ONE OF THE PROBLEMS OF TUBELESS DRILLING AND A POSSIBLE SOLUTION

L. A. Pukhlyakov

(Submitted by the Department of Combustible Minerals)

The problem of tubeless drilling was taken up in its time by A. A. Minin, A. A. Pogarskii, and K. A. Chefranov [1, 2]. It consists in the fact that the unit for breaking the rock is lowered into the borehole not on pipes, but on an elastic cable-rope. This made it possible to bring the speed of lowering and raising the tool up to 1 m/sec, and to reduce the share of time spent on this operation from 50–60% to 10% of total expenditures. Thus, if the remaining parameters of this type of drilling had remained the same as in ordinary drilling, its commercial rate would have begun to exceed the commercial rate of ordinary drilling by approximately a factor of two. However, tubeless drilling encountered new problems that nullified all the achievements of this method. First of all, difficulties arose with neutralizing the torque of the bit and with creating pressure at the bottomhole. The presence of these difficulties led to a sharp decrease in the mechanical drilling rate and, on that account, to a considerable deterioration of the remaining indicators of this type of drilling. As a result, the method of Minin et al. was not adopted for use in industry. Here, however, it should be noted that the new methods of rock destruction now being developed do not require the creation of large pressures at the bottomhole. Consequently, in the near future this will make it possible to carry out tubeless drilling on a very broad scale.

Among the other problems of tubeless drilling, the foremost is the problem of cleaning the drilling fluid, to which A. A. Minin et al., one may say, did not devote proper attention. In fact, in the unit of Minin et al. the drilling fluid was cleaned in a bailer located in the upper part of the drilling unit. The drilling fluid was sucked into this bailer by a pump located beneath it. At the same time, in its upper part there occurred a sharp change in the direction of motion of the fluid; as a result, the drilling fluid was cleaned to some extent. It should be noted, however, that A. A. Minin could not achieve a large difference between the cross sections of the bailer and the annular space, and therefore the velocity of the drilling fluid in this space differed only slightly from that inside the bailer. As a result, those particles which the upward fluid stream was capable of carrying upward through the annular space, as a rule, did not fall out of the fluid upon entering the bailer; this led to the drilling fluid becoming heavily contaminated and seizing the tool at the bottomhole.

To solve this problem, the present article proposes using a multisection bailer (Fig. 1), which is, as it were, several separate bailers (sections) standing one on top of another, each of which captures only a small part of the fluid rising in the annular space. The velocity of this fluid in each separate section must be much lower than in the annular space, and hence the conditions for settling of the sludge will be much better than with a single-section bailer. By increasing the number of individual sections of the bailer to fifty, one can achieve quite satisfactory cleaning of the drilling fluid and completely eliminate sticking of the tool at the bottom.

The walls of the individual sections of the bailer 1 must be made of pipes of the same diameter as the rest of the drilling tool, and must be connected to one another by nipples 5, not by couplings, as ordinary drill pipes are. This is connected with the fact that the bailer with the drilling assembly must be raised and lowered in the well at speeds far exceeding the rates of lifting and lowering drill strings in ordinary drilling (after all, one of the advantages of tubeless drilling is the lowering and lifting of the tool on a cable, or, more precisely, on a cable-rope). Such speeds, with a coupling connection, will cause intense eddies, which may lead to collapse of the borehole walls and, thus, to sticking of the tool. The outside cross-section of the nipple is approximately equal to the outside cross-section of the rest of the bailer, and therefore such eddies should not arise when it is lifted. To hold the assembled bailer on the elevator at the surface, longitudinal teeth are left on the nipples; in this case it is borne in mind that the ledges between the teeth enter the body of each of the nipples to the same extent as the teeth themselves protrude above the shoulders of the outer surface of the sections.

The bottom of each section 2 has a threaded opening—into it is screwed a segment of the central tube of the bailer 3. Onto the upper end of each such segment a connecting bell 4 is screwed. The bell of each lower section serves for insertion into it of the lower smooth end of the segment of the central tube of the section above. To reduce penetration of liquid into the central tube through the bell, there are several annular grooves on its inner surface. The bottom of the section is welded to the corresponding place of each section with the segment of the central tube screwed into it, which, in turn, is fitted by its bell over the smooth end of the tube of the section above.

Thanks to such a design, during disassembly and assembly of the multisection bailer (in the process of preparing it for lowering into the well or extracting it from the well), only the nipple connections need be screwed together or unscrewed in it. Otherwise, work with it must be carried out in the same order as when lowering or raising an ordinary drill string, only in smaller quantity.

In the lower part of the bailer (under the bailer) there is a turbine-type pump (an electric submersible pump), driven by a submersible electric motor with a hollow shaft. Motors and pumps of this type are produced by many enterprises in our country for the oil industry. During operation of the assembly at the bottom, the pump creates circulation—drives the drilling fluid from the central tube to the bit, and from there it rises through the annular space and is drawn into the individual sections of the bailer. Here it is freed from the drilled rock (sludge) and again enters the central tube through the special opening 8.

Fig. 1. Multi-section bailer for cleaning drilling fluid during tubeless drilling (schematic diagram): 1—side walls of an individual section; 2—bottom of the section; 3—central tube of an individual section; 4—connecting socket; 5—connecting nipple; 6—opening for the entry of uncleaned fluid into the section; 7—mesh for dispersing the stream of drilling fluid rising from the internal cavity of the section, and for preventing large fragments of drilled rock (sludge) from entering the central tube; 8—opening for the entry of cleaned fluid into the central tube of the bailer. The arrows show the direction of fluid movement; drilled rock is on the bottom of each section.

After drilling has been stopped, on a signal from the surface a special mechanism, the design of which is not given here, can close openings 8 in all sections at once and simultaneously open the upper valve of the central tube. In this state the flushing fluid will not be drawn into the internal cavities of the bailer sections, but will circulate through the central tube and the behind-bailer space. If the assembly is then raised, the velocity of the fluid between the bailer and the walls of the borehole will decrease, and it is possible to ensure that it ceases altogether. This will be of definite importance from the standpoint of reducing the load on the cable and preventing collapses during raising and lowering of the assembly.

REFERENCES

1. A. A. Minin, K. A. Chefranov. Tubeless electric drill. Neftyanoe khozyaistvo, No. 11, 1955.

2. A. A. Minin, A. A. Pogarskii, K. A. Chefranov. Technology of tubeless drilling. Gostoptekhizdat, 1956.