You will probably understand that it is really better to talk about an aqueous phase or brine phase but the misnomer "water phase" that is generally used comes from the retort measurement for the determination of the solids, oil & water content of an oilmud.
Most "water" phases usually carry a high content of either sodium chloride, calcium chloride or calcium nitrate depending upon the desired activity of the overall mud & any environmental constraints.
That said the "whole mud" salinity determination will give a concentration of chloride ions (mg/l mud) that is then divided by the decimal water fraction of the mud to indicate the water phase salinity WPS
If 15% - 30% v/v water is retorted then divide the mud salinity by 0.15 - 0.3 respectively for the WPS.
You should see here a "circular" argument.
"Corrected solids" content requires the subtraction of the soluble salt content that is deposited in the retort upon heating / evaporating the oil mud.
Likewise the water percentage requires a corresponding addition to describe the brine content.
The concentration of salt in the brine and its expansion factor for the salt volume is taken from tables using the chloride content of the brine.
Hence the field test method is a "first" approximation especially when high salinities are used in the brine phase.
For example when 30% w/w calcium chloride is being used (say with regard fior hydrate suppression not just shale inhibition) the expansion factor is 1.138.
Hence 23% water means 26% brine and the whole mud chlorides should really be divided by 0.26 not 0.23........
In absolute terms the WPS indicates a higher chloride content than the actual real brine phase salinity