Numerical simulations for the effects of normal loading on particle transport in rock fractures during shear

Abstract

The fluid flow and tracer transport in a single rock fracture during shear processes has been an important issue in rock mechanics and is investigated in this paper using Finite Element Method (FEM) and streamline particle tracking method, considering evolutions of aperture and transmissivity with shear displacement histories under different normal stresses, based on laboratory tests. The distributions of fracture aperture and its evolution during shear were calculated from the initial aperture fields, based on the laser-scanned surface roughness features of replicas of rock fracture specimens, and shear dilations measured during the coupled shear-flow tests in laboratory. The coupled shear-flow tests were performed under two levels of constant normal loading (CNL). A special algorithm for treating the contact areas as zero-aperture elements was used to produce more accurate flow field simulations by using FEM. The simulation results agreed well with the flow rate data obtained from the laboratory tests, showing that complex histories of fracture aperture and tortuous flow channels with changing normal stresses and increasing shear displacements for the flow parallel with the shear direction. The flow perpendicular to the shear direction was also predicted and normalized flow rates were compared with those for the flow parallel with the shear direction. A greater increase is observed in the direction perpendicular to the shear direction due to the significant flow channels newly created by shear. This clearly shows the shortcomings of the conventional coupled shear-flow tests in a laboratory with flow in direction parallel with the shear direction. From the obtained flow velocity fields, the particle transport was predicted by using a streamline particle tracking method with calculated flow velocity fields (vectors) from the flow simulations, obtaining results such as flow velocity profiles, total flow rates, particle travel time, breakthrough curves and the Péclet number, Pe, respectively. Analyzing breakthrough curves for the unidirectional particle transport, the transport behavior in the fracture is also anisotropic and advective transport is more dominant in the direction parallel with the shear direction. The effect of normal stress on the particle transport is significant and dispersion becomes larger with increasing normal stress. The scientific findings from these studies provided new insights to the physical behavior of fluid flow and mass transport in rock fractures that is the scientific basis for many rock mechanics problems at the fundamental level, and with special importance to rock engineering problems.

Fracture permeability under normal stress: a fully computational approach

“…2 is qualitatively similar to the stressdisplacement plots in (Koyama et al 2008;Malama and Kulatilake 2003). The displacement values represented by the solid line in Fig.…”

“…2 contain both the deformation (closure) of the fracture and the deformation (compression) of the bulk rock. As mentioned in (Koyama et al 2008), the rightmost part of the solid curve corresponds to the elastic deformation of the bulk rock and is therefore linear in this simulation. The linear component of the deformation is plotted as a dashed line with triangular markers in Fig.…”

“…It should be noted that numerical modelling of this type has been used for the evaluation of fracture permeability in many previous studies, e.g. (Brown 1987;Koyama et al 2008). From the flow simulation (step 6 in the above list), the hydraulic fracture aperture was obtained as a function of the normal stress or displacement.…”

“…2. We now follow the procedure described in (Koyama et al 2008) to extract the fracture deformation curve from these simulation data. Shifting the solid line leftwards so that it now passes through the origin produces the dashed line with square markers in Fig.…”

“…It should be noted that asperities also have a significant impact on particle transport. In particular, surface roughness gives rise to hydrodynamic dispersion during particle transport in fractures (Bauget and Fourar 2008;Cumbie and McKay 1999;Koyama et al 2008;Nowamooz et al 2013).…”