Coalbed Methane Produced Water In The Western Us


Quantitative understanding of the degree and extent of connectivity between surface water and shallow groundwater systems and methane-producing coalbeds is important when evaluating the potential effects of CBM extraction, coproduction of water, and subsequent management of the produced water. The degree of connectivity bears on the groundwater flow system, surface water and groundwater interaction, calculations or quantitative assessments of recharge rates, and discharge areas for major streams flowing into and across CBM basins. Effective management of water produced during CBM extraction is contingent on establishing to what degree surface water and groundwater resources may be depleted, degraded, supplemented, or enhanced and over what time periods.

For the western CBM basins, methane developed together with coal over millions of years from different fluvial, lagoonal, and nearshore freshwater and marine settings that contained organic material, which was progressively buried. Although these coals are regionally pervasive, individual coalbeds are discontinuous, reflecting the original meandering and discontinuous environmental setting in which plant matter was deposited and subsequent tectonic activity. Methane in the coal is held adsorbed to the coal surfaces by surrounding water pressure; water in the coal may represent original (connate) water from the environment in which the organic material was initially deposited and/or some "younger" water that has percolated from the surface or shallow groundwater into the coalbeds. Technology used to extract methane from coalbeds relies on pumping the water from the coalbed to the surface to reduce the water pressure and allow the methane to be released from the coal and up the well bore.

Variations in regional geological and hydrogeological histories for the western CBM basins have had direct bearing on the subsurface depth of the coalbeds and the differences in the volumes of methane and the volume and chemistry of the associated produced water. In the Powder River Basin of Montana and Wyoming, relatively high CBM produced water volumes with generally low dissolved salt concentration in comparison to other western CBM basins are due to the occurrence of methane-bearing coalbeds with relatively high permeability and water-filled porosity. CBM-produced water volumes are lower in the San Juan and other western CBM basins, where the methane-producing coalbeds typically occur at greater depths than in the Powder River Basin and have correspondingly lower permeabilities. The deeper coalbeds yield lower water-to-gas ratios and produced water with higher dissolved salt concentrations.

Because many of the coal seams and beds in these western basins are discontinuous, the way in which water in the coal and surrounding sedimentary rocks migrates and is replenished is more complicated than what simple hydrological systems predict. Where discontinuities and/or low permeability exist in the coalbeds, groundwater may move very slowly and natural replenishment of coalbeds after water is withdrawn may not occur in human lifetimes or even in thousands to millions of years. Such "old" or "fossil" water is considered a nonrenewable resource once it is withdrawn.

Several studies using geological, geochemical, geophysical and hydrological data indicate that the water in the San Juan Basin is probably thousands to tens of millions of years old, except at recharge areas—in other words, produced water from CBM extraction in the San Juan Basin is fossil water that will not be renewable over human lifetimes. Preliminary data from the Raton Basin indicate that some of the produced water from CBM extraction may also be fossil water. Although a few isotopic studies have suggested some of the CBM produced water in the Powder River Basin is fossil water, more detailed analyses incorporating water chemistry, isotope study, and geophysical data collection—such as those done in the San Juan Basin—would clarify the extent to which fossil water and/or recharge with younger water occurs in the Powder River Basin. Using a full suite of geological, geochemi-cal, hydrological, and geophysical data, and particularly using isotopic analyses to approximate the age of the water, will help determine whether the produced water is a resource that will be depleted by CBM production or replenished over shorter timescales.

Lack of renewability of the water resource that is extracted during CBM production is an important variable to consider in determining produced water management strategies. The renewability of water has implications for the degree of hydraulic connectivity between methane-bearing coalbeds and surrounding groundwater systems and surface waters and also the intended management of the water subsequent to extraction.

Chemical constituents in the produced CBM waters from the basins vary between and within basins and reflect variability in hydrological systems. The two primary constituents of produced water are sodium bicarbonate and, to a lesser extent, sodium chloride. TDS concentrations in the western basins range from fresh to saline (200 to 170,000 mg/L). The Powder River Basin contains primarily sodium bicarbonate-type formation water and low TDS, whereas the Piceance, Uinta, Raton, and San Juan basins contain sodium bicarbonate chloride-type water at higher concentrations than in the Powder River Basin and generally high TDS. Once at the surface, water produced with methane extraction may undergo further chemical changes associated with atmospheric equilibration and mixing with in-stream and soil-adsorbed elements. Aquifer mineral and coal composition, oxidation state, pH, sorption to aquifer mineral surfaces, and the extent to which solids precipitate along water flow paths in the aquifer all control trace element concentrations.

Although groundwater modeling may be useful for broad assessment of possible hydraulic relationships in CBM basins, current models cannot yet characterize complex waterrock interactions, differences in hydraulic properties or boundary conditions present in CBM basins. As with connectivity issues, testing the results and assumptions of groundwater models for CBM basins against complete suites of data from the basins is important to provide an appropriate level of reliability of the model results.


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