A model for off-fault plastic poroelastic deformation and its effects on permeability

Fractured reservoirs comprise finite or discrete fracture networks; if these are conductive, they
form heterogeneously distributed high-permeability streaks. These are generally referred as
fracture corridors. Unless they occur as joint swarms, fracture corridors are simply seismic or sub-
seismic fault zones with connected fractures in the near-fault damage zone. Several studies
document the decrease in rock-matrix permeability adjacent to the fault surface, within the
damage zone. Although the damage zone creates fracture connectivity and high permeability
anisotropy for reservoirs, the matrix fracture feeding mechanism is related to matrix permeability
generally described by a transfer function. This transfer function accounts for fracture properties
(i.e. fracture density, length and connectivity), relative fluid mobilities, imbibition and reservoir
properties (i.e. matrix permeability). Commonly, the matrix permeability for all transfer functions
is considered in terms of a representative rock type permeability. However, observational
evidence and our numerical model show that slip induced deformation causes significant strain on
matrix in vicinity to the fault surface causing a permeability decrease in the matrix.

In this study, we present a new approach to model strain in a porous medium and related
permeability changes due to stress perturbation from slip around pure strike slip faults. The fault
length is used to scale the amount fault slip. For given/computed dislocation (slip) the off-fault
strain is then calculated to derive porosity and permeability changes. In our study we propose an
off-fault plastic-poroelastic deformation model for any known fault length and known rock
mechanical and petrophysical properties of the surrounding material. Our modeling technique will
help to better quantify fault transmissivity in geo-reservoirs.