Porosity reduction by hydrate/ice deposit

Hi, 

I have a question regarding the porosity reduction of a rock after hydrate/ice formation. I see that there is the compressibility and expansivity option on the ROCKS data block which determines the porosity reduction of the rock. I would, however, assume that if solid ice or hydrate is formed in the porous media, the porosity of the media must decrease even if the  rock matrix doesn't expand or compress. Please correct me if I am making a wrong assumption. 

My model is showing formation of hydrates but the porosity isn't changing if I don't put the compressibility and expansivity values in ROCKS data block. Is there anything I have missed so far? I see the NAD =6 option for when ice or hydrates form but it only has the compressibility values that needs to be entered but not the porosity change due to solid formation and clogging of pores. Can someone shed some light into this issue?

George Moridis Matthew Reagan

Thanks