Factors Controlling Hole Angle and Direction
Authors: Walker Bruce H. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

Summary
An understanding of the factors controlling hole angle anddirection will contribute to hitting the drilling target more frequently. Asthe bottomhole assembly (BHA) behavior and the bit/formation interactionsbecome better understood, less expensive and safer holes can be drilled.

Introduction
Many factors affect the hole angle (vertical deviation) and direction(azimuth or horizontal deviation) of a wellbore. The effects of some of thesefactors are easily quantified, while others are not. Currently, ourunderstanding is that the factors influencing the path of the wellbore includeborehole geometry, formation conditions, operating conditions, and a number ofequipment variables. For example, is the current shape of the boreholestraight, curved, or spiral? Is the hole in gauge and round, oversized, orelliptical? What are the BHA components, sizes, and elastic properties? Is theformation massive and isotropic, or properties? Is the formation massive andisotropic, or does it have well-defined dipping bedding planes, or is itlaminated with alternating hard and soft layers? How does the particular bitinteract directionally with the formation? What are the operating parameterssuch as bit weight, rotary speed, torque, and penetration rate? Do thestabilizers cut into the hole penetration rate? Do the stabilizers cut into thehole wall and change the hole-wall geometry? What other effects in addition tobuoyancy does the drilling fluid have on the BHA? How do the dynamics of theBHA affect hole angle and direction?
Considerable effort has been made to incorporate these factors intomathematical models to understand their effect on hole angle and direction. Themodels originally were quite simple but recently have become very sophisticatedas more factors have been included. Good general bibliographies of thesefactors and the models are given in Refs. 1 through 3.
Control of hole angle and direction is applicable to both directionaldrilling and conventional rotary drilling. For both, deviation- anddirectional-control drilling techniques can be applied to drill the boreholealong a prescribed trajectory in rock space. This is achieved by influencingthe bit to build, to drop, or to hold the hole angle. Mechanical methodsinclude, for example, changing the number and location of stabilizers in theBHA, changing the stiffness of the drill-collar string elements, using a bentsub or bent housing with a downhole motor or turbine, and adding or subtractingbit weight. The hole angle or direction can be changed hydraulically with ajetting bit or naturally by new formation forces acting on the bit. Anunderstanding of how the BHA interacts with all the geological and mechanicalforces is the basis for understanding the factors controlling hole angle anddirection.

Factors Affecting Hole Angle and Direction
Formation Strength and Anisotropy. The formation is a primary factoraffecting hole deviation. Alternating formation hardnesses are a suspectedimportant cause of bit deviation. Formation hardness also affects thepenetration rate, and this determines the amount of time the bit or thestabilizer will be abrading the hole wall at any one spot. This may wear outthe stabilizer or bit, or it may wear or cut the rock and enlarge the borehole. Formations with dipping bedding planes affect the borehole path. Experience hasshown that bits generally tend to drill updip when the bedding planes have dipsof less than 45 degrees [0.79 rad] and to drill downdip when bedding dips aregreater than 60 degrees [1.05 rad]. More work is needed to generalize thesetendencies for various bit types and stabilizers in a variety of formationsbefore they can be routinely applied.
Drill Bits. The many different bit types all have individual characteristicsthat affect deviation. Primarily, these are relative to penetration rate and toPrimarily, these are relative to penetration rate and to directional drillingcapabilities. Penetration rate effects were already described. Directionally, roller bits tend to walk to the right, whereas diamond and diamond compact bitsdo not. Special bits have also been built to control the direction of bit walk. A complete quantification of the behavior of drill bits in directional drillingrequires an improved understanding of the bit/formation interaction.
BHA. Each component of the drillstring has a unique stiffness thatcontributes to the overall performance of the BHA. The stiffness or rigidity ofany component is a function of its modulus of elasticity and its moment ofinertia. The weight of each component of the BHA affects the assembly'sbehavior. This weight is a function of the size and specific weight of thecomponent and must include the buoyancy effect of the drilling fluid. Ingeneral, stabilizer placement and size are major factors in deviation control. The effects of location, size, shape, and properties of the BHA components onhole angle and direction, can be analyzed by available BHA models.
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