leakage of gas through caprocks: headspace gas evidence from the norwegian and uk north sea. selegha...
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LEAKAGE OF GAS THROUGH CAPROCKS: HEADSPACE GAS
EVIDENCE FROM THE NORWEGIAN AND UK NORTH SEA.
Selegha AbrakasaAndy Aplin
Newcastle University
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Potential Leakage Mechanisms
Along faults
Pore system after capillary failure
Along fractures resulting from tectonic stress or high pore pressures
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STUDY SITES
48 wells from 39 fields representing:
Reservoir depth range ~ 2700m – 3800m
Variety of pore pressure regimes: hydrostatic to
fracture pressure
Variety of structural settings: diapiric, tilted/rotated
fault blocks
Oil, gas and oil + gas accumulations
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Headspace Gas
Cuttings fromshale shaker
in cans(canned cuttings)
Gas sample inheadspace collectedand analysed by gas
chromatography
a) ppm of C1-C5
b) δ 13C
c) iC4/nC4
• Commonly available
• Variable quality
• Can readily differentiate thermogenic and biogenic gas
• Do profiles suggest flowpaths and leakage mechanisms?
E-1
E-2
E-3
E-4
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SUMMARY STATISTICS: THERMOGENIC GAS ANOMALIES
0
5
10
15
20
<500 500 - 999 >1000
No. OF WELLS
HEIGHT OF WET GAS ANOMALY ABOVE RESERVOIR (m)
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WET GAS ANOMALY > 500m: NO RELATION TO STRUCTURE
STRUCTURAL CONTROL WET GAS HEIGHT > 500m
ROTATED37%
TILTED27%
DIAPIRIC21%
UNKNOWN15%
ROTATEDTILTEDDIAPIRICUNKNOWN
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Pore Pressure & Fracturing
λ = Pore pressure / Lithostatic pressure
Risk of mechanical failure increases at ~ 0.8
Pressure
Dep
th
Hyd
rostatic
Lithostatic
Fracture
Pp
Lp
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PRESENCE OF THERMOGENIC GAS IN CAPROCK NOT RELATED TO PORE PRESSURE
λ≥0.810%
0.6<λ>0.434%
0.78<λ>0.633%
UNKNOWN23%
λ≥0.80.6<λ>0.40.78<λ>0.6UNKNOWN
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GAS PROFILES: VIDGIS (34/7-29S)
Res.@2705Draupne Fm. Sst.
1100
1600
2100
2600
3100
0 50 100
GAS WETNESS
DE
PTH
1100
1600
2100
2600
3100
0 2 4 6
BIOGENIC SIGNATURE
DEP
TH
SH
ET
LA
ND
GP
C.KNOLL
VIK
ING
GP
R/L
AN
DH
OR
DA
LA
ND
GP
CL
AY
ST
ON
E/
SA
ND
S
WETSANDS
LIM
ES
TO
NE
/C
LA
YS
TO
NE
MARLS
SA
ND
S/
CL
AY
ST
ON
E
VR%
0.38
0.37
0.38
0.38
0.41
0.45
600M
1100
1300
1500
1700
1900
2100
2300
2500
2700
2900
3100
0 10000 20000 30000 40000
TOTAL HCs [PPM]
DEPT
H[M
]
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SST/CLST
WETNESS
2000
2500
3000
3500
4000
4500
5000
0 50 100
DEP
TH
iC4/nC4
2000
2500
3000
3500
4000
4500
5000
0 1 2 3 4 5 6 7
DE
PT
H
2000
2500
3000
3500
4000
4500
5000
0 50000 100000 150000 200000
TOTAL HCs GAS [PPM]
DEPT
H
SH
ET
LA
ND
CR
OM
ER
KN
OL
L G
PV
IKIN
G
FANGST
BA
T G
P
LI/C
LY
/SS
T
LI/S
STONE
SST
CLST/SLST/SST
CO
AL
UN
IT
GAS PROFILE: TYRIHANS 6407 / 1 - 3
1,300 m
“HC shows in early Cret.”
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SUMMARY & CONCLUSIONS
39 N Sea fields: wide range of structural and pore pressure regimes
Almost all reservoir caprocks contain thermogenic gas; in this context
they are leaking on geological timescales
70% of reservoirs show thermogenic gas anomalies over 500m; 40% ≥
1000m
10% percent of leaking reservoirs had high potential for pressure induced
fracture (λ = 0.8)
Are the majority of reservoirs leaking via capillary failure through pores?
Or: are we observing water flow?
Next: numerical analysis of profiles: timing, rates, mechanisms