46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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Hydromechanical tests under different stress paths (hydrostatic and proportional with constant ratio of vertical to horizontal stress rates) have been performed on a weakly cemented layered sandstone, the Otter Sherwood Sandstone, outcrop analog of the Sherwood reservoir of the Wytch Farm oil field (UK). The elastic and plastic deformation regimes are well identified and the determined yield stresses are fitted using the modified Cam-clay and Elliptic Cap models for all the observed onsets of plastic yielding. Both vertical and horizontal permeability have been measured during loading. For the horizontal flow, the geometrical and anisotropy factors were determined using Finite Element simulations in order to calculate the correct horizontal permeability. Permeability evolutions follow closely the material deformation and are controlled by both volumetric and shear strains. It is possible to infer the effect of the mean pressure and/or the deviatoric stress on the permeability evolution by building isopermeability maps in the stress space. Finally, an application of elasto-plastic modeling to predict the hydromechanical behavior of this sandstone is presented. This approach allows a satisfying prediction of the permeability evolution with stresses, using an exponential function of an effective strain.
During hydrocarbon production, the decrease of pore pressure (depletion) induces anisotropic effective stress increases, which depend on the reservoir rocks properties, leading to the compaction of the reservoir. In the worst case, inelastic deformations and irreversible reduction of porosity may induce subsidence issues in the oil fields with severe consequences such as well failure, permeability reduction, and reservoir impairment . Unconsolidated or weakly consolidated reservoir rocks with high porosity and low cohesive strength are prone to experience large strains as well as drastic decrease of permeability under stress. The predictions of strain and permeability evolutions are important objectives of reservoir engineering, in order to control and optimize hydrocarbon recovery.
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