Ejector Core Barrel for Drilling Geological Exploration Boreholes

S. S. Sulakshin, V. G. Khramenkov, V. P. Rozhkov

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

In the exploration of certain types of mineral deposits, the question of sampling is an acute problem. Such types include deposits represented by rocks and minerals that are easily washed out by the flow of flushing fluid; deposits composed of highly disturbed and fractured rocks; deposits with selectively abraded veinlets of useful components. In order to obtain the necessary amount of core under these conditions, double core barrels and reverse circulation are used. The use of double core barrels for drilling through the entire borehole section reduces the rate of advance, since such barrels are intended for cutting only individual beds of the useful mineral. The use of reverse circulation makes it possible to employ simple core barrels with ordinary working bits, thereby not reducing the drilling rate and positively solving the sampling problem.

To create reverse flushing, in addition to the known method of forcing flushing fluid into the annular space from the surface and returning it through the drill pipes, various methods and means that provide local circulation are used in practice: air-lift installations, downhole pumps with pneumatic drive, pumpless drilling (with discharge of the barrel or with pulsating supply of compressed air into the drill pipes), and, finally, ejector barrels. The simplest in organization and, at the same time, quite effective is the use of ejector barrels.

Recently, to create reverse circulation in the bottom-hole zone, special barrels with a packer have been used, in which the discharge of flushing fluid from the central channel into the annular space is carried out below the packer; the flow washes the bottom and, through the internal channels of the barrel, is again discharged into the annular space, but already above the packer. The use of a packer makes it possible to obtain a more stable reverse circulation of the flushing fluid, which is in many cases a drawback of ejector barrels, especially at the end of a run. But barrels with a packer are more complicated in operation and so far have not become widely used.

Analyzing the operation of ejector barrels and taking into account the conditions of mineral exploration, it is possible to determine the basic requirements that new designs of ejector barrels must satisfy:

1) minimum pressure drop in the barrel;

2) stable reverse circulation of the flushing fluid;
3) sufficient penetration per run at a uniform and high drilling rate;
4) the possibility of collecting cuttings from the rock and the useful mineral;
5) maximum cleaning of the near-bottom circulating flushing fluid;
6) simplicity of manufacture, adjustment, and operation;
7) reliability in operation;
8) the possibility of using ordinary bits;
9) the possibility of monitoring reverse circulation during drilling;
10) the possibility, when necessary, of switching from reverse circulation to direct circulation in order to carry out effective cleaning of the borehole;
11) the possibility of feeding wedging material from the surface;
12) low cost of the tool.

Figure 1 shows diagrams of ejector core-barrel assemblies: a simple one, one with double series ejection, and one with double parallel ejection. The principle by which local reverse circulation of the flushing fluid is created when ejector tools are used is as follows. The flushing fluid pumped from the surface, leaving nozzle 1 (Fig. 1, a) at high velocity, forms a submerged jet with a depression at the point of compression. From receiving chamber “a,” the fluid, owing to the depression, enters together with the jet stream into diffuser 2 and mixing chamber 3. Owing to the continuity of the flow, fluid enters the receiving chamber from the annular zone through the internal channels “b” and “v,” creating reverse circulation. In the annular space the fluid, leaving channel “a,” is divided into a stream going to the bottom and one going to the surface.

The ejection coefficient (the ratio of the intensity of the reverse flow to the jet discharge) is determined by the geometry and dimensions of the ejector, by the jet discharge, and by the hydraulic resistances of the channels for the reverse flow.

At flushing-fluid flow rates within 120–180 l/min, the limits of variation of the geometric dimensions of the pump (ejector) elements have already practically been established [1, 2, 3, 4, 5, 6, 7, 8]. When new ejector tools are designed, the adopted dimensions of the pump elements are refined by laboratory proof tests.

A substantial drawback in the use of ejector tools is the sharp reduction of the ejected flow when hydraulic losses in the reverse channels increase, which may occur because of core wedging or silting of the near-bottom zone and which, in turn, aggravates the causes that have produced the hydraulic losses and ultimately stops the reverse flow; this logically leads to sticking of the bit.

Fig. 1. Diagrams of ejector core-barrel assemblies. 1 — nozzle; 2 — diffuser; 3 — mixing chamber; 4 — sludge pipe; 5 — core barrel.

Fig. 2. Ejector core-barrel assembly. 1—nozzle; 2—mixer; 3—screw packer; 4—gland; 5—auger-type hydrocyclone sludge catcher; 6—sludge tube; 7—grate; 8—core barrel

In this connection, all designers, when creating ejector tools, devote great attention to the maximum cross section of the return channels and to the creation of additional back pressure by, for example, installing a packer and additional channels for discharging the fluid above the packer.

In this connection, the use of sequential (Fig. 1, б) and parallel (Fig. 1, в) ejection is of interest for intensifying the ejected flow.

The use of local reverse flushing makes it possible to employ internal sludge tubes and to collect additional material for sampling. This point is especially important in cases of loss of drilling fluid, when it is impossible to collect sludge at the surface. The use of sludge tubes should even be considered mandatory in interval sampling of beds or ore bodies, when severe abrasion and washing-out of the useful component (for example, molybdenite) are observed, and when the sampling results are completely distorted if particles of the useful component are captured at the surface without the use of additional means and operations (cleaning the settling system, casing the borehole, eliminating losses, flushing the borehole before and after a run, using special settlers, separators, etc.).

Figure 1 shows two types of internal sludge tubes. The use of one or another type of sludge tube may be recommended on the basis of the condition of the maximum volume for sludge settling with the corresponding diameters and standard pipes.

Figure 2 presents an ejector tool that satisfies most of the requirements developed. Its principal advantages are: the use of screw packer 3, which creates additional resistance to the flow emerging from opening “a” and throws it toward the bottom, thereby facilitating reverse flushing; and the use of hydrocyclone auger sludge catcher 5, which promotes a sharp increase in the amount of collected sludge, from coarse to fine fractions.

Such a single-type ejector tool can be used in diamond, hard-alloy, and shot drilling with flushing and air flushing; it fits within a minimum diameter of 57 mm. In diamond drilling, the role of the packer can be performed by the reamer. An advantage of the tool is the simplicity of its design and the ease of extracting the collected

sludge; stability of the ejected flow and a sufficient run length with maximum recovery of drilled rock fines and useful mineral.

REFERENCES

1. Bagutin Yu. A., Davydov A. S. Drilling boreholes using earlift core barrels. Exploration and Protection of Mineral Resources, No. 3, 1967.

2. Drilling of boreholes, production and transportation of oil and gas. Proceedings of the Tyumen Industrial Institute, issue 2. Tyumen, 1968.

3. Grebenyuk A. A., Moiseev G. G. Results of testing a double water-jet core barrel for shot drilling VKS-1M. Exploration and Protection of Mineral Resources, No. 2, 1966.

4. Grebenyuk A. A., Moiseev G. G. Once again on the operation of ejector core barrels. Exploration and Protection of Mineral Resources, No. 5, 1968.

5. Zayonch O. L. Operation of ejector core barrels. Exploration and Protection of Mineral Resources, No. 9, 1967.

6. Sokolov E. Ya., Zinger N. M. Jet Apparatuses. Gosenergoizdat, 1960.

7. Sumaneyev N. N., Plekhanov M. I. Ejector barrels for diamond drilling. Exploration and Protection of Mineral Resources, No. 6, 1967.

8. Friedman B. E. Hydroelevators. Mashgiz, Moscow, 1960.