Case Histories Demonstrate a New Method for Well Avoidance and Relief Well Drilling
Authors: McElhinney Graham , Sognnes Roar , Smith Bob 
Published: December 2007

IADC Members

Abstract
Uncertainties affecting current wellbore surveying techniques require that new wellbores are planned with large geometric safety margins from existing wells to assure safe bypass without introducing a significant risk of hazardous collisions. This constrains development drilling in some reservoirs where new wells or laterals are desired near existing wells. It also necessitates the drilling of more complex well paths to obtain a satisfactory safe distance to existing wells, thereby adding costly and non-productive footage to new wells.
This paper describes the use of a method that utilizes MWD sensors in a new well, together with the inherent magnetic field of a nearby cased well or fish, to determine the accurate relative geometric relationship between the two. The method is shown to quantify the inclination and azimuth of the nearby cased well or magnetic fish. It is shown that the relative position of a nearby well may be monitored continuously while drilling a new borehole, allowing directional changes to be made until the nearby well is safely passed. The method do not require any access to the nearby cased well or fish, and artificial magnetic fields are not needed. The new method uses data from standard MWD directional sensors to detect the innate magnetic field of the target well. It thus has the potential to save considerable expense over previous techniques, without slowing drilling of a new well. The method increases safety while drilling to avoid existing wellbores, and enables the drilling of closely-spaced laterals and infield wells.
The method is ideally suited for relief well drilling since no access to the target problem well is required. It also provides accurate relative position between the wells, rather than use conventional survey information with its accumulated uncertainties from surface. Thus the relief well can be steered more accurately relative to the target well.

Introduction
The mechanical and electronic errors inherent in wellbore surveying devices produce significant uncertainty in the position of both new and existing wellbores. As reservoir exploitation becomes geometrically constrained, and as development wells are drilled in closer proximity, the increase in well density and complexity produce greater risk for new boreholes to collide with existing wells. Risk-based collision assessments are often performed by assuming existing wells are within certain positions.
Another source of significant uncertainty is environmental forces, which affect the various survey devices. This environmental error increases with latitude for both magnetic and gyroscopic devices, due to the alignment of a set of forces as illustrated in Fig. 1. The earth's geomagnetic field lines become near-parallel with gravity as we approach the magnetic poles. The earth's gravity field lines become near-parallel with its spin axis as we approach the geographic poles. These alignments cause greater uncertainties in the measurements of wellbore surveying instruments, resulting in an increase in wellbore positional uncertainty.
Historically, geometric uncertainties at any point along the wellbore have been represented as an ellipse, with uncertainty accumulating as measured depth increases. In fact, this assumption is over-simplified. An example of geometric positional uncertainties is illustrated in Fig. 2, showing a complex shape of uncertainties due to the accumulated errors around a wellbore at different orientations. The consequence of this is that wells which were though possible to drill with minimal risk are no longer possible to drill with acceptable safety margin using standard surveying techniques, as illustrated by comparing Fig. 2 and Fig. 3.

Well Interference Navigator Method
The new method, known as Well Interference Navigator*, or WIN*, as presented in this paper, determines the relative position to a nearby target well. It is not subject to accumulated survey errors. Accuracy actually increases with closer proximity between the new and existing wells.
P. 807^