Kristin is a HPHT gas/condensate field in the Norwegian Sea. Deep, hot wells have contributed to depth differences between wireline and LWD depths of up to 20m, as well as significant variations between different bit runs. Such differences introduce unacceptable depth uncertainty for both reservoir modeling and well operations. Special procedures were implemented to help understand observed depth differences. Radioactive markers were installed in casing strings and logged routinely with both LWD and wireline. Intervals were relogged on subsequent LWD runs to allow comparison, including logging in upwards direction. Primary depth control procedures were adopted for all WL descents. GR correlation logs were recorded during all relevant runs/passes to allow depth comparisons. The first two wells had fairly low inclination and wireline depths between runs were consistent, with simple block shifts resulting in good match between up- and downlogs. Meanwhile, LWD depths had large local variations and up to 5m difference between runs. Wireline downlogs were selected as depth reference and all other WL/LWD runs were shifted to match. Resulting depth shifts from LWD to WL showed systematic trends increasing with depth and with local features apparently coinciding with well trajectories. LWD depths were consistently shallow compared to wireline and also somewhat shallow compared with casing tallies. Our initial hypothesis was that pipe tally depths were yielding too shallow depths because pipe stretch was not accounted for. Local variations in LWD depths were believed to result from variations in drilling parameters and logging direction. With the above methodology shifts up to 20m were obtained on the third well. Because of the high well angle we questioned the validity of the downlog depths and thus the validity of our procedures. Formation tops based on wireline depths resulted in questionable local corrections to seismic time-depth maps, with significant implications for mapped reservoir volumes. Subsequent high inclination, LWD-only wells left us without a similar wireline depth reference. Given the large uncertainty involved in correcting LWD depths based on trends from earlier wells only, an alternative approach was sought. Simplified models available in literature were adopted to estimate pipe stretch/compression due to temperature, string weight and drilling parameters. Results obtained could explain only about 1/3 of earlier estimated pipe stretch. A similar methodology was applied to correct wireline depths for stretch, with estimated corrections far exceeding standard assumptions. Following run/pass-specific correction of both LWD and WL logs, depth uncertainty has been significantly reduced.
Bengt recently assumed a position as Petrophysical Discipline Advisor in Statoil. The last 3 years he has worked as a petrophysicist for the Kristin field. Prior to joining Statoil, Bengt enjoyed some quality time as a researcher/PhD student at NTNU, Trondheim, studying fluid effects on NMR measurements. Well, it was worth a try. Before that Bengt worked as a petrophysicist for Saga Petroleum in Sandvika. His initial assignment (yes, in petrophysics) was with Shell Offshore, Inc. in New Orleans, Louisiana. Bengt's destiny as a petrophysicst was obvious after having mentioned the word "gamma ray" during an interview with Shell back in 1989. Educational background includes BS and MS degrees in Petroleum Engineering from University of Tulsa, Oklahoma, U.S.A. He is a member of SPWLA, NFES, SCA, SPE, SEG.