COAL GAS ORIGINS AND EXPLORATION STRATEGIES © Andrew R. Scott Altuda Energy Corporation San Antonio, Texas USA [email protected] A LTUDA Mid-Continent Coalbed Methane Symposium Tulsa, Oklahoma November 7-9, 2004 © 2004 A.R. Scott EARTH AT NIGHT Page1of1 1,400 1,200 1,000 800 600 400 200 0 1900 1950 2000 2050 2100 Year Data from U.S. Bureau of the Census; ftuture growth estimates from U.S. Census Bureau publication NP-T1, February 2000; website www.mnforsustain.org/united_states_population.htm World view problems 1. Energy 2. Water 3. Food 4. Environment 5. Poverty 6. Terrorism & War 7. Disease 2. Education 9. Democracy 10. Population Tinker (2004) after Smalley (2003) U.S. ENERGY COMPARISON To Produce 20% of US Energy Demand (20 Quads per year) 1 million kg biomass/km2* 16,000 BTU/kg = .02 Q/4,000km2 after loss 20 ac/turbine 175,000 Kwh/yr/turbine 33 million turbines Solar Wind Biomass According to Pimentel et al. (BioScience Sept. 1994), Tinker RMS AAPG Keynote (2004) A LTUDA NATURAL GAS DEPLETION RATES 60 50 40 30 20 10 0 Rocky Texas Oklahoma Offshore Lower 48 Mountain GOM Kenderdine (2002) 1990 1999 A LTUDA COALBED METHANE PRODUCTION IN U.S. 2,000 1,800 1614 1600 1,600 1562 1379 1,400 1252 1194 1,200 1090 1003 956 1,000 851 800 752 600 539 400 348 196 200 91 10 17 24 40 0 85 87 89 91 93 95 97 99 01 2003 Energy Information Year Administration (1992, 1995,1999, 2002, 2004) A LTUDA COALBED METHANE EXPLORATION MODEL Permeability Gas content Hydrodynamics CoalbedCoalbed MethaneMethane ProducibilityProducibility Coal rank and Depositional gas generation systems and coal distribution Tectonic and structural setting A LTUDA A LTUDA EVALUATION OF GAS CHEMISTRY • Gas chromatographic analyses provides information about gas chemistry; percentages of hydrocarbons, carbon dioxide, nitrogen, hydrogen, oxygen, etc. provided. • Gas compositional analyses (gas chemistry) can be used in the classification of gases and to determine gas origins using a variety of calculated parameters including the gas dryness index and carbon dioxide content. • Gas compositional analyses provide BTU content data of gases; higher BTU (wetter) gases demand a higher selling price, whereas low-BTU gases (carbon dioxide or nitrogen-rich) yield lower gas prices. • The isotopic composition of gases, particularly the δ13C and δD values of hydrocarbon gases and carbon dioxide, as well as isotopic data from formation waters can be used to determine gas origins and possible migration pathways. • In some cases δ13C isotopic values of gases can be used to estimate distance of migration. A LTUDA GAS DRYNESS INDEX (HYDROCARBONS ONLY) C1 GDI = C1 -C5 > 0.99 Very dry 0.94 to 0.99 Dry 0.86 to 0.94 Wet ≤ 0.86 Very wet Scott et al. (1991) A LTUDA CARBON DIOXIDE CONTENT (percent) > 10 Very high 6 to 10 High 2 to 6 Moderate ≤2 Low Hanson (1990) A LTUDA FRUITLAND COAL GAS CHEMISTRY 0 20 mi COLO 030 km N. MEX CH CH N No data No data Gas Dryness Percent Carbon Dioxide C1/C1-5 > 10 1 to 3 > 0.97 0.86 to 0.90 6 to 10 < 1 0.94 to 0.97 < 0.86 3 to 6 0.90 to 0.94 A LTUDA Scott and others (1994) SAND WASH BASIN GAS CONTENT VARIABILITY W Percent Carbon Dioxide E Morgan Federal 12-12 Klein 23-11 Van Dorn No. 1 Colorado State 1-31 T8N, R93W, Sec. 12 T7N, R91W, Sec. 11 T7N, R90W, Sec. 29 T7N, R88W, Sec. 31 Datum 115 58c 353 71 70 414 21.6200c 207c 192c 76s <192c 90 140 216c 5.039s 23.8291 153 133c 26.4376 1.874c 68 2.699s 274 184s 9.7112 20.0143s 182s 117c 147s s = sidewall cores Shoreline sands c = cuttings 8.768 Drill Stem Test: 1,462 md 85c A LTUDA Gas content values are given on non-ash-free basis Scott (1993) BASIC TYPES OF COAL GASES Thermogenic • Coal gases are produced during thermal maturation of peat (organic-rich sources) • Variable gas composition: high BTU wet gases or nearly 100% methane • Brackish to saline formation water • Low to very high gas contents (<1 to 25+ g/cc; <32 to 800+ scf/ton) Secondary Biogenic • Coal gases formed by bacteria transported basinward in permeable coal seams • Occur at any coal rank after basin has been uplifted and coals exposed at outcrop • Biogenic gases are nearly 100% methane; variable carbon dioxide • Fresh to slightly brackish formation water. • Low gas contents (1 to 3 g/cc; 32 to 90 scf/ton) Migrated • Migration of thermogenic or secondary biogenic • Some coal gases not derived from coals – can be derived from shales Indigenous • Retained or adsorbed to the coal surface at time of generation. • May be thermogenic or secondary biogenic A LTUDA THERMOGENIC GASES Thermogenic gases in coal beds are derived from the thermal maturation of organic matter. Early Thermogenic gases form at low coal ranks (subbituminous to high-volatile B bituminous; VR < 0.80%) from hydrogen-rich coals. Early thermogenic methane may form in coal beds at vitrinite reflectance values of 0.40 percent. High BTU gases and some oil or paraffins are common in some areas and the peak of wet gas generation occurs between VR values of 0.60 and 0.80 percent. Gas contents are generally low, but 100 to 200 Mcf per day is possible from some low rank coals. Main Stage Thermogenic gases are formed when the threshold of thermogenic methane generation is reached at vitrinite reflectance values between 0.80 to 1.00 percent (high-volatile A bituminous rank). Gas contents are variable, but can exceed 800 scf/ton in higher rank coals. Production rates of more than 5 MMscfd are reported. COAL RANK AND THERMOGENIC GAS GENERATION Stach ASTM Scott (1975) (1983) (1995) Coal Rank lig lignite 0.38 0.38 0.38 sub subbituminous 0.65 0.49 0.49 hvCb high-volatile C bituminous 0.65 0.51 0.65 hvBb high-volatile B bituminous 0.78 0.69 0.78 hvAb high-volatile A bituminous 1.11 1.10 1.10 mvb medium-volatile bituminous 1.49 1.60 1.50 lvb low-volatile bituminous 1.91 2.04 1.91 sa semianthracite 2.50 2.40 2.50 a anthracite 5.00 5.00 5.00 ma meta-anthracite A LTUDA COAL RANK AND THERMOGENIC GAS GENERATION Methane Carbon dioxide Water Alkanes (wet gases) 0.51.01.52.02.53.03.54.0 Vitrinite reflectance (percent) Data from Burnham and Sweeney (1989); Tang and others (1991); Killops and others (1994) A LTU DA BIOGENIC GAS ORIGINS Biogenic gases in coal beds were originally believed to be preserved swamp gas. Previous explanations of gas origins evolved into exploration models and strategies that were erroneously based on the assumption that biogenic gases in coal beds were formed in peat swamps. Primary biogenic gases formed in the peat swamp were probably not preserved because (1) there is little pressure at the surface to sorb gas molecules, (2) peat has a high moisture content indicating that many potential sorption sites are occupied by water molecules that are strongly sorbed to the peat surface, and (3) compaction would drive water and dissolved methane out of the peat. It is possible some primary biogenic methane stored in larger cavities in the organic material could be preserved with burial. Secondary biogenic gases formed after burial, coalification, uplift and erosion when bacteria and/or nutrients were introduced into the coal seam by meteoric fluids migrated basinward. Most biogenic gases were formed relatively late in the coal basin evolution. Production rates of more than 1 MMcfd have been reported. SECONDARY BIOGENIC GAS GENERATION BACTERIAL CONSORTIA Fatty acids, Acetate, H2 H2-Reducing Hydrolytic Acetogenic Fermentative - CO2, NH4, HS H2-Utilizing 1 2 Acetate, H , CO Methanogens 2 2, Organic acids 3 Acetate Fermentation Carbonate Reduction - + CH3COO + H CH4 + CO2 CO2 + 4H2 CH4 + 2H2O Modified from Winfrey (1984) A LTUDA PRODUCTION HETEROGENEITY AND BIODEGRADATION Production (%) Gas Water NEBU 413 3200 Sec. 20, T30N R7W 3 19 3000 3250 36 58 Water 17 13 44 10 Gas 3050 3300 3100 0 0 3350 SJ 30-5 216 Sec. 20, T30N R5W 10 mi (16 km) How to Determine Coal Gas Origins……. …..and Develop Exploration Strategies. Sorption Isotherm Saturation Isotopic Data ISOTHERM TEMPERATURE SHIFT T2 Undersaturated T1 Saturated P1 P2 Pressure Scott and others (1994) A LTUDA RESATURATION OF ISOTHERMS Saturated Critical Secondary biogenic gas desorption pressure generation; migration of biogenic and thermogenic gases. PC P2 Pressure Scott and others (1994) A LTUDA ISOTOPIC RANGES -100 -80 -60 -40 -20 0 +20 THERMOGENIC ? BIOGENIC Acetate Fermentation ? ? Carbonate Reduction Methane Oxidation ?? IGNEOUS (ABIOTIC) ? Methane -100 -80 -60 -40 -20 0 +20 13 Carbon dioxide δ C (o/oo) Scott and others (1994) COAL GAS ORIGINS CLASSIFICATION -120 EXPLANATION -100 Early mature thermogemic associated -80 CO2 Atmospheric Bacterial Acetate Fermentation Carbonate Reduction -60 Acetate Geothermal, hydrothermal, crystalline -40 Artificial, bit metamorphic Humic -20 Abiogenic Mantle? Whiticar (1990) 0 -450 -350 -250 -150 -50 δD methane (o/oo) A LTUDA GAS CONTENT AND GAS ORIGINS SUMMARY 0.4 sub HC = 179.46 - 1088.8(VR) + Gas content is not fixed, but 1561.1(VR)2 - 280.01(VR)3 changes when equilibrium hvCb conditions in the reservoir 0.6 r = 0.995 change. hvBb 0.8 hvAb The presence of unusually 1.0 high gas contents in low rank coal beds 1.2 mvb 1.4 The presence of saturated or nearly saturated conditions Data from Tang and others (1991) lvb 1.6 0 200400 600 800 1000 C1-C5 gas generation (scf/ton) DEVELOPMENT OF EXPLORATION STRATEGIES Isotope Coal Rank Water Chemistry Geochemistry 13 O Secondary Fresh δ C CH4 < -55 /OO Biogenic 13 O δ C CO2 > +0 /OO 13 O Early δ C CH4 light /OO Thermogenic 13 O δ C CO2 -25 /OO Fresh to Saline 13 O Main-Stage δ C CH4 >-39 /OO Thermogenic 13 O δ C CO2 -25 /OO BASIC TYPES OF COAL GASES Thermogenic (updip migration or indigenous) • Coal gases are produced during thermal maturation of peat (organic-rich sources) • Variable gas composition: high BTU wet gases or nearly 100% methane • Brackish to saline formation water • Low to very high gas contents (<1 to 25+ g/cc; <32 to 800+ scf/ton) Secondary Biogenic (downdip migration or indigenous) • Coal gases formed by bacteria transported basinward in permeable coal seams • Occur at any coal rank after basin has been uplifted and coals exposed at outcrop • Biogenic gases are nearly 100% methane; variable carbon dioxide • Fresh to slightly brackish formation water.