Representative elementary surface for morphology and permeability of natural rock fractures

Fracture surface morphology, mechanical and hydraulic properties are related to fracture size. One of the core issues in the size effect is the representative elementary surface, which determines the size of the test specimens in the laboratory and facilitates the application of test results to the field. In this study, the size effect on surface morphology and permeability of rock fractures is investigated, and the representative elementary surface size for the surface morphology and the permeability was determined based on the asperity height data of fracture surfaces obtained by 3D laser scanning in the field. The roughness of fracture surfaces varies significantly at small fracture surface sizes. As the fracture surface size continues to increase, the fracture surface roughness tended toward a relatively stable state. The critical size for fracture surface roughness stability in the study, which is called the representative elementary surface, is 300-400 mm. The size effect on the permeability of rock fractures was investigated, and the results show that fracture permeability tended toward a relatively stable state with increasing fracture size, which was the same as the fracture surface roughness. The representative elementary surface size for fracture permeability is less than 100 mm in the study, which is also less than that for fracture surface roughness with the same coefficient of variation. This result indicated that a lower coefficient of variation might provide the best estimate of the representative elementary surface for fracture permeability compared with the fracture surface roughness.

Automated summary

The study investigates the size effect on surface morphology and permeability of rock fractures. The representative elementary surface size for fracture surface roughness is 300-400 mm, while for fracture permeability it is less than 100 mm.