If you start will all water after the well has cleaned up you will always get water. Gas and oil will not come later. If you get a good oil and gas zone the water may or may not come later. With a water drive reservoir, water will eventually come. With a geo-pressured reservoir you should get 0 water at first and as the oil and gas is extracted the rock will eventually crush and the water will come in. Remember this, Oil and gas comes first not last. Hydrocarbons don't drive water, water drives hydrocarbons. Hope this helps.