Introduction. Drainage is also closely akin to the well production of natural gas, the principal constituent of which is CH 4

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1 Introduction ALL coalbeds contain CH 4 : 0.01 ft 3 /t 600 ft 3 /t. Rule of thumb => increases with higher ranks & greater depth. CH 4 is adsorbed on the micropore structure of the coal matrix & compressed in the fracture system of the coalbed. Emitted wherever coal is uncovered (erosion or mining) due to disturbance of coal equilibrium press. CH 4 released during mining has been viewed as a safety hazard & general nuisance in the past, there has been an increasing awareness of the potential environmental, energy, and economic benefits for recovering CH 4. Methane drainage (coal degasification) ==> the practice of removing the gas contained in a coal seam and adjoining strata thru wellbores, drillholes, and pipelines. Similar to borehole mining, although operations may be conducted from either surface or U/G. Drainage is also closely akin to the well production of natural gas, the principal constituent of which is CH 4. It may proceed independent of or in conjunction with traditional mining. EPA estimated (1995): over 65 mines (5% of U/G mines) could be classified as large and gassy. U.S. coal mines purge over 250 million ft 3 (7.1 million m 3 ) of CH 4 daily ==> an inefficient, costly, unsafe, and archaic practice. It has been proven overseas that: (1) air dilution requirements can be drastically but safely reduced; (2) CH 4 captured by drainage can be utilized commercially. Mines employing methane drainage report the capture of 50-60% of the face emissions. Methane emissions could be reduced by approx. 60% ==> result in a total emissions reduction of approx. 1.6 to 2.7 Tg/yr. by the year Techniques for recovery: Drilling wells before, during, or after mining. Vertical wells (drilled several years in advance of mining) ==> the most expensive, but will recover up to 70% of nearly pure methane that would be emitted otherwise. 1

2 Gob wells (drilled during or after mining) can recover up to 50% of high quality CH 4, but the CH 4 may be contaminated. Such a methane/air mixture requires enrichment for power generation, not economically feasible at present. Mines in Alabama (6), Virginia (5), & Utah (1) are selling recovered methane to pipelines. A number of critical barriers be overcome before CH 4 recovery could become economically justified for a much larger number of mines in the future. Methane Emission in Coal Mining Coal mines: a sizable source of CH 4 emissions in the US, released during and after mining. EPA estimates: Tg in The majority of these emissions (70 to 80%) result from U/G mining operations. Future estimates: Year 2000: Tg/year Year 2010: Tg/year A large portion of these emissions are from the degasification systems used at gassier mines, where the methane is emitted in concentrations between 30 and 95%. Degasification system emissions account for 20 to 45% of U/G emissions. U.S. Anthropogenic Emissions (Tg/yr) U/G 3.0 to to to 6.6 Surface 0.2 to to to 0.9 Post-mining 0.5 to to to 1.2 Total - Coal 3.6 to to to 8.7 Total U.S. 25 to to to 39 Total U.S. coal methane resources: approx. 300 trillion ft 3 (8.5 x m 3 ), w/ about 200 trillion ft 3 (5.7 x m 3 ) recoverable. Candidate coal fields with large resources: Green River (WY and CO), Arkoma (AR and OK), Piceance (CO), and Northern Appalachia (PA, OH, WV, and KY). Currently, the largest commercial production comes from the Warrior Basin (AL). Individual mines now being degasified or suitable for degasification have methane emission levels of 5-12 million ft 3 /day (0.14 to 0.34 million m 3 /day). Factors control the output of methane drainage: (1) physical properties of the coal seam (diffusivity, reservoir pressure, permeability, and gas content), (2) mining method (if in progress), & (3) drainage method. 2

3 One of the best predictors ==> the product of daily coal production and depth of mining. Methane Drainage Technology Firedamp extraction from coal seams 1730 (England); first successful system 1943 (Germany); slowly becoming active in U.S. recently (> 30 mines use recovery wells as a supplement to their vent system; w/ CH 4 vented to the atmosphere) No single preferred tech. Major para- meters considered include: Natural/induced permeability of the source seam(s) & associated strata, purpose for drainage the gas, & mining method. Factors that control the output of methane drainage: physical properties of coal seam (diffusivity, reservoir pressure, permeability, and gas content), mining method (if in progress), & drainage method. In-seam Drainage Drainage can be successful only if the coal has a sufficiently high natural permeability or a fracture network can be induced in the seam by artificial methods. ==> knowledge of coal permeability is critical. If coal permeability is sufficiently high, methane can be reduced significantly by pre-draining. Gas capture eff. could be up to 50% (fig. 1) For Longwall Mining (fig. 2) Preferred spacing depends on the permeability of the seam, varying from 10 ~ 80 m. The distance from the end of each bore- hole & the opposite airway should be about 1/2 the spacing between holes. Suction on boreholes unnecessary but may be required for coals of marginal permeability or to increase the zone of influence of each borehole. Time allowed ==> minimum 6 months, preferably, over 1 year. 3

4 Coal spalling into the borehole, reduced using smooth drill rods. drill chippings removed by means of a continuous water flush, augers be used Perforated plastic liners be inserted to maintain the holes open; first 5 to 10 m of each borehole are drilled at, l00 mm dia. A standpipe is cemented into place and connected through a stuffing box into the methane drainage pipeline. The remainder of the hole drilled through the standpipe at a diameter of some 75 mm. Gas flowrate will vary with time. A high initial flow initially, diminishes fairly rapidly, but then increase again as the zone of influence in dewatered, followed by a decay as the zone of influence is depleted of gas. (fig 3) Hydraulic stimulation (hydrofracturing) used to enhance the flowrates. Injecting water or foam containing sand particles into the seam ==> to dilate the fracture network of the seam by hydraulic pressure. The sand particles are intended to maintain the flowpaths open when injection ceases. Success depends on the natural fracture network within the seam & the absence of clays that swell when wetted. Gob drainage by surface boreholes Longwall mining ==> voidage above & below the caved zone and methane can accumulate at high conc. (Gob Gas). Gob gas be removed by U/G cross-measures drainage or by drilling boreholes from the surface (fig. 4). 3 or 4 holes are drilled from surface rigs at intervals of 500 to 600 m along the centre-line of the panel and ahead of the coal face, w/ dia. at 200 to 250 mm & drilled to within some 8 to 10 m of the top of the coal seam. 4

5 Holes should be cased from the surface to a depth dictated by local geology & to extend below any beds that are likely to act as bridging caprocks. Perforated liner be used to inhibit closure from lateral shear. The initial gas ==> small; increases, as the face passes under each borehole, & CH 4 be drawn towards the borehole. Bed separation will assist drainage across the gob area. First borehole be located far enough from the face start line (about 150 m) to ensure its connection into the caved zone. When a hole is active, the rate of prod. will increase sharply and may yield > 50,000 m 3 /day (1.766 mcf/d) of commercial quality methane for several months. Reconsolidation of caved strata ==> resulting in gradual decay. Surface gas drainage pumps ==> to ensure correct gas flow direction & both the rate of flow and gas purity. Excessive pump suction can cause dilution due to ventilating being drawn into the gob. Cross-measures methane drainage Methane drainage for deep mines, w/ capture eff. ranging 20% 70%.(fig. 5) Boreholes drilled into the roof and/or also floor strata, parallel to the plane of the coal face but inclined over or under the waste. A popular method in Europe, applicable to both advancing and multiple-entry retreat systems. Drilling angle, length & spacing of boreholes ==> be decided on a site- specific basis. In general, holes should intersect the major gas-emitting horizons; spacing between holes be such that their zones of influence overlap slightly. Significant increases in CH 4 conc. along the airway ==> too great a distance between holes. If closing a borehole causes a rapid increase in the flowrate from neighbouring holes ==> too close together. strata hardness & difficulties of drilling ==> influence spacing; Typical spacing ==> m. 5

6 Computer modeling can assist in planning. But a successful drilling pattern still requires a practical study of the local geology, followed by a period of experimentation. Boreholes be drilled from a return air- way; may be required on both sides of the panel in particularly gassy situations. Cross-measures drainage can be from either current working horizon or from airways that exist in either overlying or underlying strata. U/G drilling boreholes ( mm in dia.) having smaller dia. than surface holes. Standpipes are necessary to prevent excessive amounts of air being drawn into the system. They may be grouted into place with cement, resin, rubber & sealing compounds. Typical suction: 10 kpa; For given suction, the purity of gas will vary w/ the length of the standpipe; it may need to be extended to within a few metres of the main gas-producing horizon to ensure a high conc. The suction applied to boreholes ==> a sensitive means of controlling both the flowrate & purity of gas. Increasing the suction to an active borehole will increase the flow & decrease purity. Borehole tests needed to determine the optimum amount of suction. If too high, pipelines CH 4 conc. < 15% & becomes explosive. When CH 4 conc. < 30% ==> Reduce suction automatically. If too low ==> excessive emissions can occur in vent. system. It is advisable to ensure that the inter- face (between the air and methane) remains in the strata above the level of the airways. A typical figure: 10 kpa, although large variations occur in practice. Drainage from worked-out areas Rate of emission decays w/ time ==> Total make of gas in an abandoned area may be considerable & continue for an extended period, may be years in cases, With proper design, they can often be drained by drainage pipes. Methane drainage system The infrastructure should include: 6

7 pipe ranges safety devices extractor pumps (if required) monitors controls Pipe ranges Monitoring, safety & control devices are located at strategic positions throughout the system. Either steel and/or high density poly- ethylene have been used. Steel ==> having mech. strength; be galvanized against corrosion. Heavy duty plastic piping ==> is preferred because of its lighter wt. & ease of installation. Up to 75 mm in dia. may be brought into the mine on reels, eliminat- ing most of the joints every 5 or 6 m with steel pipes. Sized according to the gas flows ex- pected (75 mm 600 mm dia.). Be colour coded for easy recog- nition. U/G pipes be suspended to reduce the impact of falling objects. Steel pipes joints should be capable of flexing to accommodate airway convergence or strata movements. No drainage pipes in airways where electrical sparking might occur; and be grounded against a build-up of electro- static charges. New pipes be pressure tested at 5000 kpa. All pipework be inspected regularly for corrosion and damage. Pipes be installed w/ a uniform gradient & water traps installed at unavoidable low points. Main vertical pipe steel & may use existing upcast shaft or a dedicated surface-connecting borehole. Shaft pipes be fitted with telescopic joints for expansion & contraction. Surface pipes be protected against frost damage, either by thermal insulation or by heated cladding. Monitors Three parameters be monitored ==> gas flow, pressure, and gas concentration. Gas flow can be determined using 7

8 Q = A X 2p ρ m 3 /s X = orifice shock loss factor A = pipe cross-sectional area (m 2 ) ρ = gas density (kg/m 3 ) p = press. drop across orifice, Pa Pressure ==> made at the mouths of boreholes and at strategic locations within the pipe network. Pressure diff. can be measured from U-tubes to capsule or diaphragm gauges. A set press. at each chosen location be maintained using mech. regulators. Spot measurements (methanometers) and permanent transducers (infrared gas analysers). Controls & safety devices Primary by manually operated gate type isolating valves in all main & branch lines, the mouth of each borehole ==> to facilitate extension of the system and pressure tests. Diaphragm & activated valves for controling or alarming signals, to vary gas pressure at the mouth of borehole in response to changes in pipeline gas conc. variations; gas flow be cut off in case of abnormal conditions (eg, the use of weak plastic tubing along gas pipelines) Use of water traps throughout network ==> from simple U-tube arrangements to automated devices. Lightening conductor, flame traps & Flame extinguishers Drainage network can be incorporated into an electronic environmental surveillance & control system. Extractor pumps For gob or cross-measures drainage controlled suction is often necessary. Two major types of extractor pumps: Water seal extractors a curved, forward-bladed centrifugal impeller located either eccentrically within a circular casing or centrally within an elliptical casing. Water is thrown radially outwards by centrifugal action 8

9 during rotation & gas from a central inlet port; then, the water is forced inwards by the converging casing, separating from gas. Advantages: gas seal is maintained without any contact between moving and stationary components ==> little risk of igniting the gas or propagating a flame. Reliable and robust, & can be used continuously or in automatic operation. Dry extractors reciprocating machines or take the form of two lemniscate (dumb-bell) shaped cams that rotate against each other within an elliptical casing. Compact and,for any given speed, produce a flowrate that is near independent of pressure differential. Subject to wear, create a substantial temperature rise in the gas and may produce considerable noise. Planning a methane drainage system Utilization of drained gas Summary 1. High installation cost of an elaborate system w/ min. operating cost 2. Delays in mining cycle when drainage equipment installed 3. Regulation and control of methane drainage inexact, empirical, and unpredictable 4. Mine safety enhanced by degasifying 5. Ventilation costs reduced in proportion to the amount of methane removed 6. Methane produced can be used locally or marketed commercially 7. An unconventional energy resource recovered in addition to the coal 8. Few U.S. installations to date 9. Application limited to coal seams 9