An introduction to unconventional gas, its geology and production with a focus on groundwater risk and hydrogeology. Examples from Australian coal seam gas are given.

1. STRATEGY | RISK | SUSTAINABILITY
Unconventional Gas
– a groundwater perspective
Nathan Littlewood
Principal Hydrogeologist, Petroc Group
www.petrocgroup.org

2. Unconventional Gas
– a groundwater perspective
Nathan Littlewood
Principal Hydrogeologist, Petroc Group
‘Unconventional’ Gas:
•Coal Seam Gas
•Shale Gas
•Tight Gas
Gas (methane) is formed from buried plant material as a result of
thermogenic (heating) or biogenic (microbiology) activity, depending on
the geological setting. The gas does not migrate to, and accumulate in, a
conventional trap, instead it is spread throughout the reservoir formation.
Gas molecules are held in situ by water pressure.
STRATEGY | RISK | SUSTAINABILITY

3. Unconventional Gas
– a groundwater perspective
Nathan Littlewood
Principal Hydrogeologist, Petroc Group
Coal Seam Gas – coal with gas
produced in situ, predominantly
methane
Shale gas – shale
formation with
more varied
hydrocarbon types
Tight gas – sandstone or carbonate
reservoirs with very low permeability.
Often older than other reservoir
types.
STRATEGY | RISK | SUSTAINABILITY

4. Langmuir Isotherm – pressure v adsorption
Capacity of the reservoir substrate to
adsorb/retain gas is directly related to the
reservoir pressure. Reducing the pressure by
pumping water releases the gas - desorption
The lower hydrostatic pressure in the
formation means more gas can mobilise and
migrate towards the pumping/production
bore along the pressure gradient.
The reservoir never becomes unsaturated with respect to water.
STRATEGY | RISK | SUSTAINABILITY

5. Typical gas and water flow in CSG production
Water and gas move to the well in dual-phase flow. Over time the
proportion of gas increases as the formation is increasingly depressurised.
Methane is not very soluble in water and is separated at the well-head.
Production Curves
STRATEGY | RISK | SUSTAINABILITY

6. Hydraulic Fracture Stimulation – “Fracking”
Fracking is a process that has
been used for many decades in
both conventional and
unconventional oil and gas
reservoirs, and in the
geothermal industry .
Reservoir rock is fractured
prior to de-pressurising the
reservoir. This increases
permeability by injecting
fluids and proppants at high
pressure.
STRATEGY | RISK | SUSTAINABILITY

7. Typical fracking fluids:
•Water
•Gelling agent
•Proppant
•Surfactant
•pH buffer
•Biocide
Hydraulic fracturing is not always
required, it is more common
where gas is deeper and the
formation under greater
overburden pressure.
STRATEGY | RISK | SUSTAINABILITY

8. Close monitoring and control of the fracking process is
necessary – in terms of fluid migration and fracture
propagation.
BTEX (benzene, toluene, ethylbenzene, xylene) compounds
are banned in some countries (e.g. Australia) for use as
additives in hydraulic fracturing. However, these
compounds may be naturally present in the target
formation and become mobilised to adjacent units.
Relatively environmentally benign chemical alternatives
are available.
Fracking Mitigation
STRATEGY | RISK | SUSTAINABILITY

9. Well Integrity
There are international and national
standards for well construction.
A poorly constructed well can act as a
vertical pathway – for gas and/or water.
Casing and cement emplacement, with
subsequent testing, is crucial.
Hundreds of production wells may be
required for a project, so the risk of some
leaky wells is real.
Even a small percentage of poor wells can
have environmental and financial impacts.
STRATEGY | RISK | SUSTAINABILITY

10. Potential Groundwater Impacts and Challenges
• Creation of vertical pathways between groundwater systems
• Increasing vertical hydraulic gradients
• Contamination from injected fluids and linked groundwater systems
• Water and brine management and disposal
• Lowering of the water table or groundwater pressures
• Time lags of impacts
• Fugitive gas
There may be significant environmental and
financial cost implications associated with these
issues, depending on the hydrogeological setting.
STRATEGY | RISK | SUSTAINABILITY

11. Potential Groundwater Impacts and Challenges
Induced fractures can create pathways through otherwise low permeability
layers. This can lead to mixing of water bodies and a drop in quality.
Depressurisation can induce flow between aquifers, if present. This can
manifest as a lowered water table, reduced yields from supply bores and
impacts to groundwater dependent ecosystems.
Baseline conditions need
to be understood, a
hydrogeologic numerical
model usually needs to be
developed to forecast and
characterise potential
impacts.
Source: NSW Dept Primary Industry
STRATEGY | RISK | SUSTAINABILITY

12. Complex Stacked Aquifer Systems
Often the hydrostratigraphic
system is multi-layered and
complex (e.g. Surat Basin coal
seam gas fields, Australia.)
Pressure gradients induced
by pumping extend across
the different aquifer and
non-aquifer lithologies. No
unit is completely
‘impermeable’.
Source: Surat UWIR, 2012
STRATEGY | RISK | SUSTAINABILITY

13. Groundwater Monitoring
Monitoring bore networks are
essential to assess lateral and
vertical changes in the groundwater
system.
Information develops and refines
the predictive model and is used in
stakeholder engagement – ‘making
good’ those impacts to water users.
Source: www solinst.com
STRATEGY | RISK | SUSTAINABILITY
Surat Basin Cumulative Management Area & Predicted Impacts

14. Numerical Modelling
Groundwater numerical
modelling is a key tool for
assessment and
management.
Used in production planning
and for environmental
protection.
Helps to understand the
hydrogeologic system and
simulate/predict impacts.
But models are only as good
as the input data.
STRATEGY | RISK | SUSTAINABILITY

15. Water Management
Depending on the unconventional gas scenario, large volumes of water can be
generated during the gas production process, with sometimes hundreds of wells
being pumped.
The end-use of this water depends on the geographic, hydrogeologic and
geochemical setting, in addition to cost and regulatory constraints.
Treatment such as desalination may be required prior to further use, this is a cost
and results in residual waste brines that require handling and disposal.
Aquifer Re-injectionBeneficial Use Evaporation Ponds
STRATEGY | RISK | SUSTAINABILITY

16. Managed Aquifer Recharge
Co-produced water can potentially be reinjected back in to the
ground after treatment. This may be in to the gas reservoir formation
but more typically it is into an overlying or underlying aquifer.
This technique requires a good
understanding of the local groundwater
system (hydraulic and geochemical) and
construction of treatment and injection
infrastructure.
•Feasibility assessments.
•Drilling of injection and monitoring wells.
•Injection trials.
•Ongoing assessment.
STRATEGY | RISK | SUSTAINABILITY

17. Summary 1
Depressurisation of the reservoir is usually required to release gas
from the formation.
Depressurisation from pumping groundwater out of the reservoir. This
can result in a major water management challenge.
Hydraulic fracture stimulation is sometimes employed to increase
permeability and gas production.
Knowledge of the hydrogeological
system is required to understand the
potential groundwater impacts
associated with unconventional gas
production.
STRATEGY | RISK | SUSTAINABILITY

18. Summary 2
There are always impacts to the prevailing system – these should be
planned and managed. An understanding of the hydrogeology is essential.
Impacts may occur in the short term or over hundreds or thousands of
years. In some cases they may be very long term and/or permanent, so it
is important to get things right first time.
How do the concepts of adaptive management, environmental
conservation and intergenerational equity fit into longer timeframes? How
to communicate realistic risk through popular media and differentiate
between natural and artificially induced phenomena? Not all planned and
unplanned impacts may be a concern, but some may last long after
development has ceased.
STRATEGY | RISK | SUSTAINABILITY