unconventional gas - a groundwater perspective (nathan littlewood)

STRATEGY | RISK | SUSTAINABILITY

Unconventional Gas – a groundwater perspective

Nathan Littlewood Principal Hydrogeologist, Petroc Group

www.petrocgroup.org



‘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.




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.


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.


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


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.


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.


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


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.


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.


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


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


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


Surat Basin Cumulative Management Area & Predicted Impacts

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.


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-injection Beneficial Use Evaporation Ponds


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.


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.


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.


unconventional gas - a groundwater perspective (nathan littlewood)

Environment