Factors Affecting the Control of Borehole Angle In Straight and Directional Wells
Authors: Bradley W.B. 
Published: December 2007

Factors contributing to borehole deviation are studied by separating the problem into two parts: drillstring mechanics and bit-rock interaction. problem into two parts: drillstring mechanics and bit-rock interaction. Methods are discussed for designing drillstring hookups to maximize penetration rates while minimizing the necessity of experimenting with penetration rates while minimizing the necessity of experimenting with drillstring hookups in the hole.

Introduction
Deviation has always been a problem. Now, with the increase in deep wells and offshore drilling, deviation assumes an even greater importance. Deviation causes crooked holes, casing wear, key seating, stuck pipe, and expensive fishing jobs. In addition, crooked holes contribute to excessive tool-joint and drillpipe body wear, resulting in premature drillstring replacement. Current methods for controlling deviation involve experimenting with various drillstring hookups in the hole to find the best combination. In addition, low weights on bit are often used. Experimenting with drillstring hookups is an expensive process, and low weights on bit result in slow drilling. This paper reviews the deviation problem and discusses methods for designing drillstring hookups to maximize penetration rates while minimizing the necessity of experimenting with drillstring hookups in the hole.

Causes of Deviation
Deviation can be defined as the departure of the path of the bit from drilling a straight hole. The hole may be either vertical or inclined, and the departure may be in any direction. Deviation may be desirable (directional wells) or undesirable (vertical wells, where dog-legs or large angles defeat objectives). It is important to recognize that the principles underlying deviation are the same in either case.
Deviation result from the flexibility of the drillstring (drill collars) and the forces acting on the string that cause it to bend. Nominally, drill collars are not considered to be very flexible; however, drill-collar flexibility must be measured against dog-leg severity limitations to get a true perspective. For example, consider a 90-ft string of perspective. For example, consider a 90-ft string of 10-in.-diameter collars lying horizontally and supported at either end. The change in slope of the collars from one end to the other amounts to a dog-leg severity of 7 degrees/100 ft. So, with respect to dog-leg practices, collars must be considered flexible.
In analyzing deviation problems, it is convenient to separate the problem into two parts.Drillstring Mechanics The analysis of drillstring behavior under the action of imposed forces.Bit-Rock Interaction The drilling behavior of bits in various rock formations under the action of applied bit loads.

Drillstring Mechanics Active Drillstring Length
The length of the drillstring involved in the deviation process depends on a number of factors; that is, it process depends on a number of factors; that is, it depends on the physical makeup of the drillstring, the geometry of the hole, and the weight on bit. Generally, not more than the bottom 150 ft of the drillstring are usually involved. At higher hole angles and weights on bit, this is often reduced to a range of 60 ft or less.
Consider, for example, the simplest case of a uniform unstabilized drillstring lying on the low side of an inclined straight hole (Fig. 1). The active length of the drillstring is the portion of the string, L, below the point of tangency. Above the tangency point, the drillstring is supported by the borehole wall, resulting in zero internal shear and bending moment.
JPT
P. 679