Depth dependent hydraulic conductivity in fractured sedimentary rocks-a geomechanical approach

Abstract

Several studies have documented a decrease in bulk hydraulic conductivity (bulk K) with depth in fractured rocks. It is commonly believed that this decrease in bulk K is a result of fracture closure that occurs because of the gradual increase in the overburden stresses with depth. Although, stress-closure behavior of a single fracture under controlled laboratory conditions had been thoroughly evaluated, field studies of fracture closure and the subsequent effect on the bulk K in common sedimentary rocks are rare. This study focuses on sandstone, dolostone, and shale rocks and presents results of a fracture closure with depth study and its effect on the hydraulic conductivity by applying a geomechanical approach. The working hypothesis that the fracture closure is a function of the fracture characteristics, rock matrix compressive strength, and the normal stress was explored further by applying Barton-Bandis (BB) method. Some of the procedures mentioned in the BB method were modified accordingly so that the methodology could be applied to the rock core samples. Continuous rock cores were obtained from the six drill holes in dolostone, sandstone, and shale. Joint roughness coefficient (JRC) and joint compressive strength (JCS) were determined by indirect methods. Fracture aperture data were obtained by physically measuring fracture aperture on the rock core samples. The data were processed by using a “closure” computer program to determine the fracture closure at various normal stresses. The cubic law was used to determine hydraulic conductivity for corresponding fracture apertures at various depths. The study concludes that an average aperture of 150 μm in sandstone would reduce to less than 10 μm under an applied normal stress of about 8 MPa, indicating a 1,000 times reduction in the hydraulic conductivity. At the same stress level, the closure was found to much more in shale and much less in dolostone.