Transcript LEAKAGE OF GAS THROUGH CAPROCKS: HEADSPACE GAS EVIDENCE FROM THE NORWEGIAN

LEAKAGE OF GAS THROUGH
CAPROCKS: HEADSPACE GAS
EVIDENCE FROM THE NORWEGIAN
AND UK NORTH SEA.
Selegha Abrakasa
Andy Aplin
Newcastle University
Potential Leakage Mechanisms
Along faults
Along fractures
resulting from
tectonic stress or
high pore pressures
Pore system after
capillary failure
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
• Commonly available
• Variable quality
• Can readily differentiate thermogenic and biogenic gas
• Do profiles suggest flowpaths and leakage
mechanisms?
E-2
E-3
E-4
E-1
Cuttings from
shale shaker
in cans
(canned cuttings)
Gas sample in
headspace collected
and analysed by gas
chromatography
a) ppm of C1-C5
b) δ 13C
c) iC4/nC4
SUMMARY STATISTICS: THERMOGENIC GAS
ANOMALIES
20
15
No. OF WELLS
10
5
0
<500
500 - 999
>1000
HEIGHT OF WET GAS ANOMALY ABOVE RESERVOIR (m)
WET GAS ANOMALY > 500m: NO RELATION TO
STRUCTURE
UNKNOWN
15%
STRUCTURAL CONTROL
WET GAS HEIGHT > 500m
DIAPIRIC
21%
TILTED
27%
ROTATED
TILTED
DIAPIRIC
UNKNOWN
ROTATED
37%
Pore Pressure & Fracturing
Pressure
λ=
Risk of mechanical
failure increases at l ~
0.8
Depth
Pore pressure /
Lithostatic pressure
Pp
Lp
PRESENCE OF THERMOGENIC GAS IN CAPROCK
NOT RELATED TO PORE PRESSURE
λ≥0.8
10%
UNKNOWN
23%
0.6<λ>0.4
34%
0.78<λ>0.6
33%
λ≥0.8
0.6<λ>0.4
0.78<λ>0.6
UNKNOWN
GAS PROFILES: VIDGIS (34/7-29S)
VR%
TOTAL HCs [PPM]
0
0
10000 20000 30000 40000
100
0
CLAYSTONE/
SANDS
1600
SANDS/
CLAY STONE
C.KNOLLMARLS
DEPTH
1900
2100
2500
2700
6
1600
0.37
2100
0.38
DEPTH
1700
2300
4
0.38
1500
DEPTH[M]
WET
SANDS
2
1100
1300
LIMESTONE/
CLAYSTONE
HORDALAND GP
R/LAND
SHETLAND GP
50
1100
1100
VIKING GP
BIOGENIC SIGNATURE
GAS WETNESS
2100
Res.@2705
Draupne
Fm. Sst.
600M
0.38
2600
0.41
2600
2900
3100
3100
3100
0.45
GAS PROFILE: TYRIHANS 6407 / 1 - 3
TOTAL HCs GAS [PPM]
50
iC4/nC4
100
0
2000
2000
2500
2500
2500
1,300 m
SST/
CLST
COAL UNIT
CLST/
SLST/
SST
“HC shows
in early
Cret.”
3500
3000
DEPTH
DEPTH
3000
DEPTH
3000
SST
BAT GP
0
50000 100000 150000 200000
2000
LI/CLY/SST
CROMER KNOLL GP
SHETLAND
0
WETNESS
3500
3500
4000
4000
4500
4500
5000
5000
4000
4500
5000
1
2
3
4
5
6
7
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