Coalbed Methane Produced Water In The Western Us

If proven more regionally correct, Raton Basin CBM water may also consist primarily of fossil water, which occurs in compartmentalized methane-bearing coalbeds.

Today, the Raton and San Juan basins receive minimal recharge to their groundwater systems as a result of arid climate conditions. Although the groundwater systems and their sources of recharge are complex and not fully understood, the presence of high TDS waters supports the lack of recharge, as does the discontinuity or compartmentalization of all coal deposits that would further minimize recharge to coalbeds. Although removal of water from coalbeds during CBM operations may induce some degree of leakage of water over time into the coalbeds from surrounding finer-grained rocks and potentially from other aquifers through deep fracture zones, CBM water at depth does not appear to be a "renewable" resource in the San Juan and Raton basins, based on the suite of data available.

Powder River Basin

Few studies have been conducted specifically to date the age of groundwater in the Powder River Basin, but existing results indicate that some of the CBM produced water may be thousands of years old. Geochemical analysis from the eastern portion of the basin has shown that much of the deeper groundwater associated with CBM has no tritium (3H), implying the water is at least 50 years old (Bartos and Ogle, 2002). Preliminary carbon-14 dating of dissolved inorganic carbon in CBM water appears to show water that is radiocarbon "dead," implying it is at least 14,000 years old.7 These results are significant since the samples were taken only a few miles from presumed coalbed recharge areas at the land surface. Brinck et al. (2008) and Frost et al. (2010) concluded from evaluation of geochemical evolution of CBM produced water in proximity to recharge areas that the influence of recharge at outcrop sites likely does not extend more than 2 to 4 kilometers (1.2 to 2.4 miles) beyond the recharge sites. Correspondingly, the water in relatively close proximity to recharge sites is likely recent in geological perspective, whereas CBM water toward the center of the basin may represent "older" water. These results provide some constraints on recharge rates being slow or relatively inhibited in the studied areas of the Powder River Basin.

Sharma and Frost (2008) further determined that the isotopic composition of dissolved inorganic carbon associated with CBM produced water can be readily distinguished from the isotopic composition of dissolved inorganic carbon found in surface water and ground-water of the Powder River watershed. This type of distinction allows for easy long-term monitoring to determine the extent to which in situ and surface-discharged CBM produced water moves within the subsurface and in receiving streams. Campbell et al. (2008) determined from strontium isotopic analyses in groundwater and produced water that some

7C.D. Frost, presentation to the committee, June 2, 2009.

coal aquifers were hydraulically confined while others were not and that faults may provide some, although limited, connectivity for fluid migration between coal formations. Sharma and Frost (2008) suggested the same, based on carbon isotope analysis. These study results are the first of their kind to attempt to describe movement of CBM produced water with these types of isotopes. These studies are explored further in Chapter 5.

Powder River Basin coalbeds also appear to contain some younger water than do the coalbeds from the San Juan or Raton basins, particularly near the outcrop areas at basin margins, which serve as recharge avenues. Additionally, the CBM produced water of the Powder River Basin has lower concentrations of solutes due to some combination of the origins of the coals in freshwater settings and subsequent interaction of the water in the coals with percolating surface water (see also "Geochemical Foundations" below).

Case Study Summary

With respect to CBM basins, isotopic and other data in the San Juan Basin demonstrate that much of the produced water may not be a renewable resource because of its great age compared to human lifetimes. Water extracted from the San Juan Basin is fossil water that has not been renewed for tens of millions of years. For the Raton Basin and at least some portions of the Powder River Basin, away from outcrop recharge areas, the data are fewer and not comprehensive, but similar results are suggested. Although regionally pervasive, the discontinuous nature of the coalbeds has led to limited ability for water to pass through the coals under gravity flow, even in cases where permeability within a coalbed may be relatively high.

GEOCHEMICAL FOUNDATIONS

In addition to geological and hydrogeological constraints on the volume of CBM produced water, analysis and understanding of the various management approaches to CBM produced water also require an introduction to CBM produced water chemistry. The two primary constituents of CBM water are sodium bicarbonate (NaHCO3, or baking soda) and sodium chloride (NaCl, or table salt; Rice and Nuccio, 2000). Constituents appearing in smaller quantities in produced water include calcium, magnesium, potassium, and barium, whereas elements such as aluminum, ammonia, selenium, arsenic, iron, manganese, boron, copper, and zinc are sometimes present in trace amounts (McBeth et al., 2003). Typically, CBM produced water contains only minimal amounts of fine, inorganic particulate matter, otherwise known as coal fines. In some instances, facultative8 iron-oxidizing bacteria and degraded methanogenic bacteria may also be present in small amounts.

8Facultative organisms are capable of respiration in the presence of oxygen but are also capable of fermentation.

COALBED METHANE PRODUCED WATER IN THE WESTERN U.S

A series of geochemical processes remove sulfate through oxidation/volatilization and calcium and magnesium by precipitation or ion exchange/ adsorption from CBM water; these processes leave the CBM water with substantially more bicarbonate and sodium (see Box 2.3). However, the TDS concentration of CBM produced water ranges from fresh to saline (i.e., 200 milligrams per liter [mg/L] to 170,000 mg/L) because of variable amounts of sodium, bicarbonate, and chloride (see Table 2.2). The recommended TDS limit for drinking water is 500 mg/L; for beneficial use, such as irrigation in Wyoming, the limit is 2,000 mg/L; and for wildlife and livestock watering in Wyoming, the limit is 5,000 mg/L (EPA, 2009; Wyoming DEQ,2005). For comparison, seawater has a TDS of approximately 35,000 mg/L. In addition to TDS, sodium in CBM produced water is of interest as it relates to the consideration of CBM produced water for irrigation. A measure that is used to determine the influence of sodium on soils and plants is related to the sodium adsorption

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