EXPERIMENTAL DATA ON THE ELIMINATION OF ACCIDENTS AND THE CASING OF BOREHOLES WITH PIPES BY MEANS OF VIBRATORS IN EXPLORATORY DRILLING

P. F. PALYANOV

In exploratory drilling, accidents in boreholes are, by their nature and complexity, extremely diverse. In core drilling, accidents connected with the sticking of the drilling tool at the bottom of the borehole may be divided into three main groups:

1) when drilling with hard-alloy bits or diamonds, the assembly is often stuck by settled sludge in the annular clearance between the walls of the core barrel and the walls of the borehole, and also by the collapse of the assembly due to pieces of rock that have fallen from the borehole walls, or by the jamming of the assembly in weakly stable beds, etc.;

2) in shot drilling, the drilling assembly is wedged by shot and shot sludge;

3) seizure of the crown bit.

The elimination of accidents connected with the sticking of the drilling tool at the bottom is at present carried out mainly by one of the following methods, or by methods used successively one after another:

1) knocking the tool free with the aid of a percussion jar;

2) pulling out the stuck assembly with the aid of a jack or a tackle system and the drilling rig winch;

3) if the preceding methods of eliminating the accident do not produce positive results, then the drill-rod string is unscrewed with a set of left-hand fishing tools, and it is extracted to the surface in sections;

4) if even the last measure proves unsuccessful, a deflecting wedge is lowered into the borehole, the borehole is deviated to the side, and drilling is continued.

Sometimes all these methods of eliminating accidents prove ineffective, and the borehole, often already drilled to a considerable depth, has to be abandoned and drilling of a new one begun.

Statistical data show that 10–12% of all the time [1] spent on drilling boreholes, and in some drilling parties this time reaches 30–40%, is expended on eliminating accidents and malfunctions in boreholes.

Work on eliminating accidents in boreholes is usually of a protracted nature, and large sums are, as a rule, spent on this work. Protracted operations for eliminating accidents lead to premature wear of the equipment, and not infrequently put drilling equipment and tools completely out of service. Moreover, the very process of eliminating accidents is entirely unmechanized and is carried out by old methods established long ago. The principal force in this case is human muscular energy, while the labor of the workers is exceptionally heavy and of low productivity. The protracted character of the process of eliminating accidents by existing methods may be shown by the following example.

In a geological exploration party, during shot drilling in category X rocks with a KA-2M-300 rig, the tool became stuck at the end of the shift when the core jammed at a depth of 53 m. It was not possible to break off the core, and therefore, in the course of time, the tool was additionally jammed by settled sludge between the walls of the core barrel and the walls of the borehole. The borehole diameter was 110 mm.

The accident was eliminated in the following sequence. When an attempt was made to break off the core, the rods were broken, and only after 2.5 hours was an overshot lowered and screwed onto the rods left in the borehole. While the tool was being knocked out with a percussion jar, the overshot was torn off. Then a tap was lowered into the borehole on rods and, after successfully screwing it into the core barrel, they again began knocking out the tool with a percussion jar. The knocking-out continued for 2.5 working shifts, with 8 people working simultaneously.

After an unsuccessful attempt to knock out the tool, a 40-ton jack was installed and they began pulling out the tool, but the rods were again broken. Then an emergency rod string with a left-hand thread and a left-hand fishing tool were lowered, and they began unscrewing and extracting, in parts, the string left in the borehole. The adapter could not be unscrewed, and therefore it was decided to cut it.

Eight days were spent on all these operations, i.e., 24 working shifts.

After cutting and extracting the adapter, the borehole inside the emergency pipes was drilled through with a tool 75 mm in diameter, to 1.5 m below the bottom, and they began cutting the core and sludge pipes with pipe cutters. The first cut was made unsuccessfully—too low; therefore, when a tap for pipes 110 mm in diameter was lowered and screwed in, it still proved impossible to pull out the cut-off part, and the rods were again broken near the tap itself. The tap lowered and screwed into the core tool had been made of steel, well heat-treated; therefore, it could not be caught by anything.

After a whole series of other unsuccessful attempts to eliminate the accident, it was decided to abandon the borehole, move the derrick alongside, and begin drilling a new borehole.

Seventeen days were spent eliminating the accident, that is, 51 working shifts. The losses incurred on this borehole amounted to 36,612 rubles. To this should also be added the cost of the tool that was put out of service: 3 taps, 1 overshot, 3 broken rods, broken clamps, and a lost 6-meter core tool in the borehole.

Thus, as this example shows, the existing methods of eliminating accidents in boreholes are of low productivity and costly; moreover, the process of eliminating accidents is technically poorly equipped, little mechanized, and based entirely on the use of human muscular energy.

Casing boreholes with pipes is also a rather labor-intensive process, taking much working time and effort from the drilling crew. Methods of casing boreholes may be divided into two main types: 1) forcing pipes into the borehole under the action of a static load (additional weight, jacks, pressure heads, anchor bolts, etc.), with rotation of the pipes or without rotation; 2) driving pipes by means of a percussion jar, also sometimes in combination with rotation.

The existing methods of casing boreholes with pipes suffer mainly from the following shortcomings: 1) the casing process is carried out by old, long-established, primitive methods; 2) in casing boreholes, human muscular energy is predominantly used, and the worker’s labor is heavy and of low productivity; 3) rather bulky devices are used for casing boreholes, which complicate the drilling process; 4) when

in the casing of boreholes, the motors installed on the drilling tower are almost never used.

It follows from this that work on eliminating accidents in boreholes and on casing boreholes with pipes is completely out of keeping with the modern level of technology. Heavy manual labor still reigns supreme here. Therefore, the question of introducing modern mechanization into these labor-intensive operations in exploratory drilling is very important and of primary importance.

Fig. 1. Installation of a vibrator on rods during accident elimination

Fig. 2. Casing boreholes with pipes by means of a vibrator

Our theoretical and experimental investigations have shown that, for eliminating accidents in boreholes and for casing boreholes with pipes, vibrations can be used with great success, since long steel pipes are an elastic system and oscillations can be transmitted through them.

The process of eliminating accidents with the aid of a vibrator proceeds as follows (Fig. 1). At the upper end of the rod string an exciter of oscillations is installed—a vibrator, which during operation creates ver-

tical alternating-sign oscillations throughout the entire rod string. These oscillations of the rod string cause the fishing tool seized at the bottom of the well to oscillate—to vibrate. If, at the same time, one begins to pull the tool upward, then the pulling force sufficient to free the tool from seizure under vibration will be many times smaller than the pulling force required to free the tool without vibration. This circumstance is explained by the fact that, under vibration, the friction between the walls of the well and the fishing tool is reduced in comparison with the friction when the tool is pulled without vibration.

Similar phenomena are also observed when casing wells with pipes by means of a vibrator (Fig. 2). A vibrator installed through a transition piece at the upper end of the casing pipes creates, during its operation, oscillations—vibrations—throughout the entire casing string. The vibrating pipe will descend into the well under a smaller axial pressure, since much less friction will arise between the pipe walls and the well walls than when pipes are lowered without vibration. Therefore, under certain conditions, for lowering pipes into a well under the action of vibrations, the weight of the casing string itself will be quite sufficient to carry out the casing of the well.

In addition, with the aid of a vibrator it is also possible to drill shallow wells in soft rocks. In this case (the work of Prof. D. D. Barkan and others), the vibrator, which causes frequent vertical oscillations in a pipe with a longitudinal slot along its entire length, forces the pipes to sink into the soft ground with a smaller axial force. After the pipe has been sunk, it is pulled out, and the occurrence of the ground is judged from the rocks extracted in the pipes.

The work of Prof. D. D. Barkan and others in this field showed that a ten-meter well is drilled in half an hour instead of the 8–12 hours that were spent on drilling the same well by hand. The drilling productivity thereby increased by 15–25 times in comparison with manual drilling, and, correspondingly, the cost of drilling the well was 3–6 times lower.

Vibrators Used for Experimental Work

For carrying out experiments on the use of vibrators for eliminating accidents in drilling wells and for casing wells with pipes, several types of vibrators were manufactured and tested, differing both in dimensions and characteristics and in the fundamental schemes of their operation. Tests of the vibrators were carried out both on the drilling rigs of the Department of Exploration of the institute and directly under field conditions, that is, in geological exploration parties.

To create harmonic excitation, centrifugal forces of unbalanced rotating masses are usually used (Fig. 3). Two identical unbalanced weights, located at the same distance from the axes of rotation, rotate in opposite directions, with the same angular velocity and with the same phase angle. The centrifugal forces arising as a consequence of the rotation of the unbalanced, eccentrically located weights—unbalances—are equal to one another in absolute magnitude, but constantly change their direction.

The horizontal components \(S_1\) and \(S_2\) of the centrifugal forces are always directed in opposite directions and will therefore be equal to one another and, at any instantaneous position of the unbalances, mutually balance each other. The vertical components \(F_1\) and \(F_2\) are directed in one and the same direction and create the disturbing force of the vibrator.

The maximum disturbing force of the vibrator will be observed when the unbalances are located on the vertical axis, that is, at \(90^\circ\) and \(270^\circ\).

Its value is determined by the formula

\[ P_{\max}=P_1+P_2=2mr\omega^2, \]

or

\[ P_{\max}=2 \cdot 0.001118\, G r n^2, \tag{2} \]

where
\(m\) is the mass of one load,
\(r\) is the eccentricity in m,
\(\omega\) is the angular velocity of rotation of the unbalances,
\(G\) is the weight of one unbalance in kg,
\(n\) is the number of rpm of the unbalances.

Fig. 3.
Diagram of operation of a two-block vibrator

Fig. 4. Two-block eccentric vibrator

1—metal plate. 2—frame. 3—shaft. 4—vibrator gears. 5—eccentrics. 6—drive gears. 7—electric motor. 8—single-sided nipple. 9—housing cover.

The maximum disturbing force of the vibrator is equal to zero when the unbalances are on the horizontal axis, that is, at \(0\) and \(180^\circ\). Such a vibrator is customarily called a directional-action vibrator.

According to this scheme, the first experimental vibrator was designed and manufactured (Fig. 4). Unbalances weighing \(1\) kg each were mounted on two identical gears at a distance from the axes of rotation of the gears \(r = 52\) mm. The gears are meshed with each other, which ensures rotation of the unbalances with the same angular velocity and with the same phase angle, but in opposite directions.

To drive the vibrator, a small electric motor with a power of \(0.3\) kW was installed. Rotation from the motor to the vibrator shaft was transmitted through a pair of gear wheels. At \(3500\) rpm of the vibrator gears, the latter developed a maximum disturbing force equal to \(1370\) kg.

This vibrator has found wide application for eliminating accidents and casing wells with pipes on the department’s drilling rigs, where the wells are predominantly of small depth. Its main shortcoming is that the maximum disturbing force developed by the vibrator is far from sufficient for eliminating accidents in deeper wells; therefore, in designing subsequent vibrators this shortcoming was taken into account.

The VP-7 vibrator (Fig. 5), like the preceding one, is made according to the scheme of an eccentric vibration exciter of directed action and creates sinusoidal alternating vibrations directed along the vertical axis.

The calculated value of the maximum disturbing force of the vibrator, equal to 7 tons, was adopted on the basis of the strength of the rods under dynamic loads. The construction of the vibrator is as follows.

Fig. 5. VP-7 vibrator

All parts of the vibrator are mounted in housing 2, which is forged from steel 25 mm thick. The housing is intended to transform the centrifugal forces of the rotating weights into vertical alternating forces; therefore it must be very rigid, so that alternating loads do not deform it. The one-sided nipple 3, installed in the bottom of the housing, receives the vertical vibrations of the vibrator and transmits them to the drill rods on which the vibrator is installed.

Two unbalances 1 are made in the form of eccentrics. The adopted form of the unbalanced weights is the simplest to manufacture, and their fabrication is within the capability of any repair shop of a geological prospecting party. The unbalance shafts are installed in the vibrator housing on ball or roller bearings No. 407 or No. 2407.

One of the shafts has an end projecting from the vibrator housing, and the drive pulley 9 is mounted on it by means of a key. The diameters of the unbalances are 90 mm, while the holes in the vibrator housing for the bearings have a diameter of 100 mm, which makes it possible to insert the weights into the vibrator housing during assembly through the bearing seats.

On the side of the housing opposite the pulley, both unbalance shafts have projecting ends, on which two identical gears 5 are mounted on keys, forming a synchronizing reducer.

Each gear is fastened on the shaft by means of locknut 11 with a retaining ring. The gears have a high circumferential speed during rotation; therefore an oil bath is provided for them, formed between

stamped cover 7 and the vibrator body. To create a seal between the cover and the body, a gasket 12 made of oiled cardboard is installed between them. The bolts by means of which the reducer-synchronizer cover is fastened to the vibrator body are carefully cotter-pinned with wire, since unpinned bolts are easily unscrewed under the action of vibrations.

The bearings are closed with covers 8, which on the reducer side have a gland seal so that as little oil as possible from the reducer gets inside the vibrator body. Covers 8 also serve for adjusting the bearings.

The vibrator drive was effected by a three-phase alternating-current electric motor with a power of 4.5 kW. The motor was installed above the vibrator on cover 10. The motor feet operate under stressed conditions; therefore rubber shock absorbers were placed under them and, in addition, the motor itself was fastened to the vibrator cover by means of clamps. Transmission from the electric motor to the vibrator pulley was carried out by means of a V-belt of grade B.

Characteristics of the VP-7 vibrator

This vibrator was manufactured in a geological-prospecting party and underwent successful tests for eliminating accidents in boreholes during shot drilling.

However, during testing of the vibrator a number of substantial shortcomings in its design were revealed. The vibrator gears proved too weak and literally after 30 minutes of operation went out of order, since they were made of St-3 steel and were not case-hardened. The keys by which the gears were connected to the shafts, under the action of vibrations, also quickly became loose and gave a large backlash. When pressing the bearings into the body, a slight misalignment was allowed, which led to rapid heating of one of the bearings, rapid burning out of the lubricant in the bearing, and even to strong heating of the vibrator body itself. The fastening of the nipple to the vibrator body also proved to be far from sufficient; therefore the one-sided nipple had to be welded to the bottom of the body. A major shortcoming of the vibrator was the form of construction of the body, which proved difficult to manufacture. After the elimination of the listed shortcomings, the VP-7 vibrator found wide application in drilling operations in some GRPs.

VP-1 vibrator. The author designed the first version of the vibrator intended for use in production conditions (Fig. 6). In designing this vibrator, all the shortcomings revealed during testing of the previous vibrators were taken into account.

One of the most important advantages of this vibrator over all the others is that, owing to the vertical arrangement of the shafts (one shaft above the other), the vibrator creates in the rod string two kinds of oscillations: vertically directed and horizontal transverse vibrations, which is very important for more effective freeing of a stuck tool in a borehole and for casing boreholes with pipes.

In addition, the vertical arrangement of the vibrator shafts makes it possible to considerably reduce the overall dimensions and weight of the vibrator, all other conditions being equal.

Fig. 6

BP-1 Selector

1. Nipple
2. Housing
3. Eccentrics
4. Roller bearings
5. Gears
6. Ratchet
7. Pivot pin
8. Shackle

conditions compared with vibrators with a horizontal arrangement of both shafts. Owing to the vertical arrangement of the shafts in the vibrator, a significant reduction is achieved in the distance between the shaft axes, a reduction in the diameter of the gears of the synchronizing reducer, which, in turn, leads to more favorable operating conditions for the gears and to a compact arrangement of the synchronizing-reducer unit.

The suspension of the electric motors of the vibrator provides for less stress in operation, since they are mounted on spring brackets.

For driving the vibrator, two electric motors with identical characteristics are used, the motors being installed on both sides of the vibrator for balance. Such a suspension reduces the vibrations of the motors themselves, and they will operate more durably.

Vibrators used for driving sheet piles have motors that are usually fastened from above, and in most cases the fastening is rigid, without any shock absorbers; therefore, with such fastening, the motors operate under exceptionally severe conditions and quickly fail.

Fig. 7. Diagram of the operation of vibrator VP-1

The vibrator VP-1 is implemented according to the scheme (Fig. 7) of a two-shaft mechanical exciter of directed-action oscillations with a vertical arrangement of the shafts (one above the other) and, during its operation, creates two types of oscillations in the rod column on which it is mounted:

a) vertical alternating sinusoidal oscillations due to the vertical components of the centrifugal forces of the rotating unbalances, and

b) transverse oscillations due to the moment of the couple of forces from the horizontal components of the centrifugal forces of the rotating unbalances.

Two unbalanced weights, eccentrically located relative to the axes of rotation, rotate in opposite directions with identical phase angle and angular velocity, which is achieved by means of two identical gears that mesh with one another and rotate together with the weights. The masses of the unbalances are equal to one another, their eccentricities are also equal; therefore the centrifugal forces created by the rotating unbalances will likewise be equal.

The vertical components of the centrifugal forces \(P_1\) and \(P_2\), for any position of the weights, will be directed in one and the same direction. These forces will create vertical oscillations of the entire vibrator; moreover, when the unbalances pass through the lower part relative to the horizontal axes (Fig. 7, diagram a), their resultant will be directed downward, and when the unbalances are in the upper part, the resultant is directed upward. The magnitude of the disturbing force of the vibrator varies from zero to plus–minus a maximum equal to the sum of the centrifugal forces, that is:

\[ P_{\max}=Q_1+Q_2=2Q. \]

The maximum value of the disturbing force of the vibrator (Fig. 7, diagram b) corresponds to the position of the centers of gravity on the vertical axis, that is, at 90 and 270°.

The disturbing force of the vibrator will be equal to zero when the centers of gravity of the unbalances are on the horizontal axis (Fig. 7, diagram c). In this case the centrifugal forces from the unbalances will be directed along the horizontal axis and in opposite directions, and therefore they mutually balance. But the pair of forces \(Q_1\) and \(Q_2\) creates a moment about the center \(O\); moreover, this moment will also be variable, and its maximum value is determined as:

\[ M_{\max}=Q\cdot c, \]

where \(c\) is the distance between the axes of the unbalances.

Owing to the presence of this variable moment, the vibrator creates transverse oscillations in the rod string. Thus, the VP-1 vibrator creates both vertical and transverse oscillations in the rod string on which it is mounted.

Characteristics of the VP-1 vibrator

If necessary, the number of revolutions of the vibrator may be increased to 2800 rpm, and the maximum disturbing force correspondingly rises to 10000 kg. Increasing the disturbing force of the vibrator is necessary when working with rods of large diameter or with casing pipes. In this case, however, the power of the electric motors must also be increased accordingly to 11–12 kW.

The VP-1 vibrator was manufactured in three geological exploration organizations and underwent tests under operating conditions. The tests showed that the vibrator can be successfully used for eliminating accidents in wells up to 200 m deep during shot drilling. The structural design of the vibrator itself proved, in principle, to be correct; all parts of the vibrator withstood the test and operated well.

A significant shortcoming of this vibrator was the fastening of the electric motors and its somewhat excessive weight. When working with rods 42 mm in diameter, under the action of the vibrator’s own weight the rods bent and buckled; with rods 50 mm in diameter this phenomenon was not observed.

The suspension of the electric motors to the vibrator proved not entirely satisfactory. The point is that an electric motor mounted on a bracket made of spring steel was in itself an individual oscillatory system with one degree of freedom. The number of natural oscillations of such a system can be calculated by the formula [3, p. 123]:

\[ n=\frac{30}{\pi}\sqrt{\frac{3E\cdot I}{m\cdot a^3}}, \]

where
\(E=2.2\cdot 10^6\ \text{kg}/\text{cm}^2\) — modulus of elasticity,
\(I=0.16\ \text{cm}^4\) — moment of inertia,
\(m=0.1\ \text{kg}\cdot\text{sec}^2/\text{cm}\) — mass of the motor,
\(a=13\ \text{cm}\) — arm of the motor suspension.

The number of natural oscillations of the motor calculated by the formula is 700 osc/min. The vibrator must operate at 2300 osc/min. Therefore, at start-up, as soon as the vibrator develops 700 osc/min, the electric motors enter the resonance zone and begin to shake very strongly, with a considerable amplitude; in this case all the power is expended on producing oscillations of the motors themselves, which begin to heat up very strongly and are no longer able to increase the vibrator speed.

To overcome this resonance point, power 3–4 times greater than the available power is required. In the present case, the resonance point had to be overcome by pressing the electric motors away from the vibrator body with wooden pads during starting of the vibrator. After the vibrator had passed this resonance point, it began to operate quite uniformly—the motors stopped shaking, and no heating of them was observed during further operation.

It is obvious that, when designing such a suspension of motors to a vibrator, the stiffness of the spring brackets must be selected so that the natural frequency of the electric motor is either higher than the operating frequency of the vibrator, or, conversely, so that resonance during operation occurs at the very beginning of acceleration, at about 200 osc/min. In the latter case, during starting of the vibrator the resonance point will be passed easily, and the vibrator will operate and start normally.

Single-block eccentric vibrator. Simultaneously with tests of double-block vibrators of directed action, tests were also conducted of a single-block eccentric vibrator of non-directed action, specially intended for lowering casing pipes into a borehole.

Compared with the double-block vibrator, the single-block vibrator is exceptionally simple in design and manufacture. It has no gears, the manufacture of which involves well-known difficulties for the repair shops of geological prospecting parties. As tests during casing of boreholes with pipes showed, however, it operates no worse than double-block vibrators.

In addition, the single-block vibrator has one more substantial advantage over double-block vibrators. When casing boreholes, the latter are mounted on the casing string from above through a sub and cover the opening in the casing pipes. Therefore, when it is necessary to lower drilling tools into the borehole in order to carry out drilling, the vibrator has to be removed.

In order to make it possible simultaneously to carry out both casing of the pipes by means of the vibrator and drilling of the rock in the borehole, it is necessary that the vibrator not cover the upper end of the casing string. For this purpose, at the Department of Prospecting, a single-block eccentric vibrator of non-directed action was manufactured and tested (Fig. 8).

The single-block vibrator is mounted on the casing string from the side and, unlike the double-block vibrator, which produces only vertical

axial vibrations in pipes, makes it possible to obtain vibrations in the casing string that are both vertical and lateral—horizontal. Naturally, the horizontal vibrations arising in the walls of the casing pipes will better contribute to reducing the friction of the pipes against the rock; that is, they will promote more rapid casing of wells with pipes than when only vertical vibrations are used.

Fig. 8. Single-block eccentric vibrator
during operation

A single-block vibrator has one unbalance rotating on a shaft, made from a pipe with plugs. Shot is poured into the pipe. The vibrator is driven by a small electric motor.

Characteristics of the single-block vibrator

The single-block vibrator was tested on the department’s drilling wells and showed satisfactory results.

Experimental Data

Experimental studies on the use of vibration mechanisms in exploration drilling were conducted both on the drilling rigs of the Department of Prospecting and in production conditions, that is, in geological-prospecting parties.

The tests went through three main stages.

1. Experimental studies of the propagation of oscillations in the rod string, clarification of the dependence of the amplitude on the cross-sectional area of the rods, and study of the damping properties of lock and nipple joints in the rod string.

2. Experiments on eliminating accidents associated with the seizure of the core barrel at the bottom of a borehole, clarification of the maximum depth of boreholes in which accidents can be eliminated with the aid of a vibrator, and establishment of the character of accidents that can be eliminated by vibrations. These experiments were carried out both on the drilling rigs of the Department and directly under field conditions.

3. Tests of vibrators for casing boreholes with pipes.

In addition to casing boreholes with the aid of a vibrator, experiments were also conducted on pulling casing pipes out of a borehole, for the most part in accident cases.

Experimental Data on the Study of Vibration in the Rod String

Experimental work in this area was carried out in three directions: a) clarification of the possibility of transmitting oscillations from an exciter of oscillations, installed at the upper end of the rod string, downward along the rods; b) identification of the dependence of the amplitude of oscillations at the lower end of the rod string, or, more precisely, the dependence of the work of impact of a ball installed in the lower part of the string, on the cross-sectional area of the rods or on the diameter of the rods; c) clarification of the damping action of locks and nipples, as well as threaded joints, on vibrations in the rod string.

The installation for testing vibrations in the rod string was assembled in a 12-meter drilling derrick (Fig. 9). In the upper part of the derrick, on four spring shock absorbers, a two-block vibrator of directed action was installed. A rod stand 9 m long and 42 mm in diameter was screwed to the nipple of the vibrator.

To measure the impact force during vibration of the lower end of the rods, a special nipple with a hardened steel ball 3.5 mm in diameter was screwed into the bottom of the stand. A metal plate of mild steel 10 mm thick was placed under the ball; the plate was laid on a solid base consisting of bricks, which, in turn, rested on a concrete floor. The plate was positioned with the calculation that, when the vibrator was not operating, the ball would press lightly on the plate.

The rod string, owing to the operation of the vibrator, entered into oscillatory motion. The nipple with the ball, screwed into the lower end of the rods, also entered into oscillatory motion, and the ball struck the steel plate, leaving impressions in the form of hemispherical indentations. Since the lower end of the rods, after each impact, rebounded slightly to the side, the impressions were not made at one and the same point, but over a certain area of the plate.

Using the law of similarity, which states that in a given material equal relative deformations are obtained at equal stresses, it is possible to determine the work performed by a single impact of the ball. For this it is necessary to calibrate the plate with the aid of such

of the same ball, for which an additional weight was screwed onto the nipple with the ball. The weight of the load with the nipple was 2370 g. This load with the nipple was dropped from various heights onto the same plate, and after this the diameter of the impression of the ball was measured. From each height, for accuracy of the measurements, the load with the ball was dropped five times. The data on measuring the diameter of the ball impression and the work performed by the load are summarized in Table 1.

Table 1

The data on measuring the diameters of the ball impressions during vibration of the rod column are given in Table 2.

Table 2

Comparing the diameters of the ball impressions obtained during free fall of the load with the diameters of the impressions obtained from the impacts of the ball under the action of rod vibrations, one may conclude that if forced oscillations are imparted to rods of diameter 42 mm with a maximum disturbing force of 700 kg and with a number of oscillations of 2500 cycles/min, then at the end of a 9-meter rod an impact work equal to approximately 2 kg·m will be obtained.

For calculating the dependence of the amplitude of oscillations—or, more precisely, of the work of impact of the ball at the lower end of the rod column—on the diameter of the rods, analogous experiments were carried out on a 22-meter metal derrick in a geological exploration crew. To set up the experiment, a two-block vibrator was used which, at a speed of 2600 rpm, developed a maximum disturbing force equal to 1500 kg.

Fig. 9. Setup for studying vibration in a rod column

For the test a special nipple was made, into the lower end of which a steel ball of diameter 11.1 mm from a ball bearing was caulked.

For the investigation, three rod stands with diameters of 42, 50, and 60 mm and lengths, respectively, of 18.22, 18.30, and 18.15 m were assembled. Each rod stand consisted of four separate rods of the standard length 4.5 m. The rod connection was by nipple.

As the screen receiving the ball impacts, offcuts of sheet mild steel 12 mm thick were taken, all the offcuts being cut from one whole sheet.

The tests were carried out as follows. A rod string with a diameter of 42 mm was screwed onto the vibrator; a nipple with a ball was screwed onto the lower end of the string. Under the ball, on a rigid base, a screen was placed in such a way that the ball lightly pressed against the steel plate. The lower end of the rod was moved aside, the vibrator was switched on, and after the vibrator had reached the normal number of revolutions, the vibrating rod was brought back to the plate, and the ball left imprints on it.

Then, instead of rods with a diameter of 42 mm, rods with diameters of 50 and 60 mm were installed. Next, in the same way, imprints of the ball on the plate were taken during vibration of these rods. It should be noted that in carrying out the experiments with rods of diameters 42, 50, and 60 mm, the frequency of oscillations and the maximum disturbing force of the vibrator remained unchanged in all three cases.

The average values of the diameters of the ball imprints for rods of different diameters were as follows:

Comparing the test results, one may draw one very important conclusion. Rods with a smaller cross-sectional area, that is, with a smaller diameter, have a larger vibration amplitude during vibration and produce a larger imprint diameter. The impact force of the ball during vibration of these rods is much greater than for rods with a larger cross-sectional area.

This circumstance is very important for the elimination of accidents by means of a vibrator, since the oscillations from the vibrator must be transmitted along the rods to the stuck tool with the greatest amplitude. On the basis of these experimental data, it may be concluded that, for eliminating accidents by means of a vibrator, it is best to use rods of smaller diameter, which exert a smaller damping effect on the vibrations due to internal friction in the material of the rods.

To determine the damping effect of nipple threaded connections on vibrations in the rod string, an experiment was carried out with two rod strings of the same diameter, 42 mm, but with different numbers of nipples.

The first rod string, 18.22 m long, consisting of 4 rods of standard length—4.5 m—had three nipple connections. The second rod string was assembled from rod cuttings of different lengths with a diameter of 42 mm. Eleven rod sections from 0.75 to 2 m long were screwed together with nipples into a string, the total length of which was 18.73 m. Thus, the string had 10 connecting nipples in its composition.

Then, similarly to the preceding case, ball imprints were taken on the screen. The measurement results showed that the string with three nipples gave an imprint 3.73 mm in diameter, while the string with ten nipples gave 3.05 mm.

It follows from this that nipple threaded connections impair the transmission of vibrations along the rod string and produce additional damping of oscillations in the rods.

Therefore, for more successful elimination of accidents by means of vibrators in boreholes, it is necessary to assemble the rod string with a smaller number of joints and nipples.

Experimental Data on the Elimination of Drilling Accidents with the Aid of Vibrators

The experiments on eliminating accidents in boreholes with the aid of vibrators, as indicated above, were carried out on the department’s drilling rigs and on the drilling rigs of geological prospecting parties. For this purpose, the three two-block directional-action vibrators described above were used.

Elimination of accidents with the aid of a vibrator on the drilling rigs of the Department of Prospecting. During student drilling practice on the department’s rigs, accidents in boreholes occur quite often because of the students’ lack of experience. At present, in the course of student drilling practice, accidents that occur as a result of the drilling tool being seized at the bottom of the borehole are eliminated with the aid of a vibrator.

In all, 13 accidents, differing in character and complexity, were eliminated with the aid of a vibrator on the department’s rigs. We shall present the most characteristic cases of accident elimination.

First case of eliminating an accident with the aid of a vibrator showed the enormous advantages of this method over all others existing up to the present time. This case is described in the journal Razvedka nedr, No. 2, 1952.

Second case. The accident occurred in a borehole 86 mm in diameter at a depth of 58.2 m during shot drilling with a KA-2M-300 rig, as a result of the drill string becoming seized.

An attempt to work the tool free with a lever and a Kremaller did not lead to positive results. To eliminate this accident, a vibrator was immediately used; at 3500 rpm it developed a maximum disturbing force of 1370 kg.

The rod string, together with the vibrator installed on it, was gripped by a cable, and when the vibrator developed its maximum number of revolutions, the cable was tensioned by the drilling rig winch. Under the action of the vibrations and the tension, at first slowly and then more rapidly, the drill string began to move upward. Thus, with the aid of the vibrator, this accident was eliminated literally within several tens of seconds.

Third case. In a borehole drilled by the shot method in hard fractured sandstones, at a depth of 41 meters, the core barrel did not always pass freely downward and upward. Sometimes, in order to lower the barrel to this depth, it had to be pushed through with the aid of a percussion jar. The same had to be done during hoisting. This interval was drilled out with pobedit core bits, since there was a constricted place in the borehole here and, probably, the opening had the shape of an ellipse. Sometimes during hoisting the barrel passed this constricted place only under the action of the tension of the rig winch, with slowed movement; but most often, during hoisting, the barrel jammed precisely at this depth, and for its further movement it was necessary to use a percussion jar.

With the aid of a vibrator, such troubles in the borehole were eliminated very easily and quickly. It was enough merely to install the vibrator on the rods and put it into operation for the core barrel to pass this constricted place upward and downward without particular difficulty.

Fourth case. During shot drilling in the borehole described in the second case, an accident occurred as a result of the core barrel being jammed by shot, settled shot and drilling sludge in the annular clearance between the walls of the core tube and the walls of the borehole, and also by drilling and shot sludge settled above the core barrel.

To eliminate the accident before using the vibrator, the following attempts were made. Two crews, 12 men each, for 2 hours unsuccessfully tried to knock out the tool with the aid of a percussion jar. After this, alongside the rods, flushing tubes of smaller diameter were lowered into the borehole and the sludge that had settled above the core barrel was flushed out. Then the percussion jar was installed again and an attempt was made to knock out the tool, but without success. With the aid of the vibrator this accident was eliminated in only 10–15 seconds.

Elimination of accidents with the aid of a vibrator under production conditions. As indicated above, experiments on eliminating accidents with the aid of a vibrator in boreholes were carried out by us in geological exploration parties; moreover, the vibrators were tested on accidents of the most varied character and at various depths.

The experimental data already make it possible at the present stage to draw a preliminary conclusion concerning the range of application of vibrators, that is, to what depth and what types of accidents can be eliminated with the aid of vibrators.

For experiments on eliminating accidents in boreholes, two-block vibrators of directed action were used; however, out of more than 40 cases in which vibrators were used for these purposes, positive results were obtained in only 23 cases.

In the initial period, experiments on eliminating accidents with the aid of a vibrator were carried out in geological exploration parties of the Kuzbass. In these parties, drilling was carried out mainly in sedimentary rocks with core bits fitted with hard-alloy cutters, and drilling was performed to depths of 500–700 m with KAM-500 and Krelius B-3 type rigs. Accidents here occurred as a result of jamming of the core tool in coal seams and in beds of aleurolites and, as a rule, at great depths of about 400–600 m. Therefore, naturally, it could be expected that the elimination of such accidents with the aid of a vibrator would be difficult, mainly for two reasons:

1) the vibrator used, with a maximum disturbing force of 1500 kg, was too weak a tool to cause a rod string 400–600 m long, with rod diameters of 50 and 60 mm, to vibrate;

2) during vibration of the core tool, the fine sludge holding the tool at the bottom of the borehole did not loosen but, on the contrary, became compacted, and the accident was aggravated still further.

Therefore a number of attempts to eliminate accidents under these conditions at depths of 400–600 m gave no positive results.

The experiments were then transferred to shallower boreholes, of the order of up to 100–200 m. The most characteristic of the accidents, both eliminated and not eliminated with the aid of a vibrator in shallow boreholes, are as follows.

Case 1. The depth of the borehole was 75.7 m; shot drilling was carried out with a KA-2M-300 rig; the inclination angle was 72°; the borehole diameter was 86 mm. The drilling tool was seized at the bottom of the borehole. One hour after the accident occurred, a vibrator was installed on the rods, and within 3 minutes the accident was successfully eliminated. The accident was eliminated in the following manner. The vibrator, installed on the rods, was switched on and was not switched off until the completion of the elimination. The first pull was applied by hand through the rig pulley; at this point it was possible to raise the tool by 5–6 cm, then, when the rope was slackened, the rod string, under the action of vibrations and its own weight, lowered back down. During the second pull the tool rose already by 18–20 cm. The third pull made it possible to raise the tool by 60 cm; then the tool again lowered when the rope was slackened. After two subsequent jerks with the winch, the tool was freed from seizure.

2nd case. In a borehole at a depth of 91.3 m, during shot drilling in hard sandstones, a tool 86 mm in diameter became stuck at the bottom. The borehole was deviated. Before the vibrator was used, the accident was being eliminated with an impact jar; in the course of 60 minutes the tool was driven out by 50 cm. After this, the tool began to be pulled out by the rig winch with vibrations. As in the preceding case, the vibrator was installed on the rods under tension, and by slackening the cable through the rig winch the tool was freed from being stuck within 12–15 minutes.

It is characteristic that when the cable was under very strong tension, the tool with vibrations moved upward more slowly than with moderate tension. The same thing was also observed when lowering the tool. If the cable was completely slackened, then the rod string, under the action of its own weight and the vibrations, slowly descended into the borehole; but if the cable was kept in a taut state and was gradually unwound from the winch drum, the rods descended into the borehole noticeably faster.

This can be explained by the presence of deviation of the borehole; namely, when the rods are taut, they are pressed with greater force against the walls of the borehole, and the vibrations of the rods are damped to a greater degree than with a weakly tensioned rod string.

3rd case. At a depth of 63.7 m, while drilling with a bit with hard-alloy cutters 76 mm in diameter, the bit seized up. It is known that this type of accident is the most serious. Such accidents are usually eliminated by means of a left-hand set of tools, by unscrewing and extracting the rods and the drilling tool in parts. With the aid of the vibrator this accident was eliminated in less than one minute.

Unfortunately, it was not possible to confirm the success of eliminating accidents involving seizure of the bit in other boreholes, since no convenient cases presented themselves. Several accidents connected with jamming of the tool in unstable rocks and with its being caught by fine rock mud, at depths of 60–180 m while drilling with hard alloys, could not be eliminated with the aid of the vibrator; nor was it possible to eliminate an accident at a depth of 36 m connected with jamming of the tool and with collapse of the borehole walls.

All the above-listed experiments on eliminating accidents were carried out with a vibrator with a maximum exciting force of 1500 kg.

Tests of the VP-7 vibrator, which at 3600 rpm developed a maximum exciting force equal to 5850 kg, gave good results; the most characteristic cases of eliminating accidents with the aid of this vibrator are as follows.

1st case. In a borehole at a depth of 75 m, a core-barrel tool became stuck. Drilling was carried out by shot with a KA-2M-300 rig, with flushing of the borehole by clay slurry. The length of the core-barrel tool was 6.5 m, the borehole diameter 89 mm, and the rocks were of category X.

For the first four days an attempt was made to eliminate the accident by ordinary methods; for this purpose they first tried to drive out the tool with an impact jar, and for 1.5 hours 15 men unsuccessfully carried out the driving-out operation. Then they tried to pull out the tool with a two-screw 40-ton jack, and this attempt led to breakage of the rods.

After these unsuccessful attempts to eliminate the accident, they proceeded to unscrew the rod string with the aid of a left-hand emergency tool. The right-hand rod string was unscrewed and brought to the surface, but it was not possible to unscrew the crossover sub.

Fifteen shifts after the accident had occurred, the VP-7 vibrator was used to eliminate it. The vibrator was installed on the left-hand rod string, which, in addition to 42 mm rods with left-hand thread, included two taps and three rods 50 mm in diameter.

Within 20 minutes of operation of the vibrator, with simultaneous working of the tool, that is, with tensioning and slackening of the cable by the winch of the rig, the accident was eliminated; the drilling tool was freed from the seizure and brought to the surface.

Upon careful external inspection of the extracted rod string, no traces of the action of vibrations on the material of the rods were found.

Case 2. In the course of eliminating the preceding accident, the vibrator failed—the gears of the synchronizer reducer were stripped. For further operation of the vibrator it was necessary to replace the worn gears with new ones, which had to be ordered again from the workshop. It was therefore decided to remove one gear and thus put one unbalance out of operation; that is, the vibrator was converted from a two-block unit into a single-block unit, with a disturbing force of 2950 kg at the same number of revolutions. This single-block vibrator was used for the subsequent elimination of accidents in boreholes.

The borehole in which the accident was eliminated by this single-block vibrator had the following data: depth—44 m, diameter—108 mm, length of the tool—6 m; shot drilling with a KD-2M-300 rig; rocks of category X; flushing—with clay mud.

The accident occurred as a result of the drilling tool being seized by shot and sludge. For its elimination, as in the preceding case, the following attempts were made over the course of 9 days.

Continuously, over three shifts, 9 men unsuccessfully beat the tool with the aid of a drilling jar; a 40-ton jack likewise proved powerless to eliminate this accident. Then, with the aid of left-hand rods and left-hand taps and bells, they unscrewed and extracted the rod string from the borehole, cut the adapter, and with a 75 mm diameter tool drilled out the core inside the core barrel left in the borehole and drilled 75 cm below the bit.

Twenty-seven shifts after the accident occurred, it was eliminated with the aid of our single-block vibrator in only 20 minutes. For this purpose, a large-diameter tap was lowered on 42 mm rods and screwed into the core tool. The vibrator was installed on the upper end of the rods and suspended from the hook of the drilling derrick. The elimination process was carried out in the same manner as in the preceding case.

We shall cite characteristic cases of accident elimination with the aid of the VP-1 vibrator.

1. During shot drilling with a KA-2M-300 rig in rocks of category IX, the core tool was seized at a depth of 132 m. This accident was eliminated quite successfully within 25 minutes with the aid of the VP-1 vibrator; the accident was eliminated while working the drilling tool with the aid of the rig winch.

2. At a depth of 200 m, likewise during shot drilling, the tool was seized at the bottom of the borehole. Drilling was being carried out with a KA-2M-300 rig; the drill rods had a diameter of 42 mm. The vibrator was installed on the upper end of the rods, and elimination of the accident was begun. In the first 10 minutes, while working the tool, it was possible to pull the tool up by 25 cm; as a result of the vibration and excessive tension, the rod string broke. After the broken rods were again caught with a tap and repeated elimination was begun, the rod string broke once more. In all, three breaks of the rod string occurred as a result of the vibrations and tension, but the tool was raised by 60 cm. After this the vibrator was removed, and the tool was pulled out fairly quickly with a jack.

As this case showed, old worn rods, under the action of the vibrator with a maximum disturbing force equal to 6500 kg, no longer withstand this dynamic load and are destroyed.

Experimental data on casing wells with pipes by means of a vibrator

For casing wells with pipes, both two-block vibrators of directed action and a single-block vibrator of nondirected action were used.

The first case of using a vibrator for casing wells with pipes was described in the journal Razvedka nedr, No. 5, 1952. Other, most characteristic cases of casing wells with vibrations are given below.

Second case. A well 42 m deep began to be cased with pipes 127 mm in diameter immediately by means of a vibrator.

The first pipe, 4.5 m long, was freely lowered under its own weight, and after a second pipe had been screwed onto it, the column lowered by itself another 1.5 m and stopped. Through an adapter a two-block vibrator was installed on the pipes and suspended by its shackle from the derrick hook. The vibrator was switched on, and the pipes at first slowly, and then more rapidly, began to descend into the well under the action of their own weight and the vibrations. The second pipe was lowered into the well with vibrations in 2.5 minutes, the pipe column during lowering being braked by the rig winch. Then a third pipe was screwed on and the vibrator was again installed. In this case the pipes were lowered another 1.7 m, after which the column would go no farther. The column was turned through three revolutions and the vibrator was switched on again; in this operation the pipes descended another 40 cm.

After this the vibrator was removed, a drilling spoon was lowered on rods, and the plug of rock that had formed near the shoe of the casing string was drilled out. The vibrator was installed again, and with its aid the pipes were cased another 1.5 m.

Thus, by alternating the action of vibrations on the casing pipes with their rotation and, from time to time, lowering the spoon to work out the plug of rock near the shoe of the column, it was possible to lower the casing pipes to a depth of 18.6 m. Further lowering of the casing string was discontinued, since it was necessary to case the well only to a depth of 16 m.

Three hours 20 minutes were spent lowering the casing pipes. The time of casing by means of the vibrator amounted in all to 20–25 minutes.

Third case. Casing of the well began from the surface with pipes 112 mm in diameter. The upper layers consisted of sands with interbeds of clay down to a depth of 15 m. This interval, similarly to the preceding case, was cased by means of a vibrator in 1 hour 20 minutes, the pipes freely descending with vibrations and stopping as soon as the vibrator ceased operating. Below 15 m there was a thick body of clays. With the vibrator it was possible to cut the shoe of the casing pipes into the clay bed by 50–60 cm, and further operation of the vibrator produced no effect.

In some cases casing wells with pipes by means of a vibrator was carried out with additional forcing down of the pipes, which also gave positive results.

Other work in wells carried out by means of a vibrator

In addition to experiments on eliminating accidents and on casing wells with pipes, other work was also carried out by means of a vibrator in order to determine the field of application of vibrators in exploratory drilling.

1. In a well 342 m deep, cased to 190 m, at a depth of 130 m the casing pipes broke off and dropped, the lower part of the column being tightly jammed by the rocks. After extracting the upper 130 m of pipes with the aid of a pipe catcher, the so-called “beard,” lowered on rods

with a diameter of 60 mm, began to seize, tear into pieces, and bring to the surface the lower pipes. The pulling was carried out by the pulley system and winch of the Krelius V-3 rig and was brought up to 40–45 tons.

At the time when the “beard” was located at a depth of 170 m, it was decided to test the vibrator, which in the course of 1.5 minutes freed the “beard,” and it was raised to the surface without pipes.

1. With the aid of the vibrator, metal piles and rods for guy wires were easily driven when installing headframes, winches, etc. A 2-meter rod 42 mm in diameter, under the action of vibrations, was sunk into the ground to 1.6–1.7 m in 2–3 minutes.

2. With the aid of vibrations it is possible to unscrew threaded joints in pipe strings when these joints can be unscrewed only with great difficulty by wrenches.

Conclusions

The results of the experiments carried out by us to study vibrations in a rod string and the use of vibrators for eliminating accidents and casing boreholes with pipes during drilling permit the following conclusions to be drawn.

1. To eliminate accidents in drill holes and to case boreholes with pipes, it is necessary to create a vibrator with a large maximum exciting force and with a low frequency. The force of the vibrator must not exceed the strength of the rods under dynamic loads.

2. Vibrations are better transmitted along rods with a smaller cross-sectional area. For more successful elimination of accidents in boreholes by means of a vibrator, rods of smaller diameter should be used.

3. The success of eliminating accidents with the aid of vibrators depends on the following factors: a) on the depth of the borehole; b) on the curvature of the borehole—in uncurved boreholes the elimination of accidents proceeds more successfully; c) on the gripping force of the drilling tool.

4. Accidents are most successfully eliminated by means of vibrations in shot drilling when they occur as a result of the tool being seized by shot or by hard pieces of rock, as well as accidents connected with the burning of crowns.

5. The elimination of accidents connected with the jamming of the drilling tool in beds of comparatively soft rocks, as well as with seizure by settled fine drilling sludge and cave-in of the tool, is almost impossible with the aid of vibrations.

6. The limiting depth of reach of vibrations established by us is 200 m, but with more powerful vibrator designs it is evident that the elimination of accidents will also be possible at greater depths.

7. According to our experiments, casing boreholes with pipes by means of a vibrator is successfully carried out in comparatively shallow boreholes.

8. The use of vibrators in exploratory drilling increases labor productivity many times over and gives large savings. Thus, on only two accidents eliminated with the aid of a vibrator, 40–50 thousand rubles were saved.

Literature

1. Kulichikhin N. I., Vozdvizhenskii B. I. Exploratory Drilling. Gosgeolizdat, 1949.
2. Astaf’ev A. F. Engineering Reference Book. ONTI, 1937.
3. Mechanical Engineering. Encyclopedic Reference Book, vol. 1, Mashgiz, 1948.