Improving Drilling Performance in the Chalk Reservoir of the Valhall Field

Abstract

Historically, drilling long horizontal wells in the Valhall field offshore Norway has been challenging. Due to high drag levels, the Chalk formation is best drilled with rotary steerable systems (RSS). However due to severe stick-slip vibrations, a high number of LWD and RSS failures have occurred. The necessity to "geosteer" in the reservoir has also contributed to tortuous well paths that have further increased drilling and liner-running difficulties. In order to achieve 2,000m long reservoir sections, BP used a multi-disciplined team to leverage improvements in both drilling and linerrunning performance. The initial focus was on reducing stick-slip vibration and frictional drag. Drill string, BHA and bit design, LWD and logging tools, well profile design and mud design were all considered in the analysis. Significant improvements in both drilling and liner-running performance have been achieved. Introduction The Valhall field is located in the North Sea approximately 290km offshore southern Norway in 69m of water. The known challenges of drilling the Valhall reservoir are:Difficulty building inclination in the soft to very soft chalk.Localised sub-seismic faulting.Higher pressured, unstable Lista shale above the reservoir. If entered, a sidetrack would be required.Higher pressured Hod formation below the reservoir. If entered the mud weight may need to be raised to keep the wellbore stable.Geo-steering through the variable thickness in the reservoir.Anti-collision issues.Depleted zones around older wells.Hole cleaning as the chalk tends to go into solution rather than remain as discrete cuttings.Torque and drag problems while drilling.Problems while running and setting the liner. Increasing difficulties encountered while drilling the overburden from a crestal location have resulted in the failure to set the 9–5/8-in. casing of several ERD well in the correct location. Failure to do so was countered by the implementation of new well design [Ref. 1]. A wellbore stability study concluded that, for the required ERD infill wells, there would be less risk if the wells were drilled at a lower angle from a flank location [Ref. 2]. To facilitate drilling at a lower angle, an unmanned wellhead platform was installed on the South Flank in 2002 and a similar wellhead platform installed on the North Flank in 2003. This approach, to a large extent, mitigated the risks involved with drilling through the overburden. However, drilling up dip towards the crest introduced several new challenges for the drilling of the Tor reservoir formation. Drilling First Well on the South Flank Before Halliburton Sperry Drilling Services was called for the job, the first well was drilled using a competitor's BHA's. A steerable motor assembly with a mill tooth bit was selected to drill out the casing shoe at 2861m and complete the build up. The motor assembly gave an average dogleg of 7.4°/30m; but the well path dipped below the base Tor, and 76m of Hod was drilled. The well was geo-steered as required to 3727m where it was necessary to circulate bottoms up to reduce the drag. From 3778m, it was not possible to set the toolface; and the run was ended.