Brownfieldbriefing Ground Gas 2018

Continuous Monitoring – the Lines of Evidence Approach to Ground-

Gas Risk Assessment

Simon Talbot - 1st Mar 2018

Presentation Content 1 History of continuous ground-gas monitoring

2 Challenges of ground-gases

3 Over ten years of continuous monitoring

4 Additional lines of evidence a) Environmental correlations

b) Purge & recovery tests

c) Differential pressure assessment

d) Concentration duration

5 New techniques

6 Case study

History to continuous monitoring - Traditional Spot Monitoring

IRP-IGM

GMGU

2006 - dti Research Project:

“Development and assessment of an in-borehole gas monitoring device as a decision making tool” Collaboration between: • Stephen Boult Salamander

• Simon Talbot GMGU • Peter Morris University of Manchester

dti Research - Expert Advisory Group • Hugh Mallett Buro Happold • Geoff Card GB Card & Partners • Peter Witherington RSK ENSR • Steve Wilson EPG Ltd

dti 2009 research outcomes: • First commercial deployment of

GasClam®

• Development of additional lines of evidence:

• Environmental correlations

• Concentration duration curves

• Purge & recovery gas-flux

• Differential pressure assessment

Also in 2009 • GGS established

• Ground Gas Handbook published making reference to the research

Christmas Borehole – case study

CL:AIRE RB 13 Boult, Morris & Talbot, 2011

• Sets out many of the different lines of evidence provided by continuous monitoring

• Moved the approach from an esoteric university research spin off into a main stream technique

• Now widely adopted and has been used on 1,000s of sites in the UK and elsewhere.

Continuous Ground-Gas Monitoring

1st Generation In-borehole device

GasClam®

Ambisense 2nd Generation In-borehole device

Gas Sentinel®

Gas Sentinel® • First British made device • Secure installation • Telemetry enabled • Continuous flow

31 Jan 02 Feb 04 Feb 06 Feb 08 Feb 10 Feb 12 Feb

When the frequency of monitoring exceeds the frequency of change of the measured parameter, the monitoring can be termed ‘continuous’

Continuous Monitoring Co

ncen

trat

ion

(v/v

%)

Solid Liquid Gas

Environmental monitoring challenges

Permeability Anisotropy

Permeability anisotropy and borehole monitoring

Importance of the Conceptual Site Model

Source - Pathway - Receptor Pollutant Linkage

Driving mechanism

Lines of Evidence - Environmental Correlations

Multi-parameter continuous data…

Identify or eliminate correlations with the environment

Identify or rule out ground-gas drivers

Atmospheric Pressure as a Ground-Gas Driver

985

990

995

1000

1005

1010

1015

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

27-Dec 28-Dec 29-Dec

Pres

sure

(mill

ibar

s)

Met

hane

(% v

/v)

Methane

Atmospheric Pressure

930

940

950

960

970

980

990

1000Pr

essu

re (m

bar)

Atmospheric Pressure

05

101520253035

20 Dec 22 Dec 24 Dec 26 Dec 28 Dec 30 Dec

Con

cent

ratio

n (%

v/v

) Methane

Atmospheric Pressure as a Ground-Gas Driver

25

30

35

40

45

50

55

16 Aug 23 Aug 30 Aug 06 Sep

Con

cent

ratio

n (p

pmv)

TVOC

05

10152025303540

Tem

pera

ture

(ºC

)

Temperature

Temperature as a Ground-Gas Driver

2.502.602.702.802.903.003.103.203.30

Wat

er le

vel (

mbg

l)

Water Level

0

5

10

15

20

25

26 Aug 31 Aug 05 Sep 10 Sep 15 Sep 20 Sep 25 Sep

Con

cent

ratio

n (%

v/

v)

O₂ CH4

Water Level as a Ground-Gas Driver

99510001005101010151020

Pres

sure

(mba

r) Atmospheric pressure

0

20

40

60

80

12-Feb 13-Feb 14-Feb 15-Feb 16-Feb 17-Feb 18-Feb 19-Feb

Con

cent

ratio

n (%

v/

v)

MethaneCarbon dioxide

What’s happening here?

Purge & Recovery Test (PRT)

Measures Gas Flux

GGS Purge & Recovery Test

Δy

Δx

PRT recovery curves

Calculating Gas Flux

• Back calculate the borehole volume then:

Where:

Q = Gas Flux

V = Volume of the internal vadose zone of the borehole

c = Change in gas concentration expressed as a percentage

t = change in time over which the change in concentration was measured

Repeatability demonstrates that the gas flux is a valid characteristic of that location

940

960

980

1000

1020

Pres

sure

(mba

r)

-20-10

0102030

Pres

sure

(m

bar)

Negative flow Positive flow

No flow

Atmospheric pressure

Borehole pressure

Differential pressure

Differential Pressure Assessment

‘Barometric Pumping’

0

5

10

15

20

25

27 Nov 29 Nov 01 Dec 03 Dec 05 Dec 07 Dec 09 Dec 11 Dec

Con

cent

ratio

n (%

)

CH4 Continuous Data

0

5

10

15

20

25

0 20 40 60 80 100

Con

cent

ratio

n (%

v/v

)

% Time

CH4 Concentration Duration Curve

Concentration Duration Curves

Characterising Gas Regimes Data Sets Conc. Dur. Curves Data Sets Conc. Dur. Curves

Families of curves describing different behaviour

New Tools - Ternary Plots

• Consider ratios with large data sets • Useful for looking at trends and source

Gas Sentinel® telemetry

Continuous Flow

Gas Sentinel®

GSV with Continuous Data

GSV with Continuous Data

CS2

CS4

CS3

Prevailing Wind

Air Vents

Receptor Monitoring (Sub-floor Void Monitoring)

Continuous monitoring on the down-wind side

Gorebridge CO2 Incident April 2014

- Incident Management Team set up in April 2014

- NHS

- Midlothian Council

- Coal Authority

- By Sept 2014, 22 people had attended A&E or local GP

- Extensive Source – Pathway – Receptor investigations carried out

2013/14 Coal Authority Investigations 87 Newbyres Crescent found to have:

• 8% C02 in downstairs toilet

• 12% C02 in Lounge (where son had been sleeping)

• 19% CO2 beneath kitchen flooring

• 21% CO2 measured in hole drilled through raft

• 23% CO2 in wall cavity

Borehole drilled to the shallowest coal seam at 13m bgl: • 25.1 % CO2 & 4.6% O2

• No grout was found in the coal seam

Coal Authority continuous gas monitoring

Highest CO2 associated with falling atmospheric pressure

2017 IMT Report Conclusions

(IMT Report – from Fairhurst)

3-4m

c13m

“Was this was an entirely preventable incident?”

• Source confirmed as worked coal seam

• Workings not fully grouted

• Grout holes beneath houses possibly not sealed

• SI boreholes beneath houses possible not sealed

• Vibro stone column foundations

• Service entries through raft not sealed

• No gas protection measures installed

• Highest CO2 associated with falling atmospheric pressure

In Summary • Ten years of experience of continuous ground-

gas monitoring

• The technique provides a range of additional lines of evidence

• It can provide higher quality information in a shorter period of time than spot monitoring

• It’s a technique that is now widely used

• Helps focus gas protection measures on those sites that needs it

Thank you

Simon Talbot - 0788 4444 272 simon.talbot@ggs-uk.com