Coalbed Methane Produced Water In The Western Us

and located at relatively shallow depths compared to the other western CBM basins. In both the Wyoming and Montana portions of the basin, water and gas volumes have increased from the late 1990s until the present, as have the number of water- and CBM-producing wells, yielding an average water-to-gas ratio (in barrels of water produced per thousand cubic feet [MCF] of gas produced) greater than 1 (Table 2.1).5 Attempts at methane extraction in one-sixth of the Powder River Basin fail because water-to-gas ratios are excessively high, compared to other parts of the basin (Table 2.1).6

CBM production in the San Juan Basin yields a much lower water-to-gas ratio than in the Powder River Basin (Figure 2.8c). Low ratios are typical of basins with deeper, less permeable coalbeds that have been producing methane for a longer period of time. A graph of production from the San Juan Basin shows a steady increase in gas production since the mid-1990s, while water production from the basin peaked in 1993 and has since followed a steady decline.

In an example from the Piceance Basin, which has the lowest CBM production levels of any of the western CBM basins, two spikes in water production are apparent (Figure 2.8d). The first peak in water production in late 1992 reflects the increase in pumping rates and number of wells as production began in the basin. The second is due to input of a large number of new wells in the 2003 to 2004 period and accounts for the relatively high water-to-gas ratio in 2006 (see Table 2.1).

Acknowledgment of these kinds of variations in water production from basin to basin and within a basin is important when considering CBM produced water management options. Any discussion of "average," "annual," or "total" water production values requires clarifying information, including the length of time over which CBM operations have been active in the basin, the total number of operating wells, the number of existing and new wells in a given part of a basin in a given year, how long those wells may be in operation, and the rate of pumping by the operator. Spacing of adjacent wells may also have an effect on how quickly or slowly water production proceeds in a CBM field (see Chapter 5). These types of data are not necessarily available in a single data repository for each state or basin but have to be compiled from numerous information sources (see Figure 2.8 and Table 2.1 for some of these data sources).

5A U.S. barrel (bbl) is equivalent to 42 gallons. One thousand MCF is equivalent to 1,000,000 cubic feet (see values in Table 2.1).

6D. Fischer, Wyoming Department of Environmental Quality, presentation to the committee, March 30, 2009.

CASE STUDIES: REGIONAL HYDROGEOLOGY AND HYDRAULICS OF THE SAN JUAN AND POWDER RIVER BASINS

San Juan Basin

Fine-grained rocks (shale) confine the Fruitland coal and sandstone aquifers in the San Juan Basin. These aquifers become unconfined at outcrops at the basin margins. The situation is similar for the coal-bearing units of the Raton Basin. Because the coal-bearing beds outcrop at the surface at elevations higher than where they occur in the interior of the basin and the coalbeds are confined, the coalbeds had been previously considered a case of a classic "confined aquifer" that recharges at the outcrops along basin margins. Regionally, this model would predict groundwater to move in a southerly direction, from topographically high recharge areas in the north to the central part of the basin and to the lower basin margins, where the groundwater would discharge.

However, data indicate that this classic confined aquifer model for the San Juan Basin is too simplistic to adequately describe the complex hydrological process that governs confined coalbed water recharge, drawdown, and discharge. Isotopic analyses, including iodine, chloride, carbon, oxygen, and hydrogen in groundwater of multiple geochemical systems, independently document that the residence time (age) of CBM water in the San Juan Basin is on the order of thousands to tens of millions of years (e.g., Phillips et al., 1986, 1989; Snyder et al., 2003; Riese et al., 2005; see Box 2.2). Within the uncertainties of isotopic analysis, these data indicate that meaningful recharge of groundwater to all coalbed aquifers with the exception of some of the peripheries of these basins, in close proximity to the outcrop areas, has not occurred within the scale of human time.

Recent data from multiple lines of geological, geochemical, geophysical, biological, and ecological investigation have further demonstrated that the last major recharge of water to the San Juan Basin coal systems occurred during Eocene time, approximately 35 million to 40 million years ago (Riese et al., 2005). The Riese et al. (2005) study sampled waters from over 100 CBM wells and examined chemical and isotopic differences across the basin. The geochemical results showed the areal distributions of different water geochemical types ("fingerprints") and a lack of coherent geochemical development along previously assumed regional flow paths from basin edges to the center of the basin (anticipated in the classical "confined aquifer" model). The geochemical patterns were consistent with compartmental-ization of the basin into discrete hydrogeological zones with different water qualities.

The data also support the idea that Eocene and post-Eocene (younger than about 34 million years) uplift of the basin may have caused hydraulic conductivities in the coalbeds to decrease even further due to gas desorption from the coals and to effectively isolate those aquifers. Geochemical and geological evidence suggests that later (Miocene to Holocene, or about 23 million years ago through more recent time) geological events changed the stress

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