Directional drilling is a critical necessity in many of today's wells. Accurate prediction and awareness of achievable build rates is vital in lowering drilling cost. The science behind controlled wellbore deviation has advanced beyond prediction based solely on BHA geometry. Although BHA configuration is an important factor, interaction between each of the four primary components (bit, BHA, operating parameters and formation) must be evaluated thoroughly. Previous papers have described how changes in bit characteristics, BHA configuration and various formations influence build rates. With regards to operating parameters, weight-on-bit (WOB) is well known to be beneficial in increasing the desired build rate under certain drilling conditions and yet does not help in other situations. We propose that most of the weight-on-bit effects are actually due to its influence on rate of penetration and bit tilt. However, the influence of operating parameters has not been fully investigated. RPM has been neglected as a significant influence on steerability. In theoretical studies, these properties have been measured and quantified with the use of a full scale drilling laboratory and commercially available PDC bits. This paper investigates the effects of RPM and ROP on build rate and illustrates the importance of these parameters. Practical guidelines for understanding the effect of operating parameters on steerable systems are also provided. Background There is general consensus in the industry that increasing the RPM of the bit provides more opportunities to cut the formation in a given amount of time. Also by slowing the forward ROP of the bit, the time allowed to side cut would be increased resulting in higher steerability. The results presented in this paper will validate and quantify these theories. Drillers have long known that controlled drilling parameters (weight-on-bit and RPM) could be used in order to effect build, walk and drop rates of a bit/BHA system. The method of limiting WOB and increasing RPM for a pendulum assembly, also known as fanning, has been recognized to promote the assembly's drop tendency. Controlling the penetration rate with a benthousing motor has been a well documented procedure for increasing build rates. However, little more than general statements and rules of thumb exist. The science behind build rate prediction has primarily been focused on computer modeling of the BHA. Most of today's software does not take into account the bit's interaction with the formation in the forward direction. Unsurprisingly, the interaction between the bit and borehole in the lateral direction has also been overlooked due to its complexity. This computer modeling encourages limitations not found in drilling lab testing or the field (i.e. limitations of the software, quality of the input data, translation/interpretation issues, unknown factors or ones that simply cannot be modeled, etc). With the use of full scale drilling laboratory, the effects of drilling parameters have been investigated. The test apparatus simulates the bit tilt and side loading normally induced by a BHA inside the wellbore. Precise lateral displacements are recorded within thousandths of an inch without the external inconsistencies usually seen in the field including BHA induced vibrations, unreliable parameter control and deviation measurement, energy deprived from the bit by the BHA, formation heterogeneity, borehole quality issues, etc. The rocks used in the drilling lab simulator are homogeneous blocks of medium and hard limestone. This allows a commercially available bit to drill several feet of formation in a "near field" environment at full scale sizes. This testing method produces well defined results that are consistent with field results.

There are four key technical issues to be addressed when drilling directional wells with steerable bits and bottom hole assemblies: maximizing hole quality, addressing tool face control, eliminating bit and assembly vibration, and predicting hole curvature or build rates. Hole quality 1 and tool face control 2-7 have been covered in several previous SPE papers, as has bit and assembly vibration elimination 8-9 . This paper covers the recent development of a fundamental model for the prediction of hole curvature based on measured bit response and modeled bottom hole assembly behavior.

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