Petroc Group: Introduction to hydrogeology and coal seam gas

STRATEGY | RISK | SUSTAINABILITY
Hydrogeology & Coal Seam Gas Nathan Littlewood
Principal Hydrogeologist
Petroc Group
www.petrocgroup.org

Introduction
• Coal seam gas (CSG) is one of a suite of ‘unconventional’ gas resources.
• Also known as coal bed methane (CBM).
• Coal beds act as both the source and reservoir for gas generated during coal
formation – typically methane (CH4).
• CSG has only relatively recently
been seriously investigated and
exploited as an energy source.
• Relatively shallow reservoirs
means production affects
groundwater.

Units!
• The USA has most experience with exploring
and producing CSG. They have been at the
forefront of the oil and gas industry.
• Petroleum geology and reservoir engineering
is dominated by the USA and their insistence
on using imperial units.
• Most countries (and
hydrogeologists) now use metric
units. However, the USA does not.

• Units Tcf, Bcf, MMcf and Mcf are
commonly used in the gas industry.
• Meaning Trillion, Billion, Million and
Thousand cubic feet.
• The American trillion has 12 zeros, in
Europe it has 18.
• 1m3 = 32 scf (standard cubic feet)
• There are many confusing pitfalls. Beware!
Imperial Metric
Volume 1 Barrel 159 Litres
Head 100 psi
70.3 m
6.89 bar
689 KPa
Permeability 100 mD
0.086
m/day
Transmissivity
1000
mD.ft
3.3
m2/day
Gradient 5 psi/ft 1.07 m/m

CSG Formation
• Tectonic activity leads to a crustal depression. This gradually fills with sediment – a
Sedimentary Basin.
• Conditions lead to plant matter depositing and degrading – Peatification
• Burial results in increasing pressure and temperature and Coalification.
• The plant type and burial
history determines the coal
type and gas generation.
• As coalification continues
there is a convergence in
chemical characteristics.
Source: Rogers et al, 2007

CSG Formation
• Temperature is the main control on
chemical change and gas generation –
Thermogenesis.
• Secondary changes may occur through
bacterial activity. Usually after uplift and
flushing of meteoric water – Biogenesis.
• Coal molecules are known as macerals.
Thermal cracking results in increased
aromatic structures and the formation of
methane.

CSG Formation
• All coal types move along a
converging path during
Thermogenesis. Losing
Hydrogen and Oxygen and
moving towards pure carbon
(graphite).
• Usually thermogenesis
doesn’t get beyond 3 main
maceral types: Liptinite,
Vitrinite, Inertinite.

Gas Adsorption
• Coal has large surface area per unit mass. 1Kg
can have similar surface area as 5 soccer
pitches.
• Gas generated during thermogenesis is held
in the structure.
• The main mechanism for gas storage is by
adsorption to the coal surface.
• Gas volumes in coal typically range between
100 and 800 scf/ton. So much gas is produced
during coalification that a large proportion
cannot be retained and escapes over time.
Source: Nunn et al, 2010

Gas Adsorption
The adsorption process between gas molecules
(A), surface sites (S) and occupied sites (SA) can
be represented as:
S + A ↔ SA
Equilibrium constant K = [SA] / [S] [A]
Constant can also be written as
K= θ/(1-θ)P
Where:
K is equilibrium constant
θ is fraction of occupied surface sites
[SA] is proportional to θ
[S] is proportional to vacant sites (1-θ)
[A] is proportional to gas pressure P
Rearranging gives expression of
surface coverage:
θ = KP / 1+KP
This is the Langmuir adsorption
isotherm

Langmuir Isotherm
The Langmuir isotherm predicts
surface coverage with changing
pressure (or concentration) for a given
temperature regime.
Langmuir with
Nobel Prize
This isotherm is critical in
understanding and exploiting gas
adsorbed in coal beds.
By reducing the pressure the
adsorption capacity decreases and
gas is released.

Langmuir Isotherm
• Movement along the methane adsorption
isotherm occurs with changing pressure
conditions.
• The position of the curve is dependent
principally on temperature.
Source: Scott, 2002
Source: Joubert et al, 1974
• Moisture content and
availability of gas also
have an effect.
• Basin history such as
periods of uplift and
burial mean shifts along
isotherm over time.
Methane adsorption to
GAC filter
Adapted from Scott, 2002

Hydrostatic Pressure
• Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to
the force of gravity – the weight of the fluid.
• Water can generally be considered incompressible and as having a constant
density. (This isn’t the case for gases where volume is proportional to
pressure).
• Hydrostatic pressure (P) can be calculated: P= ρgh
Where: ρ is fluid density
g is gravitational acceleration
h is height of fluid column
• This pressure acts on the surface of the coal and thus its capacity to adsorb
gas (remember Langmuir).

Darcy’s Law
• French engineer who undertook experiments to understand
fluid flow (water) through porous media (sand).
• He found that the water flux is proportional to the hydraulic
gradient (pressure difference) and the type of material it is
flowing through. It also depends on the fluid properties
(viscosity)
• Henry Darcy’s experiment was in a vertical column but it
applies in all directions.
• Water will flow from high to low pressure even against
gravity (artesian well).

Darcy’s Law
• Flow in an aquifer acts in the same
way as in Darcy’s experiment, but on a
bigger scale and with more
heterogeneity.
• Water moves through the aquifer at
a rate proportional to the rock
properties and the difference in
hydraulic head.
• By reducing pressures in the CSG
formation flow from other areas may
be induced towards it.
Source: Fetter, 1994
K - hydraulic conductivity
A - cross sectional area
dh/dl - head (pressure) gradient

Permeability
• The ability of a material to allow fluid flow
through it is a measure of its permeability.
•Permeability is incorporated in Darcy’s law
as a property of the porous media.
• The permeability may vary depending on
the physical properties of the flowing fluid.
• A rock type may have higher permeability
with respect to methane compared to water.
Permeability (k) = K * μ/ρg
K is hydraulic conductivity
μ is dynamic viscosity
ρ is fluid density
g is acceleration due to gravity

Permeability
• Permeability in coal beds is
determined by its fracture (cleat)
system.
• Permeability decreases with
increasing depth.
• It is highly variable in coal beds.
• More sorbed gas can be
recovered from more permeable
reservoirs.

Diffusion
• Adolf Fick (joint inventor of contact lenses) described
flux of material via diffusion in a similar way to Darcy’s
Law.
• He postulated that flux is from high concentrations to
low concentrations – a concentration gradient instead
of a pressure gradient.
• This is due to Entropy – tendency towards disorder
• Gas held within the coal (absorbed) moves according
to Fick’s Law towards larger pore spaces. Then it
switches to move according to Darcy’s Law within the
coal cleats and fractures.
Diffusion flux (J) = -D * ΔC
D is diffusion coefficient
(depends on properties of
substances involved)
ΔC is concentration gradient

• Production wells are installed into
the CSG formation.
• Pumping reduces the hydrostatic
pressure, the system moves along the
Langmuir curve and gas is released
from the coal.
• Once sufficient pressure drop is
achieved more gas flows than water.
The period of high water production
is relatively short over lifetime of
well.
• Although water pressures decline
the formation doesn’t become dry.
Gas Production - Depressurization

• Gas flows with water along pressure gradient – back towards the well.
• Water and gas are partially separated in the well and at the surface.
• Produced water goes to treatment and gas is piped to processing plant.
Gas Production - Depressurization
Image: gastoday.com

Hydraulic Fracturing
• To enhance recoverable gas volumes the
reservoir is sometimes fractured to
increase permeability.
• Hydraulic fracturing is not new, just
newly in the public arena
• Fluid with proppants is injected under
pressure to induce and hold open
fractures.
• Requires proprietary injection mixtures
that wont clog fractures and keeps coal
fines away from well.

Hydraulic Fracturing
• The cleat system and organic nature
of coal make it sensitive to complex
in-situ stress regimes.
• Fractures find the stress path of
least resistance, sometimes following
cleat systems and sometimes cutting
across them.
• Once ‘fracking’ has occurred the
applied pressure is reversed as
injection swaps to pumping.

Aquifer Connectivity & Leakage
Conceptual model of the Surat Basin (Source: UWIR, 2012)
• Definition: “A general term describing interaction
between aquifers separated by an aquitard (often
termed inter-aquifer leakage), or between different
parts of the same aquifer (intra-aquifer
connectivity).“
•Kv versus Kh and scale dependence of K
• Lateral continuity of formations
• Leakage rates and rate changes
• Hydraulic fracturing
• Need to develop a conceptual model and
then a numerical model to forecast impacts

Fugitive Gas
Source: APLNG
• Natural gas detected in water bores in Australia
since days of pioneers.
• Observed in monitoring wells away from CSG
activity and at shallow installation depths.
Sometimes seen bubbling in rivers.
• Gas volumes exceed coal adsorption capacity
and migrates away from the source unit.
• Odourless and colourless and dissipates to
atmosphere. But it’s a greenhouse gas and is lost
revenue as a well as unwanted publicity.
• Operators need good baseline data to defend
themselves.

Produced Water – Quality & Quantity
• Highest volumes of co-produced
water generated during initial stages
of gas production.
• Quality is variable but often
allowable for stock watering.
• Water and gas separated at surface
and water piped to treatment plant.
• Subsequent management of this
water is a big challenge.

• Water produced during gas extraction needs to be managed sustainably. It is one of
the major issues for the CSG industry.
• There are many options available depending on circumstances such as re-use and
re-injection.
• Managed reinjection is a favoured
approach - to re-pressurize the
groundwater system. It requires
good understanding of the
hydrogeology.
Produced Water – Managed Reinjection
Source: USGS

• Injected waters often have different
chemical assemblages than the existing
groundwater.
• Mixing of different waters may result
in chemical reactions as the system re-
equilibrates.
• Mixing could potentially result in
precipitates forming and clogging of the
aquifer.
• Chemical modelling is required, and
pre-treatment of injected water.
Source: Wilson, 2005
Produced Water – Reinjection

• Water treatment results in brine by-
product.
•‘Brine’ is not clearly defined.
• Reinjected treated water is usually less
than 10,000 mg/L.
• Seawater salinity is approximately
30,000 mg/L.
• Brines can be over 100,000 mg/L.
• Finding a beneficial use is difficult -
evaporation and landfill.
Produced Water – Brine

Monitoring bore networks are
important to assess:
•lateral and vertical variability in
the different groundwater systems
•obtain baseline water pressure
and water quality data
•Identify potential water pressure
and quality changes
Information used to build, calibrate
and refine the regional
groundwater model.
Groundwater monitoring bores
Groundwater Monitoring

Groundwater Monitoring
• Groundwater levels, pressures and chemistry
are monitored.
• Monitoring undertaken at different locations
and depths.
• Acts as baseline, early warning system.
• Existing wells and CSG-specific ones are
used, some times using remote-access
telemetry.
Source:
USGS

Summary
• Coal Seam Gas production is a
major water project.
• Understanding the science is
critical in cost-effective
production and in managing
potential impacts.
• Hydrogeology is one of the key
aspects that needs to be
understood, due to shallow
reservoirs and proximity to
groundwater.
• There are many threads that go
to enhance hydrogeological
understanding of a CSG project.

Petroc Group: Introduction to hydrogeology and coal seam gas

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (7)

Similar to Petroc Group: Introduction to hydrogeology and coal seam gas

Similar to Petroc Group: Introduction to hydrogeology and coal seam gas (20)

Recently uploaded

Recently uploaded (20)

Petroc Group: Introduction to hydrogeology and coal seam gas