STUDY OF DIRECTIONAL DRILLING TOOLS FOR DETERMINING THE ATTITUDE ELEMENTS OF GEOLOGICAL STRUCTURES

S. S. Sulakshin, B. I. Spiridonov

At all stages of geological exploration work, along with determining the qualitative and quantitative characteristics of a mineral deposit, geological structures are constructed. In doing so, the objective is to correlate in space the position of ore bodies, establish their morphology, outline the further direction of exploration work, etc. To accomplish these tasks, in a number of cases it becomes necessary to drill a large number of additional boreholes.

Often, merely to establish the dip direction of structures or of a rock sequence from a single point, two or three boreholes are laid out. This, naturally, increases the costs of deposit exploration and the time required to carry it out.

At the Department of Exploration Engineering of Tomsk Polytechnic Institute, some experience has been accumulated in using directional-drilling methods and technical means to determine the attitude elements of geological structures. Most of these methods are based on the use of the regularities of natural and artificial curvature of boreholes.

Studies have established that, in the overwhelming majority of cases, at angles of intersection with rock beds within 25–75°, boreholes deviate in a direction perpendicular to the dip of the sequence. A classic example of this regularity is the Belousovskoye deposit in eastern Kazakhstan, on the western flank of which the borehole trajectories acquire an S-shaped form, accurately describing an overturned folded structure (Fig. 1).

There are many examples in which, in a monoclinal structure, boreholes deviate in a strictly regular manner opposite to the dip of the rock beds.

Thus, when constructing geological sections, it is always necessary to pay attention to the direction of natural curvature of boreholes and to make use of its regularities [1].

In a number of cases, the intensity of natural curvature is very small. This occurs in diamond drilling, and also at comparatively shallow borehole depths.

At the early stages of exploration work, the distances between boreholes are large. With a complex geological structure of a deposit, complex tectonics, and especially when a thick cover of overburden is present, it is very difficult to correlate the section. It is all the more difficult to determine in advance...

the direction of drilling of inclined boreholes, which must be specified in order to increase the intersected stratigraphic thickness and to ensure an optimal angle of encounter.

Under such conditions, it is rational to drill vertically inclined boreholes by the method developed at the Department of Exploration Engineering of TPI.

The method is based on the use of the regularities of the natural curvature of boreholes and on increasing the degree of their manifestation. The technical means used for this is a simple single-hinge deflector [2], which is employed in a vertically drilled borehole. Following the basic regularities of natural curvature, the boreholes deviate in a direction perpendicular to the dip of the rocks. Subsequent inclinometry makes it possible to determine rather clearly the attitude elements of the rocks in the given section, as was the case at the Kamenskaya prospecting area (Fig. 2) (Kuzbass Expedition) and at the Bungursky Severny site (Shushtulepskaya GRP, Kuzbass).

Fig. 1. Borehole curving under the influence of geological factors at the Belousovo deposit:
1—borehole drilled shaft;
2—planned borehole shaft

To increase the accuracy of determining the attitude elements of rock beds and, correspondingly, to increase the reliability of structural constructions, oriented core sampling is necessary. At present, several methods and a fairly large number of technical means for obtaining it have been developed. However, all these means are complex in design and, most importantly, are not mass-produced by industry. As a result, geological exploration enterprises are deprived of the possibility of using such an important factor in the study of the geological structure of deposits as the taking of oriented core.

Fig. 2. Geological section constructed according to the regularities of borehole curvature at the Kamenskaya deposit in Kuzbass

As shown by investigations carried out at the Department of Exploration Engineering, oriented core can quite easily be taken using a simple hinged deflector, by means of which a sharp curvature of the borehole shaft is produced. Under the influence of a number of factors, the deviation of a vertically drilled borehole occurs, as a rule, toward the direction opposite to the dip of the rock beds. The interval of sharp curvature

is 5–10 m. After deflection, inclinometry is carried out on the sharply curved interval, and then a long rigid assembly with a diameter close to the borehole diameter is lowered into the borehole; with it the hole is drilled in the original direction. Over an interval of 1–3 m the main hole completely departs from the auxiliary one, and a core is recovered from the borehole that has, in cross section, the shape of a crescent (Fig. 3). The direction of the core trough is determined by the preliminary inclinometry. After this it is not difficult to orient the recovered core and determine the attitude elements of the rock mass, bedding, fracturing, etc.

Fig. 3. Scheme for obtaining oriented core when drilling an auxiliary hole: 1—the main hole; 2—the auxiliary hole; 3—continuation of the main hole; 4—core with a trough

When oriented core is taken by this simple method, additional data for constructing the structure can be obtained and used. It must be taken into account that the auxiliary, sharply curved hole deviates in a direction perpendicular to the dip of the rocks. In addition, the main and auxiliary holes at different depths may intersect some marker bed, stratum, vein, fracture, etc.

With the use of additional inclinometry when the hole is vertical, the core may also be oriented by a pilot borehole. In this case, with a small-diameter assembly, a pilot borehole 5–10 cm long is drilled eccentrically in a previously leveled bottom at the wall of the borehole. After this its position relative to the center of the bottom is determined. For this, Polyakov’s instrument is lowered into the borehole on rods; in the lower part of the instrument there is an eccentric pin, by which, when the string is rotated, the pilot borehole is felt and its direction is determined.

When drilling of the borehole is continued with an ordinary assembly, core is taken on which traces remain from the previously drilled pilot borehole; by these traces the core is oriented and the attitude elements are determined.

Additional inclinometry is necessary when boreholes are drilled vertically. In inclined boreholes, self-orienting directional-drilling assemblies are used, with which a pilot borehole is drilled in a strictly specified direction. Thus, when using the assembly for unscrewing the SVS TPI of the design of Yu. L. Boyarko, the pilot borehole is obtained at the lower wall of the borehole (Fig. 4, a). When a simple hinged deflector of small diameter is used, however, the position of the pilot borehole is obtained in the upper part of the bottom, at the hanging wall of the borehole (Fig. 4, b).

If the direction of the interval of the inclined borehole is known, then, having the traces of the pilot boreholes on the core and knowing that they are located in the upper or lower part of the bottom, it is possible to orient the recovered core at the surface. At present this method has begun to be used

in a number of geological organizations; in this case, various technical means of directional drilling are used to cut a new direction. Thus, the workers of KazIMS [3] use the SNB-KO deflecting device, and in VKGU the SO-3 tool is used.

Fig. 4. Diagram for obtaining oriented core when drilling a pilot borehole in an inclined hole:
1—pilot borehole; 2—main hole; 3—core with orienting pilot borehole

The described methods for taking oriented core with the use of directional-drilling means do not require complex instruments or equipment, and their technical implementation is accessible to workers of any qualification. Therefore, their widest possible introduction into the practice of drilling exploration is necessary; this will undoubtedly improve the quality and completeness of the geological materials obtained and the overall economic efficiency of exploration work.

REFERENCES

1. S. S. Sulakshin. Natural curvature of boreholes as a control factor in the construction of structural sections and maps. Izv. vyzov. Geologiya i razvedka, No. 4, 1964.

2. B. I. Spiridonov, S. S. Sulakshin. Experience in directional drilling of boreholes. Byull. nauch.-tekhn. inform. ONTI VIMS, No. 5 (49), 1963.

3. O. P. Leont’ev, E. I. Limanov, V. A. Bronevskii, I. N. Strabykin, I. I. Elizarov. Directional drilling of exploratory boreholes with the UNB-KO device. Alma-Ata, 1965.