Underground » Environment - Subsidence and Mine Water
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.