Shot Drilling of Boreholes at Increased Rotational Speeds of the Drilling Tool

P. F. Palyanov

At the request of one geological exploration trust, the Department of Exploration organized and carried out experimental studies in the summer of 1953 to establish the optimum regimes for shot drilling.

The need to carry out these experimental works in the geological exploration party was caused by the fact that, with a large number of operating drilling rigs, no unified drilling regime existed; each driller set the regimes on the basis of his own personal experience. Therefore, when the work of the party’s drilling rigs was analyzed, substantial discrepancies were found in the operating regimes of individual rigs when drilling under analogous conditions. These discrepancies were observed in the rotational speeds of the drilling tool, in the methods of feeding shot into the borehole, in the axial pressure on the bottom hole, and also in the flushing of the borehole.

It was therefore very important to develop a unified drilling regime for the geological exploration party as a whole, which could be recommended for drilling on all drilling rigs, while taking into account the necessity of increasing the productivity of borehole drilling.

The experimental work carried out to investigate the optimum drilling regimes in the party made it possible to generalize the operating experience of all drilling rigs and to develop a unified drilling regime for the given geological section, which ultimately led to the streamlining of the work of the individual drilling crews and to an increase in borehole-drilling productivity with the maximum possible use of equipment, tools, and the corresponding materials.

For the experimental investigations in the party, one drilling rig was equipped with the appropriate instruments and devices for measuring various characteristics. The rig was equipped with a three-legged, fifteen-meter log headframe and had three platforms with ladders.

To establish the optimum regimes of shot drilling, a KAM-500 drilling machine was used; between the machine and the electric motor a transmission with pulleys of different diameters was installed. If the normal spindle speeds recommended for drilling with the KAM-500 machine are 72, 120, and 200 rpm [2], then, thanks to the transmission, spindle speeds at no load equal to 170, 270, and 415 rpm were obtained on the experimental installation. Thus, experimental drilling was carried out at increased rotational speeds of the drilling tool.

The boreholes were flushed with a clay mud of normal composition with a specific gravity of 1.15 g/cm³.

The power installation of the drilling rig consisted of two three-phase electric motors, 220/380 volts. The first motor, with a power of 27 kW, drove the drilling machine through the transmission; the second, with a power of 14 kW—

was installed to drive the pump and the clay mixer. The power of the drill rig motor was quite sufficient for operation under the heaviest drilling conditions.

The rod string was assembled from rods 50 mm in diameter, with welded-on safety rings and with a nipple connection. The length of the stand was 9 m.

To regulate the axial pressure on the bottom of the borehole, a pressure regulator of S. A. Volkov’s system was installed on the drill rig.

A voltmeter and an ammeter were mounted on a special panel. The number of revolutions of the drill rig was measured with a revolution counter. To feed shot into the borehole, a gate valve of the K. G. Volodchenko system was used, installed on the rod stuffing box. Drilling was carried out in the interval from 80 to 210 m.

Organization of work on the experimental drilling derrick was carried out as follows. Work was conducted in three shifts, and the drilling crew consisted of three shift crews. Each shift crew consisted of 4 people: a shift foreman, senior and junior drilling workers, and one timekeeper. The latter’s duty was to keep documentation of the drilling process on a special time card.

For the period of the experimental work, the production plan was removed from the drilling derrick, and the labor of the drilling crews was paid on the basis of the average monthly wage.

The work plan included determining:

1) the most advantageous number of revolutions for shot drilling of boreholes under the given geological conditions;

2) the optimum pressure on the bottom of the borehole;

3) the effect of forced drilling regimes on core recovery.

Flushing of the boreholes was carried out with clay mud; the quantity of flushing fluid supplied was set during drilling on the basis of the recommendations of the provisional instructions for core drilling [4].

Documentation and accuracy of the results obtained. The recording of certain characteristics was carried out during experimental drilling on time cards of the established form, where the exact time of the beginning and end of a given operation was noted, as well as the quantity of shot charged (the necessary portion of shot for feeding the borehole was weighed beforehand on scales), the number of revolutions per minute of the rig spindle, and the axial pressure on the bottom in kg/cm² of the working area of the crown face.

Measurements of the advance of the drilling tool were made every 10–15 minutes and were also noted on the time card.

When processing the time cards, the rate of advance was calculated in cm/min, and graphs of borehole advance versus time, etc., were constructed.

In order to obtain more reliable results during the experimental investigation, drilling was carried out at each established regime for 3–4 runs, each run usually being brought, in duration, up to an 8-hour working shift.

On the drilling installations of the expedition, the usual rotation speeds of the drilling tool were 80 and 140 rpm. We succeeded in successfully carrying out the experimental work at rotation speeds of the drilling tool of 170 and 270 rpm. At a rotation speed of 415 rpm, it was not possible to obtain reliable results, because, owing to the high numbers of revolutions of the drilling tool, frequent rod breaks occurred, as well as failure of the drill rig and other accidents. Therefore the drilling results corresponding to a tool rotation speed of 415 rpm are not considered here.

First Stage of the Work

At the first stage of the work of the experimental drilling rig, the task was set of establishing the optimal, most advantageous rotational speed of the drilling tool during the drilling process, that is, the rotational speed of the tool at which the greatest drilling productivity would be obtained.

Drilling was carried out with bits of 110 mm diameter in rocks of categories IX and X (syenite-diorites, diorites, and silicified diorites).

The contemporary view of permissible numbers of revolutions in shot drilling is as follows. B. I. Vozdvizhensky [1] recommends that, for bits up to 115 mm in diameter, the rotational speed of the drilling tool be taken within the limits of 140–200 rpm. K. G. Volodchenko [2], when selecting the number of revolutions of the tool, recommends proceeding from the peripheral speed of the bit, which for hard rocks is taken as 0.6 m/sec, and for less hard rocks as up to 0.8 m/sec. These works also indicate that, with an increase in the number of revolutions of the tool to 300 rpm, drilling productivity increases almost in direct proportion to the peripheral speed.

Proceeding from the peripheral speed of the bit accepted by K. G. Volodchenko, equal to 0.6 m/sec for hard rocks, a bit of 110 mm diameter should operate at a number of revolutions of 105 rpm.

The “Temporary Instruction on Core Drilling” [4] indicates that the speed of shot drilling increases as the peripheral speed of rotation of the bit increases. When drilling with a KAM-500 rig, for a bit of 110 mm diameter it is recommended that the spindle speed be taken as 180–200 rpm, proceeding from a peripheral speed of the bit equal to 0.95–1.05 m/sec. With the availability of new high-speed rigs and a new equal-strength rod string, it is recommended to shift to higher peripheral speeds of rotation of the bit, up to 1.5–2 m/sec.

An increase in the rotational speed of the tool contributes to a better redistribution of serviceable shot beneath the face of the bit and to the entry of fresh, uncrushed shot to the bottom of the borehole. But at the same time, an excessive increase in the number of revolutions of the bit will entail the expulsion of shot from the inner annular space to the outside under the action of centrifugal force. Thus, at 160 rpm for a bit of 110 mm diameter, the centrifugal force [3] throwing the shot to the side will be greater than the weight of the shot itself, provided that the shot rotates at the same peripheral speed as the bit.

It is probable that the shot is dragged along by the bit and will rotate, owing to friction against the rock, with a much lower peripheral speed than the bit. Therefore the limiting numbers of revolutions of the bit at which the shot will begin to experience centrifugal forces greater than its weight will be higher than the calculated ones.

I. A. Ostroushko [3] describes original experiments in shot drilling of horizontal and slightly inclined boreholes at high numbers of revolutions of the drilling tool, up to 600–1200 rpm and higher. At such high rotational speeds of the drilling tool, the shot, under the influence of centrifugal force, was carried from the inner annular clearance to the outer one; therefore, for greater effectiveness of operation, the drill bits were fitted with special screw-type limiters with a left-hand thread and a large pitch. During operation these bits pressed the shot against the bottom and increased drilling efficiency.

The dependences of the rate of deepening of the borehole on the number of revolutions of the drilling tool, obtained by us during drilling, are as follows. Drilling was carried out in rocks of categories IX and X at tool rotational speeds of 170 and 270 rpm and at pressures of 20 and 25 kg/cm² of the working area of the bit face.

The results of the experimental data are summarized in Table 1.

Table 1

Thus, in our experiments, as the rotational speed of the drilling tool increases, the shot-drilling rate also increases.

When the rotational speed of the tool is increased from 170 to 270 rpm, the drilling rate of the borehole increases substantially both in rocks of category IX and in rocks of category X; moreover, the increase in drilling rate is more noticeable in hard rocks.

The graph of the dependence of the drilling rate on the rotational speed of the drilling tool is given in Fig. 1.

Thus, increasing the rotational speed of the drilling tool leads to an increase in drilling rate; therefore, in the process of core shot drilling it is highly advantageous to work at elevated numbers of revolutions. But, as we have already indicated above, with a large increase in the rotational speed of the bit, the shot from the inner annular clearance will, under the action of centrifugal force, pass into the outer annular clearance, and the drilling rate will decrease. To prevent this phenomenon, we used a bit with an external helical limiter (similar to the bits of I. A. Ostroushko), with the thread on the bit made with a large pitch of the order of 60–100 mm and with a left-hand cutting direction (Fig. 2). The need for an internal screw thread in the bit disappears, since here the shot will arrive at the face of the bit under the action of its own weight. Bits manufactured according to this principle and tested in the course of our experimental work gave positive results.

Fig. 1. Dependence of drilling rate on tool rotational speed

Fig. 2. Bit with an external helical limiter

It is well known that increasing the number of revolutions of the drilling tool entails yet another very undesirable consequence—in this case, the wear of rods and core barrels correspondingly increases due to their intensified abrasion against the walls of the borehole. Wear of the tool at high rotational speeds is observed especially strongly when drilling deep boreholes and leads to a reduction in its service life, to breakage of rods, accidents, and other complications during drilling.

It is obvious that, proceeding from the most reliable operation of the drilling tool, it is necessary to recognize increased rotational speeds as more expedient when drilling boreholes at shallow depths; as the depth of the borehole increases, the rotational speed of the tool should be reduced in order to avoid intensified wear of the rod string and to prevent accidents.

On this basis it may be stated that, when drilling boreholes with bits of 110 mm diameter, in the interval down to 100–120 m the rotational speed of the tool should be brought up to 210–250 rpm and higher, that is, operation should be at the 3rd speed of the KAM-500 rig; when drilling in the interval 100–200 m, the rotational speed of the tool should be reduced and operation should be at the 2nd speed, that is, at 130–150 rpm; and when drilling boreholes deeper than 200 m, the rotational speed of the tool should be reduced still further and brought to 80–110 rpm, that is, operation should be at the 1st speed.

Such a drilling regime is undoubtedly the most effective in comparison with a drilling regime over all depth intervals at the same rotational speed of the drilling tool.

In the process of drilling, special attention should be paid to preventive measures for the rod string. For more durable operation of the rod string, protective rings should be welded onto the rods by electric or gas welding; these rings prevent excessive wear of the rods from friction during their rotation in the borehole. Onto each rod 4.5 m long it is necessary to weld on at least three rings; two rings are welded at a distance of 30–40 cm from the ends of the rod, and one in the middle. In practice, two types of welding protective rings to rods are encountered (Fig. 3). In the first case (diagram a), the protective ring is welded to the rod with two seams, the seams being made from the ends of the ring and encircling the rod. In the second case (diagram b), one seam is made at the joint of the ring along the axis of the rod.

Fig. 3. Welding protective rings onto rods

The practice of our experimental work has shown that welding the rings by the second method is the most reliable in operation, since welding by the first method entails residual deformations in the body of the rod over the entire cross section of the rod and makes it less strong at the weld seam. In addition, it is often observed that, at the seam, because of excessive overheating of the rod body at the moment of welding, through slit-like holes appear, which entail intensified leakage of drilling fluid from the rods into the borehole. These slit-like holes, usually invisible to the eye, are to a greater extent characteristic of the first type of welding.

In the process of drilling, the protective rings are intensively abraded, and from time to time they must be renewed. Therefore, before beginning the drilling of a new borehole, during the dismantling and installation of the drilling derrick it is necessary to—

carefully check the serviceability of the welded-on rings and either replace worn rings or weld new ones alongside them.

Such preventive maintenance will lead not only to longer service life of the drilling tool but also to higher drilling productivity, since in this case the rotational speeds of the drilling tool can be increased.

Second stage of the work

At the second stage of the work, the objective was to establish the most advantageous optimal axial pressure on the bottom of the borehole during shot drilling at increased numbers of revolutions of the drilling tool.

The axial pressure on a shot bit is usually adopted on the basis of the specific pressure, that is, the pressure per \(1\ \text{cm}^2\) of the working area of the end face of the shot bit.

In shot drilling, the pressure on the bottom is set depending on the hardness of the rock being penetrated and the quality of the shot. The recommended values of specific pressure during drilling are as follows. B. I. Vozdvizhenskii [1] recommends taking the bottom pressure as \(15\text{—}25\ \text{kg}\) per \(1\ \text{cm}^2\) of the working end face of the bit; K. G. Volochenko [2] recommends \(15\text{—}20\ \text{kg}/\text{cm}^2\), and the “Temporary Instruction for Core Drilling” [4] recommends \(20\text{—}25\ \text{kg}/\text{cm}^2\).

The investigations we carried out in this field showed the following results. At different rotational speeds of the tool, different values of axial pressure on the bottom of the borehole were specified. Thus, at a rotational speed of the drilling tool of \(170\ \text{rpm}\), drilling was carried out with axial bottom pressures of 15, 20, 25, 30, and \(35\ \text{kg}/\text{cm}^2\) of the working area of the end face of the bit. In this case shot No. 3 was used for drilling. Then the rotational speed of the drilling tool was increased to \(270\ \text{rpm}\), and drilling was carried out at bottom pressures of 20, 25, and \(30\ \text{kg}/\text{cm}^2\) of the end-face area of the bit.

The results obtained in these tests are summarized in Table 2.

Table 2

The dependence of drilling speed on the magnitude of the axial pressure on the bottom of the borehole is shown in Fig. 4 for a rotational speed of the drilling tool of \(170\ \text{rpm}\), and in Fig. 5 for \(270\ \text{rpm}\).

The first series of experiments (Fig. 4, upper curve), carried out at \(170\ \text{rpm}\) of the tool while drilling rocks of category IX, gives a complete picture of the dependence of the rate of deepening of the borehole on the specific axial pressure on the bottom at pressures of 15, 20, 25, 30, and \(35\ \text{kg}/\text{cm}^2\) of the working area of the end face of the bit. As can be seen from the graph, when the specific axial pressure on the bottom was increased from 15 to \(25\ \text{kg}/\text{cm}^2\) of the end-face area of the bit, the drilling speed of the borehole increased from 12.5 to \(20\ \text{mm}/\text{min}\). With a further increase in axial pressure on the bottom, the drilling speed decreased, and at \(30\ \text{kg}/\text{cm}^2\) the drilling speed reached \(14\ \text{mm}/\text{min}\), while at \(35\ \text{kg}/\text{cm}^2\) it was \(12\ \text{mm}/\text{min}\).

A similar picture was also observed in drilling rocks of category X (Fig. 4, lower curve), when the drilling speed of the borehole likewise reached its maximum value at a specific axial pressure of \(25\ \mathrm{kg/cm^2}\) of the working area of the bit crown face.

The second series of experiments (Fig. 5), at a tool rotation speed of 270 rpm, also showed that the best drilling speed is obtained at a bottom-hole pressure equal to \(25\ \mathrm{kg/cm^2}\) of the working area of the bit crown face.

Thus, with an increase in the specific axial pressure on the bottom of the borehole in shot drilling, the drilling speed changes, reaching its maximum value at \(25\ \mathrm{kg/cm^2}\) of the working area of the bit crown face. With a further increase in pressure above the optimum, the drilling speed drops sharply. In this case the shot on the bottom of the borehole is rapidly crushed and, being comminuted, turns into fine, ineffective shot sludge. As reported by I. Kh. Zelenin [5], this optimum pressure also corresponds to the lowest consumption of shot and crowns.

Fig. 4. Dependence of drilling speed on axial pressure at 170 rpm.

Fig. 5. Dependence of drilling speed on axial pressure at 270 rpm.

Therefore, in shot drilling of boreholes it is very important to adhere precisely to this maximum, most advantageous specific axial pressure on the bottom, namely a pressure of \(25\ \mathrm{kg/cm^2}\) of the working area of the bit crown face, since this axial pressure ensures the maximum drilling speed. A decrease in pressure, as well as an increase relative to the optimum, leads to a decrease in borehole-drilling productivity.

During experimental drilling at increased speeds, continuous observation of core recovery was carried out. Under all drilling conditions, essentially 100 percent core recovery was observed. However, owing to insufficient attentiveness on the part of individual drillers during wedging and lifting of the core, in individual shifts the core was either not completely wedged and extracted, or, because of poor wedging and impacts of the drilling tool during lifting, it was lost in the borehole. Therefore, the overall core recovery for the experimental borehole was 70%, which fully corresponded to the planned core-recovery target.

Thus, under our conditions, operation of the drilling rig at increased speeds did not adversely affect core recovery.

Conclusion

The experimental investigations carried out on shot-drilling regimes showed that, in order to obtain the greatest productivity, the following drilling regime may be recommended.

1. Drilling should be carried out at increased rotational speeds of the drilling tool. In the interval from 0 to 100 m, drilling should be performed at the third speed with the KAM-500 rig; from 100 to 200 m, at the second speed; and above 200 m, the first speed should be used. This measure, at the highest drilling speed, will lead to a significant extension of the service life of the drilling tool.

2. To prevent rapid wear of the rods, protective metal rings must be welded onto them.

3. The axial pressure on the bottomhole must be established on the basis of an optimal specific pressure of 25 kg/cm² over the working area of the crown face.

REFERENCES

1. Kulichikhin N. I., Vozdvizhensky B. I. Exploratory Drilling. Gosgeolizdat, 1949.

2. Volodchenko K. G. Core Exploratory Drilling. Gosgeolizdat, 1953.

3. Ostroushko I. A. Destruction of Rocks during Drilling. Gosgeolizdat, 1952.

4. Temporary Instruction for Shot Drilling. Gosgeolizdat, 1951.

5. Zelenin I. Kh. Experience in drilling boreholes with steel shot. Journal “Razvedka nedr,” No. 2, 1953.