Mono Ethylene Glycol (MEG) is a compound that is used as anti freeze.
Reason for using MEGmix with water is to avoid ICE-Plug buildup in rigup and well during Pressure testing, bullheading or circulation.
MEG does not react chemically with Brines often used as Packer fluids, completion fluids or other Annulus Brines.
MEG is for this reason and lower cost than TEG(Tri ethylene Glycol), used when there is circulation to be done in contact with these well liquid chemicals.
MEG is used in Pressure testing in wireline operations. Mixed with water it is also used for bullheading the well, for cleaning out the Hydrocarbons, or killing a well.