Methods and systems for treating produced water
Produced water from a crude oil or natural gas production process is purified using a membrane purification system for petroleum production, agricultural, commercial and domestic uses. The produced water is pretreated to remove, at least, particulates and oil from the produced water. The minimally pretreated water is then purified in a membrane purification system, that is operated at conditions such that membrane scaling is reduced or prevented. In particular, the membrane purification system is operated to maintain the turbidity of clarified water feed to the system or intermediate aqueous streams that are cascading through the membrane purification system. Ensuring that the turbidity of the reject streams generated in the membrane system are useful in achieving long membrane operating life.
1. A method of treating produced water produced from an oil or gas well, comprising, in order:
recovering the produced water comprising a mixture of water, oil, solid particulates and dissolved solids from a well extending into a subterranean formation;
pretreating the produced water to remove a portion of the oil and solid particulates therefrom, to produce a clarified water having a first pH from about 10.5 to about 11.5, a turbidity of no more than 0.5 NTU units, an oil content of at most 5 ppm, an ammonia content of greater than 10 mg/L, a boron content of greater than 5 mg/L and solid particulates that are at most 5 μm in size;
passing the clarified water through a first reverse osmosis membrane module comprising a first plurality of reverse osmosis membrane elements, thereby producing a first permeate stream and a first retentate stream from the first reverse osmosis membrane module wherein the boron remaining in the first permeate stream is less than 5% the boron content of the clarified water and is at most 0.5 mg/L;
adjusting a pH of the first permeate stream to produce a second membrane module feed having a second pH in a range from 3 to 6;
passing the second membrane module feed to a second reverse osmosis membrane module comprising a second plurality of reverse osmosis membrane elements thereby producing a second permeate stream and a second retentate stream from the second reverse osmosis membrane module, wherein the ammonia remaining in the second permeate stream is less than 5% the ammonia content of the clarified water and is at most 5 mg/L; and
recovering a purified water;
wherein the method does not include an ion exchange separation step capable of removing ammonia.
2. The method of claim 1 , wherein the clarified water has an ammonia content of greater than 25 mg/L and a boron content of greater than 25 mg/L.
3. The method of claim 1 , wherein the ammonia remaining in the second permeate stream is less than 10% of the ammonia contained in the clarified water.
4. The method of claim 1 , wherein the ammonia remaining in the second permeate stream is less than 5% of the ammonia contained in the clarified water.
5. The method of claim 1 , wherein the ammonia remaining in the second permeate stream is less than less than 3% of the ammonia contained in the clarified water.
6. The method of claim 1 , wherein the purified water contains no more than 0.79 mg/L ammonia and no more than 0.41 mg/L boron.