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2.3 Conceptual modelling for the Cooper subregion

Executive summary

Cooper Creek near Innamincka, SA, 23 May 2013 Credit: Dr Anthony Budd, Geoscience Australia

Conceptual models are a simplified and generalised representation of a complex system. During development of conceptual models, the essence of how the key system components operate and interact is distilled. The Bioregional Assessment Programme (BA) conceptual models of causal pathways are developed to describe the logical chain of events ‒ either planned or unplanned ‒ that link coal resource developments to water-dependent assets.

Methods

This product details the conceptual model of causal pathways of the Cooper subregion, following the method described in companion submethodology M05 (as listed in Table 1). For the subregion it identifies:

Summary of key system components, processes and interactions

The Cooper Basin is an Upper Carboniferous – Middle Triassic geological basin in north-eastern SA and south-western Queensland. The Cooper Basin is up to 2500 m thick, at subsurface depths of between 1000 and 4400 m. The southern Cooper Basin is marked by a series of troughs (e.g. Weena and Tenappera troughs) separated by ridges against which Permian sedimentary rocks thin or pinch out. The Cooper Basin unconformably overlies the Warburton Basin. Several granite bodies intrude the older Warburton geological Basin and underlie the Cooper Basin.

The watertable is hosted in the Namba Formation and Quaternary sediments, with regional flow south-westwards towards Lake Blanche. Groundwater within the Cadna-owie – Hooray Aquifer flows south towards and beyond Lake Frome. In the hydrostratigraphic units of the Cooper Basin, the regional groundwater flow direction is mainly towards the south-west.

There is limited recharge via diffuse infiltration of sporadic rain water, flood waters or streamflow to the groundwater system. The most significant source of groundwater to the Eromanga Basin sequence in the Cooper subregion is inflow from outside the subregion. Recharge to the Cooper Basin occurs by vertical leakage or cross-formation flow.

The southern Cooper subregion is part of the Cooper Creek – Bulloo river basin. Cooper Creek is characterised by complex channel networks and numerous wetlands and waterholes. Water is derived from runoff from headwater catchments. Streamflow in Cooper Creek and its tributaries varies greatly between years from almost no flow to significant flooding. Natural discharge of groundwater to surface takes place at springs, as well as in lakes. The Lake Blanche springs are fracture or fault-fed springs sourced by the Coorikiana Sandstone and Cenozoic aquifers.

Ecosystems

The ecosystems in the Cooper subregion are classified in terms of landscape classes and their dependence on water. Based on the Australian National Aquatic Ecosystem (ANAE) Classification Framework five elements are included in the classification: topography, landform, water source, water type and water availability. The majority of the preliminary assessment extent (PAE) for the Cooper subregion is dominated by the ’Dryland’ landscape group (75.36%). Where landscapes are water-dependent, floodplain landscape classes comprise 13.10% by area of the PAE.

Coal resource development

There is no coal or coal seam gas (CSG) (i.e. stand-alone CSG) production occurring in the Cooper subregion as of December 2012. As a result, the baseline coal resource development (baseline) for the subregion does not have any coal resource developments.

As of early 2016, the only potential project in the CRDP likely to proceed to production is the Southern Cooper Basin Gas Project (Strike Energy Ltd main JV owner and operator), in the Weena Trough. Coal seams in the Patchawarra Formation of the geological Cooper Basin at depths of around 1900 to 2100 m are production targets at this prospect. It is anticipated that the project will enter production sometime during 2020 or 2021, with a reported production life of 20 years.

Water management

The Southern Cooper Basin Gas Project is located within the Far North Prescribed Wells Area (FNPWA) in SA. Groundwater in the FNPWA is managed under a water allocation plan (the Far North Water Allocation Plan, FNWAP).

In addition to the allocated volume for petroleum activities, the FNWAP also sets limits for drawdown effects at springs and at the SA border. Predicted drawdown must not exceed 1 m at the boundary of the Southwest Springs Zone, and must not exceed 0.5 m at a distance of 5 km from any individual spring. Furthermore, drawdown in excess of 10% of the pressure head at a state border is a trigger for consultation with the potentially-affected state.

Water for drilling and stimulation activities for the Southern Cooper Basin Gas Project will be sourced from shallow bores adjacent to the well site, or trucked in. Produced water will be stored in lined ponds. Pond size is dependent on predicted water production rate, predicted total produced water volumes, evaporation rates and site constraints. Ponds will be constructed according to standard Cooper Basin construction methods. Excavation and bunding will be used to elevate pond walls above ground level. Ponds will be located on existing disturbed areas as far as practicable. Impermeable liners will be installed in ponds where required.

At the completion of operations, after pond water has evaporated or been pumped out, the liner and any salt residue will be removed and disposed to an appropriately licensed facility, the ponds will be backfilled and re-profiled to match pre-existing surface contours, and the surface will be ripped to promote revegetation.

Data collected during production testing have shown that water production rates are in the order of 30 to 85 kL/day per well.

Hazard analysis

A dedicated hazard analysis, using Impact Modes and Effects Analysis (IMEA), is used to systematically identify activities that may initiate hazards, defined as events, or chains of events that might result in aneffect (change in the quality and/or quantity of surface water or groundwater).

A large number of hazards are identified; some of these are beyond the scope of a BA and others are adequately addressed by site-based risk management processes and regulation. When ranked by mid-point of the hazard priority number the top ranked hazard considered for BA in the Cooper subregion associated with CSG operations is aquifer depressurisation, occurring through the activity of water and gas extraction during the production life-cycle. When ranked by hazard score, the hazard analysis identifies disruption of natural surface drainage as the most frequent hazard associated with CSG operations in the Cooper subregion.

Causal pathways

Hazards associated with CSG operations that are considered to be in scope for the BA in the Cooper subregion are grouped according to their hydrological pathway to impact and include: (i) ‘Subsurface depressurisation and dewatering’; (ii) ‘Subsurface physical flow paths’; (iii) ‘Surface water drainage’; (iv) ‘Operational water management’. These causal pathway groups represent models linking an activity with a potential impact on the groundwater or surface water.

Subsurface depressurisation and dewatering associated with the CRDP occurs when CSG operations intentionally dewater and depressurise subsurface hydrostratigraphic units; for example, depressurising a water-saturated target coal seam to induce desorption and subsequent extraction of CSG. Groundwater level or pressure is most commonly altered, but other gradients can also be changed via this process, such as temperature, density or chemical composition (water quality).

Subsurface physical flow paths involves physical modification of the rock mass or geological architecture by creating new physical paths that water may potentially infiltrate and flow along. Just because a new physical path is created does not necessarily mean that water will start flowing along it in preference to how it flowed before – it will still follow the path of least resistance, and be governed by pressure gradients. This causal pathway group can, however, potentially lead to direct hydraulic connection between the target strata and other hydrostratigraphic units (such as regional aquifers), by creating new zones of deformation in the rock mass. This may occur when the integrity of wells drilled for groundwater or gas extraction is compromised, or may occur due to hydraulic fracturing of coal seams.

A specific causal pathway in subsurface physical flow paths is hydraulic fracturing. In the Cooper subregion, hydraulic fracturing will be necessary to liberate gas from the Patchawarra Formation coals. The available evidence from initial CSG well testing by Strike Energy Ltd has shown that hydraulic fractures are contained within target coal seams and do not propagate beyond into adjacent hydrostratigraphic units. Thus, at this stage, there is no evidence that hydraulic fracturing activity in Cooper CSG fields will create new subsurface flow paths between hydrostratigraphic units. If new flow paths were created, this would propagate depressurisation into adjacent hydrostratigraphic units. In addition, any fluids injected during hydraulic fracturing operations will be contained within the target unit within the Patchawarra Formation. The Patchawarra Formation is not utilised as a groundwater source in the Cooper subregion.

Operational water management includes water produced from CSG extraction wells. This water is recovered and stored at the surface in lined and bunded ponds. There is no provision for release to surface water or reinjection in the Cooper subregion CRDP. Within the southern Cooper subregion, the surface water feature that could potentially be affected by a loss of containment is the ephemeral, low-gradient Strzelecki Creek. Downstream effects could propagate to Lake Blanche, 40 km to the south-west. Strzelecki Creek experiences large variations in discharge and flow duration, from no flow to flooding.

Surface water drainage involves the physical disruption and disturbance of surface topography and near-surface materials (vegetation, topsoil, weathered rock). Within the southern Cooper subregion, the surface water feature that could potentially be disrupted due to infrastructure for CSG operations is the ephemeral, low-gradient Strzelecki Creek. Downstream effects could propagate to Lake Blanche. The Southern Cooper Basin Gas Project is located adjacent to existing roads and gas pipelines, so the requirement for major infrastructure development will be small. However, this will depend upon the final layout of the CSG operations including well spacing and number of wells, and location and type of support infrastructure such as accommodation, roads, gas flowlines, water management infrastructure and processing infrastructure.

Gaps

Knowledge gaps relating to the hydrogeological architecture around the Cooper subregion CRDP include a lack of detailed understanding of the three-dimensional distribution of faults and other geological structures, the hydraulic parameters of target and adjacent formations, and the inter-aquifer connectivity between the Cooper Basin and the overlying Eromanga Basin.

Uncertainties around the well spacing, depth, production timeline and size of the CSG resource hamper the assessment of the potential impact of the CRDP, but do not significantly affect the identification of causal pathways and development of conceptual models for those pathways. Similar uncertainty exists around water production rates and water requirements for the CRDP.

Further work

This is the final product for the Cooper subregion from this iteration of the Bioregional Assessment Programme. Due to the limited coal resource development potential no numerical surface water or groundwater modelling, receptor impact modelling, risk or impact analysis or associated products are being produced.

Last updated:
1 December 2017