Coalbed Methane Produced Water In The Western Us

CBM produced water in surface impoundments. These effects and their potential causes are addressed below. Although adverse effects from hydraulic fracturing have not been documented in CBM fields, the issue is of concern to the public. A brief discussion of hydraulic fracturing is included at the end of this section.

Effects of Groundwater Withdrawal on Aquifers

Research demonstrates that a principal effect of CBM withdrawals on groundwater is reduction of water volume and hydrostatic head within coalbeds from which methane is being extracted. Typically, the CBM well is pumped to reduce the hydrostatic pressure in the coalbed to a pressure approximately equal to atmospheric. However, water is still retained within the coal and generally the head level of water in the coalbed is maintained relatively close to the uppermost physical surface of the coalbed. Any effects of water withdrawal from methane-bearing coalbeds on water levels in other aquifers are a function of the depth of the target coalbeds and the degree of hydraulic connection between CBM targets and the other local or regional aquifers (see Chapter 2 for discussion of hydraulic connectivity).

Pumping water during CBM extraction in basins with deep methane-bearing coals, such as the San Juan, Raton, Uinta, and Piceance basins, is unlikely to cause lowering of the water table of shallow alluvial aquifers because of lack of hydraulic connectivity between the deep coals and shallow aquifers coupled with the great vertical separation between the coalbeds and the shallow groundwater systems (upward of thousands of feet; see also Chapter 2). Typically, methane-bearing coalbeds in these basins are bounded above and below by either aquitards or aquicludes (see Chapter 2) that are responsible for both the positive hydrostatic pressure within the coalbeds and the lack of hydraulic connectivity or communication between the coalbeds and overlying and underlying aquifers. An exception to this circumstance is that reported by Riese et al. (2005) for the San Juan Basin, in which the authors documented movement of water from below the methane-bearing coalbeds upward and into the coalbeds (see Chapter 2).

In contrast, depths to methane-bearing coalbeds in the Powder River Basin are relatively shallow and less consolidated than those of the other western CBM basins (see Chapter 2). Consequently, the coalbeds generally consist of porous and permeable formations capable of releasing large amounts of water during methane production (see Table 2.1). Some of the coalbeds or fringes of coalbeds in the Powder River Basin are also sufficiently close to the land surface that they serve as sources of domestic, residential, wildlife, and livestock water supply (Frost et al., 2010; Wheaton et al., 2005; Campbell et al., 2008). These supplies often surface as flowing springs and wells. In some instances wells are drilled into the coalbeds and the water is used for stock watering or domestic supplies. However, direct physical connections between water-bearing coalbed aquifers from which CBM is being extracted and other alluvial groundwater that supplies water wells and springs in the basin are not widely established; geochemical data suggest that coal aquifers and other alluvial groundwater aquifers do not interact to any great degree in studied parts of the Powder River Basin (see discussion in e.g., Frost et al., 2010; Bartos and Ogle, 2002; see also Chapter 2). Anecdot-ally, CBM production has been linked to some losses of drinking water or dry wells where the water wells were close to the CBM development and/or were completed in the coals which serve as a primary aquifer.

In addition to geochemical information that can help determine the degree of connectivity between CBM coalbeds and other groundwater aquifers, groundwater monitoring networks are being used to measure the degree to which CBM production may affect water levels in shallow aquifers. The Montana Bureau of Mines and Geology (MBMG) maintains and samples a regional network of groundwater monitoring wells that includes wells installed in the late 1970s and early 1980s to monitor the effects of coal mine dewatering, a separate activity from CBM operations, and more recent wells installed specifically to monitor CBM production. The MBMG receives funding from the Bureau of Land Management (BLM) in support of this monitoring program. In Wyoming, in response to concerns about potential effects to groundwater from CBM development in the Powder River Basin, BLM established a regional groundwater monitoring program that is outlined as part of the Wyodak CBM Final Environmental Impact Statement (BLM, 1999). The program was designed to collect information regarding hydraulic connectivity between producing coals and adjacent sandstone units and to measure the extent of groundwater drawdown in the CBM-producing coal zone on federally owned lands. Results from both the Montana and Wyoming groundwater well monitoring programs are briefly summarized below.

Montana

Many of the monitoring wells are completed in the Dietz (associated with the Anderson coalbeds) and Canyon coalbeds in the Powder River Basin (Wheaton and Metesh, 2002; see also Figure 2.4b). The monitoring network has been sampled for seven consecutive years (2003-2009), in addition to sporadic monitoring for nearly three decades before CBM development was initiated in the area, and the data are available in annual reports through the 2008 sampling event.1

Data from this network indicate that static water levels in the Dietz coalbeds, from which CBM is being extracted, have been lowered by as much as 150 feet. Static water levels in the Canyon coal, also a coalbed from which CBM is being extracted, have been lowered as much as 600 feet in limited areas (Meredith et al., 2008). CBM-related drawdown of 20 feet of the static water level in the Canyon and Dietz coalbeds currently extends to

1See, for example, Wheaton and Donato (2004), Wheaton et al. (2005, 2006, 2007, 2008), Meredith et al. (2008), and Wheaton and Meredith (2009).

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