Study of the Possibility of Determining the Moment of Drill Bit Entry into a Coal Seam Based on Elastic Vibrations of the Drilling Tool during Drilling
Abstract Generated abstract
This paper examines whether the moment a drill bit passes from one medium into another, such as entering a coal seam, can be inferred from elastic vibrations transmitted through the drilling tool. Laboratory experiments on a ZIF-650A drilling rig recorded vibration frequency spectra under idle operation, controlled axial loads, and drilling of sandstone, marble, and granite using several sensors, bit types, rotation speeds, and flushing conditions. The results indicate that the main vibration groups lay within 0 to 5000 Hz, were strongly affected by rotational speed, only weakly affected by axial pressure, and showed limited but observable differences among drilled rocks, especially in amplitude and frequency grouping. The authors conclude that the experimental material is insufficient to establish practical applicability for detecting rock or mineral contacts, and recommend broader laboratory and field investigations with more rock types, rigs, bits, drilling methods, and sensors.
Full Text
INVESTIGATION OF THE POSSIBILITY OF DETERMINING THE MOMENT OF DRILL-BIT ENTRY INTO A COAL SEAM FROM THE ELASTIC VIBRATIONS OF THE DRILLING TOOL DURING DRILLING
G. K. AVTEN’EV, V. G. KHRAMENKOV
(Presented by Prof. D. S. Mikov)
It is known that, when drilling rocks that differ in density and grain size, the cutters of the crowns experience different stresses, which, in turn, cause various vibrations of the crown and of the drilling tool connected with it. At small borehole depths these vibrations are transmitted through the tool to the machine; they combine with the vibrations of the working parts of the machine and the drive and produce the overall frequency spectrum of the elastic vibrations of the installation.
Experimental work on determining the moment at which the drill bit enters a coal seam from the elastic vibrations of the tool during drilling was carried out in 1959 at coal deposits of the Kuzbass by V. A. Andreev, who obtained negative results; this may have been due to the imperfection of the research procedure adopted and of the apparatus used. A continuation of the above-mentioned work was the investigations conducted in the drilling laboratory of the Department of Exploration Engineering of TPI.
In the experimental investigations, the task was to determine the frequency characteristics of the drilling installation under various specified operating conditions, with the aim of accumulating factual material for solving the inverse problem: to specify the possibility of determining, from the elastic vibrations of the tool during drilling, the moment at which the drill bit passes from one medium into another. To establish the possibility of solving the specific problem posed, laboratory investigations were carried out in the following sequence:
1) obtaining the frequency spectrum of the intrinsic background of the drilling machine under various operating conditions;
2) obtaining the frequency spectrum during operation of the drilling installation with different axial loads applied to the thrust bearing (without drilling rock);
3) establishing the dependence of frequency–amplitude changes in the spectrum, in extended frequency ranges from 0 to 5000 hertz, on changes in axial pressure and in the rotational speed of the tool when drilling samples of rocks.
Conditions of execution, procedure, and technique of the work
The investigations were carried out on a ZIF-650A machine. As rock samples, coarse-grained sandstones with clay cement were taken—
sandstone, marble, and fine-grained granite. Drilling was carried out with a TP-3 type bit and a bit reinforced with four octagonal cutters. Flushing was performed by a ZIF-200/40 pump and was kept constant within the range of 25–30 l/min. Rotation of the tool was carried out at the I, II, III, and IV speeds of the drilling rig. To reveal the regularities in the change of the frequency characteristic of the drilling installation as a function of changes in the rotational speed of the tool and changes in pressure, without the drilling process, a thrust bearing made from a roller bearing was used.
Registration of vibrations was performed by a combined setup, which consisted of a sensitive vibration element—an induction sensor of type SP-1, SPED-56 (seismic receivers), or a ribbon microphone (laryngophone), a broadband amplifier, and a frequency analyzer of type ASChKh-1.
The sensor was fastened by means of an adapter to the piston rod of the rotator cylinder. A small portion of the experiments was carried out with the sensor fastened to the driving rod.
The recording scale on the analyzer screen was set by means of a GZ-1 type audio generator.
The spectrograms were recorded either on tracing paper (by hand) or on film with a Zorkii type camera directly from the analyzer screen, separately for two ranges: 0–20000 and 0–5000 Hz, at various settings of the ASChKh-1 divider (30, 100, 300, and 1000) and at different sweep times: 3–15 sec.
Results of the Investigation and Conclusions
As a result of the investigation, about 300 documented experiments were conducted, providing definite factual material on the topic of the study. Analysis of the data obtained makes it possible to draw the following conclusions.
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On spectrograms recorded from different sensors of different types and from two sensors of the same type (SPED-56), but with their being fastened on different piston rods of the hydraulic feed cylinders and on the driving rod at different times, yet under identical conditions, no substantial changes in the frequency and amplitude of the vibrations are observed.
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Changing the type of carbide bits (all other conditions being equal) does not affect the characteristics of the spectrograms.
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Flushing also does not affect the character of the spectrograms.
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All frequency groups characterizing the operating state of the installation under the conditions of the experiments fall within the range of 0–5000 Hz.
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The amplitude of the recorded signal depends to a great extent on the rotational speed of the tool: when passing from the first speed to the fourth (the number of revolutions per minute of the tool being, respectively, 71 and 470), the recording amplitude increases by a factor of 5–10.
When the axial pressure is changed, the recording amplitude changes only slightly: when the pressure is doubled (from 500 to 1000 kgf), the amplitude of the fundamental frequency practically does not change.
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When drilling sandstones, the distinguished frequency group of 850–1050 Hz remains without substantial changes. The frequencies of 500 and 600 Hz of the main background merge into one frequency group of 400–800 Hz (Fig. 1).
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When drilling marbles, no substantial changes in frequency are observed in comparison with drilling sandstones. However, a sharp increase in the amplitude of the fundamental frequencies is noted (Fig. 2).
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When drilling granites, as in the case of marble, spectrograms are obtained with an indistinct gradation of frequency groups according to the rate of amplitude decay.
Fig. 1. Spectrogram of the frequency–amplitude characteristic of the drilling installation during drilling of sandstone.
Experimental conditions: rotational speed of the tool—IV (470 rpm); frequency range—2000 hertz; ChA-1 divider—100; SPED-type sensor; pressure 300 kg, flushing 25–30 l/min; bit reinforced with eight-sided cutters, diameter 63 mm.
Fig. 2. Spectrogram of the frequency–amplitude characteristic of the drilling installation during drilling of marble.
The experimental conditions are the same as in the preceding case, except that the bit is of type TP-3, diameter 75 mm.
However, the following groups of frequencies, which can be traced on all spectrograms, may be distinguished: the groups 480–790 and 820–1110 Hz (Fig. 3).
- On all spectrograms the principal (marking) groups of frequencies noted for idle rotation of the drill string are maintained, with some shift of these groups to the right or to the left.
In view of the small scope of the investigations carried out in this area, it is as yet impossible to judge the practical use of the elastic vibrations of the drilling tool for detecting contacts between rocks or a mineral. For a final solution of this question,
Fig. 3. Spectrogram of the frequency-amplitude characteristic of the drilling rig during drilling in granite.
Experimental conditions: drill-string rotation speed—IV (470 rpm); frequency range—0–2000 Hz; divider of the frequency-response analyzer—1–1000; SPED-type sensor; pressure 500 kg, flushing—25–30 l/min, TP-3 type bit, diameter 75 mm.
further work is required, taking into account previous laboratory investigations and a number of recommendations, namely:
1) to carry out similar laboratory investigations on a larger scale when drilling a greater number of rock samples;
2) to take a wider range of standard sizes of hard-alloy bits;
3) to carry out comparative investigations on different rigs;
4) to carry out analogous investigations during coreless drilling of boreholes;
5) to carry out investigations during borehole drilling in order to clarify the question of the need to install downhole or surface sensors;
6) to test other types of sensors.