Laboratory Study of Effects of Shear Stress on Fracture Permeability
Jeffrey Linscott, D. W. Stearns
Single-fracture hydraulic properties must be well understood before multifracture reservoir systems can be modeled with confidence. However, subsurface fractures cannot be observed or measured directly. Therefore, in-situ reservoir conditions must be simulated in the laboratory for reservoir modeling.
In the past, such studies used a uniform confining pressure while measuring fracture permeability. This pressure is adequate for determining the effects of normal stress on fracture permeability; however, since most subsurface stress states are distinctly nonhydrostatic, many fractures in natural reservoirs are subject to shear stresses in addition to normal stresses. Therefore, in this study fracture permeabilities are measured while applying both shear and normal stresses to the fracture in ratios that simulate true subsurface conditions. The resulting measurements indicate that shear stresses do affect fracture permeability. Specifically, the data show that: (1) fracture permeability reduction due to shear stresses is more likely permanent than permeability reduction due to normal stresses; (2) the effect shear stress has on fracture permeability is lithology dependent; (3) the shear stress effect depends on the magnitude of the normal stress applied; and (4) the amount of shear stress effect on fracture permeability depends on fracture surface roughness.
These data imply that in the subsurface, fracture orientation may affect fracture permeability. Further, repressuring a reservoir may affect certain fractures more than others.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.