Stress sensitivity of fractured reservoirs

A fractured reservoir has a rock mechanical character which can be described by considering it to be built up from intact rock bounded by mechanical discontinuities. These discontinuities have been formed by natural processes and comprise fractures which normally make a steep angle to bedding, and the frequently ignored bedding-plane parallel discontinuities caused by, for example, weak clay-rich layers in clastics, or stylolites in carbonates. Both the intact rock and the discontinuities exhibit sensitivity to stress. In the case of the intact rock, this expresses itself as a pore geometry sensitivity which influences permeabilities and capillary pressures, with the changes being influenced by the stress-state. In the case of the discontinuities, three types of permeability changes are proposed, depending upon the stress and strain which develops across the discontinuity. Importantly, all the constitutive laws for permeability stress-sensitivity can now be incorporated in simulations. While allowing for these effects complicates the simulation of fractured reservoirs, the enhanced realism brought to the simulation should improve the efficacy of the reservoir management.