Application of Tandem Rotary Steerable-Positive Displacement Motor Bottom Hole Assembly in Drilling Horizontal Wells: Case Study of Three Eastern Siberia Wells
Authors: Okafor Zimuzor , Buchan Andrew , Diyanov Dmitry , Rawlins Sheldon , Zhadan Grigoriy , Nikitenko Yuriy 
Published: March 2011

Abstract
The Vankor oil and gas field in Eastern Siberia is predominantly developed using horizontal wells drilled into the upper Yakovlev and lower Nizhnekhet formations. The field development scheme consists of pad clusters resulting in complex well trajectories. Due to the remoteness of the field and resulting complex logistics, any directional drilling technology deployed to deliver the complex trajectories must be robust and reliable. Traditionally, the wells have been drilled with steerable positive displacement motors; in 2008, rotary steerable tools where introduced along with logging-while-drilling (LWD) technology and this provided significant performance improvements in the 8.625 in. and 6 in. sections. To further improve on drilling performance, an engineering study was conducted in 2009 to assess other opportunities for increasing the annual cumulative meters drilled on the project. Several areas of opportunity were identified and prioritized in terms of ease of implementation and expected impact on drilling performance; a key recommendation of the study is the introduction of powered rotary steerable system (PRSS) bottomhole assembly (BHA) in the 8.625 in. section. The PRSS BHA recommended for this application is a tandem rotary steerable-positive displacement motor BHA where a straight fixed housing positive displacement motor is placed directly behind a push-the-bit rotary steerable tool.
Various configurations of the PRSS BHA design have been used in other applications including vertical control1, performance drilling1,2, coiled tubing drilling3 and directional casing while drilling4. In most cases it demonstrated the capacity to deliver better drilling performance in terms of penetration rates and directional control that outperforms conventional directional drilling systems. In the case study wells presented in this paper, about 200% improvement in rate of penetration (ROP) was achieved with an average of eleven operating rig days saved per 8.625 in. well section. The total drilling cost savings realized is estimated to be about USD 1,300,000.00; this is based on an average rig spread rate of USD 40,000.00 per day. These are good results and represent a step change in drilling performance in the field. Some limitations of the new system were also observed in the case study wells, they include inability to deliver consistent dogleg severity (DLS) in excess of 3.5°/30 m, rig hydraulics capacity constraints that limit its broad application, restrictions on bit selection and constraints on the available drillstring rotary speed caused by the unique BHA configuration that in some cases complicates hole cleaning planning.
This paper discusses these results and key performance indicators, including aspects of the BHA design, bit selection, hole cleaning management, wellbore stability and drilling practices. A new technology solution to the dogleg severity limitations is proposed: a new generation hybrid rotary steerable tool that can deliver DLS up to 15°/30 m. It can be incorporated in new tandem rotary steerable-positive displacement motor BHA designs and these new systems will have broader application and promise another step change in horizontal drilling performance.