Improving Well Position Accuracy for More Effective Resource Exploitation
Authors: Landry Bart , Poedjono Benny , Akinniranye Goke , Hollis Mike 
Published: October 2008

Abstract
In an effort to maximize resource recovery, especially in previously developed fields, drilling programs in recent years have included more ambitious well-spacing strategies and increased lateral lengths. These trends have created a growing demand for accuracy in well-position data.
New technological capabilities make positional accuracy more easily achieved in new wells being drilled, but the problem of accuracy in historical well data continues to pose a significant challenge. In this paper, the authors will outline a comprehensive approach to assessing and improving positional data in a project designed to double the density of wellbores in an established producing field. High positional accuracy was essential to ensure the effectiveness of the flood sweep strategy designed to improve resource recovery. This paper will explore the author's methodology and the challenges they faced in developing a well-survey program that produces highly accurate results in a cost-effective manner.
Program elements to be discussed include a careful analysis of historical wellbore data, evaluation of the accuracy of predictive models used to estimate wellbore position, and strategies for determining and minimizing the causes of positional inaccuracy. These strategies include the use of statistical gyro surveys both to determine the accuracy of historical data and to provide known reference points for increased positional accuracy in the new wells being drilled. Techniques for minimizing the incidence and severity of magnetically induced measurement while drilling (MWD) errors will also be discussed. By applying the methodology outlined in this paper, the authors were able to reduce anomalous inaccuracies by improving the accuracy and effectiveness of bottom hole assembly (BHA) modeling, and were able to achieve more robust resource recovery by extending the lateral lengths of wellbores by as much as 50%. As a result of this study, these strategies are now being implemented across the entire lease area, significantly improving the economics and the safety of the operator's drilling program.

Introduction
In recent years, the need for more effective hydrocarbon recovery has pushed the limits of well-spacing density and lateral wellbore length. Overcoming these limits without increasing the risk of costly wellbore collisions has required the development of a methodology for more accurately determining and effectively managing positional uncertainty. This paper outlines a study in which the authors used a comprehensive approach to determine the extent of inaccuracies in existing positional data and to mitigate the effect of those errors on well design, cost and completion.
The site for the study was two unitized fields that overlap the border of Montana and North Dakota in the northern United States (Figure 1). The units comprise a total of 114 sections with estimated recoverable reserves of approximately 225 MBO. Both units target a hydrodynamic oil accumulation on the Williston Basin-side flank of the Cedar Creek Anticline/Red River ‘B’ formation.
Figure 1 Study area on basin-side flank of Cedar Creek Anticline (available in full paper)
In 2001, in order to enhance resource recovery, the operator had downspaced from the original 640-acre spacing to 320-acre spacing and added a line-drive water flood with alternating rows of producing and injection wells (Figure 2).