Hydrology: changes in dynamics and aquifer interconnectivity

Coal seam gas extraction can reduce groundwater pressure and, subject to aquifer connectivity, has potential consequences for fresh water aquifers both in terms of quantity and quality. Connectivity and leakage between aquifers can also have implications for surface water resources. Similar concerns also apply to large coal mines, which may also disrupt surface flow through river diversions and flood protection works, and for longwall mines, through the effects of subsidence.

Consideration is given to how these issues contribute to the overall cumulative impacts of coal seam gas extraction and coal mining, according to different scales (local vs regional) and timeframes (tens to thousands of years).

Projects

Projects commissioned to date include:

Aquifer connectivity within the Great Artesian Basin, and the Surat, Bowen and Galilee Basins

The review captures the state of knowledge on aquifer connectivity, particularly within the Great Artesian Basin (GAB), and the Surat, Bowen and Galilee (geological) Basins, including:

  • types of connectivity, its implications and measurement
  • groundwater flow models, which may both rely on an understanding of aquifer connectivity, and predict the extent of connectivity between aquifers and actions that can alter groundwater flow conditions.
  • connectivity within the GAB, a large and high-priority groundwater resource, and within the Surat, Bowen and Galilee geological Basins, which are linked to the GAB and are highly prospective areas for coal seam gas development and/or coal mining.

Report: Background review

Bore integrity

This review captures the state of knowledge on bore integrity issues as they relate to coal seam gas extraction in Australia. The review examines issues associated with bore construction, integrity, monitoring and reporting, decommissioning, and legacy issues. The focus of the review is the Australian coal seam gas context, but a summary and synthesis of case histories from overseas and within other industry sectors is provided.

Report: Background review

Coal seam gas extraction: modelling groundwater impacts

Groundwater models are important at a range of scales to predict the hydrological impacts of coal seam gas (CSG) and coal development. This report describes typical Australian CSG environments and processes involved in CSG extraction, and outlines a range of modelling tools and approaches used to simulate associated groundwater impacts. It includes discussion on:

  • the effectiveness of groundwater models to represent hydrogeological conditions at specific scales
  • different approaches to modelling ranging from simple analytical models to more complex numeric regional groundwater models
  • the functionality and optimal spatial resolution of numeric groundwater models.

Areas for future research have been identified to address priority knowledge gaps and improve model performance.

The following research report is no longer current; however, can still be accessed for informational purposes.

Report:

Groundwater hydrochemical characterisation of the Surat region and Laura Basin – Queensland

Groundwater hydrochemical analysis can significantly improve understanding of processes controlling groundwater movement, recharge mechanisms, the age of groundwater, inter-aquifer connectivity and the likely chemical compositions of aquifer materials.

This desktop study focused on using hydrochemical analysis to distinguish between groundwater in coal seams and groundwater in adjacent aquifers in Queensland’s Surat Region and Laura Basin.

The project included:

  • hydrochemical characterisation of groundwater (ionic, organic, gaseous, radiological and isotopic) within aquifers to determine baseline conditions
  • comparison of groundwater quality data to relevant water quality guidelines (ANZECC/ARMCANZ 2000; NHMRC/NRMMC 2011)
  • identification of knowledge and hydrochemistry data gaps pertaining to potential coal seam gas development in the Surat Region and Laura Basin.

Report - available on Geoscience Australia's website:

Monitoring and management of subsidence induced by longwall coal mining activity

Longwall coal mining is the most common underground coal extraction method in Australia because of its relatively low cost, strong safety record and efficiency in removing coal from deep seams.

This report describes modelling tools and approaches for predicting subsidence and monitoring and measuring the surface water and groundwater related impacts of subsidence due to longwall coal mining.

Based on a long history of longwall coal mining in New South Wales, a generic approach is presented for investigating subsidence-related impacts on water resources that can be applied in Queensland where longwall coal mining is at an earlier stage. The approach uses a broad risk assessment to first identify and document the value of assets at possible risk from mine subsidence, then uses predictive techniques to identify the likelihood of impact and monitoring results to validate the assessment and refine the risk assessment. It provides guidance to proponents and regulators on the appropriate components and sequence of investigation, spanning the life of a mine (from exploration through to post-mining management stages).

Report:

Monitoring and management of subsidence induced by coal seam gas extraction

This report synthesises experiences of CSG subsidence in Australia and overseas, the assessment approaches being used to predict the scale and extent of subsidence, and monitoring and management options for subsidence induced by CSG extraction. A review of publicly available literature identified no reference to observed adverse impacts of subsidence due to CSG production, and also found that:

  • subsidence models can be used to predict the magnitude and extent of subsidence.
  • model predictions can be used to inform subsidence monitoring schemes and the management or mitigation of potential impacts on assets
  • collation of subsidence monitoring data across Australian coal seam gas developments would be useful to review critical conditions and improve subsidence prediction models.

It is recommended that subsidence monitoring be undertaken in areas of CSG development, noting that:

  • monitoring can provide an early warning of subsidence that may pose a risk to infrastructure and the environment
  • determining the potential impact of subsidence on sensitive assets such as infrastructure, water resources and ecosystems should include impact criteria specific to that asset type.

Report:

Regional hydrogeological characterisation studies in four coal basins in Queensland, South Australia and Victoria

These reports provide multidisciplinary characterisation of the detailed geology and hydrostratigraphy of the Laura, Maryborough, Otway and St Vincent Basins. They also describe the occurrence and development of coal and petroleum resources, and exploration for unconventional gas resources, including coal seam gas.

Reports - available on Geoscience Australia's website

Subsidence from coal mining activities

The review captures what is currently known about predicting, monitoring, assessing and remediating subsidence and other movement-related impacts associated with underground coal mining activities, including:

  • the different causes and expressions of subsidence from coal mining
  • existing technology and tools for monitoring and assessing the extent of subsidence
  • models to predict the scale and extent of subsidence
  • remediation options.

Background Review:

Subsidence from coal seam gas extraction in Australia

The review captures the state of knowledge on the water-related impacts of subsidence and other ground-related movements as a result of coal seam gas extraction, including:

  • the different causes of subsidence
  • models to predict the scale and extent of subsidence
  • technology and tools for monitoring, measuring and assessing the extent of subsidence
  • remediation options.

Background Review:

Research to improve treatment of faults and aquitards in Australian regional groundwater flow models to improve assessment of impacts of coal seam gas (CSG) extraction

This project focused on developing methods and techniques to underpin the risk assessments associated with deep groundwater extraction and depressurisation for energy resource development. The project aims to improve the predictive capability of regional groundwater flow models used in this context, specifically with respect to the representation of faults and aquitards. The project has three components:

  1. an examination of aquitards;
  2. an examination of faults; and
  3. an examination of the upscaling of aquitard and fault properties such that they can be adequately represented in regional groundwater flow models.

The objective of this research is to contribute to an improved conceptualisation, representation and parameterisation of aquitards and faults in groundwater flow models to reduce uncertainty in regional and local groundwater flow and pressure simulation.

Reports:

Bore and well induced inter-aquifer groundwater connectivity: consequence modelling and experimental design

This research explores some of the potential consequences from inter-aquifer connectivity caused by degraded or poorly constructed bores and wells. The project has three parts:

  1. a critical literature review
  2. local groundwater modelling to identify the types of compromised bore integrity that may be measureable in CSG-bearing basin
  3. regional groundwater modelling to assess the consequences of well and bore hole connectivity and the number of required connective pathways to create a range of noticeable impacts.

Reports: