Effects of stress on the two-dimensional permeability tensor of natural fracture networks

Abstract

S U M M A R YThe effects of stress on the 2-D permeability tensor of natural fracture networks were studied using a numerical method (Universal Distinct Element Code). On the basis of three natural fracture networks sampled around Dounreay, Scotland, numerical modelling was carried out t o examine the fluid flow in relation to the variations in burial depth, differential stress and loading direction. It was demonstrated that the permeability of all the networks decreased with depth due to the closure of aperture.The permeability approached the minimum value a t some depth below which little further variation occurred. Also, differential stress had a significant effect on both the magnitude and direction of permeability. The permeability generally decreased with increasing major horizontal stress for a fixed minor horizontal stress, but the various networks considered showed different behaviours. A factor, termed the average deviation angle of maximum permeability (A,), was defined to describe quantitatively the deviation degree of the direction of the major permeability component from the applied major stress direction. For networks whose behaviour is controlled by set(s) of systematic fractures, A, is significantly greater than zero, whereas those comprised of non-systematic fractures have A, close to zero. In general, fractured rock masses, especially those with one o r more sets of systematic fractures, cannot be treated as equivalent porous media. Specification of the geometry of the network is a necessary, but not sufficient, condition for models of fluid flow. Knowledge of the in situ stress, and the deformation it induces, is necessary to predict the behaviour of the rock mass.

Stress-dependent permeability of fractured rock masses: a numerical study

“…This result is comparable to the permeability variation with depth reported in [29] with the depth corresponding to stress changes. Similar numerical results in [19,20] with constant normal stiffness of fracture also showed the decrease of permeability corresponding to the increase of stresses. However, the application of the non-linear normal stiffness of fractures in the fracture model in this study led to more sensitive responses of permeability change at lower normal stress magnitudes, and the permeability change becomes small when the stress reaches the laboratory level of residual stress (30 MPa) of fractures.…”

“…Despite the insightful illumination regarding abrupt increase of permeability in [21], the mechanical REV issue was not considered, which could make the mechanical behavior less representative and shear displacement overestimated. The other previous studies in [19,20] did not show such sudden increase of permeability because the dilation mechanisms are not modeled. Permeability continued to decrease with the increase of stress ratio up to 8 when zero dilation angle was used and stress ratio up to 9 when a high cohesion (1.8 MPa) was used as an input for fracture parameter compared to the minimum principal stress (1 MPa), which could not capture the dilation behavior [20,19].…”

“…As far as we know, the above three aspects were not considered, at least not systematically, in the previous studies of numerical experiments for stress-dependent permeability of fractured rock masses, including those reported in [18][19][20][21].…”

“…The influence of stresses on permeability of fractured rock masses was extensively investigated considering various geometries of fracture systems [19][20][21]. Such numerical experiments will be increasingly important with the advance of computing capacity due to the strength of distinct element modeling that can incorporate both hydraulic and mechanical analysis with explicit representation of fractures.…”

A Review of Critical Conditions for the Onset of Nonlinear Fluid Flow in Rock Fractures

“…Many factors can affect the magnitude of permeability of fractured rock masses, including fracture length [28][29][30][31], aperture [32][33][34], surface roughness [35,36], dead-end [37], number of intersections [38,39], hydraulic gradient [40], boundary stress [41,42], anisotropy [43][44][45][46], scale [47][48][49][50], stiffness [51], coupled thermo-hydro-mechanical-chemical (HTMC) processes [52][53][54][55], and precipitation-dissolution and biogeochemistry [56]. The discrete fracture network (DFN) model, which can consider most of the above parameters, has been increasingly utilized to simulate fluid flow in the complex 2 Geofluids fractured rock masses [57][58][59][60], although it cannot model the aperture heterogeneity of each fracture [61][62][63].…”