Remote sealing of mine passages

Abstract

Mine passages are sealed remotely from the surface by drilling a borehole from the surface to intersect a mine passage. Fly ash is then pneumatically injected down the borehole in dilute phase at high velocity to form a partial seal obstructing more than 90% but less than 99% of the area of the mine passage. The emplaced fly ash displays the form of a conical pile with the flanking slopes curving concavely upward to the lips of a crater formed beneath the injection borehole. Sealing is then completed by injection of an expanding foam into the crater area.

Description

BACKGROUND OF THE INVENTION

Dry fly ash has been pneumatically injected through boreholes drilled from the surface into mine workings to isolate fires in abandoned coal mine workings and to stabilize the land surface by preventing subsidence. A typical delivery system utilizes a conventional bulk pneumatic truck to pump the fly ash down the borehole through a short length of surface casing set in the borehole to provide a seal and a connection to the delivery hose of the pneumatic track. Such a technique is disclosed in the Magnuson patent, U.S. Pat. No. 3,500,934. Fly ash injection is usually continued to refusal; that point at which no more fly ash can be injected down the borehole at the maximum pressure developed by the truck.

It has been proposed, and attempts have been made, to utilize this same technique to isolate a fire breaking out in an operating coal mine. The conventional way to combat a well-developed mine fire in underground workings is to seal all surface openings and maintain the seal until the fire dies from lack of oxygen and the fire area has cooled sufficiently as to preclude re-ignition upon exposure to air. Depending upon the intensity and extent of the fire, sealing must be maintained for a minimum period of 6 months to several years before the mine can be re-opened. During the time the mine is sealed, methane builds up in the mine atmosphere, often to explosive levels, and the physical condition of the mine deteriorates with roof falls being common. Both the buildup of methane in the mine atmosphere and the physical deterioration of the mine add to the hazards on re-opening the mine.

While good seals have been made in abandoned mine workings by the conventional pneumatic injection of dry fly ash down boreholes, the technique encounters a number of disadvantages when applied to fires within active mines. First, large quantities of fly ash are required. Fly ash is readily available from coal-fired power plants but the quantity of fly ash available over a short time span is limited by the rate of fly ash production and the storage capacity of the power plants. Second, premature plugging without sealing of the mine passage often occurs when there is a large amount of rubble in the passage beneath the injection borehole. Third, the technique is unsatisfactory in deep workings. When fly ash is pneumatically injected in conventional fashion down a deep borehole, friction tends to reduce the velocity at the bottom of the borehole sufficiently that plugging of the borehole occurs before sealing is accomplished.

SUMMARY OF THE INVENTION

I have discovered that passages within an operating mine may be remotely sealed from the surface much more rapidly, using substantially smaller quantities of fly ash, and at greater depths than is possible using conventional techniques of pneumatic fly ash injection. My process is especially useful in isolating a fire area within a coal mine so that the fire may be brought under control and the mine re-opened much more quickly than is possible by use of traditional methods.

In my process, boreholes are drilled from the surface to intersect mine passages at the periphery of the fire zone. Fly ash is delivered to the borehole by pneumatic tank trucks and the fly ash is pneumatically injected down the borehole in dilute phase at high velocity. Fly ash injection is continued until there is formed a conical pile of fly ash which seals or blocks more than 90% but less than 99% of the area of the passage. The conical fly ash pile exhibits flanking slopes curving concavely upward with a well developed crater below the injection borehole. Fly ash injection is then halted and the seal is completed by injecting an expanding foam, such as urethane foam, into the crater area. Quantity of fly ash used and time required to construct seals in this manner are reduced to one half or even less of that required in the conventional technique of continuing fly ash injection to refusal. In addition, my technique increases the probabilities of obtaining a perfect seal especially in those passages having an uneven roof configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view showing a preferred arrangement for injection of dry fly ash into a mine passage in dilute phase at high velocity.

FIG. 2 illustrates a system for introducing an expanding foam into the crater area of the injected fly ash so as to complete the seal of the mine passage.

FIG. 3 present graphically the differences in fly ash usage and time required to achieve a 95% seal as compared to a 100% seal as a function of passage height and width.

DETAILED DESCRIPTION OF THE INVENTION

When fly ash is pneumatically injected through boreholes into mine workings, it is generally transported to the mine site in a pneumatic truck and is discharged in the fluidized state using the air supply integral with the bulk hauler. A typical discharge rate from these trucks is approximately 1 ton of fly ash per minute at an air flow rate of 300 to 350 cfm resulting in a solids loading of about 50 pounds of fly ash per pound of air. The system works well when injection is into open passages under relatively shallow overburden. At these air-fly ash ratios, there occurs what may be considered a dense phase flow of fluidized fly ash which mixture is sensitive to choking and plugging in long lines. This is especially true when injection is attempted through boreholes into relatively deep mine workings; on the order of 500 to 1500 feet in depth. Friction in this length of borehole is sufficient to so slow the flow of the suspended fly ash that it is deposited in a cone beneath the borehole and plugs the borehole before the mine passage is completely filled.

At low solids loadings with high gas flow rates, the fly ash is distributed in a different manner. The gas jet, issuing into the mine passage from the borehole, produces a large crater under the hole and pushes the fly ash to the sides of the passage so that the corners are sealed first. Thus the seal progresses from the outer edges inwardly and finally produces a seal having a broad area from the lip of the crater to the flanking slope of the deposited fly ash. Rubble piles beneath the borehole, often caused by the bit breaking through the roof of the mine passage when the borehole is drilled, strongly interfere with the formation of a fly ash seal when fly ash is pneumatically injected in a relatively dense phase. This often leads to plugging of the borehole before a passage seal is achieved. With a high gas flow and dilute phase injection, the crater tends to spread beyond the extent of the rubble so that interference on the sealing operation by the rubble is small.

In order to obtain the dilute phase injection of fly ash and high velocities necessary for effective sealing of mine passages, the gas flow rate is selected according to the following general formula:

where G represents gas flow rate in standard cubic feet per minute and W represents width of the mine passage in feet. This is an approximate formula which has proven to be satisfactory over a wide range of passage widths. However, the calculated gas flow rate obtained by use of this formula should be increased by 10 to 20% for a passage having an unusually low height to width ratio, less than about 1:3 and for a four-way intersection.

Bulk pneumatic haulers are usually equipped with an integral air supply consisting of a blower or compressor having sufficient capacity to pressurize, fluidize and pneumatically transport the material carried by the truck. The capacity of such integral blowers is far less than that required to pneumatically inject fly ash by my method but the capacity of the truck blower is included in the gas flow rate needed as derived from the formula previously set out. Referring now to FIG. 1, there is shown a partial sectional representation of the fly ash injection stage of my process. A borehole 1 is drilled from the surface through overburden 2 to intersect a mine passage 3. Borehole 1 must be cased if it passes through aquifers in the overburden since any significant waterflow into and down the borehole tends to cause plugging when dry fly ash is injected.

Fly ash is delivered to the site in conventional bulk pneumatic trucks (not shown) which are connected to entry lines 4 of ejector assembly 5. While a single entry line may be used, it is preferred to provide dual entry lines arranged in a Y-type connection as shown in the drawing to allow alternate or simultaneous fly ash injection from two trucks. The entry lines are equipped with standard couplings allowing connection to the delivery hose carried by the pneumatic trucks. Ejector assembly 5 may be an eductor arrangement into which is introduced a supplementary gas stream 6 supplied by blower or compressor 7. Blower 7 must have sufficient capacity to deliver gas at the flow rate required to satisfy the formula set out previously. In practical terms, this requires a blower capable of supplying about 3000 to 6000 SCF per minute at pressures ranging up to about 10 psi. Intake 8 of blower 7 is connected to a source of inert gas when injection is made in proximity to the fire zone within a mine. Inert gas used may comprise nitrogen or gas produced by a standard inert gas generator. Small amounts of oxygen, below that concentration which will support combustion, are allowable in the injected gas stream.

Gas supplied by blower 7 mixes with the relatively dense phase fly ash stream delivered by the pneumatic truck to form a dilute phase suspension of fly ash. This dilute suspension is then injected by way of conduit 9 down borehole 1. In those cases where the borehole penetrates aquifers, resulting in a water flow down the hole, the borehole must be cased and conduit 9 may be coupled directly to the casing. Fly ash in dilute suspension flows down the borehole at high velocity and is distributed first to the sides of the mine passage. Sealing progress from the outer edges of the deposited fly ash inwardly toward the borehole with the formation and maintenance of a large crater 10, under the hole. Flanking slopes 11 of the fly ash deposited in the mine passage develop in a concave upward configuration with the maximum slope at the crater rim. Test observations indicated that slope of the deposited fly ash depended upon differential settling of fly ash particles and not upon the natural angle of repose. Natural cohesiveness of the fly ash permitted angles of up to 70.degree. to form on the inside slopes 12 of the crater but these steep slopes would eventually topple. Maximum stable slope obtainable was between 16.degree. and 20.degree. compared to the natural angle of repose of fly ash which is ordinarily in the range of about 8.degree. to 12.degree..

Fly ash injection is continued until more than 90%, and preferably more than 95% of the area of the mine passage is blocked. It is not possible to determine this point by surface monitoring of injection conditions such as back pressure. However, the required amount of fly ash necessary to produce a fly ash plug blocking some 95% or more of the mine passage may be calculated provided that the dimensions of the mine passage is known. This may be accomplished by application of the following formula: ##EQU1## where: h = passage height

w = passage width

.theta. = slope angle of fly ash pile

Values for .theta. depend to some extent upon the fly ash employed but a value of 15.degree. can normally be used. In this formula, a truck load is that of a standard bulk pneumatic tanker which can carry some 20-22 tons of fly ash. Alternatively, the degree of blockage of the mine passage may be continuously monitored by an acoustic method and device disclosed and claimed in copending, commonly assigned patent application, Ser. No. 504,468.

After fly ash injection has proceeded to a point where more than 90% of the passage area has been blocked, injection is stopped. Thereafter, the seal is completed by injection of an expanding foam, such as polyurethane, into the crater below the borehole. Foam injection may be accomplished in accordance with the procedure and apparatus depicted in FIG. 2. Referring now to that Figure, there is shown a system whereby a two component foam is mixed downhole directly above the crater area. The two components of the foam system, 20 and 21, are held in containers under pressure on the surface. These components, when mixed, form what is commonly referred to as a froth foam as opposed to a pourable foam. Each component contains a gas, such as a fluorocarbon, dissolved in it under pressure. Each foam component is maintained under an additional applied pressure of an inert gas such as nitrogen supplied from source 22, through pressure regulating valve 23 and branch lines 24 and 25. The two foam components are delivered downhole through conduits 26 and 27 which preferably comprise flexible hoses of about 3/4 inch diameter. Flow of each component is monitored by meters 28 and 29 and controlled by regulating valves 30 and 31.

The downhole component mixing system consists of a mixing head or chamber 32 into which component lines 26 and 27 discharge through variable pressure relief valves 33 and 34. Valves 33 and 34 maintain a back pressure on the component lines sufficient to prevent foaming of the components within the lines. Back pressure applied by valves 33 and 34 is controlled by air line 35 which is connected to a relatively high pressure air supply 36 through pressure regulating valve 37. Mounted below mixing head 32 is mixing means 38 which preferably comprises a static mixer, such as those manufactured by Kenics or Koch, wherein mixing of the two components occurs along with considerable foaming. A 3-foot long, 1-inch diameter mixer is a size generally appropriate. Connected to the bottom of mixer 38 is a short length of pipe 39 to which is attached a quick disconnect coupling 40 which may be activated by air pressure supplied by line 41. Below coupling 40 is attached a second short length of pipe 42 having mounted thereon packers 43 and 44 which prevent foam from flowing back up the borehole. The end of the second pipe 42 extends into the crater area in the injected fly ash plug and may be provided with a flat plate disperser 45 which provides a final mixing and directs foam flow horizontally.

Since the foam sets up quickly, it is highly desirable and usually necessary, to provide the system with a stop-restart capability. To achieve this, relief valves 33 and 34 are pressurized sufficiently to stop flow of the foam components. Thereafter, a solvent such as methylene chloride from storage means 46 is used to flush the mixing assembly. Solvent is pumped, via means 47 and conduit 48, to mixing head 32 whereby foam is flushed from the mixing assembly before it has time to set. Foam injection can thereafter be restarted be decreasing the air pressure on relief valves 33 and 34. After foam injection has been completed, quick disconnect coupling 40 is activated by means of air pressure applied through line 41. The mixer assembly is then retrieved from the hole leaving tail pipe 42 and packers 43 and 44 in place at the bottom of the hole.

This preferred system of foam injection requires a total of five hoses to be deployed down the hole; two hoses for the separate foam components, one small airline for the pressure regulating relief valves, one small airline to activate the quick disconnect coupling and one small line for the solvent. The hoses carrying the foam components are preferably about 3/4 inch in diameter which 1/4 inch lines are satisfactory for the other three. In operation, it is preferred to attach the mixing system assembly to a cable and winch assembly (not shown) and attach the five hoses to the cable as the assembly is lowered into the hole. Attachment of the hoses to the cable may be simply accomplished by wrapping the hose-cable bundle with tape, such as duct tape, at intervals of several feet. On retrieval of the system, the hoses may be cut free of the winch cable and wound as a bundle onto a reel.

FIG. 3 depicts graphically the relationship between the amount of fly ash needed, and time required to inject it, as a function of passage height and width for a 95% and 100% seal. As can readily be appreciated from the Figure, nearly twice as much fly ash is needed and nearly double the time is required to create a 100% seal in a mine passage as is required to create a 95% seal.

The time required to complete a seal by foam injection after a 95% seal has been created by pneumatic implacement of fly ash naturally is somewhat dependent upon borehole depth as this affects the deployment time of the mixing assembly down the hole. However, deployment of the mixing assembly can ordinarily be accomplished in less than an hour in boreholes ranging in depth to 1000 feet or more. Amount of foam required and the corresponding time necessary to generate it is relatively insensitive to passage size over that range of sizes normally found in coal mines. There are two primary reasons for this. First, fly ash fills more than 90%, and preferably about 95% of the passage so that the final open volume requiring foam topping is relatively small. The second factor which makes the required quantity of foam virtually independent of passageway geometry is the random flow pattern of the material in which the foam flows into open areas of the partial fly ash seal following generally paths of least resistence. Regardless of borehole location relative to the passageway, the general geometry of the fly ash pile or the initial flow direction of the foam, the first three or four flow channels or foam formed produce a 100% seal. Foaming time required for total sealing in no case exceeded one-half hour for a seal of at least 10 feet thick. Longer foaming times merely increases the thickness of the seal. In all cases, foam implacement was accomplished using the method described in FIG. 2 at a foam rate of 40 to 45 pounds per minute to produce a foam having a density on the order of 2-21/2 pounds per cubic foot.

Organic foams used in the process are of course flammable. Charring of the foam seal on the hot, or fire side, of the seal can be expected to occur. However, actual combustion of the foam will not occur because of the oxygen-depleted atmosphere within the fire zone. The bulk of the emplaced foam is thermally protected by fly ash thus adding to the heat resistence of the emplaced foam. In addition, the foam seal is typically a minimum of 10 feet thick and is a very effective insulating material thus inhibiting the propogation of charring through the seal.

The following examples serve to more fully illustrate specific embodiments of my invention.

EXAMPLE 1

A full scale test of the process was carried out in a straight mine passage which was accessable from both sides. The passage had a width of approximately 20 feet and the roof height varied from 9 to 11 feet with a dome about 13 feet high near the injection borehole. Three boreholes on nominal 30 foot centers were drilled through about 450 feet of overburden to intersect the mine passage. These boreholes were cased and the center hole was used as the injection borehole while the flanking holes accommodated the acoustic seal checker described in patent application Ser. No. 504,468.

Apparatus similar to that described in relation to FIG. 1 was used to inject fly ash down the center borehole. In addition to the air provided by the compressor of the bulk pneumatic truck there was supplied additional air by a blower (element 7 of FIG. 1) at the rate of 3000 scfm. An elliptical crater, whose long axis spanned the width of the passage, was formed during the early stages of sealing. The crater continued to exist throughout the sealing process and resulted in filling of the passage from the outward corners in toward the center. This kept the borehole clear of fly ash and insured filling the corners of the passage.

After injection of 14 truck loads, or approximately 280 tons of fly ash, 96% of the area of the passage was blocked as indicated by the acoustic seal checker and confirmed by physical examination. Time for actual injection of fly ash to create a 95% seal was 7 hours with a total elapsed time of about 14 hours. To increase the seal to 98% required 80 more tons of fly ash and 4 additional hours. To further increase the seal to 99% required an additional 80 tons of fly ash and another 4 hours of delivery time.

During the fly ash injection with a blower rate of 3000 scfm, the blower pressure was in the range of 5 to 6 psi and the borehole pressure fluctuated in the range of 0-1 psi. Fluctuations in pressure were due primarily to variations in the fly ash flow rate as delivered by the trucks. Surface instrumentation failed to provide any indication of the fly ash build up in the mine passage.

At the conclusion of the fly ash injection, there existed a large crater beneath the borehole extending almost to the walls. Both rib-roof corners were nearly completely sealed with fly ash. A clearance of about 4 inches between the fly ash and the roof existed on one side of the seal while the clearance on the other side varied from about 18-24 inches.

Foam injection apparatus as described in relation to FIG. 2 was lowered down the injection borehole to a point where the disperser plate extended about 11/2 feet below the mine roof. Foaming was then started using a polyurethane foam. The isocyanate portion of the formulation was a TDI prepolymer prepared with a pentaerythritol-based polyol. The polyol portion of the formulation was a blend of pentaerythritol-based polyol and an .alpha.-methyl glucoside-based polyol. Each component had about 10% of Freon 12 and Freon 22 blended into them. A foaming rate of about 42 pounds per minute was maintained with a 10% isocyanate-rich foam mixture.

Foam first filled the large crater which required about 650 pounds of foam and approximately 15 minutes of foaming time. At this point, the seal checker indicated a complete seal and underground visual observations confirmed the checker reading. Foaming was then continued for another 65 minutes to observe the behavior of the foaming operation. Foam next started to channel down the low side of the fly ash pile and later the foam finally broke out on the high side of the fly ash pile and ran down the flanking slopes. The seal checker continued to indicate a 100% seal during the second phase of the foaming operation.

EXAMPLE 2

A fire occurred in a bituminous coal mine working the Pittsburgh seam. The fire could not be controlled so the mine was evacuated and sealed. Thereafter, exploratory drilling from the surface showed that the fire area encompassed some 4 acres of mine workings.

It was determined from the data obtained in the exploratory drilling and from available mine maps that the fire area could be isolated by the construction of six seals. Thereafter, boreholes were drilled from the surface to intersect the mine workings at the seal points. Overburden thickness varied from somewhat over 400 feet minimum to 610 feet maximum. Three boreholes were drilled at each seal site on a nominal 30 foot spacing and the boreholes were cased to full depth. The center hole was used as the injection borehole while the two outside holes accommodated the acoustic seal checker described in patent application Ser. No. 504,468 and were later used to monitor the mine atmosphere on both sides of the emplaced seal.

Fly ash was then pneumatically injected in dilute phase into each of the seal sites using sufficient nitrogen in the injection gas so as to provide at all times a "safe" atmosphere; one sufficiently low in oxygen content as to not support combustion. Fly ash injection was halted at three of the seal sites at that point where the acoustic seal checker indicated that greater than 95% of the area of the mine passage had been blocked by fly ash. A minimum of 6 and a maximum of 8 truckloads of fly ash were used in each of these three seal sites. At the remaining seal sites, fly ash injection was continued until a complete seal had been obtained. A complete seal was considered to be that point at which the acoustic seal checker indicated complete blockage of the mine passage area followed by the injection of one additional truckload of fly ash. Two of these seals required 12 truckloads of fly ash each while the third seal required 26 truckloads. However, this last seal was at a 4-way intersection which accounts in large part for the difference in fly ash required. In all cases, a truckload of fly ash refers to that amount carried by a standard bulk pneumatic trailer, or about 20-22 tons. In no case did refusal occur; refusal being that point at which no additional fly ash can be injected into the hole.

The three seal sites at which fly ash injection was halted at a point where more than 95% of the passage area was blocked were then completed by injecting urethane foam in the manner described in reference to FIG. 2. Foaming rate was approximately 40 pounds per minute. The first seal required 2,324 pounds of foam; the second required 1000 pounds while the third seal required 2256 pounds. After all six seals were completed, the mine atmosphere within and without the seal area was carefully monitored to determine whether the fire area had been successfully isolated. Successful isolation was indicated by a change in the composition of the atmosphere between the two areas and a stabilization of the carbon monoxide and oxygen levels outside the fire area.

After isolation of the fire area had been established, the mine was opened and ventilated while maintaining close monitoring of the atmosphere within the fire zone. Inspection teams then entered the mine to physically examine the seals. Five of the seals proved to be accessable while one seal, that one completed with 2324 pounds of foam, could not be reached. However, monitoring indicated there to be no gas leakage across the seal. Four of the five seals examined were good.

One seal, that one completed using 2,256 pounds of foam, was found to have open communication in an area encompassing approximately 4 square feet between the fire zone and the rest of the mine. An investigation of the seal area revealed three factors were responsible for the incomplete seal. First, the injection borehole penetrated the mine passage at a loading point. Normal height of the mine passages was approximately 6 feet, the thickness of the coal seam, but at the loading point the roof had been raised an additional 4 feet. This created an irregular indentation or dome in the roof at the seal area. Second, the injection borehole penetrated the mine passage immediately adjacent the rib at one side of the borehole. Third, the borehole used for the transmitter of the acoustic seal checker had drifted some 15 feet toward the injection borehole. This resulted in the transmitter being buried, or partially buried, by fly ash during the injection process and caused inaccurate readings as to the completeness of the seal. The open area was against the rib opposite the injection borehole. However, the seal was complete enough to preclude any significant circulation between the fire area and the rest of the mine and the void area was easily plugged by the inspection team.

Claims

I claim:

1. A method for remotely constructing a seal in a mine passage which comprises:

drilling a borehole from the surface to intersect the mine passage,

pneumatically injecting dry fly ash in dilute phase down said borehole to form a partial plug of fly ash within said mine passage, said plug obstructing more than 90% but less than 99% of the area of said passage and having a crater-like depression below said borehole, and

thereafter injecting an expanding foam into said crater-like depression to fill the depression and to complete the seal.

2. The method of claim 1 wherein the flow rate of gas used to pneumatically inject said fly ash is selected according to the following approximate formula

wherein G represents gas flow rate in scfm and W represents the width of the mine passage in feet.

3. The method of claim 2 wherein said mine passage comprises a coal mine passage.

4. The method of claim 3 wherein said seal is constructed to isolate a fire burning within said mine from the remainder of the mine workings and wherein the gas used to pneumatically inject fly ash does not support combustion.

5. The method of claim 4 where said expanding foam is a two-component froth foam which is mixed at the bottom of the borehole and injected directly into the crater-like depression within the fly ash plug.

6. The method of claim 5 wherein said expanding foam is a polyurethane foam.