ANISOTROPY OF THE MECHANICAL PROPERTIES OF ROCKS AS A CAUSE OF AZIMUTHAL BOREHOLE DEVIATION

Yu. L. Boyarko

(Submitted by the scientific seminar of the Departments of Exploration Engineering, Safety Engineering, and Mining Production Technology)

Azimuthal deviation of boreholes, which in some deposits reaches considerable intensity, leads to major distortions of geological data. The causes of azimuthal deviation, however, have not yet been fully elucidated.

The works of a number of investigators have shown that the direction and intensity of azimuthal deviation depend on the type of drill bit, the direction of rotation of the drilling assembly, the magnitude of the borehole inclination angle, the mechanical properties of the rocks, etc.

Thus, in core drilling by the shot method, boreholes, as a rule, deviate to the right, and their true azimuth of direction increases. In some cases, however, boreholes turn to the left; moreover, this is characteristic not of individual boreholes but of all boreholes in certain deposits [3]. In some cases, with shot drilling, azimuthal deviation is not observed or is uncharacteristic. In addition, the intensity of azimuthal deviation, usually high at small borehole inclination angles, decreases sharply as the latter become flatter. This regularity has been noted in the works of a number of investigators and, in particular, is confirmed to some extent by the graph presented in Fig. 1.

When drilling with three-cone bits, boreholes turn to the left under right-hand rotation of the assembly [2, 4], whereas the use of blade bits leads to a turn of the boreholes to the right. For the most part, boreholes also turn to the right in diamond and carbide drilling.

An experimental change in the direction of rotation of the drilling assembly from right-hand to left-hand leads, as a rule, to a reversal of the direction of azimuthal deviation for all types of drill bits. This fact makes it possible to conclude that the dominant factor in the causes of azimuthal deviation of boreholes is the one-sided direction of rotation of the drilling assembly.

In this connection, the azimuthal deviation of boreholes may be compared with the derivation of a projectile that occurs during its flight after being fired from a rifled weapon. Just as, in the derivation of a projectile, it is deflected in the direction of rotation owing to friction against the oncoming air, so, in drilling inclined boreholes, one-sided rotation causes the occurrence of a unidirectionally oriented frictional force against the lower wall—

of the borehole, which also promotes deviation of the borehole to the right under right-hand rotation.

However, such an interpretation of the causes of azimuthal curvature of boreholes cannot explain a number of questions that arise in analyzing the regularities of azimuthal curvature. Thus, for example, it is impossible to fully explain the decrease in the intensity of azimuthal curvature with increasing zenith angle of the boreholes, although as this angle increases the frictional force of the drill bit against the low side of the borehole also increases. Nor is the deviation of boreholes to the left when they are drilled with roller bits under right-hand rotation of the drill string explained.

Fig. 1. Intensity of azimuthal curvature as a function of the zenith angle (Bakal deposit)

with increasing zenith angle of the boreholes, although as this angle increases the frictional force of the drilling bit against the low side of the borehole also increases. Nor is the deviation of boreholes to the left when they are drilled with roller bits under right-hand rotation of the string explained.

Earlier [1] we proved that one of the principal causes of zenith curvature of boreholes is the anisotropy of the mechanical properties of the rocks being drilled. Further detailed investigations in this direction show that anisotropy of the mechanical properties of drilled rocks also affects the azimuthal curvature of boreholes.

As is known, rocks for the most part have unequal hardness in different directions relative to the plane of schistosity, bedding, etc.; that is, they possess anisotropy of mechanical properties. The drillability of rocks possessing anisotropy of mechanical properties has its maximum value in the direction perpendicular to bedding or schistosity. This is explained by the fact that the principal plane of anisotropy of the mechanical properties of drilled rocks, in most cases, coincides with the plane of bedding or schistosity. When the angle of intersection between the axis of the core-barrel assembly and the principal axis of anisotropy decreases, the drillability of anisotropic rocks, for the same drilling parameters, decreases. For some anisotropic rocks, when the drilling direction changes from perpendicular to parallel to the plane of anisotropy, the hardness increases by a factor of 2 or more.

When drilling boreholes whose axes are directed at an acute angle to the plane of schistosity, objective conditions are created for nonuniform destruction of the bottomhole; this leads to regular curvature of boreholes both in the zenith and in the azimuthal planes until the borehole assumes a definite position relative to the principal plane of anisotropy.

Considering only the azimuthal curvature of boreholes in the present work—

let us consider the mechanics of bottom-hole destruction for a number of specific examples.

1. Core drilling with diamond, carbide, shot, and other bits

When the drilling bit rotates to the right (Fig. 2, a), the resultant force of pressure and torque applied to any cutter of the drilling bit is, in the general case, directed at an angle to the plane of the bottom hole. In this connection, the magnitude of bottom-hole destruction is not the same for different portions of the bottom hole and depends on the angle at which the resultant meets the principal plane of anisotropy. On the developed view of the bottom hole this is clearly visible (Fig. 2, b). The same figure gives a graph of the destruction of the developed bottom hole when anisotropic rocks are intersected at an angle of 45°. On the basis of this graph, a three-dimensional diagram of bottom-hole destruction was then constructed (Fig. 2, b).

Fig. 2. Mechanics of destruction of an annular bottom hole in anisotropic rocks:
a — diagram of the application of destructive forces;
b — developed view of the bottom hole and diagram of its destruction;
v — diagram of destruction of an annular bottom hole in dimetry.

When the drilling bit rotates to the right, the left part of the bottom hole will be destroyed more intensively (if viewed along the line of least resistance of the rock in the direction of the rise of the latter), and the borehole will be forced to curve to the right, since the drilling bit ...

the bit constantly tends to occupy a position perpendicular to the face.

For this reason, azimuthal deviation to the right will occur until the borehole has deviated from the direction perpendicular to the principal plane of anisotropy by such an angle that the deviation will be compensated for by the causes considered earlier in [1].

When drilling isotropic rocks, as well as anisotropic rocks along the planes of anisotropy, destruction of the face occurs uniformly and azimuthal deviation will not occur for this reason. This, in particular, explains the fact that at some fields azimuthal deviation is insignificant, since not all rocks are anisotropic.

Most fields, however, are represented by monoclinal structures; when they are drilled, at first the boreholes pass at an acute angle to the bedding plane. During this period the zenith angle increases and the boreholes deviate azimuthally. Subsequently the boreholes assume a direction close to the perpendicular to the principal plane of anisotropy and, accordingly, stabilize their azimuthal direction.

When drilling “down-dip” relative to the principal plane of anisotropy, azimuthal deviation of boreholes to the left will occur with right-hand rotation of the drilling assembly. But such a phenomenon is encountered rather rarely, since boreholes are most often set transverse to the dip of the rocks.

2. Drilling with a Full Face

When drilling with blade bits, the mechanics of face destruction is the same as in the case just analyzed. Consequently, when anisotropic rocks are intersected at an angle, boreholes will deviate to the right with right-hand rotation of the assembly. When drilling isotropic rocks, boreholes will not deviate azimuthally.

When drilling with roller-cone bits, the mechanics of face destruction changes in comparison with the cases considered earlier. Investigations of the motion of the cones along the face, carried out by R. A. Ioannesyan [5], show that, as the bit rotates, the cones slide along the face, having a nonsliding point \(D\) in the middle part (Fig. 3). Such sliding causes friction forces to appear; moreover, that part of the cone which works the peripheral part of the face produces a friction force \(F\) directed in the same direction in which the cone rolls. Hence we have that the destructive force \(P\), summing the forces from the pressure and rotation transmitted to the bit, will be directed at an angle to the plane of the face and toward the side opposite to the direction of rotation of the bit.

When anisotropic rocks are destroyed at an angle to the plane of anisotropy, such a direction of the forces leads to nonuniform destruction of the peripheral part of the face; the right-hand part of the face is drilled deeper, and the borehole turns to the left.

As indicated above, reversing the direction of rotation produces the opposite effect, i.e., the borehole begins to turn in the opposite direction.

This effect is used in practice to correct azimuthal deviation of boreholes, with good results. In comparison with other methods of combating azimuthal deviation, this method is the most effective and economical, in connection with which

it can be recommended for further use in combating borehole curvature and in introducing directional drilling.

Fig. 3. Mechanics of destruction of a solid bottom by a three-cone bit in anisotropic rocks: a—scheme of application of the destructive forces; b—development of the bottom and diagram of its destruction

LITERATURE

1. Yu. L. Boyarko. Mechanics of bottom-hole destruction in anisotropic rocks and the process of borehole curvature in them during shot drilling. Izvestiya Tomskogo politekhnicheskogo instituta, vol. 120, 1961.

2. A. G. Kalinin. Borehole Curvature. Gostoptekhizdat, 1963.

3. A. A. Sorokin. On the curvature of exploratory boreholes. Byulleten tsvetnoi metallurgii, No. 24, 1958.

4. V. S. Fedorov. Scientific Foundations of Drilling Regimes. Gostoptekhizdat, 1951.

5. N. I. Shatsov, R. A. Ioannesyan, et al. Drilling of Oil and Gas Wells. Gostoptekhizdat, 1961.