# 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 10000No of Wells/Bores Drilled and proposed 

## Conventional Wells

Water Bores

CSG Wells 3000030,000

# 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 cementMooga

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 Hydro

carbon Fluid Gas Hydro

carbon Fluid Gas Hydro

carbon Fluid Gas Hydro

carbon


### 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 BendBore 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 (m 3 /d) Number of Leaky Wells Leakage from wells (m 3 /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
km 2 ) Well leakage type Number of well to increase leakage by >5% Number of wells per 

## km 2 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 km 2

# 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 inter

aquifer 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 km 2 

# 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