Devex 2014

Mike Cooper

1st Subsurface Oilfield Management Limited

The technical challenges and origins of hydrogen sulphide (H2S)

in the oilfields of the Moray Firth Basin in the UK North Sea.

Hydrogen Sulphide

Hydrogen Sulfide

Rotten Eggs

Dirty Den Nasty Nick Cooked Rotten Eggs

Mercaptans

Methanethiol, Ethanethiol, Propanethiol, Butanethiol,

Pentanethiol

Hydrogen sulfide, smelling of rotten eggs

Ammonium sulphide, rotten eggs

Carboxylic acids

Propionic acid, sweat

Butyric acid, rancid dairy

Valeric acid

Caproic acid, smelling of cheese

Aldehydes & Amines

Ethylamine, fishy smell

Putrescine & Cadaverine, rotten meat

Indole

Skatole, smelling of faeces

Stink Bombs

Lower Explosive Limit 4%

Upper Explosive Limit 46%(+) Explosive

Comparable in toxicity to hydrogen cyanide

100 ppm “Immediately Dangerous To Life” H2S kills

15 ppm Short-term exposure limit

Olofactory receptors in nose paralysed at ~ 100ppm

Olofactory fatigue at very low concentrations Can’t smell it

Refractive index very close to air Can’t see it

H2S +H2O – Hydrosulphuric acid (>10ppm) Irritates eyes

Health & Safety Challenges

Recognition threshold of human smell, the concentration at which

50% of humans can detect the characteristic odour of hydrogen

sulphide.

Borderline concentration for eye irritation.

Leads to eye damage.

Olfactory nerve is paralyzed after a few inhalations, and the sense of

smell disappears, often together with awareness of danger.

Pulmonary edema with the possibility of death.

Lethal concentration for 50% of population for 5 minutes exposure.

Strong stimulation of the central nervous system and rapid breathing,

leading to loss of breathing.

Immediate collapse with loss of breathing, even after a single breath.

HSE Guidelines for H2S

0.0047

800

150-250

320-530

50-100

530-1000

>1000

10-20

PPM

Heavier than air- usually falls to low-lying, poorly ventilated areas

When mixed with lighter-than-air gasses, or at high temperature it can rise.

Auto ignition temperature 232oC, (Lit cigarette – 700oC)

H2S burns with a blue flame, producing SO2 (itself poisonous) and water.

Corrosive to metals, forming metal sulphides - characteristically dark brown or black (look

out for blackened tubulars)

Engineering Challenges

Variation in measurements – sampling errors

Reservoir Concentration

0

Adsorption and reaction with sample

chamber material shows dramatic reduction

in H2S concentration over days

Days

H2S

Concentr

ation

6

After Elshahawi & Hashem, 2005

H2S concentrations vary throughout the Process

Produced

Water / Water

Injection

16 ppmw

Inlet

6,500 ppmv

equivalent

Flare

15 ppmv Gas Lift

15 ppmv

Fuel Gas

15 ppmv

Post Amine Plant

and before

Incineration

42,600 ppmv

Amine Sweetening

5,500 ppmv

Crude Oil

Cargo

5 ppmw

Study Area – Quads 14 & 15

H2S Occurrences in Study Area

40%

92%

16%

4%

4%

0.65%

1.6%

Sour Fields v Souring Oilfields

• H2S is a common feature of mature waterfloods

• The souring phenomenon is defined as the increasing mass of H2S

per unit mass of total produced fluids.

• “Sour” typically above 3ppm. (50% of people can detect 0.0047ppm

H2S.

• AWB’s – Found above 80oC. Barotolerant to at least 9500 psi.

Formation Temperature

80oC

45oC

Sulphate-rich cold injected seawater

warms through the zone where

thermophilic AWB’s are stable

After HSE Oilfield Reservoir Souring Report 1993

Distance from Wellbore

Tem

pera

ture

H2S & CO2 content varies within an accumulation……..

6,500ppm

H2S 35%wt

CO2

Not only H2S – but also CO2

Two families not obvious

on a linear scale

H2S Origins

‘Volcanic’ OAE’s SRB “Black Sea” model 87% anoxic

Black smoker (minor) & redox

Source rock

AWB “Anoxic Wee Beasties” Sulphate-reducing bacteria

Minerals

Mantle/volcanic source??

Major crustal feature at

Top Zechstein

No major H2S or CO2 concentrations in local area

No Inert Gases (N2, Ar, He, Ne, SO2 etc)

Mid-Jurassic triple junction –

this is only leg where H2S is a

major feature

Source Rock Geochemistry – Kimmeridge Clay Formation

Moderate biodegradation – influx of meteoric waters in middle

of 750ft thick shale source rock sequence

Minor biodegradation in rich source rock interval (6% TOC)

Immature rich source rock – no GC analysis of this interval (8% TOC)

LTOBM-contamination in Piper sands extract

Sulphate-rich ground water percolation may act as

Feedstock for SRB’s.

Highest H2S concentration in OMF area Gamma Ray

After Geochem Group Limited, 1991

Base Cretaceous Unconformity Structure Map

Halibut Horst

Halibut Platform

Tartan Ridge

Brochel

accumulation is very

tight suggesting local

sourcing of H2S.

Anomalous

measure?

Gamma has very

latest Jurassic

sand stringers

with low H2S &

high CO2

Late mature source

rock in Witch Ground

Graben appears to

have low H2S content

High H2S occurrences are within or

adjacent to areas of light green to

light blue on this map:-

2.4-2.9 msec TWT

Alexandria/Brule

25

5,000

40,000

4,000

1,000

6,500

16,000

1,000

2 4,000

13

15

5

80 11

6

0

0

0?

12

H2S rich gases

from the Hot

Lens sands -

Latest Jurassic

in Tartan

Perth-Dolphin-Lowlander Hub Development Area

One billion

barrels oil-in-

place

No oil

recovered to

date.

The solution is

coming….

• No definitive single source of H2S can be discriminated from available data.

• Tectonic setting within isolated extensions of the failed arm of a triple junction may account for anomalous setting.

• No evidence for mantle origin nor proximity to evaporites

• High concentrations of H2S are found in close proximity to mature source rock (i.e. short-distance migration)

• Some evidence the kerogen in immature KCF source rock has been biodegraded

• Despite being within thick shale packages

• Syn-depositional?

• Stratigraphic component to H2S distribution due to either:

• Connectivity to ?local source kitchen

• Isolated sands in late Jurassic

Conclusions

Dave Soutter

Nick Allan

Jon Ashdown

Phil Ware

John Wood

Gordon Smith

Adrian Jones

Colin Percival

Acknowledgements

1 st Subsurface Oi l f ie ld Management L imited

Tel: 07718 385325

info@1stsom.com