Improving Wellbore Position Accuracy of Horizontal Wells by Using A Continuous Inclination Measurement From a Near Bit Inclination MWD Sensor
Authors: Berger P.E. , Sele R. 
Published: March 2010

Abstract
Wellbore position calculations are typically performed by measuring Azimuth and Inclination at 10 - 30 m intervals and using interpolation techniques to determine the borehole position between the survey stations. Input parameters are, Measured Depth (MD), Azimuth and Inclination, where the latter two parameters are measured with the MWD tool in a stationary mode (non-rotating). Output parameters are the geometric coordinates; True Vertical Depth (TVD), North and East. To maximize the exposure of a production well to the reservoir, horizontal wellbores are frequently being drilled. Furthermore, to maximize the production of hydrocarbons from these wells, their relative position within the reservoir is critical. Improving the accuracy of the Inclination measurement and thus reducing the uncertainty of the calculated TVD value will increase the confidence in wellbore position.
The NaviGator ™ ∗ geosteering tool or the AutoTrak ™ ∗ rotary steerable system are frequently used to optimize the position of horizontal wellbores within the reservoir. Both these geosteering tools use a Near Bit Inclination sensor (NBI) to help the directional driller perform directional changes smoothly and accurately. Unlike traditional directional sensors the NBI sensor is capable of accurately measuring inclination while being rotated. Consequently NBI measurements can be performed continuously during drilling. The measurements can be used to more accurately calculate the wellbore trajectory.
Results in the paper demonstrate that the NBI sensor is more accurately measuring inclination than other directional sensors in a horizontal well. Also, by continuously measuring the inclination during rotation, some error sources are reduced, resulting in improved TVD accuracy.

Introduction
The most common directional module of a Measurement While Drilling (MWD) tool (Figure 1) consists of three uni-axial accelerometers and a tri-axial magnetometers. The axes are orthogonal with respect to one another and are capable of resolving the measured local fields into three dimensional components.
The directional module of an MWD tool is located within the MWD collar itself and is typically found between 15 – 20 m behind the drill-bit, depending on the design of the Bottom Hole Assembly (BHA). Due to the geometry of the BHA and the drilling parameters, the MWD collar can be exposed to a bending effect causing the directional sensor package not to be parallel to the axis of the borehole. The effect of the BHA bend on the accelerometers/inclination measurement can be predicted and more or less compensated for. The unfavorable distance between the bit and the directional sensor has led to the development of near-bit survey sensors to enable rapid responses to inclination changes while drilling.
FIGURE 1: Directional MWD. (Available in full paper)
FIGURE 2: NaviGator. (Available in full paper)
FIGURE 3: AutoTrak. (Available in full paper)
Geosteering tools such as Baker Hughes INTEQ's Reservoir Navigation Tool (RNT) NaviGator and Rotary Closed Loop System (RCLS) AutoTrak employ near-bit inclination sensors. These sensors are typically located 1 – 4 m behind the bit and provide accurate near-bit inclination measurements in addition to the data from the directional module located further back in the BHA.
The NaviGator (Figure 2) is a combination of a steerable mud motor with the following MWD sensors; azimuthal gamma ray, multiple propagation resistivity and inclination.