Shale is a general term used for argillaceous (clay-rich) rocks which are the most abundant sediment on the earth. It is believed that clay rich rocks comprise more than 50-75% of the geologic column. Shale has very varying petrophysical and mechanical properties. Shale is in the most cases acting as a trap or seal for hydrocarbon migration, but has also in more recent years been targeted as a reservoir target in some basins. In some wells it has been observed on cement bond logs that shales in uncemented intervals have moved in and closed the annulus. Pressure communication testing has been performed on these sections and the sections has been qualified as well barrier elements (Williams et al., 2009) for plug and abandonment (P&A) purposes. The main mechanism behind the deformation process is believed to be shale creep. In this paper we will discuss shale creep and other shale deformation mechanisms and how an understanding of these can be used to activate shale that has not contacted the casing yet to form a well barrier. We have developed a numerical model based on first order principles to better understand the mechanical deformation process. We are also supporting the modeling results with laboratory experiments, before we discuss a couple of field cases where shale intervals have been activated and verified to have formed a well barrier as part of the well construction process in new wells.

As part of a major oil company's objective to save rig time as well as reduce open-hole exposure time in the Valhall overburden, steerable ream while drilling technology was utilized in their deviated and horizontal wells to eliminate under-reaming thus reducing drilling costs. The well paths require good directional control with holding capability while drilling long tangent sections. Since a single assembly is designed, the rotary steerable assembly must be capable of programmable dog leg generation when changing inclination and direction or geosteering in the horizontal section. This paper describes the application of steerable ream while drilling indifferent hole sections on several wells. Case studies will be presented from wells drilled from the Valhall platform by a jack up rig cantilevered over the wellhead platform. The authors will outline the reasons for utilizing SRWD's and the operational achievements of the system while drilling well F-8, particularly in the two largest hole sections where the tool saved 10-12 days of rig time. The problems observed while drilling directional and horizontal well paths with steerable motor assemblies will be presented in addition to the modifications implemented as solutions. These improvements should make it possible to further improve ROP and steering abilities on future wells. Additionally, the authors will discuss the improvements achieved by applying new cutter technology and design changes for stabilization of the pilot bit. Introduction The landing permit for the Valhall field, located in the southern end of Norway's North Sea sector, was awarded in 1977. Valhall is located in 70meters (m) of water with platforms for drilling, production and quarters. The first production well was drilled in 1982. Horizontal wells have been drilled from the Valhall platform since 1991. In 1996, a wellhead platform was installed to increase the oil recovery rate. A mobile drilling unit, the jack-up Mærsk Guardian was cantilevered over the wellhead platform. Wells are drilled from both the Valhall platform and the wellhead platform at present.

The overburden at Valhall consists of weak and partly fractured shale contributing to wellbore stability problems while drilling. To reduce the stability problems it is desirable to maintain a constant pressure on the formation, but this is not possible when drilling and tripping in a well. However, it is still important to minimize the pressure variation. An important factor here is to reduce the Equivalent Circulating Density (ECD) fluctuations when circulating the drilling fluid. Rotary steerable systems have been used on Valhall to improve hole cleaning and also to improve wellbore stability by keeping the ECD as low as possible while drilling. This paper forms a case history of several rotary steerable tool runs on the Valhall field, offshore Norway. The paper will, as an introduction, briefly discuss the different steering principles of available rotary steerable systems. In most fields the focus has been on using rotary steerable tools in the reservoir section; however, on Valhall they have been used in the overburden section. The reason for concentrating on the overburden at Valhall is because of the weak and fractured shale with it's wellbore stability problems. The paper will concentrate on the particular rotary steerable system used on Valhall, the Automated Guidance System or AGS Tool, which has been used to drill 12–1/4" × 13–3/4" holes. To be able to drill this hole size below a 13–3/8" casing string, a 12–1/4" bit was used below the Tool and a standard under reamer was initially used above. However in the two most recent wells, a specially designed near bit reamer, was used to drill the oversized hole. The automated guidance system has been used on 5 wells in the Valhall field and has made a total of 8 runs. Although none of these runs achieved the casing point, two of the runs set world records at the time, of more than 2000 m, and were regarded as major successes. Several runs were also planned at the limit of the Tool's specification. These runs were regarded as a combination of commercial and tool development runs, and were as such also considered successful. Comparisons will be made between the results from the different runs and the planned wellpaths. Also the actual automated guidance system wellpaths and similar mud motor runs will be compared in order to demonstrate the enhanced directional performance of the AGS Tool. Introduction The Valhall field is an initially over pressured, under saturated Upper Cretaceous chalk reservoir located in the central graben in the Norwegian sector of the North Sea. The reservoir is at a depth of approximately 2400 m subsea and consists of two oil bearing formations: the Tor and Hod. The former contains roughly two-thirds of the oil and is a soft chalk characterized by high purity (95–98 % calcite), high porosity (up to 50 %) and high oil saturation (90 % and greater). Oil and gas production from the Valhall field began in October 1982. The original 24 slot platform was first expanded to 30 slots in 1990, and then in 1996 a new 19-slot wellhead platform was installed. Plans exist to install a second wellhead platform in order to waterflood the field. The field was originally developed for recovery of 250 MM BO, but has already produced more than 450 MM BO and work is ongoing to prove and recover 1 Billion BO from the Valhall structure.

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.

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