Underground longwall mining of high-gas seams poses a significant safety risk. An uncontrolled release of methane may lead to an explosion of the coal and gas burst or buildup of explosive gases. Additionally, there will be excessive emission of methane from the seam exceeding legal ventilation levels, which will result in unplanned electrical shut-offs, hence loss of production time. Methane emissions mainly occur because of the geomechanical disturbance caused by the longwall retreat operation. Caving caused by coal extraction behind the powered roof supports leads to relaxation of the strata and formation of fractures in both the overburden and underburden. The methane trapped in the coal seam quickly desorbs from it and moves through these stress-released cracks to the working face and goaf. There is a need for a coordinated multi-pronged approach in ensuring safe and continuous longwall retreat operations. Ventilation alone cannot suffice when dealing with large amounts of methane at depth and multiple seams. Mine management needs to coordinate seam degasification, goaf methane capture, and goaf inertisation alongside ventilation methods.
Premining degasification will lower the total gas-in-place before the start of coal mining operations. The underground to Inseam (UIS) and surface to Inseam (SIS) horizontal wells will capture between 30% and 50% of in-place methane if enough lead time is given. Directional drilling makes use of drill motors with the Drill Guidance Systems (DGS).
Exploration drilling and gas reservoir modeling determine all coal seams that will contribute to the gas production. Calculations of Specific Gas Emission (SGE) determine the quantity of gas released for every ton of coal mined. Premining degassing of targeted coal seams and surrounding beds (rooftop or floor coal seams) will lower the contribution of the SGE by 30% or more. This will reduce gas contents from virgin seams above 20 m³/t to target levels below 3.0 to 5.0 m³/t.
Lower virgin seams gas contents will reduce the methane emission from coal ribs into the intake roadways, ensuring that the intake air concentration will be below regulatory standards (0.25% CH₄). The reduction of gas contents in the seams will prevent sharp increases of gas during high production shearer passes to protect from automatic power shut-off to the equipment.
Post-drainage operations produce significant amounts of gas generated due to strata relaxation during the longwall retreat operation. Longwall mining produces movement of rock strata in high permeability zones due to tension, and this promotes the vertical and lateral movement of gas. The drainage of methane from the desorption process prevents gas migration into the longwall working face and return air entries.
Long Reach Directionally Drilled (LRDD) boreholes represent an innovation technology in comparison with conventional short cross-measure boreholes. Conventional cross-measure boreholes that are drilled from gate roads have low lifetime due to wellhead and collar collapse caused by continued mining activities. The LRDD boreholes which are drilled from external roadways have 70% drainage efficiency compared to 30% efficiency in conventional cross-measure boreholes and thus retain suction all through the longwall retreat process.
The vertical and lateral positions of boreholes greatly affect the extraction quality and amount of gas. Boreholes located in the tension fracture zone at 20–30 m and up to 41 m above the working seam contain gas with a very high purity level—79–94%. Boreholes that were drilled at a depth lower than 15 meters (close to the zone of cave-in) tend to collapse because of major roof falls and can cause dilution of the air in the mine with methane concentration being equal to about 42%. Alignment of the boreholes in accordance with the direction of maximum horizontal principal stress (around 141° in the regional stress field) will allow maintaining their integrity during dynamic mining loads.
The vacuum level is responsible for the productivity of goaf wells. The continuous vacuum suction from several deep goaf wells at low-to-medium gas flow rates results in optimal extraction rate of gas. In case when the high vacuum is created in several face-adjacent wells, there will be significant air influx from the ventilation roadways into the goaf increasing the risk of explosions.
The main aim of face ventilation is to disperse methane emissions near the face. The typical ventilation regime for the U-mode is when fresh intake air flows from the longwall face to the tailgate return airways so that return-air methane levels remain below regulatory limits (i.e., 1.0% CH4).
An increased rate of face ventilation and a strong goaf suction effect may result in an oxygen inflow into the goaf. An oxygen inflow 150-300 m from the face generates an extensive airwash region. The interaction of oxygen with the remaining coal in the goaf zone leads to the occurrence of sponcom, CO generation, and an extended explosive gas fringe zone.
Preventive dual gateroads N2 inertisation decreases goaf oxidation risks. Continuous N2 inflow from the Maingate (MG) seals (1,200 L/s) and Tailgate (TG) seals (1,000 L/s) through surface boreholes ensures low levels of oxygen concentration below 5% behind the face. Oxygen-deficient conditions in the active goaf preclude spontaneous heating, limit the development of explosive gas fringe zones, and facilitate extraction of high-quality methane from deep goaf drainage wells.
An effective way to manage gas emissions during the longwall retreat requires developing an engineering system. Depending solely on face ventilation is not enough to ensure proper control over increased gas emissions in gassy seams. The safe and continuous production requires an integrated approach to pre-drainage degasification, goaf gas extraction by directional drilling, and road inertisation.
There are several important practical implications that mine designers should take into consideration and they may be reduced to three main principles:
- Proper reservoir characterization and pre-drainage: geological modeling and pre-mining seam drainage help to decrease the specific emissions before the retreat, which helps to avoid face equipment power shutdowns;
- Proper goaf borehole elevation and trajectory: placing goaf boreholes in the tension fracture zone (at about 20–41 meters above the seam) and aligning the boreholes along the maximum principal horizontal stress results in about 70% drainage efficiency and up to 94% methane concentration;
- Low-to-moderate vacuum suction and N2 inertisation: the maintenance of goaf oxygen concentration at below 5% through the use of low-to-moderate vacuum suction through multiple deep goaf wells and N2 injection in dual MG/TG.
The use of modern directional drilling technology in combination with dual gate road inertization increases workers’ safety, reduces costs associated with production shutdowns and decreases greenhouse gas emissions.
