Viscoplastic Deformation of Shale Gas Reservoir Rocks and its Relation to the In-Situ Stress Variations Observed in a Well from Barnett Shale
We have studied the viscoelastic/viscoplastic properties of various shale gas reservoir rocks through laboratory triaxial creep experiments under confining pressures representative of in situ conditions. We find that the viscoelastic stress relaxation behavior of these rocks vary considerably and is generally larger for rocks with more clay and organic content. The strain response of the rock is adequately described by a power-law function of time, and its magnitude is approximately linear against the magnitude of the applied differential pressure but insensitive to the confining pressure. Motivated by these observations, we described the rheology of the shales in the framework of linear viscoelasticity in order to calculate the differential stress accumulation/relaxation that would occur in these reservoirs over geological time scales. Variation in viscoelastic properties within the Barnett Shale can create differential stress variations on the order of one to tens of MPa, consistent with fluctuations in stress difference inferred from observations of wellbore failures. Although time-dependent deformational behaviors of intact reservoir rocks are not studied routinely in the lab, we suggest that it can have a significant impact in determining the current in-situ state of stress when reservoir deformation takes place over geological time scales.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013