Rotary Drilling In Horizontal Wells Costs Less Money And Takes Less Risks
Authors: Li Shuiquan , Zhao Zi ren , Shi Taihe 
Published: December 2007

Abstract
Based on the finite element of spatially curved and twisted rod, this paper presents a new model to analyze the BHA in curved borehole and to analyze the effect of borehole curvature on BHA performance. It suggested that the effect of borehole curvature on the BHA depends on not only borehole curvature but also the WOB. For example, when the WOB is lower, the higher the borehole curvature, the weaker the BHA's building ability is, on the other hand, when the WOB is higher, the higher, the higher the borehole curvature, the stronger the BHA's building ability is. According to this theory, the conventional BHA also could have a high building ability in curved borehole even higher than what Dyna tool could. Besides, this paper researches BHA's performance in horizontal section and horizontal extended section. Furthermore, it presents how the bit torque affects the performance of BHA, including the ability of changing inclination, especially the ability of changing azimuth and about this few literature contribute to effects of the bit torque on the performance of BHA in the past. Finally, at least four horizontal wells in west China, the great majority of building section and horizontal section as well as horizontal extended section were completed by conventional table drilling economically and safely with suitable BHA.

Introduction
Recently, more and more horizontal wells have been drilled, most of those have been finished with Dyna tool from kick point to the end of well over the world.
However, it is well known that, to compare with conventional table drilling, to use Dyna tool is very expensive and more dangerous, especially in high inclination section or horizontal section and horizontal extended section. How to complete horizontal wells with regular table drilling successfully, economically and safely is many engineers' thinking. For the sake of cost and safety, one oilfield in west China practiced at least four horizontal wells by regular table drilling with BHA, the program is just after building up an angle with Dyna tool, then to use table drilling with suitable BHA to finish subsequent building section and then horizontal section as well as horizontal extended section. At the request of the experiment, the BHA's behavior in curved borehole and in horizontal section has been researched. This paper is a part of results.
The research is based on the Finite element of spatially curved and twisted rod.

Result And Analyses
The fig. 3 presents two BHA which have been often used in directional well and horizontal wells drilling in west China.
BHA's Performance In Building Up Section. Here BHA's performance is evaluated by bit side force, positive bit inclination side force means BHA has building tendency, negative means dropping tendency; bit direction side force means BHA has directional drift tendency.

A. The Effect Of Borehole Curvature And Wob On The Bha's Performance.
Here the calculation results are based on the BHA1 in fig. 3 and following parameters: inclination angle 60, WOB is 50KN, 150KN, 250KN respectively. The result is showed in fig. 4, from the fig. 4 one could recognize following characteristics:When the lower WOB(50KN), the higher the borehole curvature, the weaker the BHA's building ability is.When middle WOB (130KN), with the borehole curvature increasing, the BHA's building ability still decreases, but it tends to a horizontal line.When higher WOB(250KN), the bigger the borehole curvature, the stronger the BHA's building ability is.
These results are a little different from those in the past, author tries to explains the result as following. P. 403