national consensus has been reached on clearly defined goals, objectives, management positions, or policies that take into account potential environmental effects of CBM produced water and allow for consideration of a range of potential beneficial use options. Resolving these gaps could increase the ability of public and private stakeholders to develop effective and environmentally and economically sound CBM development and produced water management strategies and practices.
Quality and quantity of CBM produced water, determined largely by the natural geologic and hydrologic characteristics of each CBM basin, are among the primary factors determining produced water management strategies and potential and actual effects of produced water on the environment. The degree of connectivity (“hydraulic connectivity”) among water-bearing coalbeds which are the targets of CBM production, overlying and underlying aquifers, other shallow groundwater aquifers, and surface water is also important. Hydraulic connectivity affects how water in coalbeds and surrounding sedimentary rocks moves and replenishes through time and has consequences for the effects of produced water withdrawals. Water that has not been replenished for a long time—from human lifetimes to millions of years—is termed “old” or “fossil” water and can be considered a nonrenewable resource.
The coalbeds used for CBM in the Powder River Basin of Wyoming and Montana are generally more porous and permeable and yield relatively fresher produced waters1 than the more deeply buried, methane-bearing coalbeds in the CBM basins of New Mexico, Colorado, and Utah. The high porosity and permeability in Powder River coalbeds also require larger volumes of water to be withdrawn by the CBM well operator to stimulate methane release from the coal, compared to the other western CBM basins. Large volumes of relatively fresh CBM produced water from the Powder River Basin are then primarily managed through discharge to surface storage impoundments or to ephemeral and perennial streams and rivers, with or without treatment to meet regulatory requirements. A limited amount of produced water is put to beneficial use. In contrast, smaller volumes of generally very saline CBM produced waters from basins in New Mexico, Colorado, and Utah are primarily managed through disposal by deep-well reinjection.
A suite of geological, geophysical, and geochemical data which includes “age dating” of CBM produced water is needed to establish the degree of hydraulic connectivity between
In discussing the chemistry of CBM produced water, the committee sometimes uses the qualifying word “relatively” to denote differences in the total dissolved solids (TDS), salinities, and sodicities of CBM produced waters as they vary across the western basins. For example, CBM produced water from the Powder River Basin is sometimes described as “relatively fresh,” whereas CBM produced water from the San Juan Basin may be described as having “relatively high salinity.” The report provides the background for the use of these terms.