METHODS AND TECHNIQUES FOR TESTING KUZBASS COAL SEAMS FOR GAS CONTENT

N. V. DOROSHKEVICH

Recently, in connection with the development of new mine construction, as well as with the transition of operating mines to deeper horizons, the forecasting of the gas abundance of mine fields has acquired especially important significance. For this purpose, the study of the gas content of coal seams is of great importance, since it is one of the most important factors determining the gas abundance of mines.

At one time, a whole series of calculation methods was proposed for resolving the question of the gas abundance of mines. However, these calculation methods did not have sufficient substantiation, and the results obtained did not provide confidence in the complete reliability of the forecast. Therefore, at the present time an urgent task is the development of the most accurate method for determining the gas content of coal seams, as well as the elaboration of the most rational procedure for locating boreholes intended for taking samples to determine the gas content of coal seams.

To solve this problem, in 1952 the Eastern Scientific Research Institute proposed the principle of freezing the coal core at the bottom of the borehole with a special core-freezing tube, in order to preserve more completely the gas in the samples being taken.

However, during the first tests of the core-freezing tube, serious design deficiencies were discovered. One of the main shortcomings proved to be the principle of arbitrary actuation of the tube. It became clear that certain changes had to be introduced into the core-freezing tube design proposed by VostNII. Work on improving this tube was carried out by employees of the “Kuzbassuglegeologiya” trust. As a result, in 1954 a new specimen of the tube was manufactured, called the VK-54 tube (VostNII—“Kuzbassuglegeologiya,” 1954), with the aid of which samples were taken for studying the gas content of coal seams.

At the end of 1956, in the workshops of the “Kuzbassuglegeologiya” trust, according to the design of employees of the Eastern Scientific Research Institute, the KGN-2-56 core gas sampler was manufactured. The latter operates on the principle proposed by the “Kazakhuglegeologiya” trust of capturing the gas released from coal during drilling and withdrawal of the tool.

Thus, at present we have two types of core samplers: the VK-54 core-freezing tube and the KGN-2-56 core gas sampler.

1. The core-freezing tube (Fig. 1) is the well-known Alekseenko core barrel, into which is built in.

Fig. 1. Core-freezing tube VK-54:

the so-called “gas cylinder,” holding 7–9 liters of liquid carbon dioxide, pumped by means of a compressor from a transport cylinder.

The pipe consists of the following parts: fastening adapter 1, which is connected to water adapter 2 and, in addition, connects the VK-54 pipe with the drill rods. In the center of adapter 2, by means of cuffs 7 and washer 6, piston 8 and rod 9 are fastened. In piston 8 there is a seat with a smooth surface for a D-14 mm ball. Adapter 2 has 8 cranked channels intended for supplying flushing fluid into the borehole after the piston is shifted to the lower position. Rod 9 is rigidly connected with the gas cylinder. The latter, in turn, consists of the following parts: upper head 15, body 16, and lower head 17.

The upper head of the gas cylinder 15 has stem 10 with nut 11, washer 12, packing gland 13, and plug 14, through which the gas cylinder is filled with liquid carbon dioxide. The body of the gas cylinder 16 is a pipe connected with the upper and lower heads by means of a gas thread and solder. In lower head 17 there are branch pipe 18 and hollow rod 20 with gasket 19. Nipple 26 is screwed onto hollow rod 20 with gasket 25. A throttling diaphragm 40 with gasket 39 is inserted into the upper part of the nipple. The connection of the gas cylinder with the water adapter 2 and gas adapter 27 is made by means of outer pipe 3. Lead gaskets are used to seal the adapters to one another.

Hollow rod 20 with nipple 26 and diaphragm 40 is inserted into gas adapter 27, which, in turn, is connected with distribution adapter 28, thereby forming a distribution chamber, in the lower part of which striker 30 and plug 31 are located, while into the upper part a head with a weakened section projects.

In adapter 28 there are channels for the entry and exit of carbon dioxide into coil 46. In the channel for the outlet of carbon dioxide there is ball 34 with spring 33, called a check valve. On the side of the distribution chamber the channel is closed by plug 31 with gasket 32. Inside coil 46 there is core receiver 45, which protects the coil from deformation during drilling. For the exit of flushing fluid from the core receiver during drilling, it has a channel in which nut 42 and a D-14 mm ball are fastened, closing the flushing channel.

Rivet 43 and key 44 serve to prevent the coil from rotating during drilling.

On the outside, casing 37 is placed on coil 46; it is connected with distribution adapter 38 and drill bit 47 with cutters 48, 49, and 50 arranged accordingly.

The freezing process is based on the use of the evaporation effect of liquid carbon dioxide. Because of this, the carbon dioxide pumped into the gas cylinder of the pipe must be under a certain pressure.

Initially the freezing process was as follows: when the core receiver was filled with coal, the pressure of the coal core itself had to force back a disk valve with a stem, which ended in a bevel. This bevel was to break off the nipple blocking the outlet of carbon dioxide from the cylinder. In this case the liquid carbon dioxide from the “gas cylinder,” through the distribution head, entered the coil, thus surrounding the core receiver.

Passing through the opening of the throttling washer, it began to evaporate, as a result of which a low temperature was created and around the core ob—

the crust of ice that protected the coal from degassing and increased its sorption capacity was broken. It was assumed that the low temperature and fairly high hydrostatic pressure would make it possible to reduce gas losses to a minimum with this method of sampling.

However, when coal seams were overdrilled, the column of core most often crumbled and became wedged, and the pressure on the disk plate proved insufficient to break off the head of the nipple; in most cases the tube did not operate.

The difference between the new specimen of the tube and the first is that the freezing process is controlled by creating artificial pressure on the gas cylinder. After drilling of the coal seam is completed, a steel ball 14 mm in diameter is lowered into the rods and is driven by the flushing fluid up to the piston. At the same time the pressure rises sharply to 40–45 atmospheres, the spring holding the gas cylinder is compressed, and the head of the nipple is sheared off against the bevel of the fixed striker. The discharge of liquid carbon dioxide begins.

Systematic work on studying the gas content of coal seams began in 1954, when coal samples were taken from boreholes of geological prospecting parties. In 1955, a special amount of core drilling was carried out in the field of the Krasnogorskaya mine of the Prokopyevsk geological-economic district for the purpose of taking samples to study gas content. Overdrilling of the seams was carried out directly by a master-instructor authorized to be responsible for work with apparatus operating under pressure. For each coal seam, in parallel with the taking of frozen coal cores, unfrozen samples were taken with double core barrels.

After the required interval had been drilled, the coal core (frozen or unfrozen) was placed in a special vessel, whose tightness was always checked. The well-sealed vessel was sent to the chemical laboratory, where the quantity of gas and its chemical composition were determined.

Altogether, during the period 1954–1956, 361 coal samples were taken and analyzed, of which 159 were frozen samples and 202 unfrozen.

It should be noted that in 1954, when samples were taken with the core-freezing tube of the first model, not a single frozen sample was brought up. Only when sampling with the new tube were positive results obtained. Gradually, as considerable experience was accumulated in working with the core-freezing tube, frozen and unfrozen samples (provided that all rules for overdrilling seams were observed and the boreholes were in normal condition) were obtained as needed (Fig. 2b). At the end of 1955 and the beginning of 1956 the question arose of increasing the sampling depth. During this period the sampling depth increased from 300 to 500 m (Fig. 2a).

The available results of chemical analyses for frozen and unfrozen coal samples (Fig. 3) make it possible to state that frozen coal samples retain more gas than unfrozen ones, by 20–70%. Such a difference in methane content may be explained by a whole series of causes, the principal ones being the physical condition of the core (degree of fragmentation) and the time required to bring it to the surface. The presence of these and other causes has not at present made it possible to find a “conversion coefficient” from the gas content in unfrozen cores to the content in frozen ones. How much the gas content in frozen cores differs from the true content has not yet been established; work in this direction is extremely necessary.

1. The KGN-2-56 core gas sampler (Fig. 4) operates, as already noted above, on the principle of preserving the gas released from the coal during the process of drilling and raising it through the borehole, by trapping it

a

Vertical axis: sampling depth, m
Horizontal axis: year (1954, 1955, 1956)

b

Vertical axis: number of samples taken
Horizontal axis: borehole number

Legend:
— frozen samples
– – unfrozen samples

Fig. 2.

in the body of the core gas sampler and sealing the core at the surface directly in the core receiver. In the process of drilling out the core, the gas released from the coal mass is removed from the borehole with the flushing liquid. The gas released from the core passes through the central opening of valve 12 and the two upward-diverging channels of adapter 10 into the cavity of housing 7. The water displaced by the gas from the cavity of the housing passes through the vertical channels in adapter 10 and above valve 12 into the annular channel between core receiver 13 and cylinder 14.

When raising the tool, it is also necessary to ensure that the borehole is at all times filled with water above the depth at which the core gas sampler is located.

When the core gas sampler is raised, the gas released from the core as a result of the decrease in hydrostatic pressure continues to enter the cavity of housing 7.

The last 20–40 m of hoisting should be carried out at a speed not

exceeding 1 m/sec; otherwise the rate of increase of the gas volume in the body, owing to the decrease in hydrostatic pressure, will not have time to be compensated by displacement of the solution from the body, and the excess pressure may push the core out into the borehole.

Fig. 3.

From the bell of the core-gas sampler, methane is conveyed through a rubber hose into a gas meter to determine its volume. To avoid gas losses, water is continuously supplied into the borehole. Gas is collected in control pipettes for determination of its composition.

After the gas has been removed from the body, the core-gas sampler is extracted from the borehole; cylinder 14 with crown 15 is separated; core receiver 13 is unscrewed from adapter 10 and closed on both sides with special threaded plugs. The core receiver with the sample is then sent to the laboratory for processing.

In 1955, on the basis of the extensive factual material available, employees of the Kuzbassuglegeologiya trust and the Eastern Scientific—

...research institute, the “Temporary Instruction for Determining the Gas Content of Coal Seams during Geological-Exploration Work in Kuzbass” was compiled. In 1956 an instruction was compiled for determining the gas content of coal seams during exploration of mine fields.

The instruction gives recommendations on the method of sampling coal seams for gas content during exploration of various deposits of Kuzbass; it gives detailed guidance on the sampling technique using the VK-54 core-freezing tube, the KGN-2-56 core gas sampler, and on safety measures when working with this apparatus, as well as instructions on the laboratory processing of samples for gas content and the determination of seam gas content. The instruction may also be extended to other coal basins similar to Kuzbass in geological conditions and coal quality.

The main directions of research on gas content, carried out by the trust, consist of the following:

1. The method of sampling coal seams for gas content during detailed exploration in Kuzbass.

2. The technique of re-drilling coal seams with the VK-54 core-freezing tube and the KGN-2-56 core gas sampler in order to obtain samples for gas content.

3. Laboratory processing of coal cores to determine gas content.

The method of sampling coal seams for gas content is a general scheme for obtaining information on the true gas content of each seam, providing only the stages of the design assignment. The number of sampling points for gas content within an exploration area is taken as minimal because the recommended sampling technique consists of carrying out rather labor-intensive processes. Increasing the number of samples beyond that recommended will considerably lengthen the exploration period and increase the cost of geological-exploration work.

The proposed sampling method provides for obtaining the necessary information on the gas content of coal seams over the entire stratigraphic section, with variation in the depth of occurrence of each seam, and also depending on the structure of the deposit.

Determination of the gas content of coal seams is recommended to be carried out during detailed exploration, because different tectonic conditions change the arrangement of boreholes during sampling. With a simple structure of the area, one line of boreholes is passed through its central part, re-drilling the entire stratigraphic section of the area. The gas content of the seams along this line is taken as the average, since it characterizes the coal gas content most typical for the given area for each seam.

Fig. 4. Core gas sampler KGN-2—56.

The arrangement of boreholes along a line, providing for at least a twofold intersection of each workable seam in the given section, ensures the sampling task needed to determine the change in the gas content of coals with the depth of occurrence of the seams (Fig. 5).

In the case of a large number of seams, repeated intersection and sampling of all seams is considered unnecessary. Some seams, analogous in coal quality and identical in the composition of the roof and floor rocks, may be characterized by a single sample if, at the corresponding depth, another seam similar in quality has the same gas content.

To check reliability, in addition to the central line, single boreholes are drilled along the boundaries of the area into the most gas-bearing seams (Fig. 5).

Fig. 5. Location of boreholes for taking samples for gas content. 1—exploratory boreholes for sampling for gas content; 2—boundary of the area along strike; 3—main sampling line for gas content; 4—seams from which samples for gas content are taken on the exploration line.

In areas with complex tectonics, the drilling of another row of boreholes is provided for. With an anticlinal structure, additional boreholes are placed along the axis of the anticline with the calculation that the seams will be intersected at the same depths as on the central line.

The methodology used provides for determining the dependence of the gas content of coal seams on various particular conditions of the geological structure only after establishing the general regularity in the gas content of coals.

As particular tasks, testing of nonworking coal interbeds, testing of coal seams in zones of faults, determination of gas content in the hanging wall and footwall of a disturbance, and also of the gas content of the roof rocks, floor rocks, and tectonic zones are considered. All these particular questions are clarified at a later stage of investigation—exploratory mine workings.

The number of cores extracted is determined by the thickness and structure of the coal seams being redrilled. In the intervals between samples taken with core samplers, samples are also taken by ordinary core barrels, with subsequent sealing in special vessels. Such testing presupposes determination of the effectiveness of methods of sampling coals for the study of their gas content.

Different values of the gas content of coal samples taken when redrilling one and the same seam are explained by different degrees of gas loss during drilling, and also by differences in petrographic composition and tectonics. Therefore it is recommended that the maximum value of the gas content of the core be taken as the gas content of the seam only in those cases when it does not exceed the weighted-average value of gas content by more than 30%.

The weighted-average value of gas content is determined by the formula:

\[ \frac{m_1 d_1 \Gamma_1 + m_2 d_2 \Gamma_2 + \ldots m_n d_n \Gamma_n} {m_1 d_1 + m_2 d_2 + \ldots + m_n d_n}, \]

where \(m_1, m_2, \ldots m_n\) is the thickness of the drilled interval from which the sample was obtained;

\(d_1, d_2, \ldots d_n\) is the specific weight of the corresponding sample;

\(\Gamma_1, \Gamma_2, \Gamma_3\) is the gas content of the corresponding sample in \(cm^3/g\).

Determination of the gas content of coal seams during geological-exploration work from samples taken from boreholes by core samplers is at present regarded by VostNII as the basis for forecasting the gas abundance of mines.

The technique of redrilling coal seams with a core-freezing tube and a core gas sampler, the list of their principal technical units, the principle of operation and work with this apparatus during drilling, and also the technical means and materials necessary for obtaining coal samples for gas content are not set forth here for the sake of brevity. We shall dwell only briefly on preparation of the borehole for drilling, on redrilling, and on extraction of the core from the borehole.

Before the seam designated for gas-content sampling is encountered, the master-instructor provides the drilling crew with the necessary equipment, required materials, and tools.

When the coal seam is encountered, the bottom of the borehole must be cleaned. Then the core-freezing tube or the core gas sampler is lowered on drill rods. Places where the tool is held up in the borehole are passed without jerks, with flushing and rotation. Redrilling of the seam is carried out as in ordinary drilling, but with shorter runs: \(0.35—0.50\ m\) (according to the size of the core receiver). During drilling it is necessary to avoid undercutting of the tool, since this may lead to crushing of the core and considerable gas loss before freezing (when working with VK-54).

The obtained coal core is placed in a special vessel, which is checked for tightness and sent to the chemical laboratory.

Processing of coal cores taken from geological-exploration boreholes is the second stage of work in determining the gas content of coal seams.

The purpose of processing is to determine the quantity and quality of the gas contained in the sample being processed. The essence of the laboratory processing is as follows. First, the gas released from the core is discharged, and then degassing of the coal core is carried out at 100°C and under vacuum. Next, the coal core is crushed in a ball mill and the residual gas is extracted from the crushed coal core. Finally, the gas content in the coal bed is calculated.

The study of the gas content of coal seams is at present an integral part of the detailed exploration of every site. At the same time, the state of this question is far from providing an exhaustive solution to the task that has been set. The methods and techniques now being introduced for obtaining samples for gas content still have a number of substantial shortcomings. Therefore there is no certainty as to how far the data obtained reflect the true gas content. To solve the tasks posed, it is necessary first of all to:

1. Develop a method for determining the magnitude of degassing when the seam is encountered and during its drilling.

2. Develop a flushing regime, and also select flushing solutions whose use will lead to minimal gas losses.

3. The existing methods for determining the methane content in coal from laboratory samples are extremely cumbersome, costly, and require fundamental change.

4. In view of the large amount of methane emitted from mines, it is expedient to raise the question of using it for the needs of the national economy and for domestic purposes. In this respect it is time to raise the question of exploration for combustible gases saturating the coals and of possible recoverable reserves.

5. Exploration of the gas content of seams for the purpose of obtaining industrial gas is a new direction in geological-exploration work. Therefore, a more in-depth study of this question should be undertaken.

Trust “Kuzbassuglegeologiya”