Abstract
This work aims at improving our understanding of the effect of triaxial stress on absolute permeability of homogeneous and heterogeneous rocks. Measurements of absolute permeabilities of homogenous and heterogeneous laminated rock samples under hydrostatic and different laboratory-simulated triaxial in situ stress loadings were prepared. Experiments were conducted using homogenous, standard Berea and heterogeneous laminated sandstone cores (with lamination parallel to the flow direction). The effect of in situ stress variation on absolute permeability was investigated. Absolute permeability of homogenous sandstones sample decreased in a uniform manner as the axial load increased below the ultimate shear strength. As axial stress exceeds the radial confining stress, slight increase attributed to microcracks opening was noticed. For laminated sandstones, the scenario is quite different. Permeability shows exactly a similar trend of permeability drop as axial load increases due to inferred matrix compaction. At certain axial load, the permeability starts to increase due to inferred dilatancy of microcracks at the lamina faces and then drops again as axial load increases due to lamina compaction. If the axial load is further increased, pore collapses and grain-to-grain cementing breaks down into pore space followed by microcracks development predominantly parallel to the axial load leading to permeability enhancement. It has been concluded that absolute permeability changes due to lamination opening and closure as a result of loading magnitude and orientation. It is also concluded that permeability of the formation rock is affected by heterogeneity depending on the direction of lamination as well as the state of the stresses applied and loading type.