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Sequential Methods for Coupled Geomechanics and Multiphase Flow

Sequential Methods for Coupled Geomechanics and Multiphase Flow

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6.4. STABILITY ANALYSIS VIA THE ENERGY METHOD 181<br />

which yields no additional restriction on convergence.<br />

6.4 Stability Analysis via the Energy Method<br />

6.4.1 Constitutive relations <strong>for</strong> multiphase flow <strong>and</strong> elastoplasticity<br />

We showed the constitutive relations <strong>for</strong> elastic mechanics <strong>and</strong> multiphase flow in Chapter<br />

2. Based on Coussy (1995)’s approach, we extend the constitutive relations <strong>for</strong> elasticity<br />

under isothermal conditions to those <strong>for</strong> poro-elasto-plasticity, which are written as<br />

δσ = Cdr : (δε − δεp) − bJδpJ1, (6.65)<br />

<br />

<br />

δm<br />

δpJ = MJK −bK(δεv − δεv,p) + − δφK,p , (6.66)<br />

ρ<br />

where the variation of the elastic fluid content of phase J is<br />

<br />

δm<br />

=<br />

ρ J,e<br />

K<br />

<br />

δm<br />

− δφJ,p. (6.67)<br />

ρ J<br />

Repeated indices indicate summation, again. (·)J,e <strong>and</strong> (·)J,p mean elastic <strong>and</strong> plastic<br />

quantities <strong>for</strong> fluid phase J, respectively. The plastic porosity of phase J <strong>and</strong> plastic strain<br />

can be related to each other by assuming that ˙ φJ,p = βJ ˙εp,v. Here, we assume that βJ = bJ,<br />

similar to single-phase flow, which yields<br />

δφJ,p = bJδεp,v. (6.68)<br />

For hardening, we can reuse the relation of the hardening variable ξ <strong>and</strong> <strong>for</strong>ce κ <strong>for</strong><br />

single-phase flow, namely,<br />

δκ = −H · δξ. (6.69)

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