Use of a Rotary Steerable Tool at the Valhall Field, Norway

Abstract 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 underreamer 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.