Risk Assessment of Aquifer 

Connectivity Associated with 

Well and Bore Integrity
Coal Seam Gas and Water 

What is CSG?
3

Under the EPBC Act, the Independent Expert Scientific Committee (IESC) have reviewed 

numerous EIS’s and now focusing on existing well penetrations that potentially could affect 

aquifer interconnectivity on cumulative basis
Project Background

Project Scope

• Evaluate the risk of aquifer connectivity and 

gas migration through poorly constructed or 

abandoned wells

Why are we doing this?

• It is estimated there will be >30,000 coal seam 

gas wells in QLD by 2030

• Conservative estimates suggest coal seam 

gas wells could draw >25 billion gallons of 

water from the ground each year

• With this volume of extraction, all existing 

(water and conventional) + new CSG wells 

potentially introduce mechanism for migration 

of gas and water within aquifers and to surface

• Gov’t considering requiring expensive 

diagnostic tests to assess well integrity of new 

CSG wells and a program to plug and 

abandon hundreds of old conventional gas 

wells
Current Day Location of Wells and Bores
Why the concern? Future Projection of No of Penetrations

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

No of Wells/Bores Drilled and proposed

Conventional Wells

Water Bores

CSG Wells

30000

30,000
Scope of Work

Scope of work

1. Understand mechanisms for hydraulic 

connection.

2. Develop framework for identifying at-risk 

wells.

3. Determine high risk wells and high risk 

sections within the wells.

4. Develop field-based assessment program.

5. Revise installation/abandonment procedures
Well Failure Mechanisms

a) The absence of or compromised 

surface control casing;

b) Insufficient cement during 

installation or Inadequate driller 

training, qualification or inadequate 

regulation

c) Insufficient or incorrect placement 

of cement plugs 

d) Poor casing selection relative to 

overburden pressure

e) degradation of the cement (cement 

carbonation) and casing;

f) Fissures/fractures along 

cement/native material contact

How Can a Well Fail?
Methodology

Desktop Risk Assessment

– Identify all conventional gas, CSG and water wells drilled in 

the area;

– Review well completion reports 

– Construction type

– Casing depths

– Cementing details

– Geology

– Plug and Abandonment (P&A) details 

– Apply Well Details Using RA Framework (see next slide)

– Determine “High, Moderate and Low” hazards for the following

– Aquifer cross-flow migration

– Risk of hydrocarbon migration (CSG) from the Walloon 

Coal Measures to aquifers

– Risk of conventional gas migration from the Precipice 

Sandstone to other aquifers
Risk Framework

Risk to Formation

Mooga, Gubberamunda or 

Hutton Sandstone

Low Risk

Permeable formation 

fully cased and 

cemented

OR

Cement plug extends 

across length of risk 

zone 

More than 1 cement 

plug between 

permeable formations 

and/or risk zone

OR

Moderate Risk

1 cement plug between 

permeable formations 

and/or risk zone

High Risk

Open Hole

OR

Permeable formation 

cased with no evidence 

of annulus cement
Mooga

Aquitard

Aquitard

Precipice

Base / Aquitard

Gubberamunda

Aquitard

WCM

Aquitard

Hutton

Cement plug

Cement plug

Mooga

Aquitard

Aquitard

Precipice

Base / Aquitard

Gubberamunda

Aquitard

WCM

Aquitard

Hutton

Cement plug

Cement plug

Aquitard

Precipice

Base / Aquitard

Gubberamunda

Aquitard

WCM

Aquitard

Hutton

Casing 

perforations

Mooga

Aquitard

Aquitard

Precipice

Base / Aquitard

Gubberamunda

Aquitard

WCM

Aquitard

Hutton Fully 

cemented 

annulus

Production 

casing

Casing 

Perforations

Cement plug

Cemented 

annulus

Production 

casing

No cement 

within 

annulus

A

Open hole

Open hole

B C D

Cement plug

Mooga

Aquitard

Cemented 

Casing

Aq uifer Likely hazard from A Likely hazard from B Likely hazard from C Likely hazard from D

Fluid Gas Hydrocarbon

Fluid Gas Hydrocarbon

Fluid Gas Hydrocarbon

Fluid Gas Hydrocarbon

Mooga H L M M L L L L L L L L

ubberamunda H L M M L M H L H L L L

Hutton H M H M L M H L H L L L

Example of Desktop Approach
The Bend


Bore ID Des ktop 

Gubberamunda 

Hazard (cros s 

flow)

GLNG 

Gubberamunda 

Ris k Rating

Des ktop Hutton 

Hazard (cros s 

flow)

GLNG Hutton 

Ris k Rating

BELBRI 1 Low Low Medium Moderate

BELBRI 2 High Moderate High Moderate

GRAFTON RANGE 1 Low Low Medium Low

GRAFTON RANGE 10 High Moderate High Moderate

GRAFTON RANGE 11 High Moderate High Moderate

GRAFTON RANGE 12 Medium Moderate Medium Moderate

GRAFTON RANGE 13 High Moderate High Moderate

GRAFTON RANGE 14 High Moderate High Moderate

GRAFTON RANGE 15 Medium Moderate High Moderate

GRAFTON RANGE 16 Medium Low Medium Low

GRAFTON RANGE 17 High Low High Low

GRAFTON RANGE 18 Medium Low Medium Low

GRAFTON RANGE 19 High Moderate High Moderate

GRAFTON RANGE 2 High Moderate High Moderate

GRAFTON RANGE 20 Low Low High Moderate

GRAFTON RANGE 21 Low Moderate Low Low

GRAFTON RANGE 22 Medium Low Medium Low

GRAFTON RANGE 23 Low Moderate Low Low

GRAFTON RANGE 3 High Moderate High Moderate

GRAFTON RANGE 4 High Moderate High Moderate

GRAFTON RANGE 5 Medium Low Medium Low

GRAFTON RANGE 6 High Moderate Medium Low

GRAFTON RANGE 7 Low Moderate Medium Low

GRAFTON RANGE 8 Medium Low Medium Low

GRAFTON RANGE 9 High Moderate High Moderate

Field Calibration – Does the Desktop Method Work?

Grafton Range wells were 

pressure tested in the field and the 

results listed under “GLNG Risk 

Rating

Of the 25 wells that were 

compared, most were in general 

agreeance with only Grafton 

Range 17 (Gubberamunda), 

showing distinct differences (i.e. 

desktop “high hazard” rating and 

GLNG “low risk”, or vice versa). 
Results - Hazards

Into Gubberamunda SS:

Conventional gas migration into aquifer found to be lowest risk

CSG migration into aquifers found to be moderate risk

Cross aquifer flow found to be highest risk – primarily due to old 

conventional wells (pre 1980)
Formation Exchange

Natural Leakage under 

PL8 (m3/d)

Number of Leaky Wells Leakage from wells (m3/d) Increase of leakage from wells 

(%)

Mooga Sandstone to Gubberamunda

Sandstone 17,514

2 (via annulus) 3.3 <1%-

14 (open hole) 105.4 <1%-

Gubberamunda Sandstone to Walloon 

Coal Measures 50,052

11 (via annulus) 325 0.6%

5 (open hole) 665 1.3%

Walloon Coal Measure to Hutton 

Sandstone 213,213

10 (via annulus) 443 <1%-

6 (open hole) 1197 1%

Formation Exchange

Natural Leakage (per 

km2)

Well leakage type Number of well to increase 

leakage by >5% Number of wells per km2 to 

increase leakage by >>10%

Mooga Sandstone to Gubberamunda 

Sandstone 730

Annulus 25+ 50+

open hole 6+ 12+

Gubberamunda Sandstone to Walloon 

Coal Measures 194

Annulus 25+ 51+

open hole 5+ 11+

Walloon Coal Measure to Hutton 

Sandstone 827

Annulus 16+ 32+

open hole 3+ 7+

Results – Using Hazard Assessment

Potential leakage versus natural leakage under a CSG development scenario

Number of wells required to achieve > 10% natural leakage per km2
Results

• Just under 400 conventional wells classified.

• Desktop assessment of risk compared favorably with field verification

• Historical conventional oils and gas wells posed the highest risk for interaquifer gas and groundwater migration. 

• Private landholder water wells pose a low risk due to the low consequences 

of inter-aquifer flow and the fact that most of these wells penetrate only one 

aquifer are completed above any gas target zones. 

• Cross-aquifer mixing was highest risk

• For high risk wells, >11 wells per km2 are required before leakage increases 

by 10% 

• The findings of the risk assessment were approved by the State regulator 

and is expected to significantly reduce the time and expenses associated 

with a comprehensive well integrity survey
Thank you

Tom Silverman

Principal Hydrogeologist

EHS Support LLC

tom.silverman@ehs-support.com