Continuous Ion Exchange for Wyoming CBM Produced-Water Purification: Proven Experience

Abstract

Abstract Water is pumped from thousands of shallow wells in the Powder River Basin of Wyoming to extract methane contained in coal bed seams, commonly referred to as coal bed methane or CBM. Over 2 barrels of water are pumped to produce a thousand cubic feet (MCF) of gas. The only environmental problem associated with the production of this clean burning domestic fuel is that the water's sodium bicarbonate content is so high that its discharge to the environment is harmful to both plant and animal life. EMIT Water Discharge Technology LLC and Severn Trent Services, Inc. have teamed together to apply one of STS's technologies for the purification of this produced water. The Higgins Loop Continuous Ion Exchange (CIX) System utilizes cation resins in an efficient mode to remove sodium from the water and concentrate it into a very small brine stream for disposal. The process also reduces the water's bicarbonate content by evolving carbon dioxide gas from the low sodium treated water under slightly acidic pH conditions. Purified water is then neutralized with limestone. This step increases its calcium content and makes the water more suitable for use in irrigation, human consumption, ranching and for aquatic life in rivers and creeks. The CIX process was first demonstrated in April, 2003 in a well field north of Sheridan, WY. Since then, EMIT has placed 15 more CIX Systems in operation in both Wyoming and Montana treating over 180,000 barrels per day of water. The technology has been accepted by both the gas producers and state regulators as the best available technology (BAT) and most economical process for CBM produced water treatment. This paper describes the reasons for produced water treatment, the purification process and the overall benefits to the industry and environment. Manuscript Water is pumped from thousands of shallow wells in the Powder River Basin (PRB) of Wyoming to extract methane contained in coal bed seams, commonly abbreviated CBM. At least 2 barrels of water must be pumped to produce a thousand cubic feet (MCF) of gas. Today, nearly one billion cubic feet of CBM is extracted each day in the basin, generating about 1.9 million barrels, or 80 million gallons, per day of produced water. The only environmental problem associated with the production of this clean burning domestic fuel is that the water's sodium bicarbonate content is so high that its discharge to the environment is harmful to both plant and animal life. This CBM "produced water" in the PRB typically contains 500 to 1,200 parts per million (ppm) of sodium (Na) and 1,500 to 3,600 ppm of carbonate as CO3. The primary measure of CBM water's quality is the sodium adsorption ratio (SAR) which is used to characterize salt-affected soils. Plant life is detrimentally affected by excess salts in some soils and by high levels of exchangeable sodium in others. Soils with an accumulation of exchangeable sodium are often characterized by poor tilth and low permeability making them unfavorable for plant growth. SAR is an easily measured property that gives information on the comparative concentrations of Na relative to other cations in water, calcium and magnesium. Serious soil problems occur when the SAR is over 12. CBM produced waters typically have SAR values in the 30 to 40 range as the mineral content of these waters are primarily sodium bicarbonate.