ACARP ACARP ACARP ACARP
Underground

Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels

Underground » Environment - Subsidence and Mine Water

Published: May 08Project Number: C15010

Get ReportAuthor: Yvette Heritage, Winton Gale | SCT Operations Pty Ltd

Longwall coal mining at shallow depths has many mining and environmental issues related to subsidence.  The issues range from mining impacts caused by ventilation and spontaneous combustion to environmental impacts created from the release of goaf gas and hydrological issues.  A primary cause of these impacts is subsidence which can form a connection between the goaf and surface, initiating these impacts.  Determining the magnitude and extent of surface to goaf connection can assist in reducing the impact mining has on the environment and improve mining efficiency.  

A connection is formed when the overburden subsides and new joints are created in addition to the reactivation of old joints.  This opening up of joints creates a connected fracture network from the goaf to the surface which varies in tortuosity and conductivity.  Creating a tool and method of determining connectivity and conductivity from surface to goaf would be invaluable to the coal mining industry.  

This study assessed the potential of rising helium injection into goaf and overburden strata as a tool to determine goaf to surface connectivity.  The work program was structured to assess the technical issues of flow in fractured rock and the injection and detection methods.

Laboratory studies investigated the flow mechanics and flow velocities of helium through fractures.  Field experiments trialled the helium goaf injection technique and applied the laboratory findings to the field.  

The concept of helium injection was successfully tested in the laboratory where helium was injected into artificially fractured rock samples.  From the experiments it was found that the mechanics of helium flow through fractures is by bubble flow due to the pulsed style of detected helium.  A relationship between the gas velocity and fracture aperture was then found.  This relationship allowed the determination of fracture conductivity from the first arrival of helium.  The helium flow and fracture aperture relationship was also found to be comparable with previous work on gas flow through fractured rock.  

Field trials of helium injection into the goaf of a longwall were conducted at Beltana and Ashton Underground Mines.  The flow mechanics of helium through the fracture network of the overburden was also found to be bubble flow due to the pulsed style of helium detection through the surface cracks.  

The determination of whether a surface to goaf connection exists requires a two stage process consisting of the measurement of background helium and injected helium.  The measurement of background helium rates was found to be very important in determining whether a surface to goaf connection exists.  The absence of background helium is ideal as no comparison between the background and injected rate is needed.  The issue is if background helium rates are as high as expected injected rates.  In this case, it is best to postpone the test until the background levels have considerably reduced.

In this trial, a method of helium injection and detection has successfully been used to determine whether a surface to goaf connection exists.  

The average fracture aperture is determined from the arrival time of the first injected helium pulse which takes the most direct path to the surface.  The time of arrival for the first injected helium pulse is converted into a minimum flow rate which was then used to determine the minimum average fracture aperture throughout the non caved zone of the overburden.  From the average aperture an equivalent average conductivity can be calculated.  

Injection of helium into a vertical borehole above an extracted panel was also trialled.  The borehole injection technique is a more direct approach of injecting helium into the fracture network of the overburden.  With a borehole drilled into the highly permeable caved zone of the goaf, then borehole helium injection can demonstrate more quickly if a connection to the surface exists.  

The primary advantage of the borehole helium injection is that the helium is injected directly into the fracture network as opposed to the injection of helium into the goaf which does not guarantee that the helium will rise into the fractures that have a connection to the surface.  

Helium flow through fractures and fracture networks follows the mechanics of bubble flow as shown in laboratory and field experiments.  Two techniques of helium injection into the goaf have successfully been developed and demonstrated.  The results of this study demonstrate that the method is practical and effective.  

The method and application is recommended for use and ongoing refinement.

Underground

Health and safety, productivity and environment initiatives.

Recently Completed Projects

C27039True Triaxial Strength Of Coal Measure Rocks And Its Impact On Roadway Stability And Coal Burst Assessment

Rocks in the ground are subject to a range of stresses. The stresses...

C3063Underground Vehicle Design Standards And Statutory Implications

The Australian underground diesel vehicle fleet has evolved since di...

C3064Conveyor Belting And Lagging Shear Characteristics - Drive Drum Slip

The primary aim of this project was to investigate the relationsh...

Underground

Open Cut

Safety, productivity and the right to operate are priorities for open cut mine research.

Recently Completed Projects

C26029Geological Controls On Fluorine And Phosphorus In Bowen Basin Coals

Increasing global restrictions on fluorine in product coal prompted ...

C28033Raw Ash To Yield Relationships

Correct outcomes in yield predictions for product ash from coal bore...

C27038Establishing Self-Sustaining And Recognisable Ecological Mine Rehabilitation

In recent years an increasing interest has been placed on mining ope...

Open Cut

Coal Preparation

Maximising throughput and yield while minimising costs and emissions.

Recently Completed Projects

C27064Dry Beneficiation Using FGX And X-Ray Sorters

Conventional dry processing methods engage a single beneficiation de...

C26010Multi-Sloped Screening Efficiency With Changing Strokes, Frequencies, Feed Solids And Feed Rates-Pilot Plant Study

Optimising multi-sloped screens is often described as an art and the...

C28059Impact Of Water Quality In Coal Handling And Preparations Plants

The objective of this project was to deliver a concise reference do...

Coal Preparation

Technical Market Support

Market acceptance and emphasising the advantages of Australian coals.

Technical Market Support

Mine Site Greenhouse Gas Mitigation

Mitigating greenhouse gas emissions from the production of coal.

Recently Completed Projects

C23052Novel Stone Dust Looping Process For Ventilation Air Methane Abatement

This multi‐phase project is concerned with the mitigation of m...

C27054Optimisation Of A Thermal Flow Reversal Reactor For Ventilation Air Methane Mitigation

Ventilation air methane (VAM) generally accounts for 50-85% of the t...

C28076Selective Absorption Of Methane By Ionic Liquids (SAMIL) - Phase 2 Demonstration In A Packed Bed Reactor

An alternative approach to high temperature oxidation of ventilation...

Mine Site Greenhouse Gas Mitigation

Low Emission Coal Use

Step-change technologies aimed at reducing greenhouse gas emissions.

Recently Completed Projects

C17060BGasification Of Australian Coals

Four Australian coals were trialled in the Siemens 5 MWth pilot scale ga...

C17060AOxyfuel Technology For Carbon Capture And Storage Critical Clean Coal Technology - Interim Support

The status of oxy-fuel technology for first-generation plant is indicate...

C18007Review Of Underground Coal Gasification

This report consists of a broad review of underground coal gasification,...

Low Emission Coal Use

Mining And The Community

The relationship between mines and the local community.

Recently Completed Projects

C16027Assessing Housing And Labour Market Impacts Of Mining Developments In Bowen Basin Communities

The focus of this ACARP-funded project has been to identify a number...

C22029Understanding And Managing Cumulative Impacts Of Coal Mining And Other Land Uses In Regions With Diversified Economies

The coal industry operates in the context of competing land-uses that sh...

C23016Approval And Planning Assessment Of Black Coal Mines In NSW And Qld: A Review Of Economic Assessment Techniques

This reports on issues surrounding economic assessment and analysis ...

Mining And The Community

NERDDC

National Energy Research,Development & Demonstration Council (NERDDC) reports - pre 1992.

Recently Completed Projects

1609-C1609Self Heating of Spoil Piles from Open Cut Coal Mines

Self Heating of Spoil Piles from Open Cut Coal Mines

1301-C1301Stress Control Methods for Optimised Development...

Stress Control Methods for Optimised Development and Extraction Operations

0033-C1356Commissioned Report: Australian Thermal Coals...

Commissioned Report: Australian Thermal Coals - An Industry Handbook

NERDDC