ANALYSIS OF SCHEMATIC DIAGRAMS OF DEFLECTORS

V. I. Ryazanov, B. I. Spiridonov

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

In directional drilling of core exploratory boreholes, various types of deflectors are currently used, in whose designs two basic schematic principles are employed:

1. The curvature of the borehole occurs due to the action, on the rock-destroying tool, of a deflecting force perpendicular to its longitudinal axis. In this case there is continuous milling of the side wall of the borehole in the near-bottom-hole zone.

2. Drilling in a specified direction is ensured by skewing the tool or the drilling assembly.

Tools of the first group (BSNB, design of M. P. Olekseenko; AN, KOV, ShPTTO; wedge deflectors) have a number of substantial disadvantages limiting their use, namely:

1. In addition to the axial load, a deflecting force acts on the tool, the value of which must be sufficiently large. When axial and radial forces act simultaneously, the operating conditions of the tool deteriorate. In addition, the presence of a radial force increases the frictional moments between the rotating and nonrotating parts of the tool, and this increases the disorienting moment; increased demands must therefore be placed on the deflector fastening unit.

2. Deflection of boreholes is possible only in rocks of comparatively low categories.

3. The curvature of boreholes depends on operating parameters, which often determine the magnitude of the deflecting force.

4. When the borehole is curved, drilling productivity is, as a rule, reduced.

The indicated disadvantages are characteristic to a significantly lesser degree of tools in which the curvature of boreholes occurs as a result of skewing. In this case the longitudinal axis of the rock-destroying tool is directed tangentially to the borehole trajectory. For all known directional-drilling tools of group II, several methods of skewing can be distinguished.

The simplest method consists in inserting into the drilling tool, directly above the core barrel assembly, an articulated sub (Fig. 1, a), as a result of which, in an inclined borehole, under the action of the weight of the drill-pipe string, the core barrel assembly will assume a skewed position. The advantages of tools using this method of skewing are their simplicity and the small additional expenditures of money and time associated with the artificial curvature of boreholes.

Fig. 1.

At the same time, the area of their application is limited, since stable operation of the tools is possible only in inclined boreholes at significant zenith angles, and the curvature of the borehole occurs only in the apsidal plane. An exception is the curvature of boreholes by means of such tools in strongly anisotropic rocks.

With the second method of skewing, a drilling hinge is also included in the directional-drilling tool; its diameter is close to the borehole diameter (Fig. 1, б). The hinge is connected to a small-diameter core barrel, which in an inclined borehole, under the action of its own weight, is pressed by its lower part against the lower wall of the borehole, as a result of which the borehole is drilled in the direction of unscrewing. Typical representatives of tools with this method of skewing are SVS-TPI, designed by Yu. L. Boyarko, and A. Pickard’s design (USA). The disadvantage of these tools is that their use requires considerable time expenditures for enlarging the pilot borehole, which limits their field of application.

Tools in which the third method of skewing is used (SNB-3, DD-1-TPI) consist of a nonrotating part, anchored in the borehole by various means (a stator), in which a rotor rotates (Fig. 1, в). In this case the opening in the stator is made at a certain angle to the main axis, as a result of which the rotor rotates at an angle to the axis of the borehole. The designs of tools in this case are rather complex, and, in addition, large disorienting moments act on the stator.

With the fourth method of skewing, the tool also consists of a stator and a rotor (Fig. 1, г). A deflecting device is located on the stator; with its help the upper part of the tool is pressed against the borehole wall. Tools with this method of skewing include INB-Sh, PNB-II, Thompson’s arc deflector (USA), and others. These tools make it possible to obtain borehole curvature in any direction, but they are complex in design, and the intensity of curvature is difficult to regulate.

In addition to those indicated, one can propose one more, fundamentally new method of skewing. In this case the stator, anchored from rotation...

…movement and sliding along the borehole walls as deepening proceeds, terminates in a wedge washer (Fig. 1, d), against which a short deflecting assembly, serving as a continuation of the rotor, bears. The deflecting assembly and the rotor are connected by a hinge. Under the action of the axial load, which is transmitted through the wedge washer, the deflecting assembly assumes a skewed position in the borehole, thereby ensuring curvature of the borehole during drilling. The intensity of curvature in this case is easily regulated by changing the wedge washer.

Using the described method of skewing, the Department of Exploration Engineering developed the SR-TPI directional-drilling tool, in which, with a comparatively simple design, effective operation was achieved. The simplicity of the design makes it possible to bring the deflector diameter easily to 59 mm, which is difficult with other design schemes and methods. The deflector makes it possible to curve boreholes directionally without loss of diameter. The small length of the deflecting assembly and of the entire tool (less than 800 mm) provides a curvature intensity of up to 5 degrees per 1 running meter. The SR-TPI tool has successfully passed laboratory bench tests and initial production tests.