Fluids and melts in the mantle are key to the Earth’s evolution. The main driving force for their transport is the compaction of the porous mantle. Numerically, the compaction equations can yield unphysical negative liquid fractions (porosity), and it is necessary to enforce positive porosity. However, the effect of such a treatment on liquid flow and mass conservation has not been quantified. We found that although mass conservation is affected, the liquid pathways are well resolved.