A Preliminary Analysis of Edge-effects in True-triaxially Deformed Cubic Darley Dale Sandstone Samples

Abstract

Subjecting rocks to true-triaxial deformation causes the problem of edge effects due to frictional constraint between loading ram, platen of testing apparatus and sample faces. To analyse these effects, cubic samples of true-triaxially deformed Darley Dale Sandstone were studied for crack sensitive properties. Ultrasonic wave velocities depict a clear change from the edges to the middle portion of the sample, the nature of these changes being dependent upon the loading direction. Microcrack study using SEM analysis showed no significant trend. Steady-state flow permeability measurements made from a cylindrical core cut through the middle of a sample showed that the permeability varied from 1.54×10^-15 m^2 at one edge to 1.74×10^-15 m^2 in the middle of sample and 1.63×10^-15 m^2 at the other edge, thereby showing no significant variation. So, Permeability is not significantly influenced due to edge effects. Porosity measurements using He- Pycnometer significantly displayed edge effects as 10% variation was observed from edges to the rest of the sample. Results confirm the presence of potential edge effect in true-triaxial testing which needs to be minimized using proposed methods. These analyses can be used to ascertain the mechanical failure in response to changes in the in-situ stress conditions in shale reservoirs.