Published in June 2009 Australian Longwall Magazine

There’s to be no ostrich-type behaviour when it comes to greenhouse gas emissions. Sticking one’s head into the sand simply won’t cut it when the government’s emissions reduction scheme comes into force, with conservative estimations of $500 million expected to be stacked onto coal mine costs.

Consulting firm ACIL Tasman estimated in May this year the Rudd government’s Carbon Pollution Reduction Scheme would cost up to 3300 coal sector jobs and 9900 jobs in total, with 16 mines to prematurely close in the first 10 years of implementation. It said, in its current form, the scheme would cost the sector $14.35 billion using 2008-09 coal prices.

In the public’s eye, coal mines are among the worst emitters of greenhouse gases, with coal mining accounting for 4-5% of Australia’s emissions.

“In light of such a significant additional cost, it can be expected that gas capture and emissions reduction will receive an unprecedented increase in attention and corporate support,” University of Wollongong’s Dennis Black and Naj Aziz said in a paper presented earlier this year at the Coal Operators’ Conference.

Black and Aziz have explored methods to improve both the capture and utilisation of greenhouse gases in order to reduce emissions.

“It is essential that operators and planners understand the principles of gas generation, storage and its ability to be drained from the seam.

“Given the large number of factors that impact gas generation, storage and movement, it should be no surprise that there is such a high degree of variability in gas content and composition as well as the ability to drain gas from coal seams throughout Australia,” Black and Aziz said.

Australian mines generally employ gas drainage when their gas content exceeds

6-8cu.m/t, but the researchers said the complexity and effectiveness of these drainage programs varied significantly.

Traditionally, Australian mines have used underground to inseam (UIS) drilling for pre-drainage. In gassy mines, such as those operating in the Bulli seam, more than 100,000m are drilled annually ahead of mine development, costing $4-6 million per year.

“Recent studies have been undertaken to evaluate the effectiveness of the intensive UIS gas drainage programs and it was found that some 50 per cent of the drilling effort delivered little to no benefit to gas content reduction. In such cases where the gas drainage system was not achieving optimum performance, it is not uncommon for the mine to address the problem by drilling many more holes in the area, which essentially amounts to throwing good money after bad,” the researchers said.

They attributed the poor performance to:

Insufficient drainage time prior to intersection by development gateroads;

Accumulation of water and/or coal fines within the borehole;

Accumulation of water and/or fines in sections of the range, limiting flow capacity and effective suction pressure;

Poor standard of sealing holes following intersection by development, resulting in air in the pipe range and reduced suction pressure;

Insufficient standpipe length and sealing (grouting) standard, resulting in air dilution in the pipe range and reduced suction pressure;

Boreholes not drilled in the optimum orientation for maximum drainage performance; and

Absence of in-hole dewatering where boreholes have been drilled down-dip, resulting in in-hole water accumulation which restricts gas desorption.

Black and Aziz added an inherent problem with UIS was its reliance on mine development to be finished to provide access to areas for drilling. This problem becomes magnified as longwalls become more productive. Trends show the amount of time available for drilling and draining the next gateroad in the development sequence is reducing.

“In areas with higher gas content and lower permeability, there have been many examples where the seam gas content has not been reduced sufficiently, resulting in production delays. During development production delays, the longwall typically continues to operate which erodes development lead, placing even greater pressure on development, and further reduces the available drainage lead time.

“In the extreme cases, operations have chosen to cut longwall panels short and therefore sacrifice valuable reserves rather than incur potentially significant production delays while waiting for sufficient gas to be drained.

“It is therefore extremely important that mine operators clearly understand both the drainage characteristics of the future mining areas, particularly those areas expected to be slow draining, and the expected drainage time available, based on the mine production and drilling schedule.

“Where areas are identified that drainage time is expected to be insufficient, it will be necessary to employ additional drainage methods and possibly stimulation treatments to avoid production delays or loss of reserves,” the researchers said.

To achieve increased drainage time, surface-based techniques are employed. Early drainage techniques relied on the use of vertical wells with the addition of stimulation and drainage enhancement techniques, such as under-reaming, cavity completion and hydraulic fracturing.

Advances in drilling technology led to the introduction and development of radius drilling. Radius drilling typically involves starting the drilling with a vertical or near vertical section and bending the drill string through an acceptable radius to intersect the coal seam horizontally, then continuing to drill and extend the borehole at the desired horizon to the planned borehole length.

Medium-radius drilling is now also a common method and is becoming a favoured method among Queensland coal mine pre-drainage programs, with increasing application in the Hunter Valley and consideration being given to trials in the Illawarra, according to Black and Aziz.

While pre-drainage is essential to mine a gassy seam, post-mining drainage is the major source of coal mine gas emissions. In the case of mines operating in the Bulli seam, the combined gas liberated from all affected sources during longwall extraction amounts to

35-45cu.m/t.

There have been many methods used by mines to drain gas from both the active and sealed goaf, including:

Cross-measure boreholes – drilled above and/or below the working seam located along the length of the longwall panel;

Back-of-block drainage – drilled above the working section to connect into the goaf to remove accumulated high-purity gas;

Goaf seal drainage – removal of gas from sealed goaf via pipes passing through seals; and

Horizontal directional drilling – long boreholes drilled above and/or below the working seam and oriented parallel to the longwall panel which connect to the forming goaf to drain the accumulating gas.

“Although the underground gas drainage methods are capable of removing very high volumes of gas, there are many examples where the rate of gas emission has exceeded the capacity of the drainage system, resulting in gas-related production delays.

“For mines in such situations, the use of additional surface-based goaf drainage techniques may be appropriate. One such technique is the use of vertical boreholes, located toward the tailgate side of the longwall panel and drilled ahead of the retreating longwall face,” Black and Aziz said.

To vent the gas, many mines use vertical gas drainage wells. However, this is becoming increasingly difficult in built-up areas, as each well requires infrastructure, such as an emissions reduction plant, drainage plant or reticulation lines.

Alternatives do exist though. Black said radius drilling of horizontal goaf drainage boreholes parallel to the longwall block, positioned on the tailgate side of the longwall face approximately 30-50 metres above the roof of the working section, and drilled ahead of the retreating longwall face could be used. As the longwall face passes the end of the borehole and connection to the goaf occurs, suction is applied to the goaf drainage borehole to remove the accumulating gas.

“Due to the nature of goaf formation relative to the longwall face, the position of the open end of the horizontal drainage borehole can be expected to remain relatively constant throughout the operating life of the well, resulting in a stable and overall greater gas production capacity than that which is achievable through the use of vertical goaf drainage wells,” Black said.

“A further advantage of the use of radius drilling for the formation of horizontal goaf drainage wells is the ability to drill multiple laterals to form multiple connections to the goaf, which improves both redundancy and overall gas production capability.”

Gas utilisation will also become a key component of a mine’s arsenal to minimise greenhouse gas emissions. In the past, Appin, West Cliff and Tower collieries were the

only mines to actively use gas for power generation; however, with incentive schemes

in place, there are a number of utilisation projects underway.

While flaring is the simplest, using methane for power generation has the potential to increase the financial benefits of abating emissions.

While turbines introduced at Appin and West Cliff are now decommissioned due to maintenance costs and concentration inefficiencies, they have been replaced with internal combustion engine technology that uses methane gas as the primary fuel. There are coal mine methane gas power-generation projects now operating at Appin (54 megawatt), Tower (40MW), Moranbah North (40MW), Grasstree (32MW), Oaky Creek (12-20MW), Glennies Creek (10MW), Tahmoor (7MW) and Teralba (6-8MW).

The largest source of coal mine methane is the dilute methane emitted from mine ventilation shafts, or ventilation air methane (VAM). This gas is difficult to capture and use because it has a low-methane concentration. According to Black and Aziz, it has been estimated that more than 55% of all CMM emissions originate from mine ventilation shafts, thus VAM offers both the greatest emission reduction and energy production potential.

Technical applications for VAM use include direct use as a principal energy source in oxidation units, lean-burn turbines and kilns, where it is mixed with coal fines or other combustible materials.

“With the imminent introduction of the Australian government’s carbon reduction scheme, there will be potentially significant financial incentive for coal mines to implement effective gas drainage and utilisation strategies to reduce the volume of methane gas emitted to the atmosphere,” Black and Aziz said.

That’s certainly plenty of reason for Australia’s underground coal mines to get their gas drainage and utilisation right.