Hi, I would like to know if these chemicals (demulsier, defoamer, scale inhibitor) are required for gas well? Or these chemicals are required for oil producing well only?
All of those chemicals need a continuous-phase liquid to be effective. One of the biggest problems I encounter in my business is people spritzing them into a gas stream and hoping that they'll go everywhere they need to go. They never do. The aerosols will eventually (30 seconds, 2 minutes, seldom much longer than a couple of joints of pipe) either coalesce into bigger drops that are not buoyant or attach themselves to the pipe-wall in a spotty manner that does no good.
Salt chemicals are a special case. They really work well in continuous-phase liquid (like, say, and oil well) because they will allow salts to move in a slurry. Gas wells lack the flow energy to transport the slurry and the salt becomes a bigger problem than it would have been without the chemicals.
De-foamers generally work counter to what a gas well needs (the industry spends millions of dollars pumping foamers into gas wells). De-foamers seem to be counter to that trend. I'm not a huge fan of indiscriminate use of foamers because I see unactivated soap in too many wells, and late activate soap in too many pipelines, but de-foamers in gas wells seem dumb.
In short the only way to successfully treat gas flow lines and wellbore tubulars is by batching the chemicals with pigs or plungers. Other than that, all the chemicals you mentioned are worse than worthless (they cost a lot and basically do no good).
Bottom line is that you need to be absolutely certain that you have a transport mechanism that has a chance of working before you spend a bunch of time and money on gas-well chemicals.