EP0197566B1 - Method for the controlled withdrawal of the injection point of gasification agents in the seam during the underground gasification of coal - Google Patents

Abstract

New controlled retracting gasifying agent injection point process for UCG sites. The process consists of a retraction of the gasifying agent injection point achieved by gradually plugging the tubing ends either by pneumatic injection of inert granulated material with a thermosetting binder, or by closing valves set at regular intervals inside the tubings.

Description

The underground gasification of coal deposits made of thin layers located at great depth poses a number of problems.

For economic reasons it is necessary to develop large surface gasifiers and, in the current state of the art, this implies that the gasogens develop from very long boreholes, drilled in the thickness of the layer.

In order to resist ground pressure, these boreholes must be provided with solid coatings and these coatings must not simultaneously undergo the effect of high temperatures and the stresses resulting from the lithostatic pressure. This condition can be satisfied by the use of conventional metallic casings, if a retracting operating method is adopted in which, for the entire duration of their use, the probes are used to inject gasifying agents at low temperature.

The arrangements adopted must also ensure as intimate contact as. possible between gasifying agents and coal, this condition being essential for the production of good quality gas.

Patent GB-A-2004297 A describes a process for retreating operation, in which the intimate contact between the gasifying agents and the coal is obtained by the methodical backfilling of the zones already gasified, the backfill material being a granulated material transported pneumatically to through the boreholes used for the injection of gasifying agents.

Given the large vacuum volume which remains after the gasification of the coal, this process requires the injection of very large quantities of materials and the backfilling operation can prove to be particularly expensive.

Patent US-A-4 334 579 describes a process for retreating operation, in which intimate contact between the gasifying agents and the coal is obtained without the addition of fill, by periodically carrying out the controlled retraction of the point of injection of the agents gasifiers, in such a way that a sufficiently large quantity of virgin coal remains permanently present between the point of injection of the gasifying agents and the zones already gasified ...

In one of the variants envisaged, the injection of the gasifying agents is carried out by long boreholes drilled in the thickness of the layer, the injection point being moved gradually from the end of the borehole to its point d Originally using a retractable or heat degradable injection tube for this purpose.

The present invention relates to a new method of retraction of the point of injection of the gasifying agents, injected into long vein probes.

In the process according to the invention, the retraction of the injection point does not result from the destruction or the retraction of the tube used for the injection of the gasifying agents, but it is obtained by progressive obturation of the terminal part of this tube, by pneumatic injections of sand or other materials. inert granules, added with a thermosetting binder.

As a result of this closure, a large pressure difference is maintained, permanently, between the interior of the tube for injecting the gasifying agents and the zone where the gasification reactions are developing; it follows that the gasifying agents which escape through a series of orifices provided, at regular intervals, in the walls of the injection tube can filter, through the layer, over a distance of a few meters, taking advantage of the supplement of permeability which results from the creep of the coal in the areas located along a gallery or near a cavity.

The method according to the invention can be applied according to two variants.

In a first variant, the progressive obturation of the end portion of the tube for injecting the gasifying agents is obtained by injecting into the current gasifying agents of sand or other inert granular materials added with a thermosetting binder, introduced in the gasifier and pneumatically transported supply line.

In a second variant, the progressive closure of the end portion of the gasifier injection tube is obtained by closing valves, arranged at regular intervals inside the gasifier injection tube, this closure being controlled by devices which react to the rise in temperature resulting from the progression of the gasification front.

• Figure 1 une coupe dans le plan d'une couche de charbon à faible pendage, suivant la première variante d'application ;
• Figure 2 une coupe verticale, suivant le plan XY de la figure 1 ;
• Figure 3 une coupe à travers un élément du tubage utilisé pour l'application de la deuxième variante du procédé ;
• Figure 4 une coupe dans le plan de la couche qui illustre l'utilisation de la deuxième variante du procédé, et
• Figure 5 une coupe dans le plan de la couche, qui illustre l'utilisation de l'une ou l'autre variante du procédé pour l'exploitation d'un panneau de grande largeur.

The process according to the invention is illustrated, by way of example only, by the appended schematic drawings, in which are shown in:

• Figure 1 a section in the plane of a layer of low dip coal, according to the first application variant;
• Figure 2 a vertical section along the XY plane of Figure 1;
• Figure 3 a section through an element of the tubing used for the application of the second variant of the method;
• FIG. 4 a section in the layer plane which illustrates the use of the second variant of the method, and
• Figure 5 a section in the plane of the layer, which illustrates the use of one or the other variant of the method for the exploitation of a very wide panel.

In the diagrams of Figures 1 and 2, layer 1, located in a virgin deposit more than 800 meters deep, is intersected by a deviated borehole with large radius 2, which is extended by a rectilinear section from 200 meters to 300 meters in length, drilled into the thickness of the layer.

A vertical borehole 3 intersects the same layer, near the end of borehole 2.

From the surface to the roof of the layer, the two boreholes are coated with casings connected to the rocks by cementation.

In the parts located in layers, the two boreholes are coated with perforated casings (liners) which allow the passage of gases, while opposing the creep of the coal.

The casing of borehole 2 is closed at its end (point 5).

Hole 2 is intended for the injection of gasifying agents and Hole 3 for the recovery of the gas produced.

The operation begins by igniting the coal, at the bottom of the borehole 3, using one of the known methods for this purpose: self-ignition of the coal by injection of hot air, by injection of air enriched with oxygen or through the use of auto-flammable chemicals such as silane or triethylborane.

During a period of a few days, the combustion of the coal is maintained around the well 3, alternating periods of air injection, at a pressure higher than the minimum fracturing pressure of the deposit, and periods of decompression of the well, to the evacuation of combustion gases.

This first operation has the result of creating, around the well 3, a scree zone 4, of great permeability, which corresponds to the vacuum produced by the combustion of the coal and filled by the abundance of coal located on the periphery and by the rock fall from the layer roof.

The connection between boreholes 2 and 3 is carried out by backburning, by injecting through borehole 2 an oxygenating gasifying agent such as: air, oxygen mixture + water vapor or oxygen mixture + C0 ₂ .

During this operation, the pressures prevailing at the bottom of boreholes 2 and 3 are controlled by direct measurement or by a calculation taking into account the pressures measured at the surface, the flow rates in circulation and the pressure drops in the two wells.

As soon as the pressure difference between the bottom of the well 2 and the bottom of the well 3 falls below a certain set value (of the order of 5 bar to 10 bar), the point of retraction is carried out injection of the gasifying agents by sealing the terminal part of the borehole 2.

For this purpose, there is installed on the surface next to the well 2 a pressure silo 6, containing a reserve of granulated materials, and a cellular distributor 7, whose rotation makes it possible to inject these granulated materials into the flow of gasifying agents. .

The reduction in the difference between the pressures measured at the bottom of boreholes 2 and 3 causes the alveolar distributor to move and the injection of a first quantity of granulated materials. This first quantity injected reaches the bottom of the borehole a few tens of seconds later and a new movement of the alveolar distributor is triggered if, at this time, the difference between the two pressures has not yet reached its setpoint.

When this set value is reached, a certain length of the end part of the borehole 2 is blocked by the granulated materials and as a result the point of injection of the gasifying agents is moved from point 5 to point 8, there where the end of the closed part is located.

The circulation of the gasifying agents, between point 8 and the cavity 4, is done by filtration through the carbon, benefiting from the increase in permeability which results from the creep of the carbon, in the direction of the empty spaces. The gasification front progresses from the cavity 4, moving against the flow direction of the gasifying agents. This mode of operation has the effect of ensuring the production of a high quality gas, thanks to the large development of gas-solid contact surfaces and to the very uniform dispersion of the flow of gasifying agents.

The operation of the granulated material dispenser can be made fully automatic by the use of a microprocessor whose program provides, at any time, the value of the difference between the pressures prevailing at the bottom of wells 2 and 3.

The granulated materials injected consist, for the most part, of inert materials such as: sand, corundum or glass powder. To these products are added 20% to 30% of a thermosetting material chosen from the range of epoxy resins or from any other range of chemicals with equivalent characteristics. This addition can be carried out by mixing inert grains and granules of resinous material or by coating the inert grains in a thin layer of resin.

These products are intended to react, when the part of the borehole, in which the obturation was carried out, is reached by the thermal wave, which advances in front of the gasification front. Under the effect of heat, the injected granular materials will transform into a resinous concrete, adhering to the walls of the casing inside which they have been injected, so that the obturation of the terminal part of the borehole is not questioned by the progression of the gasification front and by the thermal destruction of the end of the casing.

In the second variant of application of the method, the casing of the borehole, in the part drilled in the thickness of the layer, is divided into elements a few meters in length.

Figure 3 shows a middle section through one of these elements.

At the upstream end of this element, the casing can be closed by the displacement of the valve head 9, which comes to rest on the valve seat 10.

The closing of the valve is caused by the pressure of the spring 11, which acts on the tail of valve 12.

In the absence of any rise in temperature, the valve is held, in the open position, by the rod 13, which slides inside the sheath 14 but whose displacement is prevented by the presence of the plug 15.

In the center of this plug is a fusible cylinder, made of an alloy of lead and tin, the melting point of which is around 200 ° C to 300 ° C. When the gasification front approaches the plug 15, the rise in temperature causes the fusible cylinder to melt. Consequently, the rod 13 can slide freely in the sheath 14 and this releases the valve, which closes under the action of the spring 11.

The sheath 14 is held in the axis of the casing by one or more centralizers, such as 16.

Each element of the casing has a perforated part 17 and one or more packers such as 18, made up of flexible strips of metal or plastic, which can move apart under the effect of pressure and seal between the outer wall of the casing and the coal which constitutes the inner wall of the borehole.

FIG. 4 illustrates the use of the second variant of the method.

As in the first variant, the operation begins by igniting the coal at the bottom of borehole 3 and by creating, around this borehole, a scree zone 4 of high permeability

The connection between probes 2 and 3 is carried out by back-combustion, by injecting - through probe 2, an oxygen-based gasifying agent.

At the start of this operation, the gasifying agent is injected through the perforated part of the last casing element.

The heat released by the back-combustion operation causes a temperature rise in the coal located in the vicinity of the reaction zone and this temperature rise propagates, in front of the reaction front, progressing from well 3 towards well 2.

When the temperature of the end of the gasifying agent injection tubing reaches 200 ° C to 300 ° C, the fusible cylinder located at the end of the last casing element melts, which causes the valve 19 to close. , located at the upstream end of this element. From this moment the gasifying agent is injected into the vein through the perforated part of the fore-casing element.

When the temperature, in the vicinity of the valve 19, reaches 200 ° C to 300 ° C, the fusible cylinder located at the end of the Savant-last casing element in turn melts, which causes the valve 20 to close.

Thanks to the repetition of this process, the point of injection of the gasifying agent is maintained, at all times, a few meters in front of the gasification front and this ensures the dispersion of the flow of gasifying agent by filtration through the coal.

FIG. 5 illustrates the use of the method according to the invention for the exploitation of a very wide panel.

For the preparation of this panel a number of parallel boreholes 21 are drilled in the thickness of the layer, at intervals of 20 meters to 30 meters, and a gallery 22, connected to the gas evacuation borehole 23, is traced in a direction perpendicular to the direction of the boreholes 21. These preparatory works are carried out from an underlying mining gallery by a method described in patent GB-A-2,086,930 or from the surface by the technique of deviated soundings .

The underground gas generator is fired all along gallery 22.

The gasifying agents are injected simultaneously by all the boreholes 21, each of them being provided with an injection device for granulated materials or with a series of valves making it possible to ensure controlled retraction of the injection point.

The gasification causes the gradual widening of the gallery 22 and the displacement of the gasification front, counter-current to the direction of flow of the gasifying agents.

The pressure wave of the rocks, which moves in front of the gasification front, causes the progressive crushing of the coal pillars which separate the holes 21, which leads to a progressive widening of the coal zones, through which s' performs the filtration of gasifying agents.

Curves 24 and 25 show two successive positions of the gasification front and the corresponding injection points.

• - Il simplifie la réalisation de l'opération de rétraction du point d'injection, qui peut être entièrement automatisée et qui n'entraîne plus d'interruption dans le processus de gazéification.
• - Il permet de réaliser les opérations de rétraction du point d'injection, par petits pas successifs, uniformément répartis dans le temps, ce qui évite les fluctuations dans la composition et dans les caractéristiques du gaz produit.
• - Il maintient une importante différence de pression entré les sondages d'injection et la zone de gazéification, ce qui permet d'assurer la dispersion des agents gazéifiants par filtration à travers la masse du charbon, avec pour conséquence un contact très intime entre gaz et solides, qui favorise la production d'un gaz de bonne qualité.

If we compare it to the methods previously used to carry out the controlled retraction of the point of injection of the gasifying agents, the method according to the invention has the following advantages:

• - It simplifies carrying out the injection point retraction operation, which can be fully automated and which no longer results in any interruption in the gasification process.
• - It allows retraction operations of the injection point to be carried out, in successive small steps, uniformly distributed over time, which avoids fluctuations in the composition and in the characteristics of the gas produced.
• - It maintains a significant pressure difference between the injection probes and the gasification zone, which makes it possible to ensure the dispersion of the gasifying agents by filtration through the mass of coal, with the consequence of very intimate contact between gas and solids, which promotes the production of good quality gas.

Claims (4)

1. Method for the controlled withdrawal from the point of injection of gasifying agents distributed through one or a plurality of bore holes (2) sunk into the thickness of the layer of coal (1) and covered with perforated liners, the gasifying agents being displaced from the point of injection by progressively occluding the end portion (5) of the liners, characterised in that such occlusion is obtained by progressive filling of the end of the liner with no flow towards the outer cavities, such filling being carried out by pneumatic injections of inert granulated materials such as sand, corundum of crushed glass, with the addition of 20 % to 30 % of a thermosetting binder, the rate of these injections being regulated so that there is at all times between the interior of the injection bore holes (2) and the gasification area a sufficiently high difference in pressures that the falsifying agents are able to filter through the coal over a distance of some meters.

2. Method for the controlled withdrawal from the point of injection of gasifying agents according to Claim 1, the injection of the granulated materials being regulated by a compartmented distributor (7), characterised in that movement of the said distributor (7) is controlled by an automatic device which maintains constant the offset in pressure between the end of the injection bore hole and the end of the bore hole for recovering the gas produced.

3. Method for the controlled withdrawal from the point of injection of gasifying agents according to Claim 1, in which the progressive occlusion of the end portion (5) of the liners is achieved by the closure of valves (9) disposed at regular intervals through the interior of the liners, characterised in that the closure of the valves is controlled automatically by devices which react to the rise in temperature resulting from the progression of the gasification front.

4. Method for the controlled withdrawal from the point of injection of gasifying agents according to Claim 3, characterised in that the device which controls the closure of each valve (9) comprises a metal rod (13) adapted to slide inside a sheath (14) a few meters in length, the end of this sheath (14) being occluded by a plug (15) the centre of which consists of an alloy of which the melting point is around 200 to 300 °C.

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