WATER FLOW IN A NATURAL ROCK FRACTURE AS A FUNCTION OF STRESS AND SAMPLE SIZE

Abstract Five granite cores (0.100, 0.150, 0.193, 0.245 and 0.294m dia), each containing part of the same natural fracture oriented normal to the core axis, were used to study the effect of changes in sample size on the normal stress-permeability properties of natural fractures. Each fracture sample was subjected to three uniaxial compressive loading and unloading cycles with maximum axial stresses of 30 MPa. In each loading and unloading cycle, fracture deformation and steady-state flow rate through the fracture plane from a central borehole were measured for specific levels of normal stress. Fracture flow rate was found to decrease with increasing sample size and with each additional loading cycle. In addition, the deviation of the relation between fracture flow rate and fracture deformation from behaviour predicted by a parallel plate model increased with sample size and number of loading cycles. These properties are assumed to be related to the roughness characteristics of the fracture samples. Lacking detailed measurements of the roughness characteristics, these properties were deduced from the experimental deformation and flow-rate data using theoretical load-deformation and stress-permeability models. The data indicate that the smaller fracture surfaces may be characterized by a few high asperities unevenly distributed over the sample, while the larger fracture surfaces may be typified by a greater number of more variable height asperities more evenly distributed over the sample. With the application of normal load, the larger samples have more asperities in contact and therefore lower contact point stresses and more tortuous flow channels. This results in less permanent fracture deformation, less flow-rate and deformation hysteresis between loading and unloading cycles and greater deviation of the data from the cubic flow law.