Formation of a shale barrier using numerical and analytical methods
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The petroleum industry is looking for less expensive solutions regarding Plug & Abandonment. Earlier studies have shown incidents that shale formation around a well sometimes deforms and establishes sufficient seal around cas- ing. This may reduce costs of plugging operations significantly, and it is therefore valuable to learn more about this time dependent deformation phenomenon. This thesis investigates creep and plastic strain as possible causes for this deformation. Analyses on creep are simulated in PFC (Particle Flow Code). The simulations include calibration of strain from a real shale specimen and simulations of creep around a borehole. The calibrated parameters are then applied for the simulations on creep around a borehole to investigate how this real shale behaves during creep in well-scale. Then, creep threshold stresses in the code are evaluated, and it is discovered huge uncertainties regarding this parameter. The plastic strain deformation part includes a derivation of an expression of total strain around a borehole using the Mohr-Coulomb criterion with altered plastic parameters. Due to its effective stress dependency, the Mohr-Coulomb criterion is well suited to evaluate plastic strain deformation in a borehole surrounded by a shale subjected to long term drainage. After deriving the expression, parameter analyses show that the unconfined residual strength C0* has large impact on a rock s ability to deform around a well.