Methods for treating produced water to remove boron and ammonia

Abstract

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 contaminants are removed. In particular, the membrane purification system is operated with pH adjustments to allow boron and ammonia to be effectively removed. In some embodiments, greater than 95% of the boron content and greater than 95% of the ammonia content are removed. Some method embodiments include no separate ion exchange separation step capable of removing ammonia.

What is claimed is: 
     
         1 . A method of treating produced water produced from an oil or gas well, comprising:
 a. recovering the produced water comprising a mixture of water, oil, solid particulates and dissolved solids from a well extending into a subterranean formation;   b. pretreating the produced water to remove a portion of the oil and solid particulates therefrom, to produce a clarified water having a turbidity of no more than 0.5 NTU units, an oil content of at most 5 ppm, solid particulates that are at most 5 μm in size, a boron content of greater than 25 mg/L, and an ammonia content of greater than 10 mg/L; and   c. removing boron content and ammonia content from the clarified water using a first reverse osmosis membrane module and a second reverse osmosis membrane module to form purified water; wherein:   the clarified water is passed through one of the first or second reverse osmosis membrane modules at a first pH from such that the boron content is reduced to less than 5% of the boron content of the clarified water; and   the clarified water is passed through one of the first or second reverse osmosis membrane modules at a second pH such that the ammonia content is reduced to less than 5% the ammonia content of the clarified water.   
     
     
         2 . The method of  claim 1 , wherein the first pH is from about 10.5 to about 11.5. 
     
     
         3 . The method of  claim 1 , wherein the second pH is from about 3 to about 7.9. 
     
     
         4 . The method of  claim 1 , wherein the first plurality of reverse osmosis membrane elements and second plurality of reverse osmosis membrane elements comprise polymeric membrane elements. 
     
     
         5 . The method of  claim 1 , wherein pretreating the produced water includes filtering the produced water using microfiltration, ultrafiltration, nanofiltration and combinations thereof. 
     
     
         6 . The method of  claim 1 , further comprising passing the produced water through an ultrafiltration membrane, wherein the produced water contains greater than 100,000 ppm TDS. 
     
     
         7 . The method of  claim 1 , wherein pretreating the produced water comprises partially softening the produced water to a hardness in a range from 1 to 50 ppm. 
     
     
         8 . The method of  claim 1 , wherein pretreating the produced water comprises partially softening the produced water at a temperature of from 150° F. to 200° F. 
     
     
         9 . The method of  claim 1 , wherein pretreating the produced water comprises partially softening the produced water by passing the produced water through an ion exchange softening process for reducing the hardness of the produced water and wherein the partially softened produced water has a hardness of greater than 50 ppm. 
     
     
         10 . The method of  claim 1 , wherein pretreating the produced water does not include a warm lime softening process, and further does not include an ion exchange softening process. 
     
     
         11 . The method of  claim 1 , wherein pretreating the produced water includes precipitating particulates in the produced water in a clarification module, wherein precipitating particulates in the produced water includes supplying a sufficient amount of an alkaline chemical, selected from sodium hydroxide, soda ash, sodium carbonate, calcium oxide, calcium hydroxide, calcium carbonate, magnesium oxide and combinations thereof, to the produced water to increase its pH by at most 2 numbers, and further comprising filtering the pretreated water from the clarification module. 
     
     
         12 . The method of  claim 1 , wherein the produced water has a TDS content of greater than 1000 mg/l. 
     
     
         13 . The method of  claim 1 , wherein the clarified water has a TDS content of greater than 1000 mg/l. 
     
     
         14 . The method of  claim 1 , wherein the purified water contains no more than 0.004 ppm hardness, no more than 0.1 ppm silica and no more than 19 ppm TDS. 
     
     
         15 . The method of  claim 1 , wherein the ammonia content is reduced to less than 3% the ammonia content of the clarified water. 
     
     
         16 . The method of  claim 1 , wherein the purified water contains no more than 0.5 mg/L boron. 
     
     
         17 . The method of  claim 1 , wherein the purified water contains no more than 5 mg/L ammonia. 
     
     
         18 . 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. 
     
     
         19 . The method of  claim 1 , wherein a feedstream for the second reverse osmosis membrane comprises a permeate stream of the first reverse osmosis membrane module. 
     
     
         20 . The method of  claim 1 , wherein a feedstream for the first reverse osmosis membrane comprises a permeate stream of the second reverse osmosis membrane module.