Directional Survey and Proximity Log Analysis of a Downhole Well Intersection
Authors: Warren Tommy M. 
Published: December 2007

Summary
A relief well was drilled to intersect the casing of a blowout well at 13,500 ft (4115 m) total vertical depth (TVD). During drilling, numerous directional surveys were run on the relief well. These surveys have been analyzed to determine the error associated with each. Both systematic and random errors were found in the surveys. The systematic errors caused much larger position errors than did the random errors. Magnetic and position errors than did the random errors. Magnetic and resistivity proximity logs were run to determine the distance between the blowout well and the relief well. Comparison with the directional survey data indicated that the magnetic proximity log could detect casing at least 42 ft (13 m) away and gave reasonably accurate distance and direction to it. The resistivity logs gave qualitative indications of casing when the wells were 100 ft (30 m) apart. but the quantitative interpretation was not as good as the magnetic log.

Introduction
The R.L. Bergeron 1 was being drilled as a Tuscaloosa development well in the Moore-Sams field near Baton Rouge, LA, when it blew out at a depth of 18,562 ft (5658 m). The Bergeron 2 relief well was designed to intersect the blowout well at 9,500 ft (2896 m) TVD and to parallel it to a depth of 13,500 ft (4115 m) TVD. This parallel it to a depth of 13,500 ft (4115 m) TVD. This was estimated to be the shallowest depth that a successful kill could be made because of the high pressure in Bergeron 1, as indicated by the kick data. Since Bergeron 1 was cased at 13,500 ft (4115 m), it was necessary that the relief well be within a few inches (centimeters) of the original casing for perforation. The relief well was to be back to vertical and parallel to Bergeron 1 at 9,500 ft (2896 m), thus taking advantage of the relatively clean shale through this interval for proximity log analysis. This plan allowed 4,000 ft (1219 m) in which to locate Bergeron 1 and to position Bergeron 2 within perforating distance. The shallow intersection plan also took advantage of the reduced uncertainty of plan also took advantage of the reduced uncertainty of the bottomhole location of Bergeron 1 at the shallower depth. The proposed well plans for Bergeron 2, as well as a wellbore schematic of Bergeron 1, are shown in Fig. 1. The original Bergeron 2 is referred to as Bergeron 2-A, with the first and second sidetracks designated Bergeron 2-B and 2-C, respectively.
Methods of Determining Proximity to Bergeron 2
The plan for drilling Bergeron 2 depended on the ability to detect the casing in Bergeron 1 because directional surveys alone would not be sufficient to get the wells close enough. Two commercially available proximity logging techniques were accessible for homing in on Bergeron 1 after the wells were reasonably close together.
A long-spaced electric log was to be used as the primary means of detecting the presence of casing. The primary means of detecting the presence of casing. The electric log, which gives information about distance but does not indicate direction, previously had been used successfully by Amoco Production Co. in the U.S. gulf coast area. The technique uses known formation resistivity from conventional resistivity logs to make a layered resistivity model of the formation in the vicinity of the intercept point. This layered model can be used to predict the expected response of the long-spaced electric predict the expected response of the long-spaced electric log when no casing effects are present. The observed log resistivity will be lower than the model predictions because of the short-circuiting effect of the casing when the blowout well is near the relief well. The ratio of the observed log resistivity to the predicted resistivity can be used to determine the distance between wellbores. Ref. 1 explains the technique in more detail and gives departure curves for converting the resistivity ratio to distance.
JPT
P. 2351