Because of a localized decrease in pressure, water may rise to a height above the oil-water inteerface and be produced through a horizontal well. The localized increase in the water-oil interface elevation creates a cone of water near the well. If a large enough drawdown is exerted on the reservoir, water will start replacing the oil in the production stream. The instant when water first enters the well is known as the critical drawdown or threshold drawdown. As the producing water-oil ratio increases, the operating cost of the well as well as the oil production cost will start increasing. At some point, the well may need to be shut-in to allow the water-oil interface to subside. In this situation, an optimum shut-in time exists, and it is dependent upon the subsidence time of the water cone.
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