LEAKAGE OF GAS THROUGH CAPROCKS: HEADSPACE GAS EVIDENCE FROM THE NORWEGIAN AND UK NORTH SEA.
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LEAKAGE OF GAS THROUGH CAPROCKS: HEADSPACE GAS
EVIDENCE FROM THE NORWEGIAN AND UK NORTH SEA.
Selegha AbrakasaAndy Aplin
Newcastle University
Potential Leakage Mechanisms
Along faults
Pore system after capillary failure
Along fractures resulting from tectonic stress or high pore pressures
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
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
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)
WET GAS ANOMALY > 500m: NO RELATION TO STRUCTURE
STRUCTURAL CONTROL WET GAS HEIGHT > 500m
ROTATED37%
TILTED27%
DIAPIRIC21%
UNKNOWN15%
ROTATEDTILTEDDIAPIRICUNKNOWN
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
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
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
]
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.”
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