by David Husen
Strong U.S. demand for natural gas and concomitant high market prices are driving tremendous exploration and development of new gas fields. Gas fields can be characterized by the type of underground formation in which the gas resides. One type, known as Coal Bed Methane (CBM), is among the most active currently being developed and presents some unique water treatment challenges and opportunities.
As the name suggests, CBM gas is interspersed with a highly fractured bed of coal, typically at a rate of 50-70 cubic feet of gas per ton of coal. In the Rocky Mountain region, CBM fields are found primarily in Wyoming, Colorado and Utah. According to the U.S. Geological Survey (USGS), this region has extensive coal deposits bearing 30-50 trillion cubic feet (tcf) of recoverable CBM. Nationwide, the total volume of CBM is estimated at over 700 tcf, although only a quarter may be economically recoverable. An aquifer above and within the coal formations seals the gas in place. To access it, the producer must first remove much of the water.
CBM Water Challenges
Challenges in treating CBM produced water lie in the large quantities of water produced over a very broad geography. It's estimated the planned 6,500 CBM wells in Wyoming's Powder River Basin alone will produce nearly 90 million gallons per day (mgd) of produced water that must be treated for reuse or disposal.
Handling this water is a critical economic, environmental, and practical consideration for all gas producers, who must manage it in a way that allows this valuable natural resource to be re-used in the gas field or for another beneficial application. Today, much of this water is transported by truck or pipeline and disposed of by deep-well injection, even while other water is brought to the sites to support the gas production operations. In many cases, a gas producer's inability to efficiently manage the produced water inhibits gas production capacity. Existing wells are throttled or shut down, and development of new wells is delayed until solutions to the produced water problem are found.
Design challenges of CBM water treatment are unique in that the flow capacity requirement varies significantly during the relatively short treatment period. Initially, water flow from a new well is low but rises rapidly to its peak during the first few months. The flow may then remain constant for six to 18 months before it begins to slowly decline over one to three years, ultimately flattening out at less than 10% of its peak flow. This flow rate variability demands scalable solutions to match installed treatment system capacity with demand at each project phase. This is important to minimize a producer's costs during gas production. It's even more critical during the initial phases when water alone is being pumped, and thus there's no revenue stream associated with the well.
Process challenges are determined by the character of the produced water and economics of wastewater management. Produced water varies depending on the field where it's produced but, typically, it contains very high total dissolved solids (TDS). Organic content, total suspended solids (TSS) and heavy metals are usually very low, often below regulatory discharge levels. The treatment objective is to produce water that's acceptable for surface discharge and irrigation use. This water quality is most often defined in terms of Electrical Conductivity (EC) and Sodium Absorption Ratio (SAR), which is a ratio of the concentration of sodium relative to the concentrations of calcium and magnesium. Water that cannot be economically treated for discharge or irrigation must be re-injected or hauled from the site. Due to the very high cost of this alternative, waste minimization is a primary objective when designing the treatment process.
Treatment Solutions
In the past, reverse osmosis (RO) or ion exchange (IX) technologies were used to treat CBM produced water. It's now apparent these technologies, when applied individually in large, fixed installations, cannot meet the economic and operational challenges of the CBM market. They lack the flexibility to treat the variable water quality and quantity, and the capital cost is too high to be economically feasible for CBM producers. The best solution, then, is a combination of technologies that incorporate advanced chemistry, implemented in scalable equipment configurations. This solution can include specialized softening processes, nanofiltration and other membrane technologies, all designed and operated to minimize the final waste volume. Technology will be implemented in modular, mobile packaged systems that can be easily moved from site to site as water production changes from each well.
The advantages of mobile treatment to CBM producers are rapid deployment to the well site, minimal capital investment, scalable pre-engineered systems, and treatment equipment/trains that can be easily moved on- and off-site to meet changing capacity demands. Standard mobile water treatment options include mobile filtration, IX, microfiltration (MF), ultrafiltration (UF) and RO. Additional technologies and processes can be incorporated in a mobile water treatment system to meet specific needs.
Mobile filtration trailers – such as those from Siemens – can hold up to six vessels in a single automated trailer, capable of filtering 600 gallons per minute (gpm). The vessels can be loaded with various media and used for filtration, carbon dechlorination, total organic carbon (TOC) reduction, iron/manganese removal, or softening. For treating CBM produced water, softening trailers would be the primary choice, where local regeneration is desirable.
Mobile IX trailers can contain different configurations of cation, anion or mixed-bed exchange vessels, depending on the customer's feed water quality, flow rate requirements and final effluent quality requirements. With mobile IX, there is no on-site waste generation or hazardous chemical handling. When the resin in the vessels exhausts, the trailer is replaced with one that contains freshly regenerated resin, and the exhausted resin is returned to a local regeneration facility.
A membrane separations trailer is a self-contained system that may incorporate pretreatment with a combination of MF, UF or RO technologies in a single trailer. Depending on the vendor, it produces 200 gpm of low-conductivity product water under a variety of raw water conditions. The trailer can include multi-media, carbon or softening pretreatment equipment to remove common impurities in raw water. These mobile systems contain instrumentation and equipment for fully automatic and monitored operation. Chemical feed skids and fail-safe shutdown controls also can be included.
Service Options
A comprehensive service offering benefits CBM producers by optimizing labor at multiple sites, eliminating the need to hire, train and supervise on-site service personnel, and expediting delivery of spare parts and consumables. These services include: outsourced operations, where a water treatment vendor such as Siemens operates the water treatment equipment at the site; and a scheduled maintenance or preventive maintenance contract to keep the equipment running at peak performance. For those vendors with local branch locations throughout North America, service personnel can be on site quickly. Likewise, local stocking and distribution centers ensure fast delivery of spare parts and consumables when needed to maintain high system reliability and low downtime.
Conclusion
The wide range of water treatment technologies and services for treating gas field produced water can help CBM producers efficiently and cost-effectively manage their water, while meeting environmental regulations. Depending on the level of treatment, the purified produced water could also supplant existing water supplies for re-injection, cooling towers and other industrial uses, or it could be used for agricultural purposes or discharged to the surface to create a stream.
About the Author: David Husen is responsible for services business development in the global oil and gas markets for Siemens Water Technologies. With a bachelor's degree in chemical engineering from UCLA, he began his professional career as a project engineer contracted to Aramco in Abquaiq, Saudi Arabia, then spent 10 years designing and building wastewater treatment plants for Unocal, followed by 12 years of managing regional operations and service for USFilter and Siemens. Contact: www.siemens.com/water/
