Outburst Research Needs

Hanes ACARP Workshop

DMR Wollongong

12th February 2003

Ray Williams

GeoGAS

Issues Mechanism Thresholds

– Circumstance (structure/mining)

– Target outcome

– Types

– Values, Reliability, Practicality

– Combinations

Barrier Sample Frequency

If the gas content is low enough, an outburst will not occur

regardless of the state of other contributing factors.

While outbursts almost always occur on geological structures, at

this stage it has not been proven that such structures cannot be

adequately predicted.

The gas content must be reduced to below the defined threshold

value (DTV) within the roadway to be driven and including a barrier

surrounding that roadway.

The DTV is designed such that no uncontrolled rapid gas emissions

occur, regardless of whether they are outbursts or GDI’s.

Current Basis for Outburst AlleviationCurrent Basis for Outburst Alleviation

A Couple of Quandaries

What gas pressure are we talking about?

Is CO2 more outburst prone than CH4 or not?

Disturbed and Normal Coal

Hard to drain zones have low permeability, due to:– stress effects associated with a structure (high

outburst proneness), inability to drill, hole collapse, or

– an almost total lack of structure (eg much of Tahmoor Colliery’s hard to drain zones).

Define unstructured coal with high level of certainty (borehole to borehole RIM) as a means of applying a higher gas content threshold.

In applying thresholds what are we aiming to achieve?

First Barrier is Zero Initiation

InitiationInitiation involves - – Sudden failure of a barrier and

reduction in pore pressure ConsequencesConsequences

– Gas and coal are projected into the working place

– Gas type (effect on humans)

Desorption Rate and Gas Content Threshold

GeoGAS Desorption Rate IndexGeoGAS Desorption Rate Index

The GeoGAS DRI is calculated from the quantity of gas desorbed after 30 seconds of crushing a 200 g sample

Desorption Rate Bench Mark CoalsDesorption Rate Bench Mark Coals

0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9

10 10

Ga

s co

nte

nt (

m3

/t)G

as

con

ten

t (m

3/t)

0 0 200 200 400 400 600 600 800 800 1000 1000 1200 1200 1400 1400 1600 1600 Gas volume 30 sec crushing 200 g (Gas volume 30 sec crushing 200 g (mlml))

>90% CO2 >90% CO2 <10% CO2 <10% CO2

GeoGAS Fast Desorption method GeoGAS Fast Desorption method

Comparison Test Coal with Bench Mark CoalComparison Test Coal with Bench Mark Coal

South CoastSouth Coasty = 0.0104xy = 0.0104x

R22 = 0.9679 = 0.9679

Test CoalTest Coaly = 0.0084xy = 0.0084x

RR2 = 0.9712 = 0.9712

0.000.00

1.001.00

2.002.00

3.003.00

4.004.00

5.005.00

6.006.00

7.007.00

8.008.00

9.009.00

10.0010.00

00 200200 400400 600600 800800 10001000

GeoGAS DRIGeoGAS DRI

Gas

co

nte

nt

(m3/

t)G

as c

on

ten

t (m

3/t)

South Coast CH4 CoalSouth Coast CH4 Coal Test CoalTest Coal

Comparison of Gas Content ValuesComparison of Gas Content Values for a Desorption Rate Index of 900for a Desorption Rate Index of 900

4

5

6

7

8

9

10

11

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Ratio CO2/CO2+CH4

Mea

sure

d G

as C

on

ten

t Q

m (

m3/

t)South Coast Bench MarkBowen Basin CokingMt Davy, NZBowen Basin mixed gas CokingSouth Coast Non Bench MarkHunter Valley 1Hunter ValleyBowen Basin SteamingHunter Valley 2

South Coast "Bench Mark" mines have an established history of outbursting with proven effectiveness of the gas contentthreshold limits

DRI regional.xls

Weaknesses in the Desorption Rate Weaknesses in the Desorption Rate “Bench Mark” Approach“Bench Mark” Approach

Assumes that other than desorption rate, all other factors are equal - which is not the case.– Eg for equivalent types of faulting, higher strength

Hunter Valley coals would be less mylonitised than South Coast coals.

– No account taken of differences in seam thickness, stress, permeability differences, gas sorption capacities.

So…Is CO2 more outburst prone So…Is CO2 more outburst prone than CH4?than CH4? Measurement errors

in CO2 coals -

experience at

Collinsville

CO2 versus CH4

experience in Poland

Confusion in the

literature, especially

basing comparisons

on equivalent

desorption

pressures.

0

2

4

6

8

10

12

14

16

0.00 0.20 0.40 0.60 0.80 1.00

Ratio CH4/CO2+CH4

Gas

Co

nte

nt

(Qm

m3/

t)

OutburstGasContents.xls

Gas Content Characterisation at Outburst SitesGas Content Characterisation at Outburst Sites - GeoGAS Tests to 1996- GeoGAS Tests to 1996

Gas Content Gas Content Sample - LOCATIONSample - LOCATION

Where gas content is likely to be the highest

Gas drainage efficiency diminishesGas drainage efficiency diminishestoward the end of boreholes,toward the end of boreholes,Not from the end…Not from the end…

0

2

4

6

8

10

200 250 300 350 400 450 500

Distance from previous maingate (m)

Gas

con

tent

(m

3/t)

25 m spacing, 95 days drainage

50 m spacing, 240 days drainage

100 m spacing, 720 m drainage

Line of "barrier"10 m off B Hdg

Initial gas content

Cross panel borehole A Hdg B Hdg

Effective end of borehole 30 moff B Hdg rib

A H

eadin

g

B H

eadin

g

Lin

e o

f barr

ier

EffectEffectOf HoleOf HoleSpacingSpacing

0

2

4

6

8

10

200 250 300 350 400 450 500

Distance from previous maingate (m)

Gas

con

tent

(m

3/t)

Permeability 10 mD in X& 2 mD in Y directions

Permeability 10 mD in X& 10 mD in Y directions

Permeability 2 mD in X &10 mD in Y directions

Line of "barrier"10 m off B Hdg

Initial gas content

Cross panel borehole A Hdg B Hdg

Effective end of borehole 30 moff B Hdg rib

A H

eadin

g

B H

eadin

g

Lin

e o

f barr

ier

Effect OfEffect OfDirectionalDirectionalPermeabilityPermeability

xx

yy

Compliance

barriercore location

end of borehole

A Heading

B Heading

Compliance core locationCompliance core location

Design length of borehole over-drill Design length of borehole over-drill according to:according to:

Barrier width Borehole “end effect”

– Directional permeability– Hole spacing and orientation– Gas content magnitude

Hole sump/dewatering tube What you are trying to achieve

Gas Content Gas Content Sample - FREQUENCYSample - FREQUENCY

Sufficient to prevent inadvertent mining into coal above the threshold

Considerations…

Examine past history

Uniformity of results

Closeness to threshold

Abnormalities (drilling, geology, drainage)

Familiarity and understanding

Define triggers for increasing frequencyDefine triggers for increasing frequency

Eg Uniformity

0

5

10

15

0 20 40 60Time on drainage (days)

Ga

s F

low

(l/

m/m

in)

0

5

10

15

20

25

0 20 40 60 80 100 120 140Time on drainage (days)

Ga

s F

low

(l/

m/m

in)

Regular flowRegular flow

Irregular flowIrregular flow

Some Points to Remember - Some Points to Remember -

Don’t argue over definition. Uncontrolled gas events require careful consideration.

The biggest outburst you have had is not the biggest outburst you will ever have.

When using gas content data, make sure it is Measured Gas Content (Qm) calculated to 20°C and 101.3kPa.

The rate of increase in gas pressure is the key ingredient, governed by desorption pressure, desorption rate and permeability.

Some Key Points (cont)Some Key Points (cont)

Reduce the gas content low enough (threshold) and outbursts will not occur, regardless of other conditions.

You can’t define geological structures with the required degree of certainty.

Drilling conditions are a highly important but fallible means of indicating outburst proneness.

Gas drainage is least effective in outburst prone coal

Some Key Points (cont)Some Key Points (cont)

Coring for gas content testing is difficult or impossible in outburst prone coal.

Maximise gas drainage time. Keep on top of gas drainage, by knowing how the

system is performing. It’s a big mistake to think that because gas emission

is low, or you are getting low gas flows from boreholes, that the gas content is low and therefore the risk of outbursts is low.

Investigate abnormal results.

Suggested Research PrioritiesSuggested Research Priorities

Define geological structures with high level of certainty. Rational design of barrier widths. Provide a better means of quantifying outburst risk in

coals of differing properties. Challenge is to be both reliable and practical.

Sample location and frequency issues. Early identification of hard to drain coal. Methods of mining in undrainable coal.

Be able to tell the difference between normal and

abnormal – challenges for Queensland (RTMS?).

Reduction in operator discretion in setting minimum

standards for sample location and frequency