Mass Transfer & Porous Media (MTPM) - Andra

P/MTPM/30GAS INTRUSION IN SATURATEDBENTONITE – A THERMODYNAMICAPPROACHMartin Birgersson, Mattias Åkesson, Harald HökmarkClay Technology AB, Ideon Research Centre Lund, SwedenA thermodynamic description of the response of a saturated bentonite system to changes in externalvariables has successfully been adopted in several different cases. Examples include correspondencebetween swelling pressure and water retention properties of unconfined bentonite ( Bucher and Müller-Vonmoos, 1989), pressure response due to changes in salt concentration of an external solution(Karnland et al., 2005) and the description of the behavior of confined bentonite below 0˚C. In thepresent work the same thermodynamic framework is used as a starting point for describing gasintrusion in saturated bentonite.The response to changes in external variables (water pressure, gas pressure, temperature, salinity etc) of aconfined bentonite system penetrable to water is a matter of equalization of the chemical potentials of waterin the clay and water external to the system. The chemical potential of clay water under isothermalconditions and without added solutes isµ clay( w) = µ 0 + RT ln RHunconf.( w)+ P⋅v wWhere µ 0 is a reference chemical potential for bulk water in equilibrium with its vapor pressure atthe considered temperature, RHunconf. ( w ) is a function describing equilibrium relative humidity forthe unconfined clay with water content. Here the molar volume of clay water is assumed to be equalto the bulk value, v w , and independent of pressure.A homogenous, isotropic model of bentonite is presented which is completely defined by the curveRHunconf. ( w ) , and the water content at saturation, w sat . Total pressure at equilibrium in this model iswhereP g > 0⎧ ⎪ Ps+ Pl, saturatedPTOT( Pg, P l)= ⎨⎩⎪ Pg, unsaturatedRTP s =− ln RH ( w sat ) ,v wis gas pressure and the chemical potential of external water isµ = µ 0 + Pl⋅ v w,which defines liquid pressure, P l , when µ − µ 0 < 0 . The criterion for saturation is PS > Pg −Pl≡ s ,where s denotes suction.It should be noted that the present model is based on thermodynamic equilibrium in a rather strict sensewhich is not achieved in experiments where the initial state is unsaturated. E.g. the stress build-up in ainfiltration test do not follow the equation for total pressure from above. On the other hand, it can be arguedbased on experimental facts that the model is relevant to adopt when describing gas entry events, wherethe initial state is a water-saturated system.INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCECLAYS IN NATURAL & ENGINEERED BARRIERSFOR RADIOACTIVE WASTE CONFINEMENTPage 481