈ 1 for whole rock. The order of application of P c and P f was also important. Permeability hysteresis and an ultimate decrease in permeability in both whole and jointed rock resulted when internal fluid pressure was cycled. This effect seems to diminish with increasing confining pressure. At a particular P c, the volume flow rate, q, is proportional to (P c − P f) −n. Increasing the surface roughness of the joints decreased the value of n, which was smallest for the tension fracture and the whole rock. Within the uncertainty of joint aperture measurements, a flat plate model of the joint seem inadequate.