1 Chapter L Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction: An Example from the Ferron Sandstone Member of the Mancos Shale, Utah National Coal Resource Assessment Click here to return to Disc 1 Volume Table of Contents By Todd A. Dallegge 1 and Charles E. Barker 1 Chapter L of Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Edited by M.A. Kirschbaum, L.N.R. Roberts, and L.R.H. Biewick U.S. Geological Survey Professional Paper 1625 B* 1 U.S. Geological Survey, Denver, Colorado 8225 * This report, although in the USGS Professional Paper series, is available only on CD-ROM and is not available separately U.S. Department of the Interior U.S. Geological Survey
2 Contents Overview... L1 What Is Coal-Bed Methane?... 2 Importance of Coal-Bed Methane Production... 2 How Much Coal-Bed Methane is Available?... 3 How Do Coal Beds Generate and Store Methane?... 4 Details About Coal Cleat... 5 Production of Coal-Bed Methane... 6 Indtroduction to Ferron Sandstone CBM... 7 Regional Geology... 8 Structural Setting in the Western United States During the Late Cretaceous... 9 Depositional Environment for the Ferron Sandstone Member... 1 Formation of Ferron Coal Ferron Coals Characteristics of Lower Ferron Coal at Drunkards Wash Characteristics of Upper Ferron Coal at Emery Similarities of Upper and Lower Ferron Coals Procedures for Estimating Gas-In-Place Direct Method Direct Measurements of Coal-Bed Methane Content Methods for Determining Sorption Isotherms Experimental Method Experimental Method of Estimating CBM Content Theoretical Method Theoretical Method of Estimating CBM Content: Burial History Reconstruction Biogenic Gas An Unknown Quantity Estimates of Total Coal Mass for Drunkards Wash Area... 2 Method for Determining Gas-In-Place Estimating the GIP for Drunkards Wash Direct Method Experimental Method Thoeoretical Method Summary Caveats References Cited Glossary 24
3 Figures 1. Photograph of the Ferron Sandstone Member showing thick sandstones and the Sub-A coal zone at the Interstate-7 roadcut south of Emery, Utah... Frontispiece 2. Photograph of coal-bearing Ferron Sandstone Member of the Mancos Shale and the Marysvale volcanic field... L1 3. Map showing major U.S. coal-bed methane resources Graph showing potential future availability of U.S. coal-bed methane Coal cleat terminology Gas generation from sapropelic and humic coals Photograph of outcrop showing pyrite filling cleat fracture Photograph of cleats in the A coal horizon at the I-7 roadcut south of Emery, Utah Coal-bed methane release process Measured, calculated, and hypothetical gas production curves for River Gas Corp well Map showing location of Drunkards Wash study area Depositional setting for Ferron Sandstone Member of the Mancos Shale Block diagram showing depositional environments of Upper Cretaceous rocks in the foreland basin of eastern Utah Late Triassic Late Cretaceous depositional pattern in the Western United States Diagrammatic cross section of the Sevier foreland basin Generalized cross section showing Ferron Sandstone Member stratigraphy Diagram showing depositional environment for Ferron Sandstone Member of the Mancos Shale Diagram showing peat environments and preservation of peat by a transgressive event Location map for coal-bed methane development in eastern Utah Generalized cross section from the Wasatch Plateau to the San Rafael Swell showing position of Ferron coals Surface outcrop pattern of the Ferron Sandstone Member Photomicrograph of an upper Ferron coal showing vitrinite, the major gas-sorbing component of a coal Map showing vitrinite reflectance values for the Ferron Sandstone Member Adsorption isotherm of lower Ferron coal from Utah well Diagram of upper Ferron experimental gas-in-place estimate Drunkards Wash burial history reconstruction Muddy Creek (Emery) burial history reconstruction Map showing isopachs of total coal for Drunkards Wash area... 2
4 Table 1. Average analyses of Ferron coals... L14 Figure 1 (Frontispiece). Photograph of the Ferron Sandstone Member of the Mancos Shale showing thick sandstones and the Sub-A coal zone at the Interstate-7 roadcut south of Emery, Utah.
5 Coal-Bed Methane Gas-In-Place Estimates Using Sorption Isotherms and Burial History Reconstruction: An Example from the Ferron Sandstone Member of the Mancos Shale, Utah By Todd A. Dallegge and Charles E. Barker Overview This chapter describes coal-bed methane resource assessment and production practices to non-specialists. The chapter takes the reader through the basics of coal-bed methane (CBM). A glossary of technical terms follows the References Cited section. After a brief overview of coal-bed methane, a comparison is made between gas-in-place (GIP) estimates derived from direct gas-content measurement with those estimates made from experimentally determined and modeled theoretical values. This comparison uses the exposed and buried coal-bearing Ferron Sandstone Member of the Mancos Shale. The Ferron coals offer a unique field laboratory where analysis of exposed coals are used to estimate GIP and model CBM productivity trends in subsurface coals. Figure 2. Photograph of coal-bearing Ferron Sandstone Member of the Mancos Shale (foreground) and the Marysvale volcanic field (background). L1
6 L2 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah What is Coal-Bed Methane? Coal-bed methane (CBM) is an economic source of pipeline-quality methane that is generated and stored in coal beds. It is a widely occurring, exploitable resource that can be easily recovered and used near the well or where any gas-pipeline infrastructure currently exists. Major U.S. Coal-Bed Methane Resources Importance of Coal-Bed Methane Production Coal-bed methane production started as a way to keep coal mining safe from explosions. Not only does it provide the same service now, it also decreases emissions of greenhouse gasses from mines, decreases air pollution because it is a clean-burning fuel, decreases the need for conventional fossil fuels, and further utilizes the vast coal resources that are already known. CBM potential could double our natural gas reserves in the United States (Rogers, 1994). Figure 3 shows the major areas and amounts of U.S. coal-bed methane resources as of CBM is of particular interest to regions without a pipeline infrastructure because the gas is usable at the wellhead or through a local pipeline for domestic or industrial use. Greater Green River (31 TCF) Uinta (1 TCF) Piceance (84 TCF) Alaska (~1 TCF) San Juan (88 TCF) Powder River (39 TCF) Forest City Raton (18 TCF) Arkoma (4 TCF) TCF = trillion cubic feet Cherokee Illinois (21 TCF) Northern Appalachian (61 TCF) Black Warrior (2 TCF) Southern Appalachian (5 TCF) Modified from Schraufnagel and Schafer (1996) Figure 3. Map showing major U.S. coal-bed methane resources.
7 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L3 How Much Coal-Bed Methane is Available? Prior to 1982, there was little coal-bed methane production in the United States. Potential future availability of U.S. coal-bed methane Between 1983 and 1993, industry drilled ~ 6,6 coal-bed methane wells (Schraufnagel and Schafer, 1996). 3 Billions Cubic Feet Per Day Billions Cubic Feet Per Day By 1994, annual production had grown to 858 BCF (billion cubic feet) (Schraufnagel and Schafer, 1996). In 1995, coal-bed gas supplied 5 percent of U.S. natural gas production. CBM is thought to be an important potential resource available anywhere coal is found worldwide. CBM for the top 2 coal-bearing countries is estimated at m 3 (1,8 TCF trillion cubic feet) (Kuuskraa and others, 1992). Figure 4 shows the estimated future availability of U.S. coalbed methane Year Existing resource production and projected future production of these resources Production projection for new resources modified after Kuuskraa and others (1992) Figure 4. Graph showing potential future availability of U.S. coal-bed methane.
8 L4 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah How Do Coal Beds Generate and Store Methane? Coal acts as both source rock and reservoir rock for methane. Methane is generated by microbial (biogenic) or thermal (thermogenic) processes shortly after burial and throughout the diagenetic cycle resulting from further burial. Much of this gas is physically sorbed on coal surfaces in areas with coal microporosity. One gram of coal can contain as much surface area as several football fields and therefore is capable of sorbing large quantities of methane. One short ton of coal can produce ~1,3 m 3 of methane. 2 5 in cm Approximate Scale Master Cleat Secondary Cleat Primary Cleat Non-coal interbed Coal Tertiary Cleat Modified from Tremain and others (1994) Hydraulic pressure, rather than a pressure seal or closed structure (common for conventional oil and gas fields), is the major trapping force for CBM. Coal is extremely porous (openings), but has low permeability (connected openings). Most coals contain methane, but much of it cannot be economically produced without the presence of natural fractures (cleats) to connect the pores. Cleats allow the desorbed gas to flow to the well. CH 4 (methane) and CO 2 (carbon dioxide) are the major components of CBM. Temperature, C Catagenesis 15 2 Metagenesis Saprolelic Source Humic Source Relative Yield of Gas Figure 6. Gas generation from sapropelic and humic coals Temperature, F From Hunt (1996) CH 4 CO 2 C 2 H 6 + N 2 H 2 S Figure 5. Coal cleat terminology.
9 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction Master Cleat Primary Cleat Non-coal interbed Coal Details About Coal Cleat Cleat develops in a coal from devolatilization occurring during coalification, as well as during regional and local structural events. Cleat patterns are placed in a hierarchy of sizes. Secondary and tertiary cleats are the small-scale microfractures that develop within the coal seam. Primary cleat develops across coal layers and provide connections with non-coal interbeds. Master cleat crosses both coals and non-coal interbeds. These cleat patterns are crucial for gas production because they allow for the release of sorbed gas within coal beds and migration to the well. Cleat can be destroyed by further burial in a process called healing or filled by secondary mineralization. Secondary Cleat Tertiary Cleat 2 in 5 cm Approximate Scale Modified from Tremain and others (1994) Non-coal interbed Master cleat Secondary cleat Primary cleat Figure 8. Photograph of cleat in the A coal horizon at the I-7 roadcut south of Emery, Utah. Figure 7. Photograph of outcrop showing pyrite filling cleat fracture. L5
10 O L6 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Production of Coal-Bed Methane Methane sorbed within coal beds is regulated by the hydrodynamic pressure gradient. Methane is maintained within the coal bed as long as the water table remains above the gas-saturated coal. If the water table is lowered by basin or climatic changes, then methane stored within the coal is reduced by release to the atmosphere. Coal-bed gas content must have reached near-saturation either by biogenic or thermogenic gas-generation processes to be economically viable. Cleats must be present to allow for connectivity between sorption sites. If the coal-bed horizons are buried deeply (~2 or more km), cleats are closed because of overburden pressure acting on the structurally weak coal bed. To initiate gas production, water must be pumped out of the saturated coal zone. Dewatering reduces the cleat pressure allowing gas to desorb from the coal matrix and diffuse to the cleat. Some wells may not be economical if too much water has to be pumped. Pumping requires energy and the net gain from burning the gas may be too low to be profitable. Some coal beds may never be dewatered, depending on the hydrology. If CBM fields are associated with a conventional gas trap, like the Drunkards Wash area in Price, Utah (Burns and Lamarre, 1997), gas flows freely upon completion of the well without the need for dewatering. CBM, unlike conventional oil and gas production, usually shows an increase in the amount of production at first, then steadily declines. See figure 1. Normal gas production is pressure driven and shows a decline in production from the initial tapping of the reservoir. In the case of CBM, gas is sorbed within the microporosity of the coal matrix. As the partial pressure within the coal bed is reduced, methane is produced as the gas desorbs the microporosity of the matrix and flows through the cleat to the well. As a coal is dewatered, the cleat system progressively opens farther and farther away from the well. As this process continues, gas flow increases from the expanding volume of dewatered coal. Water production decreases with time, which makes gas production from the well more economical. In 1997, production at Drunkards Wash was 43.7 MMCF/d (million cubic feet per day m 3 /d) of gas from 89 wells (Burns and Lamarre, 1997). Methane concentrations are from 95.8 percent to 98.3 percent (Burns and Lamarre, 1997). CO 2 makes up the other fraction. Cleat Methane gas Water Nitrogen gas Carbon dioxide gas COAL MATRIX Figure 9. Coal-bed methane release process. mcf/day Reduce methane partial pressure + in cleat + Reduce cleat pressure by producing water Methane desorbs from matrix and diffuses to cleat Methane and water flow to wellbore Wellbore, Master Cleat, or Fault Modified from Puri and Yee (199) Curves compare production from a CBM well and a convential gas well. TIME (Years) Measured Calculated Decline Hypothetical Conventional Gas Production Curve Modified from Tabet and others (1996) Figure 1. Measured, calculated, and hypothetical gas production curves for River Gas Corp well.
11 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L Introduction to Ferron Sandstone CBM The U.S. Geological Survey (USGS) has been assessing coal-mass resources in the United States. This coal assessment data could also be used to make an assessment of coal-bed methane resources in the United States. To do this, a method is needed to take coal mass resource data and convert it, using estimates of gas content per unit mass, to gas-in-place (GIP) estimates. Estimates of gas content per unit mass can be made from (1) direct measurements of the gas content of fresh samples, (2) values determined experimentally, and (3) values derived from theoretical models. This study compares these three different methods and the resulting GIP estimates. The comparison is made using data from coals of the Ferron Sandstone Member of the Mancos Shale. Near Emery, Utah, the upper Ferron coals are exposed in a 2- by 5-km outcrop band that gives a three-dimensional exposure of these deposits. The lower Ferron coals in the northern portion of the coal production trend near Price, Utah, are still buried. A portion of this production trend, located within the Drunkards Wash area, has been intensively developed for CBM, and a nearly complete grid of data is available to calculate the coal mass in-place. Further, many of the coal cores were measured directly for gas content. This situation offers an optimal setting to test various methods of estimating GIP resources Salt Lake City Study Area UTAH 11 Drunkards Wash Study Area Emery Price Colorado River 8 Km 4 Mi Figure 11. Map showing location of Drunkards Wash study area.
12 L8 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Regional Geology The Upper Cretaceous (~ 9 million years old) sediments of the Ferron Sandstone Member of the Mancos Shale were deposited in the shallow Mancos sea. The Ferron Sandstone Member was deposited in a series of deltas along the western edge of a basin that formed when thrust faulting during the Sevier orogeny thickened the crust and caused the crustal area ahead of the faulting to sag. Depositional Setting for the Ferron Sandstone Member of the Mancos Shale 14 W W 42 N UTAH Sevier Orogenic Belt Foreland Basin 5 MI Lower Ferron Ferron Emery Upper Ferron Marysvale Volcanic Deposits Price San Rafael Swell Cretaceous Epeiric Sea Pz Block Diagram of the Depositional Environments of Upper Cretaceous Rocks in the Foreland Basin of Eastern Utah Thrust Front Coal Sevier Alluvial Plain Coal Swamps Delta Mancos Shale Coastal Bar Shallow Sea 37 N 5 Km Modified from Ryer, 1991 Modified from Hintze (1988) Figure 13. Block diagram showing depositional environments of Upper Cretaceous rocks in the foreland basin of eastern Utah. Figure 12. Depositional setting for Ferron Sandstone Member of the Mancos Shale.
13 Structural Setting in the Western United States During the Late Cretaceous Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L9 The Sevier thrust belt system formed a foreland basin in central Utah during Turonian time (~9 Ma ~9 million years ago). Sediments from the Sevier Highlands to the west were shed to the east and formed many clastic wedges that intertongued with marine intercontinental Mancos seaway. As sea level rose and fell and the Sevier thrust front advanced, transgressive and regressive cycles are represented by clastic and carbonate deposition that filled the continuously subsiding foreland basin. The Ferron Sandstone Member of the Mancos Shale represents one episode of deltaic sedimentation into this foreland basin. The Ferron Sandstone Member of the Mancos Shale was deposited in the Vernal and Last Chance deltas. LATE TRIASSIC-LATE CRETACEOUS DEPOSITIONAL PATTERN WEST COAST TRENCH WEST COAST TRENCH KULA PLATE SEVIER OR OGENIC OGENIC BELT Thicknesses of Mesozoic Strata < 1 ft 1-5 ft > 5 ft Modified from Hintze (1975) Western Utah West Blackhawk CBG Sevier Orogenic Belt Fairway Foreland Basin Cretaceous Paleozoic Strata Deposits Found in the Sevier Foreland Basin Ferron Fairway Emery Sandstone Mbr Blue Gate Shale Mbr Ferron Sandstone Member Figure 15. Diagrammatic cross section of the Sevier foreland basin. Mancos Shale Seaway Mesaverde Group Tununk Shale Member Dakota Sandstone Western Colorado East Fox Hills Sandstone Modified from Armstrong (1968) Niobrara Formation Figure 14. Late Triassic Late Cretaceous depositional pattern in the Western United States.
14 L1 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Depositional Environment for the Ferron Sandstone Member These deltaic environments produced substantial coal deposits. The deposits created by the Last Chance delta can be observed in surface outcrop exposures. The deposits of the Vernal delta are only found in the subsurface. The deposits from the Ferron Sandstone Member show a series of stacked, small-scale, transgressive-regressive cycles that are defined in outcrop by cliff-forming delta-front sandstones (Ryer, 1991). Drunkards Wash gas field, located west of Price, Utah, is currently being exploited for coal-bed methane production. Depositional Environment for the Ferron Sandstone Member Blue Gate Member of Mancos Shale Ferron Sandstone Member A Sub- A M C G Tununk Member of Mancos Shale Marine shale Beach and open-shelf sandstone Delta-front and fluvial sandstone Delta-plain and fluvial deposits Coal C Named coal beds I J?? Washboard unit of Cotter (1975) Clawson unit of Cotter (1975) Lower Ferron Sandstone Member Modified from Ryer (1991) Gas-Producing Lower Ferron Coals Non-Gas-Producing Upper Ferron Coals Emery Ferron outcrop belt Vernal Delta Price Shoreface Last Chance Delta Offshore Bars 2 km 1 Mi N Modified from Cotter (1976) Clawson and Washboard Units Figure 16. Generalized cross section showing Ferron Sandstone Member stratigraphy. Figure 17. Diagram showing depositional environment for Ferron Sandstone Member of the Mancos Shale.
15 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L11 Formation of Ferron Coal The Ferron Sandstone Member contains as many as 14 coal horizons. These coals formed in swampy areas behind the delta-front of the prograding shoreline. They generally mark the onset of each transgressive event (Garrison and others, 1997) because peat preservation is enhanced under these conditions. Peat forms when plant debris is preserved by a rise in the water table caused by a relative rise in sea level that occurs during a transgressive event. This preserved peat is transformed to coal by thermogenic and biogenic processes resulting from burial. A combination of the rapidly subsiding foreland basin and sea-level changes provided accommodation space for the accumulation of relatively thick coal beds. Many of these coal beds are laterally extensive or can be correlated to carbonaceous mudstones that were deposited on the alluvial plain (Garrison and others, 1997). It is possible to study the upper Ferron coals across an outcrop band that is ~2 km wide and ~5 km long. Peat Environments 111 3' 111 Sphagnum Peat Forest peat Swamp Peat Reed peat Sapropel Marine Muds Sea level RIVER GAS EIS STUDY AREA Price Preservation of Peat by a Transgressive Event Sphagnum Peat Bright coal, low ash Forest peat Swamp Peat Bright coal, higher ash Reed peat Sapropel Dull Coal Coals Forming After Burial Sea level Transgression Marine Muds Cannel coal Carb. Shale Modified from Diessel, 1992 Figure 18. Diagram showing peat environments and preservation of peat by a transgressive event. 39 3' Ferron Coal Trend Emery Ferron Location map for coalbed methane development in eastern Utah Source: UGS Castle Dale Figure 19. Location map for coal-bed methane development in eastern Utah.
16 L12 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Ferron Coals Near Emery, Utah, Ferron coals are exposed on the western flank of the San Rafael Swell. Along the eastern front of the Wasatch Plateau near Emery, Utah, the Ferron coals are deeply buried and form a production fairway (fig. 2). To the north, near Price, Utah, Ferron coals are only found in the subsurface. Average depth to the Ferron coals in Drunkards Wash, west of Price, is 732 m (2,4 ft) with an average net-coal thickness of 7.3 m (24 ft) (Burns and Lamarre, 1997). WEST Generalized Section from the Wasatch Plateau to the San Rafael Swell showing position of the Ferron Coals Wasatch Plateau 3,48 1,524 SL -1,524 Meters Cretaceous Jurassic Joes Valley Fault System Production Fairway Paleozoic Ferron Coals San Rafael Swell Precambrian EAST 1, 5, SL -5, Feet Salt Lake City Study Area UTAH Surface Outcrop Pattern of the Ferron Sandstone Member 11 Drunkard's Wash Study Area 8 Km 4 Mi Emery Price Colorado River Emery Hunington Clawson Ferron Moore WASATCH PLATEAU Castle Dale Miller Cyn Ivie Crk. Willow Sprs. Price Dry wash Muddy Crk. Ferron Mbr. Outcrop CASTLE VALLEY N I-7 Section is approximately 5 miles (8 km) across Modified from Hintze (198) Figure 2. Generalized cross section from the Wasatch Plateau to the San Rafael Swell showing position of Ferron coals. Marysvale Volcanic Deposits 1 Miles 16 Kilometers From Tabet and others, 1995 Figure 21. Surface outcrop pattern of the Ferron Sandstone Member.
17 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L13 Characteristics of Lower Ferron Coal at Drunkards Wash Vitrinite Reflectance Values for the Ferron Sandstone Salt Lake City Price Drunkard's Wash EIS Study Area.7 Price Ferron ss. Outcrop Figure 22. Photomicrograph of an upper Ferron coal showing vitrinite, the major gas-sorbing component of a coal. ad lor AU 8 Km 4 Mi LE PL AT E Huntington.6 Castle Dale ST Clawson Ferron.6 Moore H AT C.5 Dry Wash Muddy Crk. I Emery.5 Miller Cyn N Ivie Crk..45 Willow Sprs. WA S Near Emery, Utah, the upper Ferron coals generally have a lower rank, Ro values of.4 percent to.6 percent, than the lower Ferron coals at Drunkards Wash. Our proximate analyses indicate the following averages: ash yield 35. weight percent (dry basis), volatile matter content 48.7 weight percent (d.a.f.), fixed carbon content 51.3 weight percent (d.a.f.). Burial and gas-generation models with identical heat-flow histories indicate that the upper Ferron coals have generated about.1 cm3 gas/g carbon, whereas the lower Ferron coals have generated about 8 cm3 gas/g coal (256 standard cubic feet per short ton [scf/short t]). UTAH Co 38 or ive r Characteristics of Upper Ferron Coal at Emery Emery Y Study Area LLE 4 VA 114 CA At Drunkards Wash, the lower Ferron coals have a mean random vitrinite reflectance value (Ro ) of.7 percent. This classifies them as high-volatile B bituminous coal (Burns and Lamarre, 1997). Proximate analyses of the coals indicated the following averages; ash yield 14.6 weight percent (basis not reported), volatile matter content 42 weight percent (dry, ash-free basis [d.a.f.]), fixed carbon content 48.6 weight percent (basis not reported) (Burns and Lamarre, 1997). Permeability in Ferron coals is estimated between 5 and 2 millidarcies (md) (Burns and Lamarre, 1997). 1 Miles 16 Kilometers Marysvale Volcanic Deposits Ro from Tabet and others (1995) and Burns and Lamarre (1997) with data from this study Figure 23. Map showing vitrinite reflectance values for the Ferron Sandstone Member.
18 L14 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Similarities of Upper and Lower Ferron Coals The upper and lower Ferron coals were deposited in a similar geologic setting. Consequently, the coals had similar starting compositions, but changes occurred because of burial history (rank) and grade (syndepositional mineral matter). The coals near Emery have a higher ash content (lower grade) and lower rank indicated by higher moisture and generally lower vitrinite reflectance values. This would account for much of the difference in the chemical properties between lower and upper Ferron coals. Because the lower and upper Ferron coals are similar, we used average-gas-content estimates from different methods and applied them to both lower and upper Ferron coals. These gas-content estimates were used with coalmass estimates to compute GIP for the lower Ferron coals at the Drunkards Wash area. Table 1. Average analyses of Ferron coals. [ar, as-received; d.a.f., dry, ash-free; mmmf, moist, mineral-matter-free. Modified from Doelling and Smith (1982)] Location Analysis Price Emery Basis Moisture (wt. %) ar Volatile matter (wt. %) d.a.f. Fixed carbon (wt. %) d.a.f. Ash (wt. %) ar Sulfur (wt. %) d.a.f. Btu/lb 12,322 9,453 mmmf R o (%).5 to.7.4 to.6 mean random
19 Coal-Bed Methane Gas-In-Place Resource Estimates Using Sorption Isotherms and Burial History Reconstruction L15 Procedures for Estimating Gas-In-Place Direct Method Direct measurements of gas content of the lower Ferron coal at the Drunkards Wash area have been reported by Burns and Lamarre (1997). The other parameters necessary for this method are proximate analyses and total coal mass. We produced an estimate of the total coal mass from coal-thickness maps of the Drunkards Wash area. The GIP estimate for the Drunkards Wash area was calculated by multiplying the desorption data (gas content) by the total coal mass of the lower Ferron coals. Direct Measurements of Coal-Bed Methane Content The direct method for measuring CBM content involves coring the coal, immediately placing the coal in a gas-tight container, and then measuring the gas evolved over time. The gas evolved, when corrected for gas lost after core drilling and before placement in canister, is a direct measurement of gas content. Figure 24 shows the adsorption isotherm for coals at Drunkards Wash EIS area. The red curve indicates the maximum amount of gas (at saturation) that the coal can hold at a given pressure. These measurements were made at equilibrium moisture conditions and at 38 C (1 F), the apparent reservoir temperature. The curve predicts a maximum gas content for the lower Ferron coals of ~9.4 cm 3 /g (~3 scf/short t) for a pressure of 5,275 kpa (765 psi). The average total gas content from desorption tests at Drunkards Wash is m 3 /g (438 scf/short t). The measured experimental isotherm (red curve) at the reservoir temperature (38 C, 765 psi) should intersect at the measured desorption content. Burns and Lamarre (1997) interpreted this condition as oversaturation of the coals. CBM gas content of coals is not considered to reach oversaturated conditions. We believe the discrepancy is due to conducting the experimental sorption isotherm tests at an excessively high temperature, thus depressing the isotherm. The adsorption isotherm generated at a reservoir temperature of 21 C (7 F) would yield a curve (blue line) near that indicated by the measured gas content of the lower Ferron coals at the Drunkards Wash EIS area. Methods for Determining Sorption Isotherms Sorption iostherms indicate the maximum volume of methane that a coal can store under equilibrium conditions at a given pressure and temperature. The direct method of determining sorption isotherms involves drilling and cut- Adsorption isotherm of lower Ferron coal from Utah # well in sec. 25, T. 14 S., R. 9 E. Gas Content (scf/short t) Depth feet Dry, non-ash free V=425*P/(P+357) Eq. moisture = 2.4% Temperature = 1. F 438 SCF/T 4 6 Pressure (psi) 765 PSI 1. F 8 Inferred 7. F Modified from Burns and Lamarre, 1997 Figure 24. Adsorption isotherm of lower Ferron coal from Utah well. ting core that is immediately placed in canisters followed by measurements of the volume of gas evolved from the coal over time. The indirect method takes advantage of core or cuttings that have been stored and does not require fresh core, thus making this method more economical. Sorption isotherms are experimentally measured using a powdered coal sample whose saturated methane content at a single temperature is measured at about six pressure points (Mavor and others, 199). Moisture content in a coal decreases the sorption capacity of a coal. Because coal loses moisture at a variable rate subsequent to removal from the bore hole, a standard moisture content is used when measuring sorption isotherms. Sorption isotherm data are useful in predicting theoretical gas-production characteristics. For example, desorption of gas will not occur at pressures above the critical desorption pressure in a coal that is undersaturated with gas. The reservoir pressure must be reduced by pumping and dewatering the coal until the critical sorption pressure is reached. In a case where the coal is saturated with gas, the reservoir pressure is equal to the critical desorption pressure, and dewatering causes immediate onset of gas production. 1 5 Gas Content (cm 3 /g)
20 Air-Dry Gas Content, (cm 3 /g) L16 Geology and Resource Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah Experimental Method We used adsorption isotherm analyses of coal-seam samples from the upper Ferron coals near Emery, Utah, to estimate the gas content of the coals and applied this value to the coal mass estimate of the lower Ferron coals within the Drunkards Wash EIS area. The experimental maximum gas content was derived from adsorption isotherm data by computing the gas content at present reservoir temperature and pressure conditions. Our experimental GIP estimate for the Drunkards Wash EIS area was calculated by multiplying the estimated gas content of the upper Ferron coals by the total coal mass (from the direct method above) of the lower Ferron coals. Experimental Method of Estimating CBM Content 4 Upper Ferron Experimental Gas-In-Place Estimate Six channel samples from various exposures of the upper Ferron coals were used to estimate the experimental saturated gas content from sorption isotherm analyses. The six samples were analyzed for adsorption capacity at 25 C (77 F) over a range of pressures from to 12 MPa ( to 1,74 psi). Adsorption isotherms were plotted from these experimental runs. A maximum burial depth of 73 m (2,395 ft) was determined from vitrinite reflectance values and burial history analysis. A hydrostatic gradient of 1.4 kpa/m (.46 psi/ft) was used to calculate a pressure of 7.6 MPa (1,1 psi) at maximum burial depth. The maximum burial pressure of 7.6 MPa was used in each adsorption isotherm plot to determine the experimental gas content at saturation (cm 3 /g [scf/short t]) for that sample. Proximate analyses of the six samples determined ash and moisture content. The ash + moisture fraction was plotted against the adsorbed values (at 7.6 MPa) to determine the average pure coal (at percent ash + moisture). The gas content of the pure coal is 5 cm 3 /g (16 scf/short t) Samples Plot 1 Regr Plot 1 Pred Ash+Moisture Content, fraction Figure 25. Diagram of upper Ferron experimental gas-in-place estimate.
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