The Role of Rock Strength Anisotropy in Natural Hole Deviation
Authors: McLamore R.T. 
Published: December 2007

The influence exerted by the interaction of rock and bit is a major factor preventing a comprehensive solution to the problem of natural hole preventing a comprehensive solution to the problem of natural hole deviation. Described here is an analysis of the bit-tooth chipping mechanism in dipping laminated anisotropic rock. There are encouraging indications that the tendency of a bit to deviate in such rock can be diminished by proper bit-tooth design.

Introduction
The inability to drill a straight vertical hole economically in dipping bedded rock constitutes a major problem in the drilling industry today. The crooked problem in the drilling industry today. The crooked hole frequently impedes drilling progress and raises drilling costs when light bit weights are needed to control hole angle. Often remedial action must be taken, such as runs with the Dynadrill, the Bit Boss, or a whipstock to overcome natural hole deviation tendencies, so that the hole will penetrate the target area.
Since the early 1950's many theories have been advanced to explain hole deviation. The significant work to date has been in drillstring mechanics and has resulted in the design of drillstrings to control the rate of change of hole angle. The major factor yet to be fully understood with regard to natural hole deviation is the interaction of the bit and the rock. Once this factor can be put in analytical form, it should be possible to design bits and drillstring hookups for straight hole drilling.
It has been shown that one of the causes of hole deviation is the nature of the rock failure under the bit. When a conventionally shaped wedge penetrates a dipping laminated anisotropic rock the chips formed on each side of the wedge are not equal in size. This is in contrast with the behavior of the homogeneous and isotropic rocks as revealed in studies by various authors. This phenomenon, referred to here as "preferred chip formation," has been correlated with natural hole deviation tendencies as observed in the Canadian foothills. In this paper, a theory is presented that relates this phenomenon to hole deviation. presented that relates this phenomenon to hole deviation. Theoretical expressions have been derived that predict the occurrence of preferred chip formation in terms of
the anisotropic strength characteristic of the rock, the dip of the beds, and the included angle of the bit tooth.
The theory has been used to design a wedge geometry for preventing preferred chip formation in dipping laminated rock. The tooth design has been incorporated into a bit, and to date, 27 field tests have been run. The results of the tests are encouraging.

Discussion
To illustrate in a meaningful manner the role of rock strength anisotropy in natural hole deviation, a number of individual concepts must be discussed. In the most logical sequence, these concepts are:
the nature of preferred chip formation, the role of preferred chip formation in natural hole deviation, preferred chip formation in natural hole deviation, the strength characteristics of anisotropic rock, the proposed wedge penetration model for anisotropic rock, and the effect of wedge geometry on preferred chip formation.
Then the results of tile preferred chip formation. Then the results of tile initial field test of the experimental deviation-control rock bit will be discussed.
JPT
P. 1313