Mechanics of Bottomhole Failure in Anisotropic Rocks and the Process of Wellbore Deviation in Them During Shot Drilling

Yu. L. Boyarko

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

In exploration practice three types of drilling are used: diamond, shot, and hard-alloy drilling. The most widespread in the Soviet Union are the latter two types. Until recently, diamond drilling was not widely used because of the absence of domestic diamond deposits. Hard-alloy drilling is used in rocks from categories I to VII in drillability. Rocks of category VIII are usually drilled with hard-alloy bits only with difficulty, and here it becomes necessary to switch to shot drilling. Rocks of category VIII and higher are drilled with shot.

When shot drilling is used, the accident rate rises sharply, the power expenditure for drilling increases, and the self-deviation of boreholes increases very strongly in those rocks where, under hard-alloy drilling, it had been insignificant. Thus, for example, in the Bakal Geological Exploration Party, when shales are drilled with shot, the degree of self-deviation is 7--8° per 100 meters of advance. When switching to drilling with pobedit bits, the degree of self-deviation drops sharply and is only 1.5--2° per 100 meters of advance. In order to take effective measures to prevent such intense deviation, it is necessary to know its true causes, so that a number of countermeasures can be opposed to the phenomena causing deviation.

In educational literature, all discussions of the causes of deviation usually refer to hard-alloy drilling; at least, in all drawings attached to the descriptions, hard-alloy bits are shown. The causes of deviation in shot drilling are either not mentioned or are discussed very little. In some cases, the instability of the rectilinear form of equilibrium of a long twisted rod is taken as the main cause of borehole deviation [2], as a result of which the borehole usually tends to assume, in the subsurface, a position close to a spiral. More commonly, however, deviation from the specified direction is explained by accumulation of shot at the bottom of the hole on the lower side during inclined drilling, as a result of which the bit is pressed toward the upper part of the bottomhole and the borehole flattens out. This is all that can be found about the causes of deviation in shot drilling, apart from mention of a large clearance between the bit and the walls of the borehole.

The causes of deviation in drilling with hard-alloy bits cannot be mechanically transferred to shot drilling, since in the first case the de-

curvature is explained mainly by deviations of boreholes when passing from soft rocks into hard ones and back; in shot drilling, however, many-meter-thick masses of homogeneous rocks are usually present. What, then, is the reason that causes boreholes to curve in rocks that are outwardly homogeneous by 8–10° for every 100 meters drilled, and to be almost rectilinear in other rocks lying next to the former, often even conformably? The cause of curvature cannot be explained by the distribution of shot at the bottomhole, since with an inclined bottomhole in one case intensive curvature toward the “flattening” of the borehole occurs, while in another such curvature is absent. In some cases curvature is even observed toward the “twisting-out” side. The conclusion suggests itself that, in shot drilling, the self-deviation of boreholes depends chiefly on the properties of the rocks being drilled.

E. F. Epshtein, in his book Wear of Hard Alloys during Friction against Rocks in Core Exploratory Drilling, considering the mechanical properties of rocks and touching on anisotropy, says: “Comparing the well-known facts of borehole curvature with data on the anisotropy of the mechanical properties of rocks, contrary to existing views, we arrive at the following conclusion: the geological prerequisites for borehole curvature when drilling in metamorphic rocks consist chiefly in the anisotropy of the mechanical properties of rocks.” The cited work deals with carbide drilling, but, in our opinion, a similar conclusion may also be expressed for shot drilling. For example, an analysis of the regularities of borehole curvature during core shot drilling at the Bakal deposit shows that the main cause of borehole self-deviation is the very high anisotropy of the mechanical properties of certain rocks of the Bakal complex.

The entire complex of rocks composing this area may be roughly divided into three groups: schists, quartzites, and dolomites. The schists are characterized by a layered texture. Quartzites and dolomites have a massive texture. Rocks with a layered texture are characterized by the greatest anisotropy of mechanical properties. In indentation tests with the cutter along the schistosity and perpendicular to it, the schists give a threefold divergence of results. Dolomite and quartzite, although they have a massive texture, are relatively isotropic only in the case of quartzite. Dolomite, however, is anisotropic, which can evidently be explained by regional metamorphism. Data on determining the degree of anisotropy are given in Table 1. The method for determining the degree of anisotropy was borrowed from E. F. Epshtein [7].

Comparing the degree of anisotropy of mechanical properties and the degree of self-deviation for all three types of rock, one may confidently say that the main cause of wellbore deviation at the Bakal deposit is the anisotropy of the mechanical properties of the rocks. Quartzite, for example, is mechanically almost isotropic, has a massive texture, and self-deviation during its drilling is almost entirely absent \((1^\circ\) per 100 m). Conversely, shales, having a layered texture and strongly pronounced anisotropy, cause enormous self-deviations of boreholes \((7—8^\circ\) per 100 m). Dolomites and the siderites occurring in them, although they have a massive texture, nevertheless, when tested for anisotropy, show a certain difference in hardness in different directions; accordingly, during drilling this also causes a definite degree of self-deviation of boreholes. Similar arguments can be adduced for the rocks of the Zlatoust geological-prospecting party and other parties of the Southern Urals.

However, all that has been said above is too general a conclusion and does not make it possible to develop any measures for preventing self-deviation in such rocks. To develop measures for preventing self-deviation of boreholes under such conditions, it is necessary to carry out a more detailed analysis of the mechanics of bottomhole failure in anisotropic rocks, which is done below by the author. For clarity, let us take the case of a borehole intersecting layered (or anisotropic) rocks at an angle of \(45^\circ\) (Fig. 1). Shot, distributed beneath the end face of the bit uniformly around the circumference of the bottomhole, transmits destructive forces to the rock. If one neglects the lateral forces created by skewing of the tool in the borehole and small in comparison with the axial pressure on the bottomhole, then to some extent an analogy can be drawn between the work of the shot at the bottomhole and the work of balls in a ball bearing. The greatest forces are transmitted by shot balls 1 and \(1'\), located at the very bottom. Balls located higher transmit smaller forces, the magnitude of which depends on the angle between the borehole axis and the direction of the transmitted force. Thus, shot balls 2, 3 and \(2'\), \(3'\) transmit equal forces to the rock at an angle of \(45^\circ\) with respect to the borehole axis. But because the rock is anisotropic, on the two sides of the end face of the bit unequal failure will result from equal forces. In our case, for shales, shot balls 3 and \(3'\) will do three times more work than shot balls 2 and \(2'\).

Fig. 1. a) Diagram of bottomhole failure in core drilling of anisotropic rocks. b) Failure epure of the bottomhole.

Proceeding from this principle, one can depict the epure of rock failure beneath the end face of the bit, which will have the form of an oval whose axis does not coincide with the axis of the bit end face (Fig. 1, a). The failure epure is displaced in the direction opposite to the dip direction of the layering. If the failure epure of the entire bottomhole is depicted in a plane—

[[unclear: beginning of sentence]], perpendicular to the borehole axis, then it will appear in the form of circles (Fig. 1, b), the centers of which are located at one point and are displaced from the center of the borehole in the direction likewise opposite to the direction of dip of the bedding. The curvature of the borehole occurs as follows (Fig. 1, a): in the drilled-out space, along the contour of the failure support, the face of the bit will be forced to occupy a middle position, i.e., the axis of the bit face must coincide with the axis of the support—the shaft. But in the next cycle of failure there will again be one-sided treatment of the bottomhole, and the bit will again move in the same direction, etc. Thus, together with forward advance along the borehole axis, the core-barrel assembly is continually forced to make a plane-parallel displacement in the direction opposite to the dip of the bedding and anisotropy. This process will continue until asymmetric failure of the rock under the bit face ceases. And this will occur when the borehole axis is located perpendicular to the bedding or to the axis of anisotropy. In this case the shot pellets will work in symmetrical positions with equal destructive force, as a result of which the axis of the failure support will coincide with the axis of the bit face.

Fig. 2. Diagram of the anisotropy of shales.

The investigations carried out make it possible, by constructing a polar diagram of failure, to analyze visually the degree of self-deviation as a function of the angle at which the axis of the core-barrel assembly meets the principal axis of anisotropy. An example is the diagram constructed for the above-mentioned shales (Fig. 2). The diagram is a contour close to an ellipse. The first quadrant is constructed from points obtained in testing anisotropic rocks for cutter indentation at different angles to the axes of anisotropy. The entire remaining contour is built according to the principle of symmetry. The inner contour in this case is constructed for belts of shot working at an angle of \(45^\circ\) to the axis of the bit face. The construction is performed by reducing all data of the outer ellipse by the value \(\cos 45^\circ = 0.707\). In the ellipsoidal contours constructed, the major axes coincide with the principal axis of anisotropy, and the minor axes are arranged parallel to the bedding. The polar diagram constructed in this way ...

makes it possible, in turn, to construct diagrams of bottomhole failure for any angles of encounter between the axis of the tool and the bedding. In our case, five diagrams were constructed with angles of encounter of 90°, 67°, 45°, 22°, and 0°; from these it can be seen at which angles of encounter the most intense deviation will occur.

An analysis of the mechanics of bottomhole failure when drilling anisotropic rocks makes it possible to understand correctly the causes of spontaneous deviation of boreholes and to approach, in a technically competent manner, the problem of preventing borehole deviation. Polar diagrams of rock anisotropy and the diagrams of bottomhole failure constructed with their aid for different angles of encounter clearly show how deviation occurs and which angles of encounter are most dangerous with respect to deviation.

REFERENCES

1. Vozdvizhenskii S. G., Volkov S. A. Exploratory core drilling. Gosgeoltekhizdat, 1957.

2. Dinnik A. N. Papers on mining engineering. Ugletekhizdat, 1957.

3. Lekhnitskii S. G. Theory of elasticity of an anisotropic body. GTTI, 1950.

4. Ostroushko I. A. Drilling of core blast holes. Metallurgizdat, 1956.

5. Rabinovich A. L. On the elastic constants and strength of anisotropic materials. Proceedings of TsAGI, No. 582, 1946.

6. Shamshev V. A. et al. Exploratory drilling. Gosgeoltekhizdat, 1958.

7. Epstein E. F. Wear of hard alloys during friction against rocks in core exploratory drilling. Gostoptekhizdat, 1952.