Journal of Mining Science, Vol. 32, No. 5, 1996

PROBLEMS AND POTENTIALITIES

OF INTENSIFIED

DEGASSING OF COAL SEAMS

E. V. Kreinin

UDC 553.98

STATE O F T H E P R O B L E M Most Russian coal deposits are gassy. The methane content of such highly metamorphosed coal seams rises with depth and reaches 45-50 m3/ton [1]. On the one hand, sorbed methane of coal measures as well as methane of free piles is a source of explosion in coal mines causing death of miners, and, on the other hand, it is a valuable gaseous energy carrier. The traditional coal seam degassing technology now in use in Russia [2] is inefficient and is based on drilling, from mine workings and from the surface, of a large number of boreholes of different types (fan, clustered, parallel, intersecting, etc.). Degassing boreholes generally range from 50 mm to 100 mm in diameter and 5 m to 40 m in length. The degassing factor (degree of methane extraction) of coal seams in the traditional technology varies from 10 to 40%. Since the 1950's coal methane with a concentration of as much as 90-95% has been actively extracted in the USA employing industrially adopted technology of the company Amoco. This technology includes the following major stages: drilling of vertical boreholes with a network of up to 500 m, hydraulic fracturing 0aydrofracturing) of the coal seam with water, injecting into the hydrofracturing slit quartz sand and a gelatinous carrier having surface-active and chemical substances, evacuation of the underground water from the secured slit, and, finally, extracting coal methane from the depression region created. The degassing factor in this technology reaches 70%. Since the indigenous coal seam degassing technology [3] has fallen behind, it has become necessary to develop new nontladitional technologies whose industrial application would have helped not merely to increase the safety of the miner's work, but at the same time to extract commercial volumes of high-quality gaseous hydrocarbons. Studies in this direction conducted at the IPKON, Russian Academy of Sciences, are well known [4]. In this paper we have discussed two new technologies, a distinctive feature of which is the creation of highly permeable reservoirs having a developed lateral infiltration surface. Both technologies, in one form or another, were applied in underground gasification of coal seams.

RESERVOIR A F T E R H Y D R O F R A C T U R I N G OF COAL SEAM The earliest studies of hydrofracturing of coal seam in gasifying it underground were conducted as far back as in the 1950's. Fracturing slits were opened up immediately by mine workings. Inspection of the slit (40 m in length) revealed [5] that its path was formed in the bedding plane along the interlayer contacts. No fracturing was observed along the normal or any other angle relative to bedding. The fracturing slit measured (width x height) (20-30) cm x (4-7) era, the average slit cross section being 0.01-0.02 m 2. The sand is unevenly dispersed in the slit (across the length and breadth). It has been shown as well that the slit is not necessarily to be secured by sand became as a result of removal of the coal from the hydrofracturing slit, the stress on the mass surrounding it is effectively relieved. Further refinement of the coal seam hydrofracturmg technology was directed to creating methods to control them and to expand the slit [6]. Application of technical means to control the direction of the hydrofracturing process made it poss~le to connect five vertical boreholes (depth 240 m, total linear length 87 m) into a single channel.

A. A. Skochinskii Institute of Mining, Lyubertsy. Translated from Fiziko-tekhnicheskie Problemy Razrabotld Poleznylda Iskopaemykh, No. 5, pp. 106-111, September-October, 1996. Original article submitted February 20, 1995. 1062-7391/96/3205-0433515.00 ©1997 Plenum Publishing Corporation

433

Air 2~

Combustion )roducts 4

I Combustion products

Y I U__I, Air 2t

4

QFig. i If in the untouched state the coal seam had a permeability of 4-5 mD (gas permeability factor of the enclosing rocks 0.I roD), then after hydrofracturing the channel allows 4800 m 3 of air to pass through per hour at a pressure of 3 bar. Tt~ corresponded to an average equivalent channel diameter of 0.35-0.40 m. The primary distinction of the new hydrofracturing technology is pulsed flushing of the hydrofracturing slit formed. As the water injection pressure drops to 1-2 bar, the slit is purged with air whose consumption surpasses water consumption in the process by 10-15 times. The leakproofness of the vertical boreholes makes it possible to periodically raise the pressure in the discharge borehole while switching from water to air and vice versa. Variable (rising) water velocity in the slit and dynamic impacts upon periodic pressure elevation and release facilitate active breaking of coal in the slit and removal of coal pieces from the discharge borehole. The main attention in future refinement of this technology will be focused on increasing the distance between boreholes to as much as 100 m, which will render such a degassing reservoir competitive with the current foreign technologies.

R E S E R V O I R A F T E R T H E R M A L EXPANSION OF L O N G D R I L L I N G CHANNELS Use of long drilling channels for degassing of coal seams is highly promising because they may be as long as several hundred meters and their cross section is comparable with the cross section of hydrofracturing slits. However, much more effective is thermal expansion of drilling channels by moving the combustion center toward the air blast injected into the channel [7]. A line diagram of the module consisting of two boreholes on horizontal (a) and inclined (b) coal seams is shown in Fig. 1. Vertical-horizontal or vertical-inclined boreholes 2 are drilled into the coal seam 1 being degassed. The uncased part of these boreholes 3 may be horizontal or inclined. A vertical borehole 4 is drilled at the far end of the drilling channel. This borehole is connected with borehole 3 by hydrofracturing, whereafter the coal seam is ignited i~ borehole 4. Air blast is injected into borehole 2, and borehote 4 is opened to the atmosphere. The combustion center begi~ 434

TABLE 1 S1. No.

I 2 3

Methane influx relative, absolute,

Degassing channel

m3pa-linear m3/hDrilling channel of degassing boreholes ( ~ s 0 m m l = 20m) Hydrofracturing slit of traditional technology (.cross section 20 × 5 cm, l = 20 m) Hydrofracturing slit after pulsed flushing ~technology (cross section 4Crux 40 cm, 1 = 50 rn) Hydrofracturing slit in Amoco technology (cross section 20 x 5 crn, l = 400 rn) Extended drilling channel through the coal ~seam (O 150mIn,l = 400 m) Extended drilling channel with thermal working up (O 750m/rl, l = 400 m)

( ,8

16

1,5

30

1,25

0,16

~,0

250

0,41

0,01

1,5

600

0,47

0,18

1,7

680

2,35 (9,4)

0,59

8,5 (34)

3400 (13 600)

to move through the drilling channel toward the air blast being injected into borehole 2, leaving thereafter a reservoir of a fairly large diameter. For instance, at an air blast rate of 1740 m3/h the drilling channel expanded from 150 to 750 ram. Such an artificially created reservoir (especially after its cooling) has numerous extended (in keeping with the depth of heating of the coal seam on account of its thermal conductivity) cracks and is an effective drain for underground water and coal m e t h a n e .

The module depicted in Fig. 1 may be assembled into various combinations which greatly facilitate degassing of a section of the coal seam.

EFFICIENCY O F T H E N E W T E C H N O L O G I E S The efficiency of the proposed coal seam degassing technologies can be assessed only by comparing them with the current technologies. Scarcity of factual data (technical and economic) hinders comparison of various methods, but we ventured into making such an attempt using the practical data on the widely employed technology of the American firm Amoco as well as the experimental data accumulated in Russia. In Table 1 we have summarized the data on six degassing channel construction technologies at various stages of practical realization. The geometric parameters of the channels to be constructed were determined on the basis of actual drilling and hydrofracturing slit opening data as well as of calculated thermal expansion data. Expected methane influx (calculating on 1 m3/h'linear meter, the total on 1 m3/h) was taken as the principal criterion of channel efficiency. The channel (the 4th in Table 1) built by the Amoco technology was taken as the basic version. It is known [1] that in the USA 13 billion m 3 of methane was extracted in 1992 through 5000 boreholes. This corresponds to an average commercial channel (borehole) discharge of 600 m3/h (in some cases the discharge was as much as 12-16 thousand m3/h through certain boreholes). The channel length was taken to be 400 m, whence the relative channel discharge was 1.5 m3~-linear meter. The technologies being used in Russia (the 1st and 2nd channels in Table 1) also have small relative influxes and, because of the small length of the channels, have very small total methane discharge (16 and 30 m3/h, respectively). The channel created by horizontal drilling (the 5th channel in Table 1) is similar in geometrical dimensions to the channel created by the Amoco technology (the 4th channel in Table 1), and so their methane discharge is also practically the same.

It is somewhat more difficult to predict methane discharge from the degassing channels created by the new technologies (the 3rd and 6th channels in Table 1). In keeping with the linear infiltration law, methane influx into the channel can be determined by the Dupuy equation q-----

/~

R " Po " In~ r

c

435

~.

\

/

\

\ /

~'\

I I~

~ ',

Years Fig. 2 where k is the effective gas permeability factor, lc is the channel length, r c is the channel radius, R is the distance up to the supply line, and Pl is the injection pressure. Taking this into consideration, we see that the channel radius hardly affects methane influx. But by taking account of the increased effective gas permeability factor in the wall-adjoining region, we took the risk of assuming relative influxes directly proportional to the lateral surface of the channels. Hence, the total predicted methane influxes into the channels (the 3rd and 6th channels in Table 1) are, respectively, 250 and 3400 m3/h. Moreover, after thermal working up of the lateral channel and its subsequent cooling numerous cracks appear having depths of 0.5-1.0 m. The infiltration law changes, and methane discharge begins to depend much more on the cross secti0a of the channel and its lateral surface. In a specially arranged test bed experiment we noted a fourfold increase in the channel discharge on account of thermal working up. This permitted us to indicate in parentheses the expected methane influx into the worked-up drilling channel. By the way, the hydrofracturing slit may also be thermally worked up, which should lead to a corresponding increase in its discharge. Thus, based on the assessment made, the proposed two new technologies for construction of degassing channels are quite competitive with the US Amoco technology which is globally recognized as the most advanced. Importantly, the technologies of intensified methane extraction make it possible to significantly reduce the time for drying and subsequent degassing of coal deposits. Figure 2 shows the predicted change in the water (1) and coat methane (2) influx with time when traditional technologies (--) are replaced by the new ones (--). It is worthwhile to make a technoeconomic appraisal of the new technologies. If we take a 500 m x 300 m sectio~ of a coal seam 5 m in thickness, coal reserves in it will be 900 thousand tons and methane reserve will be 27 million m3 for an average methane content of 30 m3/ton in the coal seam. In order to degas this section it would be necessary to drill three modules (Fig. 1). At 1993 prices the cost of preparinl the section for degassing and of the requisite technological equipment (compressor, pipelines, etc.) would be about 30 milli0~ rubles, but in the old technology it would be roughly 5 million rubles. The price of natural gas in Russia in September 1993 was 20,000 rubles/1000 m~ (the global price is about $60/1000 m3). The cost of methane upon extraction by the new technology should not exceed 20,000/1 + 0.15 = 17,400 rubles/1000 m 3, where 0.15 is the standard ratio of capital investment in the coal industry. In that case, the annual profit will be (20,000 - - 17,000) x 27,000 x 0.8 = 56.56 million rubles, where 0.8 is the degree of methane extraction from the coal seam. The economic return from additional capital investments in the new technology will be 56.16/(30 - - 5) = 2.?A rubles/ruble. The annual profit from methane extraction in the chosen section was 56 miUion rubles and the relative value w~ 62 rubles/ton or 2600 rubles/1000 m 3. CONCLUSION In the old technology, all capital investments for degassing of coal seams were dictated by the need for accident-fief mining. The methane extracted had a low concentration (up to only 30%), and all the concerns boiled down, for the most part to search for methods of its combustion, but not of its use as an energy source. 436

The new technology of intensified extraction of coal methane simultaneously solves two problems: degassing of the coal seam for its accident-free mining and obtaining a valuable gaseous energy carrier. Highly permeable reservoirs with a developed lateral infiltration surface are presumed to provide for a methane (concentration 90%) discharge of up to 10 thousand m3/h. Commercial extraction of coal methane will facilitate creation of a new energy sub-branch, which is particularly important primarily for coal basins far removed from oil and gas deposits. Degassing of coal deposit by the new technologies is possible preliminarily as well as in the course of mining of the coal seams. Both degassing technologies discussed above can be used in various combinations. By the year 1994 it became necessary to look for means for pilot-scale testing of both of the proposed technologies. Capital investments for testing each of them is estimated at 30-50 million rubles.

REFERENCES I,

2. 3. 4. 5. 6. 7.

International Symposium "Nontraditional Sources of Hydrocarbons and Problems of Recovering Them" [in Russian], October 12-16, St. Petersburg, 1992, Vol.2. A Guide to Degassing of Coal Mines [in Russian], Moscow (1990). Yu. N. Malyshev, E. V. Kreinin, and I. V. Sergeev, "Preliminary degassing of coal seams--an essential condition for safe work of miners," Bezopasn. Trud. Prom., No. 4, 24-25 (1993). B. M. Zimenkov, et al., Real Problems of Recovery of Deposits and Utilization of Minerals [in Russian], Moscow (1993), pp. 185-204. F. G. Tyutin, "Underground inspection of hydraulic fracturing zone in I.,7 seam at the Lisichan.~k station Podzemgaz," Podzemn. GazifLkats. Ugl., No. 4, 22-25 (1956). E. V. Kreinin and M. K. Rewa, Underground Gasification of Coals [in Russian], Kemerovo (1966). E. V. Kreinin, "Coal seam as a source of nontraditional hydrocarbons," Gorn. Vestn., No. 2, 61-63 (1993).

437