Transcript Reporting Status or Progress

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
Current Basis for Outburst Alleviation

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.
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

Initiation involves – Sudden failure of a barrier and
reduction in pore pressure

Consequences
– Gas and coal are projected into the
working place
– Gas type (effect on humans)
Desorption Rate
and Gas Content Threshold
GeoGAS Desorption Rate Index

The GeoGAS
DRI is
calculated from
the quantity of
gas desorbed
after 30 seconds
of crushing a
200 g sample
Gas content (m3/t)
Desorption Rate Bench Mark Coals
10
9
8
7
6
5
4
3
2
1
0
GeoGAS Fast Desorption method
0
200
400
600
800
1000
1200
Gas volume 30 sec crushing 200 g (ml)
>90% CO2
<10% CO2
1400
1600
Comparison Test Coal with Bench Mark Coal
Gas content (m3/t)
10.00
9.00
South Coast
y = 0.0104x
R2 = 0.9679
8.00
7.00
6.00
5.00
4.00
3.00
Test Coal
y = 0.0084x
R2 = 0.9712
2.00
1.00
0.00
0
200
400
600
GeoGAS DRI
South Coast CH4 Coal
Test Coal
800
1000
Comparison of Gas Content Values
for a Desorption Rate Index of 900
11
South Coast Bench Mark
Bowen Basin Coking
Mt Davy, NZ
Bowen Basin mixed gas Coking
South Coast Non Bench Mark
Hunter Valley 1
Hunter Valley
Bowen Basin Steaming
Hunter Valley 2
10
Measured Gas Content Qm (m3/t)
9
8
7
6
South Coast "Bench Mark" mines have an
established history of outbursting with
proven effectiveness of the gas content
threshold limits
5
4
0
0.1
0.2
0.3
0.4
0.5
0.6
Ratio CO2/CO2+CH4
0.7
0.8
0.9
DRI regional.xls
1
Weaknesses in the Desorption Rate
“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
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.
Gas Content Characterisation at Outburst Sites
- GeoGAS Tests to 1996
16
14
Gas Content (Qm m3/t)

12
10
8
6
4
2
0
0.00
0.20
0.40
0.60
Ratio CH4/CO2+CH4
0.80
1.00
OutburstGasContents.xls
Gas Content Sample - LOCATION

Where gas content is likely to be the highest
Gas drainage efficiency diminishes
toward the end of boreholes,
Not from the end…
10
2 5 m s p a c in g , 9 5 d a y s d ra in a g e
5 0 m s p a c in g , 2 4 0 d a y s d ra in a g e
1 0 0 m s p a c in g , 7 2 0 m d ra in a g e
6
L in e o f " b a rrie r"
1 0 m o ff B H d g
4
E ffe c t ive e n d o f
b o re h o le 3 0 m
2
o ff B H d g rib
C ro s s p a n e l b o re h o le
A Hdg B Hdg
0
300
350
400
D is t a n c e fro m p re vio u s m a in g a t e (m )
Line of barrier
250
B Heading
200
A Heading
Gas content (m3/t)
Effect
Of Hole
Spacing
8
In it ia l g a s c o n t e n t
450
500
10
P e rm e a b ilit y 1 0 m D in X
In it ia l g a s c o n t e n t
& 2 m D in Y d ire c t io n s
P e rm e a b ilit y 1 0 m D in X
& 1 0 m D in Y d ire c t io n s
P e rm e a b ilit y 2 m D in X &
6
1 0 m D in Y d ire c t io n s
L in e o f " b a rrie r"
1 0 m o ff B H d g
4
E ffe c t ive e n d o f
b o re h o le 3 0 m
2
o ff B H d g rib
C ro s s p a n e l b o re h o le
A Hdg B Hdg
0
300
350
400
450
500
D is t a n c e fro m p re vio u s m a in g a t e (m )
Line of barrier
250
B Heading
200
A Heading
Gas content (m3/t)
Effect Of
Directional
Permeability
8
x
y
Compliance core location
end of borehole
barrier
Compliance
core location
B Heading
A Heading
Design length of borehole over-drill
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 Sample - 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 frequency
Eg Uniformity
Ga s Flow (l/m /m in)
15
10
Regular flow
5
0
0
20
40
60
T i m e o n d r a i n a g e (d a y s)
25
Irregular flow
Ga s Flow (l/m /m in)
20
15
10
5
0
0
20
40
60
80
T i m e o n d r a i n a g e (d a y s)
100
120
140
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)




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)





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 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