AU2014203426A1 - Method for Methane Recovery from Coal Seams - Google Patents

Abstract

Said method for methane recovery from coal seams consists in the creation of acoustic, electric, mechanical and hydrodynamic tensile-compressive stresses by excitation 5 using repetitive short-interval pulses produced by an explosion of a calibrated conductor of a vibration source arranged in the operating interval of a well the energy of which is applied to a coal seam. In this case, a slotted perforation is made in the well to be oriented in the directions of principal stresses in a coal seam, an additional slotted perforation is made in permeable coal seam enclosing rocks, and the direction of the additional slotted 10 perforation is oriented in the directions of principal stresses in the coal seam enclosing rocks that intensify the acoustic and hydrodynamic cavitation of gas bubbles emitted from coal, cracks, microcracks, pores, micropores, capillaries, microcapillaries of the coal seam, as well as cracks and microcracks created in the permeable coal seam enclosing rocks. The technical effect of the inventive method is to increase production of the coal-bed methane, 15 to reduce energy consumption, and to improve safety and environmental friendliness of the process. Method for Methane Recovery from Coal Seams CosMecTHOe witno, sbaeH4e CTO4H Ka KoneOaHWA Aeo r epoopau Uwenes PTen Joint use of vibration source and slot perforation Casing string Dispersive medium Slot leakage Plasma emitter Microfracturing Fig. 1

Description

P/00/011 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL TO BE COMPLETED BY APPLICANT Name of Applicant: "Georesonance" Ltd Address for Service: A.P.T. Patent and Trade Mark Attorneys PO Box 222, Mitcham, SA 5062 Invention Title: Method for Methane Recovery from Coal Seams The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 METHOD FOR METHANE RECOVERY FROM COAL SEAMS Disclosure 5 This invention relates to the methods for methane recovery from coal seams and permeable enclosing rocks by periodic excitation with the energy of plasma applied to a productive coal seam and to permeable enclosing rocks through a slotted perforation orientated with an allowance for the direction of vectors of principal stresses caused by an explosion of a calibrated metal conductor resulting in generation of directional short 10 interval broad-band high-pressure pulses of a plasma impulse generator arranged in the operating interval of a vertical well bore opened up by the slotted perforation for the initiation of tensile-compressive stresses in a coal seam, onset of acoustic and hydrodynamic cavitation which fosters formation of a vast pattern of anomalous microfracturing and that creates conditions for maximum desorption of methane from coal, 15 cracks, microcracks, micropores, capillaries and microcapillaries, as well as from the permeable enclosing rocks. All prior art methods for methane recovery consist in gas recovery from coal seams only and do not provide for the methane recovery from the permeable enclosing rocks, and that fails to provide for the maximum occupational safety of miners in the future. There are 20 known employed methods for: - seam washout around a well by making use of spontaneous coal and gas outbursts; - induction and sustention of auto-destruction with the formation of a reservoir zone through a hydrodynamic drag; - injection of water and air (as well as carbon dioxide) into a coal seam; 25 - methane gas recovery from monobore and multilateral wells; - formation of cavities around a well; - methane gas recovery through degassing holes; - hydraulic fracturing of coal seams. However said methods are expensive, labor-consuming, environmentally unsafe, 30 energy-intensive and inefficient as evidenced by a large number of both vertical and horizontal wells producing no methane. There are also known methods disclosed in a US Patent 2005/009831 Al and a US Patent 2006/0108111 Al suggesting that a coal seam should be stimulated physically and acoustically from the daylight surface and by sound-emitting devices arranged in a vertical 35 well. 2 However, stimulation from the daylight surface (US 2005/009831 Al) is energy intensive, and energy of broad-band vibrations subsides with the seam depth. Moreover, such stimulation is environmentally unsafe and can have unpredictable effects in the vicinity of fracture zones. 5 Sound-emitting devices (US Patent 2005/009831 Al and US Patent 2006/0108111 Al) arranged in a vertical well with the view of increasing the permeability emit one frequency, whereas a methane-bearing coal bed is a multifactorial nonlinear dynamic system with ongoing self-sustained erratic multi-frequency oscillations, it does not seem possible to separate the dominant frequency and thus to solve a problem of the 10 permeability increase at a considerable distance from the exitation source. There is a known method for a slot perforation of wells disclosed in a Patent RU2254451, IPC E 21/B 43/112, as well as in a Patent RU2369728. However, the slot leakage occurs only within the well bore zone and does not spread to the entire coal seam. There is a known method for a plasma impulse excitation of hydrocarbon producing 15 formations disclosed in Patents RU2248591; RU2373386; RU2373387, as well as in a US Patent 2014/0027110A 1. However, all said methods provide for stimulation of hydrocarbon producing formations through cumulative perforation or in an open well bore. The cumulative perforation impairs the effectiveness of an initiated plasma pulse and, when in an open well bore, can cause caving of the bottom-hole zone and sticking of 20 plasma pulse equipment due to coal plasticity and brittleness. Furthermore, all said methods do not provide for the methane recovery from the permeable enclosing rocks. The technical effect of the inventive method is to increase production of the coal-bed methane, to reduce energy consumption, and to improve safety and environmental friendliness of the process. 25 The technical effect is achieved by that said method for methane recovery from coal seams consists in the creation of acoustic, electric, mechanical and hydrodynamic tensile compressive stresses by excitation using repetitive short-interval pulses produced by an explosion of a calibrated conductor of a vibration source arranged in the operating interval of a well the energy of which is applied to a coal seam. In this case, a slotted perforation is 30 made in the well to be oriented in the directions of principal stresses in a coal seam, an additional slotted perforation is made in permeable coal seam enclosing rocks, and the direction of the additional slotted perforation is oriented in the directions of principal stresses in the coal seam enclosing rocks that intensify the acoustic and hydrodynamic cavitation of gas bubbles emitted from coal, cracks, microcracks, pores, micropores, 3 capillaries, microcapillaries of the coal seam, as well as cracks and microcracks created in the permeable coal seam enclosing rocks, which fosters development of a pattern of anomalous microfracturing in the coal seam and more cracks and microcracks in the permeable coal seam enclosing rocks and maximum methane desorption and diffusion. 5 The claimed technical solution will now be illustrated by Figs 1-5. Fig. 1 is a schematic view of the result of periodic action of plasma energy on a coal bed. Fig. 2 is a portion of a seam (sample) before and after excitation by a method as claimed in the invention. 10 Fig. 3 is a sectional image of a seam (sample) after excitation by a method as claimed in the invention. Fig. 4 is well performance parameters before and after excitation by a method as claimed in the invention. Fig. 5 is an effect of repetitive pulses on the stress condition in a coal seam. 15 A combination of the slotted perforation in the operating interval of a well (see Fig. 1) across a productive coal seam of any metamorphism and, simultaneously, across more permeable enclosing rocks enables the blast wave produced by the plasma initiation to penetrate radially into the seam and enclosing rocks without hindrance and, subject to periodic pulse repetition, to create repeatedly tensile-compressive stresses, and that allows 20 (due to a synergetic effect: microfracturing, cavitation, heat-and-mass transfer, relief of surface tension in capillaries, emergence of a concentration-diffusion force accumulated by an external energy) methane recovery to the greatest possible extent without resort to other additional production enhancement operations. Said method provides a direct access to a coal seam and permeable enclosing rocks 25 through a slotted perforation, takes into account physical, mechanical and geotechnical peculiarities of coal seams and permeable enclosing rocks, and, as a result of a directional periodic broad-band impulse excitation according to a developed program and a mathematical model, produces an effect of natural modulation of coal seams accompanied by active methane desorption and diffusion. 30 The program of the periodic broad-band plasma impulse excitation applied to a coal seam through a slotted perforation to maximise the methane smoothly uses the following natural specifics: - an enclosing rock-bound coal bed under an overburden load is essentially a porous system, often less consolidated than the rock mass; 4 - vertical encroachment of a fluid (water) penetrating through a coal bed is controlled by the capillary and gravity forces; - coal seams with a lower permeability feature a higher capillary pressure; alternatively, coal seams and rocks with a greater permeability feature a lower capillary 5 pressure; - the capillary pressure rises as the coal seam water saturation reduces and facilitates the process gas desorption and diffusion; - the mechanical strength of coal is much lower than that of other rocks, and it is unable to withstand a steeper gradient of excitation without rupture. There is a paradox 10 known as the Bridgman effect which is that bond breaking in coal occurs when the stress is relieved and not applied. Under such conditions, coal breaks into wafer-like laminae; - while stressed and having a higher than usual sound conductivity, a coal seam exhibits properties of a nonequilibrium, dissipative transmission medium in which the natural frequency chaos is maintained by an external energy (tides, distant earthquakes, 15 blasting operations at distant areas under development); - according to the electrical properties, most of the coals pertain to semiconductors and conductors. The plasma impulse excitation of a coal seam or permeable enclosing rocks produce mechanical and concentration diffusion forces associated with the charged fluid movement in a porous fluid-saturated medium. External forces emerge that are of 20 electrokinetic origin and that produce an electric field with each pulse; such field evolves into the energy of a different field; and as the impulse excitation ceases, the accumulated external energy recovers, with some losses, its original form. The gas saturation of methane-bearing seams is made up of four components: -- non-associated gas filling up pores and cracks - 5-6 %; 25 -- gas adsorbed on the walls of micropores, capillaries and cracks (physisorption and volume filling) - 28 to 35 %; -- gas contained in the coal mass in a dissolved form - 40 to 50 %; -- gas partially dissolved in water films, in which case, according to the Henry's law, gas solubility in aqueous solutions increases in direct proportion to the pressure with the 30 depth - 3 to 8 %. The basic mass of methane molecules in gas-bearing beds is distributed throughout the coal mass, so a notion of an interstitial solid solution is applicable to the methane-coal system. The methane molecules that intrude into the volume take up not the voids in the 5 crystalline lattice but vacancies in a solid body in accordance with the coal seam sorption curve. There is the only method of gas liberation which is a diffusion mechanism. To initiate it, it is necessary to subject coal during stress release to the dispersion of particles 5 of approximately 10-6 cm in size. The methane concentration in coal will drop manyfold, and it will be released as free gas. The only mechanism that can lead to coal dispergation and to the development of a pattern of anomalous microfracturing is the explosion of gas bubbles impregnated into the coal seam structure which will start actively evolving during periodic directional broad 10 band plasma impulse excitation that has direct access to the coal seam through the slotted perforation thus creating the acoustic and hydrodynamic cavitation. Water penetrating a coal seam with the dissolved gas is weak which is attributed to the existance of cavitation seeds therein. They are poorly wetted coal surfaces and coal particles with gas-filled cracks and microcracks. 15 When plasma is created in the vicinity of the operating slot interval, a sound is radiated into the fluid with an acoustic pressure of over 100 dB resulting in the formation of cavitation bubbles during half-periods of discharges on the cavitation seeds of gaseous inclusions contained in the fluid and on the oscillating surfaces of the sound-emitting device. The bubbles collapse during the compression half-cycles producing a short-time (in 20 a microsecond) pressure of up to 10,000 kg/sq. cm which is capable of destroying a stronger than coal material. Bench tests of the direct periodic broad-band plasma impulse excitation of coal samples placed within the blast zone confirmed the dispergating effect as well as coal splitting into wafer-like laminae (see Fig. 2). 25 The tomographic examination of samples subjected to the periodic broad-band plasma impulse excitation through the slotted perforation showed the development of microfracturing in a sample, and most of the microcracks ran orthogonally to the direction of bedding (Fig. 3). Employment of the plasma impulse excitation technology in well UM-5.9 with a 30 slotted perforation at the Taldinsk field in Kuzbas confirmed a post-excitation increase in the permeability of six methane-bearing beds (see Fig. 4). Employment of the plasma impulse excitation technology in the Pingdingshan district, China, in beds with the permeability of 0.014 mD confirmed an increase in the bed permeability by methane entry into the well and propagation of tensile-compressive 6 stresses to a distance of more than 200 metres, which accompanied by active methane release (Fig. 5). An economic effect achieved by working of this invention comes down to the maximum volume of gas produced both from coal seams and from more permeable 5 enclosing rocks with minimum energy, high safeness and environmental friendliness of the process. The effectiveness is achieved by that the inventive method involves: - vertical drilling at a pre-explored methane-bearing bed (or an old developed/undeveloped well is used), 10 - determination of bed thickness along the well-bore, - determination of the coal grade composition, reservoir pressure, temperature, hydrology, porosity and permeability of coal seams and enclosing rocks; - gassing test of coal seams, - bringing of a source of repetitive directional broad-band short-interval high 15 pressure pulses to a methane-bearing bed, including directly to a coal seam and permeable enclosing rocks through a slotted perforation in the operating interval of a vertical well, - excitation of the bed and permeable enclosing rocks with the energy of plasma produced by the explosion of a calibrated metal conductor in the form of repetitive directional compressing and tensile short-interval high-pressure pulses; in this case, the 20 number of high-pressure pulses and the duration of excitation in each interval of the methane-bearing bed is determined by the bed thickness along the well-bore, petrophysical and grade composition of coals, as well as by geotechnical characteristics of the permeable enclosing rocks. 25 The methane production by a method as claimed in the invention is carried out at a methane-bearing bed being under an overburden load through vertical wells drilled from the daylight surface and cased with casings of different diameters with a slotted perforation in the area of the operating interval which unloads both the coal seam and the permeable enclosing rocks. 30 Fig. 1 is a schematic view of the result of periodic action of plasma energy on a coal bed. In this case, a turnkey (pre-bored) well is taken, the seam thickness along the well bore is determined, the grade composition of coal and permeable enclosing rock characteristic are found, whereupon a source of repetitive directional broad-band short interval high-pressure pulses is brought to a methane-bearing bed through a slotted 7 perforation in the operating interval of a vertical well and the bed excitation begins in the form of repetitive directional short-interval high-pressure pulses, in which case the number of high-pressure pulses and the duration of excitation in each interval of the methane bearing bed is determined by the bed thickness along the well-bore, grade composition of 5 coals and by the characteristic of the enclosing rocks. The source of repetitive directional broad-band short-interval high-pressure pulses excites with the energy of plasma produced by the explosion of a calibrated metal conductor. In and of itself, the source of repetitive directional broad-band short-interval high-pressure pulses is essentially a plasma impulse generator. Normally, such source operates as follows: a high voltage current of 3000 10 5000V from a bank of energy-storage capacitors is applied to electrodes, which are closed by a calibrated conductor resulting in its explosion and confined plasma creation. During the explosion, the energy releases and turns into a state of a strongly heated gas with a very high pressure, which, in its turn, builds up a shock wave affecting the environment with a great force and causing its compression, which lasts until the shock pressure equals the 15 seam pressure, whereupon the bed starts expanding towards the well with the source of excitation. Multiple repetition of recurrent short-interval broad-band pulses in a medium with good electrical and sound conductivity that cause compressive and tensile stresses leads to the development of a pattern of anomalous microfracturing in the seam, cavitation, heat and mass exchange, and natural modulation of the bed, which fosters maximum 20 methane desorption. If there are more permeable enclosing rocks, the plasma impulse excitation is carried out in such rocks as well, since methane diffuses into more permeable rocks in which case its volume may exceed the volume of methane in the coal seam. The permeable enclosing rocks behave like an oil and gas producing reservoir which is free of coal dust, and hence, 25 the gas recovery will be as high as possible. 8

Claims (1)

1. A method for methane recovery from coal seams consisting in the creation of acoustic, electric, mechanical and hydrodynamic tensile-compressive stresses by 5 excitation using repetitive short-interval pulses produced by an explosion of a calibrated conductor of a vibration source arranged in the operating interval of a well the energy of which is applied to a coal seam, wherein a slotted perforation is made in the well to be oriented in the directions of principal stresses in a coal seam, an additional slotted perforation is made in permeable coal seam enclosing rocks, and the direction of the 10 additional slotted perforation is oriented in the directions of principal stresses in the coal seam enclosing rocks that intensify the acoustic and hydrodynamic cavitation of gas bubbles emitted from coal, cracks, microcracks, pores, micropores, capillaries, microcapillaries of the coal seam, as well as cracks and microcracks created in the permeable coal seam enclosing rocks, which fosters development of a pattern of 15 anomalous microfracturing in the coal seam and more cracks and microcracks in the permeable coal seam enclosing rocks and maximum methane desorption and diffusion. 9