EXPERIMENTAL STUDIES OF A PERIODIC METHOD FOR FEEDING SHOT TO THE BOTTOMHOLE OF WELLS

P. I. STEPANOV

(Presented by Professor, Doctor A. A. Belitskii)

This article attempts to examine the factors that make it possible to increase the rate of penetration per hour of net drilling when wells are fed periodically with cast-iron shot, as compared with trip feeding. For this purpose the author carried out experimental studies in the drilling laboratory of the S. M. Kirov Tomsk Polytechnic Institute on a special installation.

In the literature, and also among practicing drillers, there is no single opinion on the question of effective deepening as a function of the condition of the shot at the bottomhole. Some believe that the maximum rate of penetration during drilling is ensured by crushed particles of shot. According to these views, the maximum penetration during a run occurs after no whole shot remains on the bottomhole and only large particles of crushed shot are present. As the crushed shot is further comminuted, the rate of penetration decreases. In order to create a reserve of crushed shot at the bottomhole, in their opinion, it is necessary to pour a large portion of whole shot into the well at once. According to the ideas of other investigators, the maximum rate of penetration is obtained when, at the bottomhole, there is basically only whole spherical shot. Still others believe that the maximum rate of penetration occurs when both whole and crushed shot are present together at the bottomhole. However, the proponents of this point of view do not indicate at what ratio of crushed and whole shot the maximum rate of penetration occurs, nor what practical measures may be recommended for maintaining this state of the shot at the bottomhole.

It is known that with trip feeding the rate of penetration increases and then, having reached maximum values, begins to decrease. Therefore, the experimental studies were aimed first of all at clarifying why, with trip feeding, the rate of penetration during a run is not constant, and at what state of the working shot the maximum penetration during the run occurs. Further, we set ourselves the task of finding conditions under which it would be possible to maintain the maximum possible rate of penetration during a run at one level. For this purpose, a periodic method was used for feeding wells in small portions. Then experiments were carried out to determine the influence of the weight quantity of shot present at the bottomhole

wells, on the rate of advance per hour of net drilling, both with run-by-run and with periodic feeding.

The experimental drilling was carried out on a “GP-1” rig with a bit having an outside diameter of 65 mm and a peripheral speed of 1.4 m/sec in a block of medium-grained granite. To study the process and the nature of the work of the shot at the bottomhole, the shot was removed from the well by means of a cylindrical electromagnet after every 10 minutes of net drilling time. The shot was dried, screened on sieves with aperture diameters of 2.5; 2.0; 1.5; 1.0 mm, and each fraction was weighed. The quantity of flushing fluid supplied to the well was measured with a graduated vessel.

Fig. 1. Dependence of the rate of advance on the amount of the portion loaded per run.
Portion weight per run: 1—200 g; 2—400 g; 3—800 g.

As a result of the experimental observations it was established that the maximum values of the rate of advance during a run, as well as the average run rate of penetration, are almost independent of the weight quantity of the portion of shot loaded into the well. Fig. 1 shows the results of drilling with portions of 200, 400, and 800 g. Here the abscissa gives the drilling time, and the ordinates give the penetration in centimeters over the observation interval. From the graphs, and also from Table 1, it is evident that maxi-

Table 1

The maximum deepening rates over a time interval equal to 5 minutes are, for 200 g, 3.7 cm; for 400 g, 4.0 cm; and for 800 g, 3.9 cm (points O₁, O₂, O₃ in Fig. 1), while the average trip rates are, respectively, 0.49, 0.47, 0.5 cm/min; i.e., their absolute values are practically equal in all cases. Similar results were obtained by V. I. Molchanov in studies at the Kaz GRES on the dependence of the average trip drilling rate on the weight quantity of shot charged into the borehole for portions weighing 5, 10, and 15 kg.

It is also evident from Table 1 that, with an increase in the weight of the portion per trip, the consumption of shot per 1 running cm of drilling increases almost directly proportionally.

It was further established that, with trip feeding, the maximum deepening rate during a trip occurs after less than 50% of whole shot remains at the bottomhole of the well, and is maintained at approximately one level until 5–10% of whole shot remains at the bottomhole. Thus, for example, it is evident from Table 1 that when 200 g of shot is charged per trip, after the maximum (3.9 cm) 10 g of whole shot, or 6%, remained. Thereafter, when drilling with split particles, as they are crushed, the drilling rate decreases. When 400 g of shot is charged per trip, as the whole pellets split and the split fraction increased, the drilling rate increased. Then, before the beginning of the observation interval that gave the maximum drilling, there were 110 g, or 35%, of whole shot, and at the end there were 10 g, or 4%. After this the drilling rate also begins to decrease. Fig. 2 shows the results when drilling began with shot charged in a ratio of 50% whole and 50% split shot. As a result, in the very first observation interval a drilling advance was obtained that was even somewhat greater than the maximum in the preceding example (Table 1).

The increase in the deepening rate is apparently achieved through an increase in the pressure on one whole pellet, as a result of which it crushes the rock at the bottomhole more intensively. The increase in pressure on one whole pellet is achieved owing to a reduction in their total number beneath the end face of the bit, due to the entry there of split particles. Indeed, in the drilling process, whole and split shot are at all times in a mixed state and, consequently, both the former and the latter get under the end face of the bit. It goes without saying that, as it rotates, the bit will rest on whole pellets, since they have the largest diameter. The split particles, however, also perform a role that is far from secondary. First, they clean out the crushing trace formed by the motion of the round shot; second, they prevent the latter from emerging from under the end face of the bit, i.e., they act as “separators” for the round shot; and finally, during their rolling, they themselves perform effective work in destroying the rock.

On this basis, it becomes clear why advanced drilling crews, using periodic feeding, achieve higher production indicators than with trip feeding. These crews, by periodically charging ever new portions of whole spherical shot into the well, obtain either several maxima of drilling advance in a trip (instead of one with trip feeding), or systematically maintain at the bottomhole a definite ratio of whole and split shot, and then it is possible to maintain the maximum drilling advance throughout the entire trip. The obtaining of several maxima of drilling advance in a trip occurs with periodic feeding in large portions, while the maintenance of a definite ratio of whole and split shot is achieved through a periodic method of feeding in small portions or a continuous (trickle) method of feeding.

Thus, the maximum rate of deepening does not decrease and is maintained at one level if, after the addition of each new portion of whole shot to the bottomhole of the well, the ratio of whole to broken shot becomes established at approximately one to one, and if at the end of the interval (period of time) between additions no less than 5–10% of whole shot remains at the bottomhole. This circumstance is of great importance for the practical regulation of the regime of periodic feeding of wells with shot. Of particular importance here is the lower limit of the presence of whole shot at the bottomhole of the well. If it is allowed to decrease to zero, and even to a state in which drilling is carried out by broken shot—though with coarse particles—then, in addition to a decrease in the rate of deepening, nonuniformity will occur in the treatment of the core and the walls of the well. In this case, by the end of the period between additions the core will have been treated less than at the beginning; therefore, when the next portion of whole shot is added, it will become wedged in the bit crown, and the process of deepening the well may cease. If, however, whole shot is present at the bottomhole of the well even in an insignificant amount (5–10%) of the total quantity, no nonuniformity is observed in the treatment of the core and in the development of the well.

In the process of drilling, whole cast-iron shot is continuously crushed and replenishes the fraction of broken shot. Therefore, in order to maintain a definite ratio of whole and broken shot at the bottomhole, during drilling it is necessary continuously to add whole shot to the bottomhole and at the same time remove, by means of the flushing fluid, the excess amount of broken particles. The ideal way of solving this problem is the continuous replacement of the shot grains that are being crushed by new ones; that is, as many whole shot grains as are crushed are added, with the simultaneous removal into the sludge pipe of the excess quantity of comminuted particles. This will be most closely matched by a continuous (trough) method of feeding by means of an automatically operating shot feeder.

With periodic feeding in small and large portions, the process of replenishing the whole shot proceeds somewhat differently. Here the replenishment of whole shot is not continuous, but periodic, at definite intervals of time and in definite portions—doses. Hence, with periodic feeding, the question of determining the weight of the portion and the interval between additions is of great importance. Laboratory and field investigations have established that, when drilling with cast-iron shot, the weight of the portion and the interval between successive additions must be determined by the rate of crushing of whole shot grains at the bottomhole of the well. The rate of crushing depends on many factors. Chief among them are: pressure on the bottomhole, rotational speed of the bit crown, quality of the shot, weight of the portion, ratio of whole to broken shot, etc. For example, if at a peripheral speed of the bit crown equal to 1.4 m/sec and at a pressure of 22.5 kg/cm² a portion of whole shot weighing 200 g is crushed in 15–20 min, then at the same peripheral speed but at a pressure of 45 kg/cm² it is crushed in 4–5 min; the same portion of shot, at a pressure of 22.5 kg/cm² but at a peripheral speed of 2.6 m/sec, is likewise crushed in 4–5 min. Vibration has a great effect on the rate of crushing. Thus, according to the data of S. S. Sulakshin and V. M. Matrosov, when drilling in a block of granodiorite with a peripheral speed of 0.5 m/sec and a pressure of 22 kg/cm², a portion of whole shot weighing 100 g is crushed in 15–20 min; with a vibrator, with a disturbing force equal to approximately 60 kg/cm², the same portion is crushed in 2–3 min. In the latter case, the average rate of deepening was twice as great as without the vibrator.

Interesting results were obtained in studying the effect, on the rate of splitting of shot, of the weight of the charge and of the ratio of whole to broken shot. Thus, if a charge of ordinary cast-iron shot weighing 100 g is poured into the borehole and drilling is carried out with a crown peripheral speed of 1.4 m/sec at a pressure of 22.5 kg/cm², 10–15 minutes are required for all the shot to split. If the same charge of whole shot is poured in together with broken shot, the weight of which is also 100 g, it splits in 5–7 minutes. Charges of whole shot alone weighing 400 g and 800 g split in 20–30 minutes, whereas in a mixture with broken shot they split in 8–10 minutes, and so on. It follows from this that, in determining the weight of a charge of whole shot, it is necessary to establish its minimum required amount, which would ensure a normal process of deepening the borehole.

The physico-mechanical properties of rocks have almost no substantial effect on the rate of splitting of whole shot. To study the process of the rate of splitting of whole shot as a function of the physico-mechanical properties of the rocks being drilled, drilling was carried out in sandstone and granite. It turned out that a charge of whole shot weighing 200 g, when drilling granite, splits in 15–20 minutes, and when drilling sandstone, in 20–25 minutes. In this case, in the first instance the average penetration rate was 0.49 cm/min, and in the second, 2.0 cm/min, i.e., the drillability of the sandstone was four times greater than the drillability of the granite.

Observations of the work of leading drilling crews of the “Altaĭtsvetmetrazvedka” trust also established that, when determining the regime for feeding the bottomhole of wells with small charges, these crews proceed, as a rule, not from the category of the rocks by drillability, but from the consumption of shot per hour of net drilling (hourly consumption). The hourly consumption of shot does not depend on the category of the rocks and is approximately the same for category VIII as for category XII, but it depends on the diameter of the crown, the bottomhole pressure, and the peripheral speed of the crown. Table 2 gives the results of observations of one of the drilling crews of the Belousovskaya GRP, which drilled a borehole in microquartzite with different regimes of periodic feeding in small charges.

Table 2

It is evident from the table that the best indices for penetration per hour of net drilling and for shot consumption were obtained in shifts 1, 3, and 5, and the worst in shifts 4, 6, and 7. In the first case, 150 g was fed to the bottomhole every 5–7 minutes, and in the second, 300 g was fed at the same intervals. When drilling rocks

For categories VIII–IX in drillability, shot is also fed to the bottom at 150 g every 5–7 minutes, which likewise gives the highest drilling productivity indices.

It should be noted that the results given above from experimental observations of the crushing rate of ordinary cast-iron whole shot were obtained for one grade and diameter of shot. With a change in the quality characteristics of whole shot, the above dependencies of the rate of its crushing also change. Therefore, the regime of periodic feeding of the bottom with small portions of shot must in each case be established for shot of a definite quality. As was already indicated, the factor for checking the correctness of the established feeding regime in small portions should be the uniformity with which the core is treated by the shot. If the core is treated unevenly and there are even places where it has self-chipped in the bit, this indicates an incorrectly established regime for feeding shot to the bottom of the wells. It is necessary either to increase the weight of the portion of shot fed to the bottom, or to reduce the interval between additions.

To obtain the maximum possible drilling productivity, along with determining the optimum magnitude of the dose and the time interval between additions, a correctly established flushing regime is of great importance. With excessive flushing, more crushed shot will be carried away from the bottom than is necessary. Consequently, with subsequent additions a definite ratio of whole and crushed shot will not be maintained. With insufficient flushing, as whole shot is added and crushed, more and more crushed shot accumulates at the bottom, which ultimately leads to a disruption of the rational ratio of whole and crushed shot, and hence to a reduction in the rate of deepening. In addition, with insufficient flushing, very fine fractions of crushed shot accumulate at the bottom, which interfere with the more productive work of the coarse particles. Experimental and production observations have established that, in order to obtain the maximum possible rate of penetration with periodic feeding in small portions, i.e., when a rational ratio of coarse and crushed shot is maintained at the bottom, it is necessary to remove from the bottom of the well all crushed particles smaller than one to one and a half millimeters.

Fig. 2. Dependence of the rate of penetration under periodic feeding on the quantity of flushing fluid supplied to the well. Quantity of flushing fluid, in l/min per 1 linear cm of bit diameter: 1—2.0–2.5; 2—4.0–4.3.

In Fig. 2, constructed according to the same principle as Fig. 1, and also in Table 4, the results of drilling on the experimental installation are shown.

with a bit of 65 mm diameter, with periodic feeding in small portions (50 g every 5 minutes) and with different amounts of flushing fluid. Here, as already mentioned above, at the beginning of the run, crushed and round whole shot was immediately poured onto the bottomhole in a ratio of one to one, i.e., 50 g of whole shot and 50 g of crushed shot. As a result, during the very first observation interval the penetration obtained was even somewhat greater than with run-by-run feeding (points \(O_1\), \(O_2\), \(O_3\), in Fig. 1).

Table 4

Line 1 in this figure characterizes the penetration during drilling with flushing of 2–2.3 l/min per 1 sq. cm of outside diameter of the bit (recalculated for a bit of 110 mm diameter, this is equal to 22–25 l/min). From the graph and Table 4 it is evident that, as portions of whole shot were added, the penetration continually decreased and, having reached a certain level, became more or less constant. Analysis of the extracted shot showed that the portion of whole shot is almost completely split during the interval between additions, while the amount of crushed shot becomes greater and greater; moreover, even fragments with a diameter of 0.25–0.5 mm are not carried away from the bottomhole. At the moment when the next portion was added at point \(O_4\), there were 60 g, or 20%, of whole shot on the bottomhole, and 220 g, or 80%, of crushed shot. However, even in this case the average run penetration rate is greater than the average run rate with run-by-run feeding. Thus, if with run-by-run feeding it is on average 0.48–0.49 cm/min (Table 1), then in the indicated experiment the average run penetration rate is 0.65 cm/min, which amounts to 133%. The shot consumption in this case was 18 g per 1 linear cm of penetration.

Line 2 characterizes the penetration under the same regime of feeding the bottomhole of the well with shot, but with increased flushing. The amount of flushing fluid was supplied at the rate of 4.1–4.3 l/min per 1 sq. cm of outside diameter of the bit (recalculated for a bit of 110 mm diameter, this amounts to 45–47 l/min). As can be seen from the diagram in Fig. 2 and Table 4, the penetration rate remains almost at one level for some time, and then, although only slightly, also decreases. Analysis of the extracted shot showed that at the moment whole shot was added at the beginning of the run, a ratio of whole to crushed shot of one to one was established, while before the next charging there was 5–10% whole shot, and the remaining part of the shot was crushed. But as whole shot was added and split, crushed shot, even with this flushing, accumulated more and more. At the end of the run (point \(O_5\), Fig. 2), before the next charging, the weight of crushed shot was 160 g. Thus, this amount of flushing fluid is also insufficient to maintain a definite ratio of whole and crushed shot at the bottomhole. In this case the average run ...

the rate of penetration was \(0.83\ \text{cm/min}\), or 173% relative to the rate of penetration with run-by-run feeding, and the shot consumption was 15 g per 1 linear cm of penetration.

The necessity and possibility of using a comparatively large flushing flow when using periodic feeding in small portions is also confirmed by theoretical calculations.

Thus, for example, if in Rittinger’s formula \(\kappa\) is taken as equal to 5.11, then the falling velocity of shot particles with a diameter of \(1.0\ \text{mm}\) and, consequently, the jet velocity will be \(0.4\ \text{m/sec}\). With a bit diameter of \(110\ \text{mm}\) and shot of \(3\ \text{mm}\), the borehole diameter will be \(125\ \text{mm}\); consequently, the area of the annular clearance will be \(27.6\ \text{cm}^2\). Hence the flow rate of the flushing fluid is \(Q = 27.6 \times 40 = 1.1\ \text{l/sec}\), or approximately \(66\ \text{l/min}\). If \(\kappa\) is taken as equal to 4, i.e., for rounded particles, then \(Q\) will be \(60\ \text{l/min}\).

Conclusions

1. The most effective destruction of rock when drilling with cast-iron shot is achieved when both crushed and whole shot are present at the bottomhole together, approximately in the ratio of 50–90% of the former and 10–50% of the latter.

2. A definite ratio of whole and crushed shot can be maintained only by using a continuous feeding method or periodic feeding in small portions, as well as by using a comparatively high flushing flow.

3. The weight of the portion and the time interval between additions must be determined on the basis of the crushing rate of the whole shot grains in that portion. The crushing rate of whole shot grains depends on the pressure at the bottomhole, the peripheral speed of the bit, vibrations, the quality of the shot, and the ratio of whole to crushed shot, and is almost independent of the physicomechanical properties of the rocks drilled with cast-iron shot.

Corrections and Noted Misprints

In the article by S. A. Stroitelev, “Study of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.