Water treatment process for reducing the hardness of an oilfield produced water

摘要

A multi-stage process is provided for treating a water to reduce its hardness. The feed water is initially heated to a thermal reaction temperature in a heating stage. The heated water is then maintained at or near the thermal reaction temperature for a sufficient residence time to thermally react the water, producing a first portion of a scale from the scale-forming ions in the water while discharging dissolved gases from the water. After the thermal reaction stage, one or more chemical additives in the form of a hydroxide ion donor and optionally a bicarbonate ion donor are added to the heated water in a chemical reaction stage. The chemical additive is chemically reacted with the water in the chemical reaction stage, producing a second portion of the scale from the scale-forming ions in the water. A treated water having a reduced hardness is recovered from a sludge settling stage and the scale produced by the thermal and chemical reaction stages is discharged from the water and accumulated in a sludge accumulation stage for withdrawal from the process.