1 research outputs found
Permeability of matrix-fracture systems under mechanical loading – constraints from laboratory experiments and 3-D numerical modelling
The permeability of single fractures is commonly
approximated by the cubic law assumption, which is however
only valid under the condition of a single phase laminar flow
between parallel plates. Departure from cubic law are related
to many features like aperture fluctuations due to fracture
surface roughness, relative shear displacement, the amount
of flow exchange between the matrix and the fracture itself,
etc. In order to quantify constitutive relationships among the
aforementioned aspects, we have conducted a flow-through
experiment with a porous rock sample (Flechtinger sandstone)
containing a single macroscopic fracture. Based on
this experiment, we obtained range of variations of intrinsic
rock parameters, permeability and stress-strain relationships
of the combined matrix-fracture system under hydrostatic
loading. From the measured deformation of the matrixfracture
system, we derived the evolution in the mechanical
aperture of the fracture. In order to quantify the processes
behind the laboratory observations, we carried out coupled
hydro-mechanical simulations of the matrix-fracture system.
Navier–Stokes flow was solved in the 3-dimensional open
rough fracture domain, and back-coupled to the Darcy flow
and the poroelastic behaviour of the rock matrix. The results
demonstrate that the elastic behaviour and the related permeability
alteration of the fracture domain could be captured by
the numerical simulation. Furthermore, the stress-strain values
obtained in the vicinity of the fracture asperities suggest
that inelastic deformation develops at low mechanical load.
An attempt was made to quantify the inelastic deformation
by using the failure envelope obtained by laboratory experiments
(whether tensile, shear, compaction, or a combination
of those). However, change in permeability observed in the
experiments are significantly larger than that in the simulation
showing the importance of plastic deformation during
opening and closure of the fracture and its impact on the cubic
law approximation