THERMAL MATURITY

1
UNCONVENTIONAL OIL AND GAS RESERVOIRS
PETE 689/602 FALL 2003 Lecture 9 Coalbed Gas
LECTURE Dr. Walter Ayers
2
Outline

• Importance of coalbed gas sands
• Locations of USA coalbed gas basins
• What is coalbed gas
• Controls on coalbed gas occurrence and
• producibility
• Thermogenic, biogenic and mixed gas systems
• Examples from several US CBM plays

3
U.S. COALBED GAS RESOURCES
4
(No Transcript)
5
1998 GAS RESERVES, USA
UNCONVENTIONAL GAS 31
Data from EIA
6
1998 GAS PRODUCTION, USA
UNCONVENTIONAL GAS 23
Data from EIA
7
U.S. COALBED METHANE PRODUCTION AND RESERVES,
1989 THROUGH 2001
Ayers, 2002 data from U.S. DOE / EIA
8
TOTAL U.S. AND SAN JUAN BASIN COALBED GAS
PRODUCTION,1989 - 2000
9
COMPARISON OF COALBED GAS WITH CONVENTIONAL
NATURAL GAS
10
COMPARISON OF SANDSTONE AND COAL RESERVOIRS
11
Sorption isotherm for Gulf Coast Wilcox coal is
similar to that for Fort Union coal. (Pratt and
other, 1999 Warwick and others, 2000)
12
SCHEMATIC PRODUCTION HISTORY FOR A COALBED
METHANE WELL
From Kuuskraa and Brandenburg, 1989
13
INTEGRATED COALBED GAS PROJECT
Ayers, 1994
14
CRITICAL FACTORS FOR COALBED GAS EXPLORATION AND
PRODUCTION

• Gas Composition
• Source Rock
• Gas Content
• Thermal Maturity
• Coal Occurrence
• Structural Setting
• Cleat Characteristics
• In-Situ Stress
• Hydrologic Setting

Fairways or sweetspots occupy less than 10 of a
producing basin. The roles critical factors
in fairway delineation may vary between high- and
low-rank coal beds.





15
COAL OCCURRENCE

• RESOURCE DELINEATION QUESTIONS
• What is total coal thickness?
• What is thickness of the thickest (maximum)
• coal?
• Does the coal occur in a few, grouped beds,
• or is it in many, thin beds distributed
  over
• an immense interval?
• How continuous are the coal seams?
• What is the depth of the prospective interval?

Ayers, 1994
16
STRUCTURAL SETTING

• QUESTIONS
• What is structural complexity?
• Can compartments accommodate enough wells
• to support infrastructure?
• Are there fault blocks isolated from recharge?
• Are the faults sealing or transmissive?
• Is there evidence of structural traps?
• Is coal degassed below unconformities?

Ayers, 1994
17
From Ayers and Ambrose, 1990
18
STRUCTURE ON HUERFANITO BENTONITE, SAN JUAN BASIN
OVERBURDEN Fort Union 150 to 2,000
ft (Ayers, 1986) Gulf Coast Calvert Bluff 150
to 3,500 ft Thick coal gt 2,000 ft deep (Ayers
and Lewis, 1985)
From Scott and others, 1994 after Ayers and
others, 1991 and Palmer, 1992
19
Net Coal Thickness Fruitland Formation, San Juan
Basin
From Ayers et al., 1994
20
POWDER RIVER BASIN STRUCTURAL ELEMENTS
Face Cleat Orientation
Modified from Tyler and others, 1995, after
Ayers, 1984
21
POWDER RIVER BASIN, FORT UNION FM
TOTAL COAL THICKNESS Fort Union 50 to 300
ft Gulf Coast Calvert Bluff 3 to 20
ft Ayers, 1984 Ayers and Lewis, 1985
THICKNESS
From Ayers, 1984
COAL ISOLITH, TONGUE RIVER MEMBER
22
(No Transcript)
23
POWDER RIVER BASIN, FORT UNION FM
NUMBER OF COAL BEDS Fort Union 1 to 32
Gulf Coast Calvert Bluff 1 to 16 Coal beds gt
2 ft thick 0 to 4 Coal beds gt 5 ft thick Ayers
and Lewis, 1985
COAL BEDS
COAL ISOPLETH, TONGUE RIVER MEMBER
From Ayers, 1984
24
COAL STRATIGRAPHY
Ayers, 1998
25
Depositional Model Fruitland Fm / Pictured Cliffs
Ss
Pictured Cliffs Tongue
Lewis Shale
Wave-Dominated Delta
Swamp
26
RELATIONS BETWEEN FRAMEWORK SANDS AND COAL BEDS,
SANDOW MINE
From Ayers, 1986
27
JACKSON GP LIGNITE, SAN MIGUEL MINE, S. TEXAS
Photo by W. Ayers
28
GAS RESOURCES

• QUESTIONS
• What is the gas content of the coal?
• Is biogenic or migrated thermogenic
• gas present?
• What is the relation between gas
• content and depth?
• Is gas content measured or inferred

Ayers, 1994
29
THERMAL MATURITY

• QUESTIONS
• Are coal-rank maps available or are there
• sufficient data to make these maps?
• Has the coal produced thermogenic gas
• is it high-volatile A bituminous or greater
• (Ro gt 0.78)?
• What is the relationship between maturation
• level and present burial depth?

Ayers, 1994
30
Gulf Coast Coal Rank Lignite to Subbituminous
31
FRUITLAND COAL RANK, SAN JUAN BASIN
0.78
Modified from Scott, 1993
32
ADSORPTIVE CAPACITY OF COAL AS RELATED TO RANK
AND DEPTH

• Gulf Coast Coal is low-rank (lignite to
  subbituminous)
• Coalbed gas origin is
• biogenic or migrated
• thermogenic
• Kaiser and others, 1980 Tewalt, 1986
• Mukopadhyay, 1989

33
SORPTION ISOTHERM, FRUITLAND COAL, NEBU WELL 403
From Mavor, 1990
34
GAS STORAGE CAPACITY OF WILCOX LIGNITE, PA 1
WELL
From Warwick and others, 2000
35
ADSORPTION CAPACITY OF COAL FOR METHANE AND ETHANE
Modified from Levine, 1993 After Ruppel and
others, 1972
36
PERCENT FUSINITE vs. DESORPTION
RATE, SELECTED WESTPHANIAN COALS, UK
Percentage of methane desorbed in first 10 hours
Fusinite ()
From Creedy, 1991
37
GAS COMPOSITION, TOP HARD COAL, WITHAM, UK
Modified from Creedy, 1988
38
GAS IN-PLACE, FRUITLAND FM COAL BEDS, San Juan
Basin
BASED ON a. Coal thickness, and b. gas content

• GIP in Gulf Coast Coal
• is unknown
• Limited gas content
• measurements range from
• a trace to 11 Scf/t (daf)
• Warwick and others, 2000

From Ayers and Ambrose, 1990
39
SOURCE ROCK AND RESERVOIR

• QUESTIONS
• What are the maceral types?
• Are they liquid- or gas-prone?
• Have they generated hydrocarbons?
• What is the moisture content?

Ayers, 1994
40
GAS COMPOSITION

• QUESTIONS
• What is the gas origin?
• What is its composition?
• Will carbon dioxide or other
• noncombustibles have to be removed?

Ayers, 1994
41
MACERAL GROUPS AND HYDROCARBON POTENTIAL
From Boreman and Powell, 1993, after Stach and
others, 1975
42
GAS GENERATION FROM SAPROPELIC AND HUMIC MATTER
From Rightmire, 1984 after Hunt, 1979
43
Gulf Coast Coal Rank Lignite to Subbituminous
44
COMPOSITION AND CONTENT OF GAS IN COAL BEDS
Ayers, 2000
45
ISOTOPIC CHARACTERIZATION OF GASES
Modified from Kotabarba and Rice, 1993 after
Whiticar and others, 1986
46
GAS DRYNESS AND CARBON DIOXIDE RANGES FOR COALBED
GAS
From Scott, 1993, after Scott and others, 1991
Hanson, 1990
47
TYPICAL COMPOSITION OF BRITISH COALBED GAS
After Creedy, 1988
48
FRUITLAND COAL GAS, DRYNESS INDEX
From Scott and others, 1994
49
CARBON DIOXIDE CONTENT, FRUITLAND COALBED GAS
From Scott and others, 1994
50
CLEAT CHARACTERISTICS

• QUESTIONS
• What are the face and butt cleat
• orientations?
• What are the face and butt cleat
• spacings?
• Are cleats open?
• Are the cleats mineralized?
• Do cleat properties change areally
• and vertically?

Ayers, 1994
51
CLEAT CHARACTERISTICS
FACE CLEAT
PALEOSTRESS
BUTT CLEAT
Sh
SH
TONSTEIN
BRIGHT
THICK COAL
C O A L
DULL
BRIGHT
THIN, BRIGHT COAL
MASTER FACE CLEAT
From Ayers, 2002
52
CLEATS IN FRUITLAND COAL
Photo by W. Ayers
53
VITRINITE REFLECTANCE vs. MEAN SPACING, FACE
CLEATS
54
FACE CLEAT DOMAINS, SAN JUAN BASIN
2 face cleat domains are present in the San Juan
Basin
Face cleat characteristics are poorly known
for Gulf Coast low-rank coals
From Tremain and others, 1994
55
IN-SITU STRESS

• QUESTIONS
• What is the coal depth (overburden stress)?
• Is the regional stress regime compressional
• or extensional?
• If compressional, what is the magnitude
• of minimum horizontal stress relative to
• overburden stress?
• What is the orientation of maximum horizontal
• stress relative to face cleat

Ayers, 1994
56
COMPLEX FRACTURE
Complex (T-shaped) fractures may occur where
minimum horizontal stress exceeds overburden
stress
Modified from McKee, 1989
57
(No Transcript)
58
BLACK WARRIOR BASIN, POTTSVILLE COAL
From Sparks and others, 1993
59
BLACK WARRIOR BASIN, POTTSVILLE COAL
From Sparks and others, 1993
60
(No Transcript)
61
(No Transcript)
62
(No Transcript)
63
CONTROLS ON OVERBURDEN STRESS
Ayers, 1995
64
GAS AND WATER PRODUCTION

• MANY coalbed gas projects require
• Water production
• Water handling (and possibly, treatment)
• Water disposal

65
HYDROLOGIC SETTING

• QUESTIONS
• Is the water fresh, brackish, etc.?
• Are there hydrochemical maps or data?
• Is there a potentiometric surface map?
• What is reservoir gradient?
• Is the ground-water system dynamic?
• What is the relationship between ground-water
• flow paths and coal-rank?
• Is there evidence of reservoir compartments?

Ayers, 1994
66
GROUND-WATER FLOW, COMPLEX MODEL BASED ON
WASHAKIE BASIN
Modified from Scott and Kaiser, 1993 after Law
and others, 1989
67
FRUITLAND COALBED METHANE - HYDROLOGIC ELEMENTS
From Scott and others, 1994 after Kaiser and
others, 1991
68
(No Transcript)
69
(No Transcript)
70
MAXIMUM DAILY WATER PRODUCTION, NORTHWEST SAN
JUAN BASIN
From Scott and others, 1997
71
GROUND-WATER FLOW AND GAS ORIGINS, SAN JUAN BASIN
From Kaiser, 1993
72
GAS SATURATION OF COAL
From Scott and others, 1994
73
HOGBACK MONOCLINE, NORTHERN SAN JUAN BASIN
74
TRAPS AND EXPLORATION CONCEPTS
Ayers, 2000
75
STRUCTURE ON HUERFANITO BENTONITE FRUITLAND CBM
FAIRWAY
76
FRUITLAND FORMATION, SAN JUAN BASIN
From Kaiser and others, 1990
77
FRUITLAND COALBED GAS PRODUCTION
78
(No Transcript)
79
INTEGRATION DEFINING POTENTIAL FAIRWAYS AND
COMPARTMENTS AND OPTIMIZING RECOVERY
Ayers, 1994
80
FORT UNION FORMATION, POWDER RIVER BASIN

• Powder River Basin - Fort Union Coal
• Production History
• Coal Occurrence
• Structural Setting
• Hydrology
• Tongue River Coals - Reservoir Characteristics
• Powder River Basin Coalbed Play
• Primary Economic Factors
• Conclusions

81
POWDER RIVER BASIN, COALBED GAS PLAYS
Activity outline modified from Montgomery,
1999 Base map is Maximum Coal Thickness Map,
Ayers, 1986
82
NUMBER OF COALBED GAS WELLS, POWDER RIVER BASIN
Data from WOGCC, 2002
83
COALBED GAS AND WATER PRODUCTION, Powder River
Basin
Data from WOGCC, 2002
84
COAL OCCURRENCE IN THE TONGUE RIVER MEMBER,
FORT UNION FM
Sand
From Ayers, 1986
85
POWDER RIVER BASIN, FORT UNION FM
TOTAL COAL THICKNESS Fort Union 50 to 300
ft Gulf Coast Calvert Bluff 3 to 20
ft Ayers, 1984 Ayers and Lewis, 1985
From Ayers, 1984
86
POWDER RIVER BASIN, FORT UNION FM
NUMBER OF COAL BEDS FORT UNION 1 to 32
Gulf Coast Calvert Bluff 1 to 16 Coal beds gt
2 ft thick 0 to 4 Coal beds gt 5 ft thick
Ayers and Lewis, 1985
87
POWDER RIVER BASIN, FORT UNION FM
From Ayers, 1984
88
GAS QUANTITIES GENERATED DURING COALIFICATION
89
POWDER RIVER BASIN STRUCTURAL ELEMENTS
Face Cleat Orientation
Modified from Tyler and others, 1995, after
Ayers, 1984
90
POTENTIOMETRIC SURFACE, TERTIARY STRATA, POWDER
RIVER BASIN
Modified from Lobmeyer, 1985
91
COALBED GAS EXPLORATION IN THE POWDER RIVER BASIN
Ayers, 2000
92
FLOWING ARTESIAN WELLS, POWDER RIVER BASIN
Modified from Choate and others, 1984
93
ISOTOPIC CHARACTERIZATION OF GASES
Gulf Coast Wilcox USGS-PA-2 (Warwick and
others, 2000)
Fort Union CBM (Warwick et al., 2000)
Fermentation
Thermal
Modified from Kotabarba and Rice, 1993 after
Whiticar and others, 1986
94
Sorption isotherm for Gulf Coast Wilcox coal is
similar to that for Fort Union coal. (Pratt and
other, 1999 Warwick and others, 2000)
95
SCHEMATIC THICK COAL COMPLETIONS, DEEP vs
SHALLOW

• Biogenic gas
• Low gas content
• Depressurization of deep coal

NO SCALE - GREAT VERTICAL EXAGGERATION
Ayers, 2000
96
STRATIGRAPHIC AND STRUCTURAL COMPLEXITY IN COAL
RESERVOIRS, POWDER RIVER BASIN
Ayers, 2000
97
POWDER RIVER BASIN COALBED GAS DEVELOPMENT
STRATEGIES
98
POWDER RIVER BASIN, COALBED GAS PLAYS
Activity outline modified from Montgomery,
1999 Base map is Maximum Coal Thickness Map,
Ayers, 1986
99
BIOGENIC ENGINE - MAP VIEW(Gas-Cap Recharge
Thin vs Thick Coal)
100
BIOGENIC ENGINE - CROSS SECTION(GAS CAP RECHARGE)
101
TONGUE RIVER COALBED RESERVOIR CHARACTERISTICS

• Low rank (subbit. 8,200 - 9,600 BTU, as recd)
• Low ash (4 - 11)
• Low Sulfur (0.4 - 0.6)
• High moisture (22 - 30)
• Maceral Content Vitrinite 69 - 78 inertinite
  19 - 26 (Canyon coal) (Pratt et al., 1999)
• Thick (commonly gt 50 ft), laterally continuous
  coal
• Well cleated for low-rank coal
• Shallow low overburden stress
• High permeability (100 - 1,000 md) (Pratt et al.,
  1999)

102
COALBED RESERVOIR CHARACTERISTICS (cont.)

• Gas content lt 70 Scf/t
• Biogenic gas
• Produced gas lt 1 carbon dioxide (Montgomery,
  1999)
• Desorbed gas composition (Pratt et al., 1999)
• Limited data, Canyon coal bed
• Approximately 90 methane
• 8 Carbon dioxide?
• 1-2 nitrogen

103
DRILLING COMPLETION METHODS

• 80-acre spacing
• Depths commonly 200 to 900 ft
• Truck-mounted rigs drill in 2 days
• Drill 9 7/8-inch hole 1 ft into coal and set
• 7-inch surface casing
• Drill coal with 6 1/4-inch bit using air,
  air-mist, or water
• Under-ream coal to 10 or 12 inches and clean hole

104
DRILLING COMPLETION (cont.)

• Use service rig to set tubing and submersible
  pump
• Produce for 1-2 months to clean near-wellbore
  area
• Pull tubing and pump
• Stimulate with 500 bbl water at rate of 30 - 40
  bbl/min. and surface injection pressure of 130
  psig
• Use no proppant
• Clean up
• Pratt et al. (1999)

105
WELL SCHEMATIC
106
POWDER RIVER BASINCOALBED GAS PRODUCTION

• 3,390 producing wells - August 2000
• Average gas production - 135 Mcf/d per well
  (August 2000)
• 1.28 MMbw/d (average 518 bw/d per well March
  2000)
• Water/gas ratio is 3-4 bbl/Mcf early, dropping to
  1-2 bbl/Mcf
• Reserves are 200 - 400 MMcf/well

Data from Dwights, after Pratt et al. ,1999
Montgomery, 1999 Rice and others, 2000
107
PRIMARY ECONOMIC FACTORS

• Drilling and completion costs 45,000 - 65,000
• Facilities costs 20,000 - 30,000
• Low Water disposal costs
• Produce at low wellhead pressures
• Compression costs may be significant
• Gas prices 1.15 - 2.00 / MMBTU at wellhead
  (early 2000 higher in late 2000 and early 2001)

108
ACCOUNTING FOR PRODUCTION RATES AND VOLUMES

• HIGH GAS PRODUCTION RATES IN PRB
• Gas rates are high owing to high values of kH
• Coal is very thick
• permeability is high because (1) coal is
  relatively well cleated (low ash) and (2) in-situ
  stress is low
• HIGH CUMULATIVE GAS PRODUCTION
• Drainage area is greater than well spacing?
• Migration updip through high permeability coal?
• Low-rank coal has gas stored in pores that is
  unaccounted for in desorption?
• Exsolution from water (Pratt et al., 1999
  Bustin and Clarkson, 1999)?
• Active gas generation by microbial bacteria?

109
CONCLUSIONS

• CBM activity is robust in the Powder River Basin
  in post-tax credit era
• Reserves per well are low compared to those in
  other basins
• The play is economic because
• Coalbeds are shallow and stress is low
  open cleat
• Permeability is high and coal beds are thick and
  continuous
• Shallow wells - low drilling costs
• Completions are simple
• Stimulation is minor and relatively inexpensive
• Produced water is fresh - disposal costs are low
• Pipeline access to markets has improved.
• Powder River Basin results have encouraged
  assessment of CBM potential in other low-rank
  coals, including those in Texas
• Powder River Basin is not an analog for Texas
  Gulf Coast CBM