The more mature the fields are, the more water you will need to squeeze the oil out of the reservoir. It is the water that contains ions such as calcium, barium and sulphates which form scale. The scale types that are experienced in the wells are calcium carbonate, iron sulphides and barium sulphates.
Acid wash, where acid is pumped into the well to remove the scale, is one of the oldest and most used methods. However, the acid only dissolves the calcium carbonate and iron sulphide scales; it does not prevent scale from forming. Acid wash is an expensive mitigation measure as production has to be shut in while the operation is being carried out plus the handling of the acid is a potential safety risk.