STRATEGY | RISK | SUSTAINABILITY

Hydrogeology & Coal Seam Gas Nathan Littlewood Principal Hydrogeologist

Petroc Group www.petrocgroup.org

STRATEGY | RISK | SUSTAINABILITY

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.

STRATEGY | RISK | SUSTAINABILITY

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

STRATEGY | RISK | SUSTAINABILITY

STRATEGY | RISK | SUSTAINABILITY

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.

Source: Rogers et al, 2007

STRATEGY | RISK | SUSTAINABILITY

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.

Source: Rogers et al, 2007

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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

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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

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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.

Source: Rogers et al, 2007

STRATEGY | RISK | SUSTAINABILITY

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

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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).

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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).

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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

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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

STRATEGY | RISK | SUSTAINABILITY

Permeability

Source: Rogers et al, 2007

• 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.

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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

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• 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

Source: Rogers et al, 2007

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• 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

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Hydraulic Fracturing

Source: Rogers et al, 2007

Source: Rogers et al, 2007

• 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.

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Hydraulic Fracturing

Source: Rogers et al, 2007

Source: Rogers et al, 2007

• 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.

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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

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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.

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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.

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• 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

STRATEGY | RISK | SUSTAINABILITY

• 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

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• 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

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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

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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

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Summary

Source: Rogers et al, 2007

• 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.

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