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