METHOD FOR EXTRACTING METHANE FROM COAL BEDS AND FROM PENETRATING ROCK ENCLOSING A COAL BED

Abstract

 A method for extracting methane from coal beds includes creating acoustic, electrical, mechanical and hydrodynamic compressive/tensile stresses by applying periodic short pulses generated via the explosion of a calibrated conductor located in the operating range of a well of a source of oscillations, the energy of which is fed to the coal bed. A slotted perforation is created in the well, said perforation being oriented along the directions of the main stresses in the coal bed, an additional slotted perforation is created in penetrating rock which encompasses the coal bed, and the additional slotted perforation is directed along the directions of the main stresses of the rock encompassing the coal bed, enhancing the acoustic and hydrodynamic cavitation of gas bubbles emitted from the coal, cracks, microcraks, pores, micropores, capillaries and microcapillaries of the coal bed, and also from the cracks and microcracks cre ated in the penetrating rock encompassing the coal bed. The technical result of the proposed method consists in increasing coal methane production, in reducing energy consumption, and in increasing the safety and environmnental-friendliness of the process.

Description

Background

[0001] The invention pertains to methods of extracting methane from coal beds and permeable enclosing rock by the periodic action of plasma energy brought up to the producing coal bed and to the permeable enclosing rocks through a slit perforation, oriented in regard to the direction of the vectors of the principal stresses, produced by the explosion of a calibrated metallic conductor, resulting in the creation of directional short broadband pulses of high pressure of a pulsed plasma generator situated in the working interval of the vertical well shaft which is opened by the slit perforation for initiation of compressive and rarefactive stresses in the coal bed, and the occurrence of acoustic and hydrodynamic cavitation encouraging the formation of an extensive network of anomalous microfractures, which creates conditions for maximum desorption of methane from the coal, cracks, microcracks, micropores, capillaries and microcapillaries, and also from the permeable enclosing rocks (Fig. 1).

[0002] All of the known methods of extracting methane involve the extraction of gas solely from coal beds and do not consider extracting methane from the permeable enclosing rocks, which does not fully ensure the future working safety of the mine operators. Among the known methods used are:

• washing out of the bed/well around the borehole with the aid of spontaneous emissions of coal and gas;
• provocation and maintaining of self-destruction with formation of a collector zone by means of hydrodynamic action;
• injecting water and air, as well as carbon dioxide gas, into the coal bed;
• extraction of methane gas from single-shaft and multiple-shaft horizontal wells;
• formation of cavities around a well;
• extracting of methane gas through degasification wells;
• hydraulic fracturing of coal beds.

[0003] However, these methods are costly, labor-intensive, ecologically unsafe, energy-intensive and inefficient, as shown by the large number of both vertical and horizontal wells with no inflow of coalbed methane.

[0004] There are also known methods described in patent documents US 2005/009831 A1 and US 2006/0108111 A1. The above-mentioned methods propose physical and acoustic action on the coal bed from a day surface and by acoustic emitters placed in the vertical well.

[0005] However action from the day surface (US 2005/009831 A1) is energy-intensive and the energy of generated broadband oscillations is decreasing with the greater depth of beds. Besides this action is ecologically unsafe can result to unpredictable damages near the fracture.

[0006] Acoustic emitters (US 2005/009831 A1 and US 2006/0108111 A1) placed in the vertical well in order to increase permeability emit a single frequency while a methane coal bed is multifactorial nonlinear dynamic system wherein there are multi-frequency continuously sustained disordered oscillations. It is impossible to select a dynamic frequency and therefor to solve the problem of permeability increase for a great distance from the excitation source.

[0007] The method of hydromechanics wells perforation is known from the patent RU #2254451 and also from RU #2369728. However a slit discharge appears only in the near the borehole area and doesn't spread to the whole coal bed.

[0008] The method of plasma-pulsed action on producing beds is disclosed in patents RU #2248591, RU #2373386, RU #2373387 and in US patent application #61/684,988. However all mentioned methods don't provide action on producing beds of hydrocarbons via cumulative perforation or in open well borehole. A cumulative perforation reduces efficiency of generated plasma pulse and in the open borehole can result in to self-destruction of bottom-hole borehole zone and tacking of plasma-pulsed facility because of plasticity and brittleness of coal. Besides all mentioned methods don't provide extracting methane from permeable enclosing rock.

Detailed Description

[0009] The combination of slit perforation of the working interval of a well along the producing coal bed of any given metamorphism and at the same time along the more permeable enclosing rock allows the shock wave produced after the formation of plasma to penetrate radially without obstruction into the bed, as well as the enclosing rock, and also under periodic repetition of the pulses to repeatedly create compressive and rarefactive stresses, which enables maximum extraction of methane thanks to a synergistic effect (microfracturing, cavitation, heat and mass exchange, elimination of surface tension in capillaries, appearance of a concentration-diffusion force and accumulated outside energy), without resorting to other supplemental geological and technical measures.

[0010] This method has direct access to the coal bed and the permeable enclosing rocks through the slit perforation, and it allows for the physical, mechanical and geological technical peculiarities of the coal beds, as well as the permeable enclosing rocks, and as a result of the directional periodic broadband pulsed action according to a developed program and a mathematical model it creates an effect of self-modulation of the coal beds, accompanied by active desorption and diffusion of methane.

[0011] The following specific natural features are utilized by the program of broadband periodic pulsed plasma action applied to the coal bed through a slit perforation for the maximum extraction of methane:

• the coal deposit not relieved of the load of the rock pressure and compressed by the enclosing rocks constitutes a porous system, often less dense than the rock strata;
• the fluid (water) penetrating the coal deposit and its distribution along the vertical is controlled by capillary and gravitational forces;
• coal beds with less permeability are distinguished by greater capillary pressure, and vice versa, coal beds and rocks with greater permeability have lower capillary pressure;
• the capillary pressure increases with decreasing water saturation of the coal bed and promotes the process of desorption and diffusion of the gas;
• the mechanical strength of coal is much lower than that of other rocks, and it is not able to withstand a high action gradient without being crushed. The paradox known as the P. W. Bridgeman effect has been established, namely, the breaking of bonds in the coal occurs upon releasing of the stress, and not upon its application. In these circumstances, the coal is broken up into wafer-like sheets;
• the coal bed, being in a stressed state and having an elevated sound conductivity, has the properties of a nonequilibrium dissipative transmission medium, in which a natural frequency chaos is sustained by replenishment of outside energy (the tides, distant earthquakes, explosion work at remote sites being developed);
• with regard to electrical properties, the majority of coals are semiconductors and conductors. Upon pulsed plasma action on the coal bed or enclosing permeable rock, mechanical and concentration-diffusion forces are produced, related to the displacement of the charged liquid in the porous fluid-saturated medium. Outside forces of electrokinetic origin appear, which create an electric field during each pulse. This passes into the energy of another field, and when the pulsed action ceases the accumulated outside energy returns, with certain losses, to its original form.

[0012] The gas saturated state of methane coal beds is made up of four components:

• free gas filling the pores and cracks 5-6 %;
• gas adsorbed onto the walls of micropores, capillaries and cracks (physical sorption and volume filling) 28 - 35 %;
• gas located in the coal volume in dissolved form 40 - 50 %;
• gas partly dissolved in films of water, while according to Henry's law the gas solubility in aqueous solutions increases in direct proportion to the pressure with depth, 3 - 8%.

[0013] In gas-bearing beds, the main mass of the methane molecules is distributed in the coal volume and the concept of an interstitial solid solution is applicable to the system of methane and coal. The methane molecules interpenetrating the volume do not occupy voids in the crystal lattice, but rather vacancies in the solid in accordance with the sorption curve for coal beds.

[0014] There is only a single method for gas removal - the diffusion mechanism. In order to carry this out, the coal upon relieving the load must be subjected to dispersion with formation of particles approximately 10"6 cm in size. The methane concentration in the coal will decrease several-fold, and it will pass into the free state.

[0015] The only mechanism capable of bringing about a dispersion of the coal and the development of an anomalous network of microfracturing is the bursting of gas bubbles interspersed in the structure of the coal bed, which begin to be actively released under periodic directional broadband pulsed plasma action having direct access to the coal bed through a slit perforation, creating acoustic and hydrodynamic cavitation.

[0016] The water penetrating into the coal bed with dissolved gas has low strength, due to the presence in it of cavitation nuclei: poorly wettable coal surfaces, coal particles with cracks and microcracks, which are filled with gas.

[0017] Upon formation of a plasma in the region of the working slit interval, sound is emitted into the liquid with sonic pressure of more than 100 db, which results in the formation of cavitation bubbles during the half-periods of rarefaction on the cavitation nuclei of the gas inclusions contained in the liquid and on the oscillating surfaces of the acoustic emitter. The bubbles collapse during the half-periods of compression, creating briefly for the time of one microsecond a pressure of as much as 10,000 kg/cm2, which is able to break up stronger materials than coal.

[0018] During testbed-based testing of a broadband pulsed plasma direct periodic action on coal specimens placed in the zone of the shock wave, the dispersing effect as well as the destratification of the coal into wafer-like sheets was confirmed (Fig. 2).

[0019] Tomographic X-raying of specimens undergoing the pulsed plasma periodic broadband action through a slit perforation revealed the development of microfracturing in the specimen, the majority of the microcracks being situated orthogonally to the direction of stratification (Fig. 3).

[0020] The use of the pulsed plasma technology at well UM-5.9, having a slit perforation, at the Tallinn field in the Kuzbas has confirmed the increased permeability after action on 6 methane coal beds (Fig. 4).

[0021] The use of the pulsed plasma technology in China, in the Pin Din Shan district in beds having a permeability of 0.014 mJ has confirmed the increased permeability of the bed by the passage of methane into the well and by the propagation of the compressive and rarefactive stresses to a distance of more than 200 meters, accompanied by active excretion of methane (Fig. 5).

[0022] The economic effectiveness achievable by realization of this invention comes down to a maximum volume of extracted gas both from coal beds and from more permeable enclosing rocks, with minimum energy expenses, good safety, and an ecological process.

[0023] The technical result is accomplished by:

• drilling a vertical well at a previously inspected methane coal bed (or using an old developed or undeveloped well),
• determining the thickness of the bed in the well profile,
• determining the grade composition of the coal, the stratal pressure, the temperature, the hydrology, the porosity and permeability of the coal beds and enclosing rocks;
• determining the gas saturation of the coal beds,
• bringing up a source of periodic directional broadband short pulses of high pressure to the methane coal deposit, including directly the coal bed and permeable enclosing rocks, through a slit perforation of the working interval of the vertical well,
• acting on the bed and the permeable enclosing rocks with the energy of a plasma formed by the explosion of a calibrated metallic conductor, in the form of periodic directional compressive and rarefactive short pulses of high pressure, the number of the high pressure pulses and the length of action in each interval of the methane coal deposit being determined by the thickness of the bed in the well profile, the petrophysical and grade composition of the coals, and also by the geological technical characterization of the enclosing permeable rocks.

[0024] The extraction of methane by the proposed method is done on a methane coal deposit not relieved of the load of the rock pressure by means of vertical wells drilled from the top surface, encased with production casings of different diameter and having a slit perforation in the region of the working interval, relieving the load on both the coal bed and the permeable encasing rocks.

[0025] Figure 1 shows a diagram of the result of the periodic action of plasma energy on the coal deposit. In the present case, a ready-made well is used (previously drilled), the thickness of the stratum is determined in the well profile, the grade composition of the coal is determined and the permeable enclosing rocks are characterized, after which there is brought up to the methane coal deposit through a slit perforation of the working interval of the vertical well a source of periodic directional short broadband pulses of high pressure and the action on the bed commences in the form of periodic directional short pulses of high pressure, the number of high pressure pulses and the length of action in each interval of the methane coal deposit being determined by the thickness of the bed in the well profile, the grade composition of the coals and the characterization of the enclosing rocks. The source of periodic directional broadband short pulses of high pressure acts by the energy of the plasma formed by the explosion of a calibrated metallic conductor. By its nature, the source of the periodic directional short pulses of high pressure represents a generator of pulsed plasma action. Usually such a source works as follows. High-voltage current (3000-5000 V) from a bank of storage capacitors is applied to electrodes, which make a circuit via the calibrated conductor, resulting in its explosion and the formation of a plasma in the enclosed space. During the explosion, energy is released, passing into the state of a highly heated gas with very high pressure, which in turn forms a shock wave, acting with great force on the surroundings, causing them to be compressed, which continues until the pressure in the shock wave is equalized with the stratal pressure, after which the process of rarefaction of the stratum occurs in the direction of the well with the source of excitation. The multiple repeating of the periodic broadband short pulses in a medium having good electrical conductance and sound conductance, bringing about compressive and rarefactive stresses, results in the development of a network of anomalous microfracturing in the bed, cavitation, exchange of heat and mass, and self-modulation of the bed, which promotes maximum desorption of the methane.

[0026] In the event that more permeable enclosing rocks are present, the pulsed plasma action is also carried out in these rocks, since the methane diffuses into the more permeable rocks and its volume may exceed the volume of methane in the coal bed. The permeable enclosing rocks behave like an oil and gas producing collector, not having any coal dust, and therefore the gas output will be maximum.

Claims

1. Method of extracting methane from coal beds, including the creation of acoustic, electrical, mechanical and hydrodynamic compressive and rarefactive stresses by the action of periodic short pulses, produced by an explosion of a calibrated conductor of a source of oscillations placed in the working interval of a well, whose energy is supplied to the coal bed, characterized in that, in order to increase the extraction of coalbed methane, a slit perforation is created in the well, oriented along the directions of the principal stresses in the coal bed, an additional slit perforation is created in the permeable rocks enclosing the coal bed, the direction of the additional slit perforation being oriented along the directions of the principal stresses of the rocks enclosing the coal bed, said rocks intensifying the acoustic and hydrodynamic cavitation of the gas bubbles being released from the coal, the cracks, the microcracks, the pores, the micropores, the capillaries, and the microcapillaries of the coal bed, as well as the cracks and microcracks created in the permeable rocks enclosing the coal bed, which promotes the development of a network of anomalous microfracturing in the coal bed and additional cracks and microcracks in the permeable rocks enclosing the coal bed and maximum desorption and diffusion of the methane.

Drawing