MODELING ADDED COMPRESSIBILITY OF POROSITY AND THE THERMOMECHANICAL RESPONSE OF WET POROUS ROCK WITH APPLICATION TO MT HELEN TUFF

This paper is concerned with modeling the response of a porous brittle solid whose pores may be dry or partially filled with fluid. A form f or the Helmholtz free energy is proposed which incorporates known Mie- Gruneisen constitutive equations for the nonporous solid and for the f luid, and which uses an Einstein formulation with variable specific he at. In addition, a functional form for porosity is postulated which de pends on two material constants that control the added elastic compres sibility of porosity observed in porous rock. The irreversible process of pore crushing is modeled using compaction and dilation surfaces to determine a scalar internal variable that measures unloaded porosity. Restrictions on constitutive assumptions for the composite of porous solid and fluid are obtained which ensure thermodynamic consistency. E xamples show that although the added compressibility of porosity is de termined by fitting data for dry MI. Helen tuff, the predicted respons es of saturated and partially saturated tuff agree well with experimen tal data.