An experimental study of the texture of deformation bands: effects on the porosity and permeability of sandstones

Abstract

We have investigated the texture and formation of deformation bands in relation to permeability and porosity. Video image analysis of the Brumunddal sandstone showed a decrease in the number of large pores in the deformed zones. The frequency of small pores is increasing in the intermediate zone, compared to the undeformed rock and the central zone of a deformation band. Triaxial compression tests were performed on Red Wildmoor sandstone with constant confining pressure (8 MPa). Axial P-and S-wave velocities measured during loading showed structural changes in development of a deformation band: Stage I and II – closure of micro-cracks and pores and tighter grain packing parallel to the maximum stress direction and simultaneously dilation perpendicular to the maximum stress direction. Stage III – both the P-and the S-wave velocities decreased, reflecting tighter grain packing and development of micro-fractures. These observations are supported by permeability measurements taken before, under and after triaxial compression, with recovering of permeability due to elastic effect and static reduction due to tighter packing and ultimately grain size reduction. NMR images of oil-saturated samples after loading to failure show: stage III – grain size reduction; stage IV – secondary fracturing; and stage V – development of a slip plane.