SOME RECOMMENDATIONS FOR SELECTING INDIVIDUAL PARAMETERS OF THE TECHNICAL CHARACTERISTICS OF INSTALLATIONS FOR VIBRATION-ROTARY DRILLING

V. M. MATROSOV

(Presented by the scientific seminar of the Department of Exploration Engineering)

Increasing the efficiency of technological processes carried out with the aid of machines is often achieved at the present time through the broad use of various physical and chemical phenomena. In some cases they accelerate or slow down a process; in others they improve the quality of work, reduce or prevent accidents, etc. Phenomena of this kind include mechanical oscillations, or vibrations, i.e., periodic, sufficiently small displacements of systems.

Vibrating machines, i.e., machines in which oscillations are imparted to the working member, have in recent years become very widely used in many branches of industry and agriculture and are employed in performing the most varied technological operations, such as compaction, separation, loading, transportation, grinding, plowing, driving piles, sheet piles, and pipes into the ground, drilling rocks, drilling metals and pouring them into molds, relieving stresses in castings, testing various monitoring and measuring instruments, and others [1]. In mining enterprises alone, according to A. O. Spivakovsky and I. F. Goncharevich, vibrating machines are used to perform 35 different technological operations [2].

Oscillatory processes are also becoming widespread in drilling practice, both for the direct sinking of boreholes and for auxiliary purposes: casing wells and extracting pipes, eliminating certain accidents in wells, and cleaning drilling fluids of sludge (Table 1).

Depending on the working conditions, two cases may be distinguished for the use of oscillatory processes directly in drilling wells: 1) vibration drilling (driving) in soft rocks; 2) vibration drilling in combination with rotation of the tool (vibration-rotary drilling) in rocks of medium hardness and hard rocks.

As vibration exciters in these cases, vibrating mechanisms of two types are used: surface vibrators and downhole vibrators. Mechanisms of the first type include mechanical centrifugal vibrators of directed and undirected action, vibratory hammers, and others, distinguished by high power and considerable overall dimensions. The field of application of such mechanisms is limited, since

as the oscillations excited by them rapidly attenuate with depth. In practice, such vibrators find application only in drilling shallow boreholes. For drilling with surface vibrators, the Central Design Bureau of the State Geological Committee of the USSR has created a whole series of installations: BUV-1, UVB-3A, UVB-15, UVB-25A.

Mechanisms of the second type (turbo-downhole, hydraulic, etc.) are more promising, since they are located in the borehole and are directly connected with the working element—the core bit or the drill bit. However, to date there are still no serviceable industrial designs of vibrators of this type. Therefore, in drilling practice only submersible hydraulic impactors are still being used.

The high efficiency of vibration-drilling mechanisms gives grounds to believe that they will take one of the leading places in drilling technology in the near future. For the creation of high-productivity drilling

Table 1

Field of application of oscillatory processes in drilling

machines of vibration-rotary action and their broad introduction into exploratory drilling practice, it is necessary to solve a whole series of important scientific and technical problems. Until recently, in the practice of designing vibration-drilling installations, their optimal parameters were not calculated. At best, only calculations of individual assemblies and elements of the installation for strength under the action of alternating dynamic loads were performed. Therefore, during tests of prototypes, individual parameters of the technical characteristics of vibration installations did not correspond to the optimal drilling regimes.

The difficulty of solving the stated problem in designing vibration-rotary installations lies in the large number of parameters that directly or indirectly affect the efficiency of this drilling method. Some of these parameters are associated with the technology of rotary drilling, while others are associated with the design of the vibration exciter—the vibrator. Creating optimal conditions for the operation of a vibration-rotary installation requires an organic interrelation of all these parameters. Only if this condition is met is it possible to achieve the most effective results.

The objective of the present work does not include the development of structural diagrams of vibration mechanisms for particular operating conditions. Here, an attempt is made only to give recommendations for selecting the principal parameters of the technical characteristics of a vibration-rotary drilling installation, both with surface and with downhole vibrators, and the modes of its operation as a function of the physicomechanical properties of rocks. These recommendations have been compiled on the basis of materials from studies of the vibration-rotary method of drilling with an annular bottom hole, carried out in 1957–1961 at the Department of Exploration Engineering of the Tomsk Polytechnic Institute [3, 4]. Materials on vibration-rotary drilling published in periodical literature were also used.

The principal parameters of the vibration-rotary drilling mode include:

1) the exciting force of the vibrator $P_{\max}$, kg;
2) the axial load on the bottom hole $Q$, kg;
3) the vibration frequency of the vibrator $n$, cycles/min;
4) the circumferential speed of the bit $W$, m/sec;
5) the intensity of flushing the borehole.

There are also a number of other parameters (the moment of the eccentrics of the centrifugal vibrator, the amplitude of oscillations of the drilling tool, the ratio of the frequencies of natural and forced oscillations in the “drilling tool—vibrator” system, etc.) that also affect the efficiency of vibration drilling. However, these parameters are closely related to the principal ones or are predetermined by their combination.

Below, each of the listed parameters of a vibration-rotary installation is considered from the standpoint of assigning its optimal values for the conditions of core drilling.

Exciting force of the vibrator. This parameter of the vibro-installation depends on the moment of the vibrator eccentrics and the vibration frequency and is related to them by the following dependence:

\[ P_{\max}=\frac{M\cdot n^2}{44750}, \tag{1} \]

where $M$ is the moment of the vibrator eccentrics, kg $\times$ cm.

As studies [3, 4, 5] have shown, the mechanical drilling speed by the vibration-rotary method is directly proportional to the exciting force of the vibrator. Consequently, other conditions being equal, to achieve high productivity it is necessary to strive for the maximum values of the exciting force of the vibration installation.

The upper limit of the values of the exciting force of the vibrator must be established in each specific case depending on the strength characteristics of the drilling equipment used, in particular, the drill pipes. In this respect it should be noted that, up to the present time, there is no generally accepted method for calculating drill pipes for such loads.

loads, and this circumstance leads to a poorly substantiated selection of equipment for vibratory drilling.

For surface vibratory mechanisms, high values of the disturbing force should be achieved by increasing the moment of the vibrator eccentrics, since in this case there are no major restrictions with respect to the dimensions of the latter. To proceed here by increasing the vibrator’s disturbing force through a high frequency of oscillation is hardly advisable, since in this case energy expenditures increase significantly, while the period of action of the rock-breaking tool on the bottom decreases, which can lead only to fatigue failure of rocks, a process of low effectiveness.

In downhole vibratory mechanisms of the centrifugal type, located in the borehole and constrained by its dimensions, large values of the disturbing force can be obtained only as a result of increasing the frequency of oscillations. In this case the unit impulse created by the vibrator must be no less than the destructive impulse for the given rock [3]. In other types of downhole vibratory mechanisms, for example hydraulic, hydraulic-ram, and others, the magnitude of the impact dynamic impulse is closely dependent on the flow rate of the flushing fluid. The limiting value in this case will depend on the magnitude of the hydraulic resistances arising when the maximum amount of fluid is supplied through the adopted cross section of the drill pipes.

When working with drill pipes of the core-drilling assortment, the maximum value of the vibrator’s disturbing force may be recommended within the limits of 1500–2000 kg. If a special drilling tool is available, or in the case of drilling shallow boreholes, the vibrator’s disturbing force may reach 10,000–15,000 kg. Of great importance in selecting the magnitude of the vibrator’s disturbing force for conditions of core exploratory drilling will be the service life of the bit used, and this factor must not be overlooked.

Axial load on the bottom. In drilling by the vibration-rotary method, this parameter increases the energy of the unit impulse, reduces the power consumption required by the vibrator, and stabilizes the operation of the entire vibratory installation as a whole.

At small values of the axial load, the drill string will perform oscillations with a large amplitude, but with small velocities and accelerations within one cycle. The impact energy in this case will be small. If, however, the axial load exceeds the value of the maximum disturbing force of the vibrator, then the displacements of the drill string often disappear entirely, and the effect of vibrations will not appear. Therefore, when selecting values of axial load for conditions of core exploratory drilling, as a first approximation one may use the recommendations of V. A. Kruzhkov [8]. According to these recommendations, the impact velocity and the effect of the action of the rock-breaking tool on the borehole bottom reach a maximum when the ratio of the axial load to the maximum disturbing force of the vibrator is equal to 0.4. When this ratio is decreased, the impact velocity decreases; when it is increased, pauses arise in the operation of the vibratory installation, and it begins to “stall.” In our investigations the vibratory installation operated most stably when the values of the indicated ratio were within the limits of 0.3–0.4.

It should be noted that when working with downhole vibrators it is advisable to create the axial load with the aid of a heavy bottom assembly.

Frequency of vibrator oscillations. As was indicated above, by changing the frequency of oscillations of the vibrator one can regulate the magnitude of the disturbing force (for vibrators of the centrifugal type). In addition, the frequency of oscillations of the vibrator is the principal factor-

…which determines the frequency at which impact pulses are applied to the bottom of the borehole (usually it is equal to, or a multiple of, the frequency of oscillation of the vibrator).

As the frequency with which impact pulses are applied to the bottom of the borehole increases, the drilling rate, as a rule, increases. In view of this, increasing the frequency of oscillation of the vibrator, other conditions being equal, has a positive effect on the productivity of vibration-rotary drilling. At the same time, however, it should be remembered that, in parallel with the increase in the vibrator oscillation frequency, its disturbing force also increases, and in the second power. Therefore, for large values of the eccentric moment, the vibrator oscillation frequency should be limited to within 1500–2000 oscillations/min. For small values of the eccentric moment of the vibrator, the oscillation frequency may be increased considerably, but in this case it is necessary to carry out a careful calculation of the equipment for alternating loads.

When the contact pressure is insufficient for a volumetric process of rock destruction to occur at the bottom of the borehole (at small values of the disturbing force of the vibrator), increasing the oscillation frequency is the only way to maintain the rate of borehole deepening at a high level. In this case, fatigue failure of brittle rocks will occur.

The upper limit of the vibrator oscillation frequency may be restricted on the basis of considerations of the optimum time of contact between the crown cutter and the rock at the bottom of the borehole [6]. As is known, an increase in the vibrator oscillation frequency correspondingly entails a decrease in the duration of the crown’s action on the bottom during one cycle. Therefore, at a high oscillation frequency of the drilling assembly, the duration of contact between the crown and the rock may prove to be less than the optimum time (for roller-bit drilling, according to R. M. Eigeles, 5–8 milliseconds). This will entail a sharp decrease in the efficiency of rock destruction. To prevent such phenomena, the vibrator oscillation frequency should be limited:

\[ n_{\mathrm{в}} \ll \frac{60}{\tau_0} = \frac{60}{(0{,}005 \div 0{,}008)} = 12000 \div 7500\ \text{osc/min}. \tag{2} \]

where \(\tau_0\) is the optimum contact time, sec;
\(n_{\mathrm{в}}\) is the upper limit of the vibrator oscillation frequency, osc/min.

The lower limit of the vibrator oscillation frequency must ensure that the disturbing force of the vibrator exceeds the value of the axial load. This condition is expressed by the following relation:

\[ n_{\mathrm{н}} \gg 212 \cdot \sqrt{\frac{Q}{M}}, \tag{3} \]

where \(n_{\mathrm{н}}\) is the lower limit of the vibrator oscillation frequency, osc/min.

From the standpoint of ensuring stable operation of the vibration installation

\[ \left(\frac{Q}{P_{\max}} = 0{,}4\right), \]

the vibrator oscillation frequency may be found from the expression:

\[ n_{\mathrm{с}} = 334 \sqrt{\frac{Q}{M}}. \tag{4} \]

Circumferential speed of crown rotation. In vibration-rotary drilling, the phenomenon of oblique impact is observed: depending on the ratio of the circumferential and vertical velocities, the crown cutters penetrate the rock at one angle or another to the surface.

face. The volume of rock chipped in one cycle and the mechanical drilling rate reach a maximum at the optimal angle of attack \(\rho_0\), which complements the friction angle for the given “cutter—rock” pair to \(90^\circ\) [3, 4].

The parameters considered above provide a certain value of the vertical velocity of the bit. To ensure the most effective operation of a vibration-rotary installation, the value of the circumferential rotational velocity of the bit must be selected so that the resultant direction of motion of the bit cutters coincides with the optimal angle of attack for the specific drilling conditions. In this connection, the circumferential rotational velocity of the bit must be closely coordinated with the other parameters of the drilling regime by the vibration-rotary method. In doing so, it is necessary to know the magnitudes of the optimal angles of attack for various “rock—cutter” pairs.

The analytical coordination of the circumferential rotational velocity of the bit with the other parameters of the vibration installation, as well as the values of the optimal angles of attack, were given in previously published works [3, 4]. Here it should be noted that, when calculating the required circumferential rotational velocity of the bit, one must first specify the values of the axial load and the frequency of oscillations of the vibrator. Then, for the given moment of the vibrator eccentrics and the value of the optimal angle of attack for the specific conditions, the circumferential velocity of the bit is calculated.

With the circumferential velocity of the bit thus found, the number of revolutions per minute of the drilling tool is calculated by the formula:

\[ n_0 = \frac{60 \cdot w}{\pi \cdot D_{\mathrm{cp}}}, \tag{5} \]

where \(n_0\) is the number of rev/min of the drilling tool;
\(D_{\mathrm{cp}}\) is the mean diameter of the bit, m.

Figure 1 presents a graph of the dependence of the circumferential rotational velocity of the bit on the frequency of oscillations of a vibrator having an eccentric moment equal to \(50\ \text{kg}\cdot\text{cm}\), constructed for various axial loads and optimal angles of attack. From analysis of this graph it follows that the circumferential rotational velocity of the bit must increase with increasing frequency of vibrator oscillations and decrease with increasing axial load and values of the optimal angle of attack. For other values of the eccentric moment, the character of the change in the circumferential velocity of the bit will be preserved, and it will differ only in absolute value.

When using existing machines with known numbers of revolutions of the circumferential velocities of the bit for vibration-rotary drilling, these should be taken as given, and the other parameters of the optimal technological operating regime of the vibration installation should be calculated. An example of a practical calculation of the parameters of the optimal operating regime of a vibration-rotary installation is given below.

Example. Determine the frequency of oscillations of the vibrator and the magnitude of the axial load for optimal operation of the vibration installation under the following conditions:

1) drilling is carried out in marble, \(\rho_0 = 78^\circ\);

2) circumferential velocity of the bit, \(w = 0.15\ \text{m/sec}\);

3) moment of the vibrator eccentrics, \(M = 50\ \text{kg/cm}\);

4) ratio

\[ \frac{Q}{P_{\max}} = 0.3. \]

The relationship between the parameters of the optimal regime of vibration drilling…

...with a vibration-rotary installation is expressed by the equation derived by the author [4]:

\[ w=\frac{294.3}{n\cdot \operatorname{tg}\rho_0} \left[ 0.64\sqrt{\left(\frac{P_{\max}}{Q}\right)^2-1}+(1+2K) \right], \]

where

\[ K=\frac{1}{\pi}\arcsin\frac{Q}{P_{\max}}. \]

Substitute the numerical values into the equation and solve it with respect to \(n\).

\[ K=\frac{1}{3.14}\arcsin 0.3=0.095; \]

\[ n=\frac{294.3}{0.15\cdot \operatorname{tg}78^\circ} \left[ 0.64\sqrt{\left(\frac{1}{0.3}\right)^2-1}+(1+2\cdot 0.095) \right] =1350\ \text{rev/min}. \]

We find the magnitude of the maximum exciting force of the vibrator at \(n=1350\ \text{rev/min}\):

\[ P_{\max}=\frac{M\cdot n^2}{44750} =\frac{50\cdot 1350^2}{44750} =2000\ \text{kg}. \]

We calculate the magnitude of the axial load:

\[ Q=0.3\cdot P_{\max}=0.3\cdot 2000=600\ \text{kg}. \]

It follows from the example given that, using the proposed recommendations and formulas, one can quickly select the best parameters for the operating regime of a vibration installation.

Intensity of borehole flushing. Flushing of boreholes in vibration-rotary drilling is determined by the same factors as in ordinary drilling without vibrations, i.e., by the drilling method (shot or carbide) and by the volume of rock destroyed per unit time. The amount of flushing fluid supplied to the borehole can be determined in each specific case in accordance with the existing instructions and recommendations [7].

If the flushing fluid is used to actuate a downhole vibrator, special calculations must be made.

Arrangement of the bottom-hole drilling tool. The recommendations given for selecting individual parameters of the technical characteristics of vibration-rotary-action installations, of course, do not cover all possible cases and conditions encountered in conducting exploratory drilling operations. Therefore these conclusions should be refined in each specific case. For example, when using bottom-hole vibrators, it is advisable to abandon the rigid connection of the drilling tool with the drill-pipe string in order to improve the working conditions of the latter. In this respect there already exist a number of recommendations for bottom-hole hydraulic hammers (VINIBT, TsKB MG and ON). Figure 2 presents a diagram of the proposed arrangement of a drilling tool for vibration-rotary drilling with a bottom-hole vibrator. The tool is assembled in the following sequence: bit, core barrel, adapter (sludge pipe), bottom-hole vibrator, weighted drill pipes, coupling with a hexagonal opening, hexagonal rod, string of ordinary drill pipes. Owing to the presence of the coupling and the hexagonal rod, the core barrel with the vibrator and heavy bottom section, receiving rotation from the surface, can move under the action of vibrational oscillations in the axial direction relative to...

of the drill-pipe string. In this case the drill pipes will be isolated from the vibrational oscillations of the drilling assembly, remaining in a suspended state. The axial load on the bottom of the borehole must be produced entirely by the weight of the weighted drill pipes.

Fig. 1. Graph of the change in the peripheral rotational speed of the bit as a function of the vibration frequency of the vibrator at various axial loads and certain optimal angles of contact.

\(1\) — \(\rho_0 = 76^\circ\); \(2\) — \(\rho_0 = 84^\circ\).

Fig. 2. Arrangement of the lower part of the drilling assembly.
1 — bit; 2 — core barrel; 3 — adapter; 4 — bottom-hole vibrator; 5 — weighted drill pipes; 6 — coupling; 7 — six-sided rod; 8 — string of ordinary drill pipes.

The bottom-hole vibrator should be placed directly above the core barrel. If it is placed, for example, above the heavy lower assembly, then in the event of failure of the vibrator it will be much more difficult to eliminate the resulting accident. In addition, the vibrational oscillations transmitted to the bit through the heavy lower assembly will be attenuated.

Conclusions

The selection of the optimal combination of individual parameters of the technical characteristics of a vibration-rotary installation is a very complex problem, the solution of which requires analyzing

a large number of different variants. A variant selected on the basis of the most efficient operation of the installation (at optimal angles of contact of the drilling tool with the bottom-hole surface) may be unacceptable from the standpoint of steady, stable operation of the vibration installation. Finally, a variant satisfying the indicated conditions may prove unsuitable because of insufficient strength of the drilling equipment, etc.

The complex and painstaking work of selecting the best variant of the technical characteristics of a vibration installation can be successfully carried out by using modern research methods. Great prospects in this respect are opened up by the development of methods of electronic modeling for determining rational parameters of vibration-drilling installations [9].

The creation of installations of vibratory-rotary action possessing the most rational parameters of the technical characteristics will, to a significant degree, increase labor productivity in drilling operations, reduce the cost and labor intensity of the work, and facilitate the possibility of automating the drilling process.

REFERENCES

1. Artobolevsky I. I., Bessonov A. P., Shlyakhtin A. V. On Machines of Vibratory Action. Publishing House of the Academy of Sciences of the USSR, 1956.

2. Goncharevich I. F., Spivakovsky A. O. The state and tasks of scientific research in the field of the development of vibration engineering in mining. In: Problems of Mining, IGD, Academy of Sciences of the USSR, 1958.

3. Matrosov V. M. Investigation of the Vibratory-Rotary Method of Drilling Wells with an Annular Bottom. Abstract of Candidate’s Dissertation. Tomsk, 1961.

4. Matrosov V. M. Some questions of the theory of vibratory-rotary drilling with an annular bottom. Proceedings of TPI, vol. 120, 1962.

5. Voskresensky F. F. et al. Vibratory and Percussion-Rotary Drilling. Gosgortekhizdat, 1961.

6. Eifler F. M. On certain regularities of the dynamic penetration of bit teeth into rock. Neftyanoe khozyaistvo, No. 8, 1956.

7. Vozdvizhensky B. I., Volkov S. A. et al. Exploratory Core Drilling. Gosgeoltekhizdat, 1957.

8. Kruzhkov V. A. Calculation of the basic parameters of a vibratory-rotary drilling rig. Gornyi zhurnal, No. 9, 1957.

9. Ashavsky A. M., Litvinov N. N. Application of electronic modeling installations for calculating the optimal parameters of the drilling process. Razvedka i okhrana nedr, No. 5, 1960.