Fractures often control subsurface fluid flow and efforts to predict near-surface fluid flow typically focus on quantifying the relative influence of aperture and surface roughness on transmissivity. At greater depths, lithostatic and tectonic stresses play an increasingly important role in controlling fracture transmissivities. For the case of well-correlated fracture surfaces, such as those expected in new tensile fractures, fracture transmissivities quickly become negligible as a normal stress is applied to the fractures. However, if these fracture surfaces are displaced in shear, the fractures remain open and conductive at much higher stresses. Shear displacement of two well correlated or perfectly mated surfaces leads to anisotropy in the correlation structure of the fracture aperture field. In the presence of large normal stress, it is likely that only fractures subjected to some shear displacement will remain conductive, and thus, they are likely to exhibit some degree of anisotropy. Predicting the degree of anisotropy requires quantifying the combined influence of surface roughness, shear displacement of the fracture surfaces and the applied normal stresses. We use previously tested computational models to explore the influence of shear displacements of fracture surfaces subjected to a sequence of normal displacements (increasing stress) on transmissivity both parallel and perpendicular to the shear displacement of the surfaces. Results suggest that normal deformation of displaced fracture surfaces can lead to an order of magnitude increase in the anisotropy ratio from that observed in the unstressed fracture. These results suggest that the influence of normal stress on fracture anisotropy must be considered when implementing effective-continuum or discrete-fracture-network representations of fluid flow through fracture rock masses.
Medina, R. (University of California) | Elkhoury, J.E. (University of California) | Detwiler, R.L. (University of California) | Morris, J.P. (Schlumberger-Doll Research Center) | Prioul, R. (Schlumberger-Doll Research Center) | Desroches, J. (Services Pétroliers Schlumberger)
We conducted experiments in which a high concentration (50% v/v) of granular solids suspended in a non-Newtonian carrier fluid (0.75% guar gum in water) flowed through a parallel-plate fracture. Digital imaging and particle-imagevelocimetry analysis provided a detailed map of velocities within the fracture. Results demonstrate development of a strongly heterogeneous velocity field within the fracture. We observed the highest velocities along the no-flow boundaries of the fracture and the lowest velocities along the centerline of the fracture. Computational fluid dynamics (CFD) simulations using a recently developed model of the rheology of dense suspensions of mono-disperse solids in Newtonian carrier fluids closely reproduced experimental observations of pressure gradient versus flow rate. Results from additional simulations suggest that small (3%) variations in solid volume fraction within the fracture could lead to significant (factor of two) velocity variations within the fracture with negligible changes in observed pressure gradients. The variations in solid volume fraction persist over the length of the fracture, suggesting that such heterogeneities may play a significant role in the transport of dense suspensions. Our work suggests that a simple average conductivity parameter does not adequately represent the flow of high solid content suspensions in a fracture, as the flow develops strong three-dimensional structure even in a uniform rectangular channel.