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Management and Effects of Coalbed Methane Produced Water in the Western United States CHAPTER TWO Coalbed Methane Produced Water in Western U.S. Basins: Hydrogeological and Geochemical Foundations A fundamental challenge regarding management of coalbed methane (CBM) produced water is determining to what degree surface water and groundwater resources may be depleted, supplemented, degraded, or enhanced and over what time periods as consequences of CBM extraction and management of produced water. To understand these consequences this chapter reviews the features of western CBM basins including (1) the hydrogeological characteristics of the basins; (2) the nature of connections between water in methane-bearing coal deposits and surface water and groundwater systems in the basins; and (3) the chemistry and age of the waters in the coalbeds. The chapter focuses primarily on two basins—the San Juan Basin in Colorado and New Mexico and the Powder River Basin in Wyoming and Montana. These basins capture and contrast the currently known range of CBM produced water quality and quantity and produced water management approaches throughout the western CBM basins. The Uinta, Piceance, and Raton basins of Utah, Colorado, and New Mexico are also briefly discussed (see Figure 2.1). At present, no CBM production occurs in North Dakota. HYDROGEOLOGICAL FOUNDATIONS Origins of CBM and Associated Water Coal is formed from plant matter (organic material) that has undergone burial, consolidation, and heating over millions of years under younger sediments. In the western United States, the wetland areas that provided the organic material for present-day coal basins existed between about 145 million and 56 million years ago. The plant matter formed either within alluvial systems of streams, lakes, and peat swamps, all of non-marine origin (northern Rocky Mountain area of the United States), or behind barrier islands and in back bays, lagoons, and deltas along the midcontinental seaway with waters of marine or
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Management and Effects of Coalbed Methane Produced Water in the Western United States FIGURE 2.1 Map of western CBM basins within the six states that are the subject of this study. Only those basins with cumulative production to date greater than 40 billion cubic feet (BCF) are included in the discussion in this report. SOURCE: Adapted from EIA (2007). brackish origin (southern reaches of the Rocky Mountains; see Figure 2.2). Because of the naturally discontinuous distribution of these wetland settings and the tectonic processes that affected buried coals during and after their formation, most of the coal deposits now in these western basins, although regionally pervasive, are also discontinuous. The coal deposits occur as seams or beds that are often distributed as discrete “lenses” or layers that pinch out, terminate, or branch (see descriptions of individual basins below). Discontinuities within these coalbeds or seams (hereafter referred to as “coalbeds”) are important in that they affect the way in which water in the coalbeds and surrounding sedimentary formations migrates and is replenished.
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Management and Effects of Coalbed Methane Produced Water in the Western United States FIGURE 2.2 Illustration of the Cretaceous interior seaways, including the Western, Hudson, and Labrador seaways. The Cretaceous Period lasted from about 145 million to 65 million years ago. Coal-bearing basins in the western United States that are the subject of this report formed from organic-rich sediments (plant material) deposited in and along the wetlands of the Western Interior Seaway. The organic-rich sediments were deposited through Cretaceous and Paleocene (ca. 65 million to 56 million years ago) times during the rise and fall of intercontinental sea levels. SOURCE: W.A. Cobban and K.C. McKinney, USGS. Available at esp.cr.usgs.gov/research/fossils/ammonites.html. Coalbeds can serve as aquifers or subsurface rock layers that are sufficiently permeable1 to conduct groundwater and can provide sufficient water for human use. Other less permeable materials (e.g., siltstones, shales, clays) above and below the coal seams—sometimes 1 A permeable geological material, or a material’s “permeability,” refers to its ability to transmit fluids and is generally associated with the degree of connectivity between pores in the material. A higher degree of pore connectivity would indicate higher permeability or ability of the material to transmit fluids.
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Management and Effects of Coalbed Methane Produced Water in the Western United States called “aquitards”2—can inhibit upward or downward water flow from coalbeds. This inhibited flow essentially causes a water-saturated coalbed to be “confined” with respect to contained water. When water in a confined coalbed connects to the water table at an elevation above the elevation of the coal seam, the water in the coalbed may become overpressured with respect to the pressure exerted by a static column of water in the overlying rock (termed “hydrostatic pressure” or “hydrostatic head”). When a well is placed in a confined coal seam, the water level will rise to an elevation above the seam. Because of the discontinuous nature of coalbeds, not all groundwater flow in coal-bearing basins can be described by simple hydrogeological models. These models usually describe water as moving from higher elevation “recharge” areas into lower elevation discharge areas from which the water may flow out as streams and springs. Groundwater “recharge” is a process by which water moves downward from the surface to groundwater and can occur naturally (e.g., rainwater percolation) or through artificial (human-induced) means. In some basins where natural recharge areas are located far from downgradient portions of a coalbed or other aquifers, replenishment of these aquifers by infiltrating precipitation may not occur within human lifetimes or even thousands to millions of years when water is removed from the aquifer. In essence this “old” or “fossil” water in a coalbed aquifer can be considered a “nonrenewable” resource once removed from the coalbed. The “age” of the water, or its residence time in the coalbed, thus also indicates the degree to which the CBM water is connected to surface water and shallow groundwater. “Old” water would suggest slow or inhibited connections to surface water or shallow groundwater that otherwise might serve as a source of “new” water to a coalbed. Determining the connections between coalbeds and surrounding aquifers (hydraulic connections) and the renewability of the water resource in the coalbed is important to understanding the consequences of water removal from the coalbed during CBM production (described in detail later in the chapter). The age of the water in coalbeds from which CBM is being extracted thus can become an important factor in determining how produced water is managed. The next section outlines the development of methane and associated water in coalbeds. Subsequently, geological and hydrogeological characteristics of each basin are briefly described because of the role they play in determining both the volume and quality of water produced during CBM extraction. 2 An “aquitard” is a confining bed or geologic material that retards but does not prevent the flow of water to or from an adjacent aquifer, does not easily yield water to wells or springs, and may serve as a storage unit for groundwater. An “aquiclude” is a body of relatively impermeable geologic material that can absorb water slowly but does not transmit it rapidly enough to supply a well or spring (Bates and Jackson, 1987).
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Management and Effects of Coalbed Methane Produced Water in the Western United States Production of Methane and Water from Coalbeds Methane associated with buried coalbeds is originally formed from one of two processes: thermogenesis or microbial methanogenesis. Thermogenesis involves the degradation of organic matter by temperatures usually greater than 120°C (248°F) associated with pressure from burial at depths greater than about 1,000 feet. The San Juan Basin contains coals with themogenic methane. Microbial methanogenesis is the decay of organic matter through microbial activity at relatively shallower depths and lower temperatures than those related to thermogenesis; the Powder River Basin coals contain methane generated in this way. In addition to genesis of methane during compaction and heating of organic material, coals develop systematic fractures or “cleats,” roughly analogous to cleavage planes in minerals (Riese et al., 2005). The presence of water in the coalbeds keeps the methane adsorbed on the surfaces of the coal and within the cleats and adsorbed to walls in the micropore structure of the coal matrix (see Figure 1.1). Water in the coalbeds may derive from (1) original water (“connate” water) associated with freshwater or marine settings in which the organic material was originally deposited, and/or (2) water (e.g., rainfall) that later percolated from the surface through to the coals as they were progressively buried. To extract methane adsorbed to the coal, water must be pumped out of coal seams to lower the water pressure (head) and allow the methane to desorb, coalesce, and bubble into the pumped water, analogous to the formation of bubbles of carbon dioxide in a bottle of carbonated beverage when the cap is removed (see Figure 1.1). The amount of water that must be removed from the coalbeds in order to release methane depends on the original water pressure in the coal, the physical capacity of the coal to hold and release water, and the extent to which coals may be hydraulically connected to adjacent geological formations. Water production records show that the volume of water pumped from individual CBM wells generally decreases exponentially with time, with a corresponding increase in the rate of methane production (see Figure 2.3). In many cases, water pumping may discontinue within 10 to 20 years of initial pumping, while methane production may continue. In contrast to conventional oil and gas fields where produced water is sometimes reinjected into the producing formation to enhance oil and gas recovery, CBM produced water is not returned to the coal seams from which it was extracted because doing so would hinder additional methane recovery. Thus, other options are considered with respect to storage, disposal, or use of the CBM produced water. The generalized trend shown in Figure 2.3 for water and methane production related to CBM extraction is useful for discussion of long-term predictions for water and gas volumes from a particular basin. However, the volume of water produced per year, the ratio of water to gas extracted from a well, and lifetime water production within and between the western CBM basins do not follow a common trend. Hydrogeological properties and operational practices affect the volume of water produced. For example, the rate at which
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Management and Effects of Coalbed Methane Produced Water in the Western United States FIGURE 2.3 Schematic production curves for typical CBM wells show that operator-controlled water production rates decrease exponentially over time while methane production increases before moving into a stage of decline. Water production is a function of initial, operator-controlled pumping rates that aim to reduce pressure and stimulate flow of water and gas to the well. Once gas flow has been achieved, over time, the operator will gradually reduce the water production rate until the gas production rate is maximized. SOURCE: Nuccio (2000). pumped water enters wells during production depends on the natural hydraulic properties and water-filled porosity of the coal seam containing the methane and the operator-controlled water-pumping rate. Shallow, weakly-consolidated coalbeds may have extensive internal fractures and interconnection of fractures that produce a porous and permeable formation capable of releasing large amounts of water during methane production (e.g., the Powder River Basin). In other areas where the methane-bearing coalbeds lie at much greater depths, the amount of water that must be pumped from the coal and the rate at which that water can be pumped to facilitate the release of methane are often limited by the effective water-filled porosity and permeability of the coal (e.g., the San Juan Basin). The limited interconnectivity between fractures and cleats in these deeper coals often requires use of hydraulic fracturing to stimulate release of the methane (Box 2.1; see also Chapter 5). CBM production and associated produced water volumes are also a function of economic conditions. Generally, if the price of natural gas goes below a certain price point, the CBM operator will begin to “shut in” (cease to produce from) wells, which will reduce the quantity of produced water generated by the industry. When natural gas prices are above a certain level, CBM operators will generally increase production to generate more income and profit. The total volume of CBM produced water generated by a CBM operator will thus vary as a result. Other factors such as contract deliverables, reservoir management requirements, and reservoir energy may also affect the decision to shut in a well or keep it in production.
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Management and Effects of Coalbed Methane Produced Water in the Western United States BOX 2.1 Hydraulic Fracturing Hydraulic fracturing is a technique used in many oil and gas production settings to help release oil or gas from the geological formation and allow the hydrocarbon to flow more freely and consistently to the well bore. The technique injects fluids and sand under pressure into the formation of interest to open and stimulate the growth of new fractures, thereby increasing the number of pathways through which oil or gas can reach the well. Among the CBM basins examined in this study, hydraulic fracturing is used to enhance the flow of methane gas in the San Juan, Raton, Piceance, and Uinta basins. Hydraulic fracturing is used very infrequently in CBM operations of the Powder River Basin due to the high natural permeability of the shallow, methane-bearing coal seams. The standard industry practice for oil and gas operators to fracture a formation hydraulically is to fill the space between the outside of the steel casing of the well pipe and drill hole with cement along some or all of the well bore to the top of the target rock unit from which oil or gas (including methane from coalbeds) is going to be recovered. Holes or perforations are then blasted through the well casing opposite the target rock formation (for CBM production, the target is the coal seam). Fracturing fluids, if used, are pumped under high pressure through these holes into the target formation and are then pumped, together with the oil and gas and/or any produced water, back to the surface for recovery and disposal. In the Montana portion of the Powder River Basin, water, rather than other fluids, may be injected into CBM wells by some operators to improve conductivity around the well bore. The well casing and encasing concrete in a CBM well are designed to maximize recovery of all types of fluids from the target formation and to minimize loss of fluid, whether hydraulic fracture fluid, oil, gas, or water, to other geological formations along any part of the well bore. Western CBM Basins This section provides an overview of the variations in regional geological and hydro-geological histories for the western CBM basins. These variations have direct bearing on the subsurface depth from which methane is extracted and the volume and chemistry of associated produced water. 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)3, 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 section on “Geochemical Foundations” 3 TDS (total dissolved solids) is an expression for the combined concentration of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular suspended form, and which will pass through a sieve opening of 2 micrometers (Water Systems Council, 2007).
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Management and Effects of Coalbed Methane Produced Water in the Western United States later in this chapter provides the background for the use of these terms throughout the report. POWDER RIVER BASIN The Powder River Basin of Wyoming and Montana covers approximately 25,800 square miles (see Figure 2.4). CBM in the basin is derived from coals in the Tongue River and FIGURE 2.4 (a) Powder River Basin of northeastern Wyoming and southeastern Montana. Major drainages flow north or east into the Missouri river system. Location of cross-section B-B’ in Figure 2.4b is shown within the purple-brown shading that indicates coal of subbituminous grade. (b) Northwest-southeast geological cross section through the basin depicts a major coal-bearing and CBM-producing rock unit, the Fort Union Formation, and the overlying Wasatch Formation. Although oil and gas production began in the Powder River Basin in the 1920s, the first CBM well was not drilled there until the late 1980s (in the Wyoming portion of the basin). By the end of 2008, approximately 18,000 CBM wells were extracting methane from the Tongue River and Lebo Shale members of the Fort Union Formation, mostly at shallow depths ranging from approximately 450 to 4,500 feet (USGS, 2005). Wells shallower than 450 feet have produced methane from coalbeds in some localized areas. The Wyodak coal zone, including the Lower and Upper Wyodak and Wyodak Rider zones, contains the Canyon, Anderson, Smith, and Big George coals, which are CBM production targets. Vertical and lateral correlations in the Fort Union Formation show successive splitting of thick coal beds resulting in overlapping coal zones (Flores et al., 2010). This effect has played a role in the use of slightly different nomenclature to identify coal horizons in the basin. For example, the Anderson coal is sometimes referred to as the Wyodak coal; in the northwestern part of the basin and in Montana, the Anderson and Canyon coals are interleaved with the Dietz coal (sometimes referred to as the “Anderson-Dietz coal”). The Smith and Big George coals are not easily differentiated in every part of the basin and are sometimes referred to as the Smith/Big George coal, or as in the case of this cross section, only as the Smith coal. Elsewhere in the basin, the Big George coal occurs in the same part of the Wyodak Rider zone and is identified as such (Copeland and Ewald, 2008). The Lebo Shale Member is not depicted on this cross section but lies below the Tongue River Member. SOURCES: (a) ALL Consulting (2003); (b) Adapted from Copeland and Ewald (2008).
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Management and Effects of Coalbed Methane Produced Water in the Western United States
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Management and Effects of Coalbed Methane Produced Water in the Western United States Lebo Shale members of the Fort Union Formation, which formed about 65 million to 56 million years ago (Paleocene time). The Fort Union Formation, a heterogeneous geological unit of sandstone, shale, and coal deposits, is overlain by the Wasatch Formation in many locations. These formations outcrop extensively around the east-central margin of the Powder River Basin near Gillette, Wyoming, and around the west central margin of the basin near Sheridan, Wyoming. Open-pit and strip-mining commercial coal operations are common in the outcrop areas. Thickness of individual coalbeds in the Fort Union Formation ranges from a few inches to over 200 feet, with an average thickness of 25 feet. The Fort Union Formation originally was deposited on the margins of an ancient interior seaway as part of river (freshwater, fluvial) systems with braided, meandering, and dissected streams in the center of the basin and alluvial plains along the basin margins (USGS, 1999; Copeland and Ewald, 2008). The irregular spatial and vertical distributions of coalbeds (laterally and vertically discontinuous) reflect shifts of these fluvial and alluvial systems through time. The Tongue River Member of the Forth Union Formation contains thick, laterally extensive coalbeds that vary unpredictably in thickness and geometry, terminating and merging abruptly. The Tongue River Member, including the Wyodak coal zone4 and the Canyon and Anderson coals within this zone, contains most of the recoverable CBM in the Wyoming portion of the Powder River Basin (Figure 2.4b; Copeland and Ewald, 2008). In the eastern part of the basin, regional groundwater flow moves from the south and east toward the northwest and into the central part of the basin (Daddow, 1986; Martin et al., 1988). In the southeastern part of the basin, regional groundwater flow is to the north, although local flow often varies from this overall pattern (BLM, 2003; USGS, 2005). The generally northward regional groundwater flow in the basin moves slowly because of pinching out of sandstone units, which are the principal water-conducting deposits contributing to groundwater flow. Water in sandstone aquifers associated with the coalbeds can be hydraulically confined, particularly in deeper, isolated beds far from recharge areas. Individual coalbeds in the Wasatch, Fort Union, and Lance formations (e.g., the Anderson coal) can also constitute important aquifers. The Wyodak and Wyodak Rider coal zone of the Fort Union Formation is the most hydrologically continuous unit in the Powder River Basin and, together with its related coalbeds (the Anderson, Canyon, Big George, and Smith coals; Figure 2.4b), constitutes a regional aquifer. Limited recharge to the Wyodak and Wyodak Rider coal zone occurs at outcrops along the eastern margin of the Powder River Basin (e.g., Daddow, 1986). Recharge water flows downgradient within the coalbeds that outcrop at the surface. These 4 A “coal zone,” according to Copeland and Ewald (2008), is a stratigraphic interval containing a suite of coalbeds that vary in thickness, have stratigraphic proximity to one another, and split apart or merge from a single coalbed.
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Management and Effects of Coalbed Methane Produced Water in the Western United States coalbeds then act as independent isolated aquifers. Flow into or out of the coalbeds along fault and fracture lines takes place to a limited extent (Frost et al., 2010). Because the origin of the coals in the Wyodak and Wyodak Rider coal zone was in a freshwater setting, as opposed to a marine setting—which was the case for the coal deposits of the San Juan, Raton, Uinta, and Piceance basins—the connate waters associated with the Powder River coals were probably fresher from the outset compared to the connate coalbed waters in the other basins. In cases where the Powder River Basin coals are also connected hydraulically to natural recharge areas, the higher relative permeability of the coals would facilitate flow and contribute further to water chemistry in the coals having relatively few dissolved solids compared to water in coalbeds of other western CBM basins (see section on “Geochemical Foundations” later in this chapter). As a result of some combination of these natural circumstances, relatively fresh connate water and/or higher relative permeability, produced water from the Powder River Basin coalbeds is generally less saline than waters produced from other western CBM basins. The low TDS content and low salinity allow management of the CBM produced water either through direct discharge to ephemeral and perennial streams (either with or without treatment) or storage in surface impoundments (see later in chapter for water chemistry and Chapter 4 for details of water management practices in the basin). The degree to which water in the coals of the Wyodak and Wyodak Rider coal zones represents original (“old” or “fossil”) connate water and/or younger water that percolated into the coal from surface recharge areas is not well constrained with geologic, geophysical, geochemical, or hydrologic data. SAN JUAN BASIN The San Juan Basin covers about 7,500 square miles in the Four Corners region of the adjoining states Utah, Colorado, Arizona, and New Mexico (Figure 2.5). The basin strikes west-northwest to east-southeast and is asymmetrical in shape, with the deepest and thickest sedimentary rocks located in the north-central portion of the basin. The major coal-bearing and methane-producing unit is the Cretaceous Fruitland Formation, underlain by the Pictured Cliffs Sandstone. The layered and discontinuous Fruitland coals have three-dimensional complexity, reflecting the original complexity of the back-barrier lagoonal wetland ecosystems from which they originated (Snyder et al., 2003; Riese et al., 2005). Production of CBM from the San Juan Basin occurs at depths ranging from 550 to 4,000 feet in three distinct and geographically discrete hydraulic pressure and permeability zones: (1) a central, high hydraulic head, high-permeability “fairway” (primarily in the gray shading of the basin in Figure 2.5a); (2) a northern, high hydraulic head, low-permeability area (primarily in the green and purple-brown shading of the northern part of the basin in Figure 2.5a); and (3) a southern, low hydraulic head, low-permeability area (primarily in the purple-brown shading of the southern part of the basin in Figure 2.5a). Although the
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Management and Effects of Coalbed Methane Produced Water in the Western United States Potentially toxic metals, such as arsenic, lead, and chromium, are generally found at concentrations less than most water quality standards in certain locations (Jackson and Reddy, 2007; see Table 2.3). Once at the surface, CBM produced water also undergoes 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 macro- and trace element concentrations. Patz et al. (2006) documented changes in concentrations of trace metals in surface water in the Powder River Basin as such waters moved downgradient, below produced water discharge points. McBeth et al. (2003) determined that soluble salt and trace metal concentrations in surface storage ponds may increase or decrease depending on time, the underlying soil and rock material, and the TABLE 2.3 Concentrations of trace elements (g/L) in CBM waters in outfalls and surface impoundments in the Powder River Basin: Little Powder, Powder, and Tongue River watersheds. Little Powder River Powder River Tongue River Outfalls Disposal Ponds Outfalls Disposal Ponds Outfalls Disposal Ponds Aluminum 304±132 573±266 181±80.7 251±95.0 1,817±1,174 361±162 Arsenic 0.75±0.75 9.74±6.74 0.75±0.75 3.75±0.75 0.75±0.75 1.50±0.75 Barium 614±49.4 334±35.7 514±98.9 284±61.8 271±26.1 130±23.3 Boron 99.3±4.76 126±19.2 141±21.5 164±12.3 109±6.81 124±6.81 Cadmium <1.12±1.12 <1.12±1.12 <1.12±1.12 <1.12±1.12 <1.12±1.12 <1.12±1.12 Chromium 8.84±1.04 8.84±1.56 12.0±2.60 11.4±1.56 8.32±1.56 9.36±1.56 Copper 10.8±1.27 19.1±4.45 7.63±2.54 19.7±1.91 10.8±1.91 17.2±1.91 Iron 124±11.2 217±107 81.0±13.4 203±89.9 71.5±34.1 145±46.4 Lead <2.07±2.07 <2.07±2.07 <2.07±2.07 <2.07±2.07 <2.07±2.07 <2.07±2.07 Manganese 12.6±3.85 11.5±8.24 8.24±2.20 3.30±1.10 7.69±2.75 7.14±2.20 Molybdenum <0.96±0.96 2.88±1.92 0.96±0.29 1.92±0.96 <0.96±0.96 1.92±0.96 Selenium 1.58±0.79 1.58±0.79 1.58±0.79 2.37±0.79 0.79±0.79 0.79±0.79 Zinc 7.85±1.96 9.15±1.96 7.19±1.96 10.5±3.27 10.5±3.92 18.3±5.23 NOTE: Outfalls refer to direct discharges of from water separated from individual methane wells. Disposal ponds are containment structures that store the discharge water from multiple outflows (see Chapter 4). Mean values (g/L) plus one standard deviation are shown for each constituent. SOURCE: Jackson and Reddy (2007). Figures converted from micromoles per liter in the original data source to micrograms per liter in this table.
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Management and Effects of Coalbed Methane Produced Water in the Western United States degree to which mixing occurs in holding ponds (see also Chapters 4 and 5). However, in many circumstances—particularly in Colorado and Wyoming where produced water is present on the landscape—the spatial distributions, concentrations, and fate of trace elements in the water remain uncertain given the minimal sampling and analysis available (see also Chapter 5).9 In contrast to the studies outlined above that examined inorganic carbon, trace concentrations of dissolved organic substances may also be present in some CBM produced waters, although these substances in CBM produced waters are neither well documented nor researched. Phenols, biphenyls, heterocyclic compounds, polycyclic aromatic hydrocarbons (PAHs), and other organic constituents have been measured in some produced waters, with PAHs being the most common organic substance detected or measured. Orem et al. (2007) report microgram per liter (µg/L) concentrations of organic compounds in CBM produced waters in the Powder River Basin, with PAH values up to 23 µg/L. The committee was unable to find other data regarding organic substances dissolved in CBM produced waters of the other western basins. GROUND- AND SURFACE WATER CONNECTIVITY AND GROUNDWATER MODELING: DATA GAPS AND UNCERTAINTIES Concern over management of CBM-produced water stems largely from two factors: water quantity and quality on local and regional watershed scales. Litigation during the past decade has been extensive, with plaintiffs registering concerns over numerous water quality and quantity issues and their effects (see also Chapters 3 and 5). Additionally, a number of research projects have involved either monitoring and data gathering or modeling in an attempt to define the extent of local or regional water resource responses to CBM produced water withdrawals and discharges. However, for the purposes of planning CBM produced water management, questions remain with regard to the effects of large-scale, localized, regional, and/or basin-wide withdrawals; deep-well reinjection; discharge for disposal through infiltration or evaporation; and release of treated or untreated CBM water to ephemeral and perennial streams. For purposes of evaluating these various management options on water quality and quantity (discussed in detail in Chapters 4 and 5), data to determine the connectivity of groundwater and surface water and groundwater modeling are necessary. The gaps and uncertainties related to connectivity and modeling conclude the discussion in this chapter. 9 D. Baldwin, Colorado Oil and Gas Commission, personal communication, January 6, 2008.
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Management and Effects of Coalbed Methane Produced Water in the Western United States Data Gap to Establish Surface Water and Groundwater Connectivity Establishing quantitatively the extent to which CBM-producing formations hydraulically connect to surface waters and major aquifers is necessary to predict the effects of CBM water withdrawal and management on surface water and groundwater quantity and quality. As discussed above, the only study that included sufficient geochemical, geological, geophysical, hydrological, and other data to establish the degree of hydraulic connectivity between methane-bearing coalbeds and surface and shallow groundwater was conducted in the San Juan Basin (Riese et al., 2005). Such data are needed to assess fundamental aspects of the groundwater flow system, the water level (potentiometric surface) and how it changes, surface water and groundwater interaction, calculations or quantitative assessments of recharge rates, and discharge areas for major streams flowing into and across CBM basins. Because comprehensive data and analyses of this nature are lacking for other western CBM basins, the committee considers this a significant information gap. Gaps with Modeling Groundwater Flow Although natural systems are complex, numerical models of groundwater flow in CBM basins have used fairly simple approaches in which water is modeled to move uniformly within relatively homogeneous aquifers. Thus, interactions that might occur between local, shallow streams and groundwater and deep CBM-associated waters may not be adequately represented by the model parameters. Independent and comprehensive data are needed to test and confirm the validity of the results of groundwater models for CBM basins beyond calibration to water level (the potentiometric surface). In some cases where groundwater models are used to characterize groundwater flow, the model results have not been rigorously examined through a combination of sensitivity analysis, history matching, and using multiple lines of calibration (e.g. Anderson and Woessner, 1992; ASTM, 2000). Understanding model limitations and uncertainties becomes particularly important when results of models may be used to assess the longer-term consequences to groundwater levels from CBM-related water-pumping activities. In the Powder River Basin, for example, one modeling study indicated effects from CBM pumping that included depression of the potentiometric surface of coal aquifers, which serve as local water sources, and potential loss of stream flow for as long as 50 years (Meyers, 2009). Simple mathematical models (e.g., the Glover-Balmer method) related to the effects of regional CBM withdrawals in the San Juan Basin have also been employed and model results interpreted to suggest stream depletion and drawdown of the potentiometric surface of coal-bearing formations within 20 miles of their outcrop area (e.g., S.S. Papadopulos & Associates, Inc., 2006; Hathaway et al., 2006). In the case of the San Juan Basin where other studies yielded results with sufficient isotopic age dating (Box 2.2), the
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Management and Effects of Coalbed Methane Produced Water in the Western United States data show CBM produced water is primarily fossil groundwater that has not been recharged for thousands to tens of millions of years, which contradicts the model results for stream depletion and drawdown. Similarly comprehensive data to test the results from the Powder River Basin modeling study are not currently available. The Glover-Balmer method incorporates assumptions based on pumping water from a well constructed in an artesian aquifer. Many of these assumptions are violated when applied to pumping from multiple wells in a complicated watershed (e.g. Spalding and Khaleel, 1991; Sophocleous et al, 1995). For example, the method assumes that the aquifer is “isotropic” (permeability the same horizontally and vertically) and “homogeneous” (the same material everywhere). In CBM basins, interlayered coarse- and fine-grained rocks occur and generate notable heterogeneous and non-isotropic conditions. The method also assumes that the aquifer extends to infinity. This assumption can be valid locally when a well is pumped, but does not apply to a basin where geologic units pinch out or disappear over short distances as they do in the western CBM basins. The Glover-Balmer method is only useful as a first approximation, at best, if at a watershed scale. Although modeling may be useful for broad assessment of possible hydraulic relationships in CBM basins, numerical models of hydrogeological systems currently do not yield unique results. Different, multiple combinations of input parameters can produce the same overall results for measurements of water levels and other hydrological data typically used to calibrate the model (e.g., Oreskes et al., 1994; McDonnell et al., 2007). Furthermore, current models cannot yet characterize complex water-rock interactions, differences in hydraulic properties, or boundary conditions in CBM basins. Thus, testing the results and assumptions of numerical and other groundwater models against data from the field or area being modeled is important in order to establish a level of reliability that is suitable for making management decisions. For example, if a model predicts decreasing flow in streams because of CBM production, then low-flow measurements in the rivers presumed to be affected are necessary to test the model results. Similarly, if modeling suggests that streams receive water from coalbeds to maintain baseflow, then chemical measurements in the streams are necessary to determine if CBM “fingerprints” (chemical constituents typical of CBM formations) are present in the water. Despite these limitations, groundwater models of basins can predict general travel time of groundwater along flow paths, and these predictions can be tested by age dating the water. Until the gap is filled between the results of groundwater models and the necessary data to test them, care is urged with regard to using model results alone to make regulatory or other determinations regarding produced water management. The ability to place more reliance in the future on outputs of models that more closely resemble natural complexities of the hydraulic conditions of CBM basins necessitates demonstrating better convergence between existing model results and data collected and analyzed from the basins.
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Management and Effects of Coalbed Methane Produced Water in the Western United States CHAPTER SUMMARY 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
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Management and Effects of Coalbed Methane Produced Water in the Western United States 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, geochemical, 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 instream 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 water-rock 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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