New Drilling System Cuts Drilling Days In Half

Four wells have been drilled in the Obiafu field, Nigeria, since late 2000. Obiafu 29 was drilled in 2000, and Obiafu 30 to 32 were drilled during 2001. Three of the four wells were build-and-tangent sections, with a build to 45° and hold. Obiafu 29 had a build, hold, and drop profile (S-shape) with a maximum hole angle of 35° and a drop off to 12°. Operational and wellbore condition problems were present while these Obiafu field wells were drilled. The operational problems included poor bottomhole assembly (BHA) efficiency; i.e., the BHA needed to be changed out too often; high consumption of drill bits, poor sliding performance, and, in some cases, excessive sliding required to hold a well trajectory. Difficulties with running wireline logs to total depth (TD) were also experienced. The wellbore condition problems included well deterioration. On Obiafu 30, the 9 5/8-in. casing had to be run prematurely, and the well was drilled to TD in 8 ½-in. hole. On Obiafu 31, the BHA became stuck and the well had to be sidetracked. On Obiafu 32, 9 5/8-in. casing was stuck 1,500 meters from well TD and a 7-in. liner had to be run and cemented in 12¼-in. open hole. In view of these difficulties, the operator selected Obiafu 33 as a candidate for a new combination of matched steerable positive-displacement motor (PDM) and extended-gauge bit. Obiafu 33 was to be similar in design to Obiafu 29, with a build, hold, and drop profile (S shape), a maximum hole angle of 35°, and a drop off to 12°. The operator felt that this new steerable motor/long-gauge bit technology could alleviate many of the operational problems. The offset data also suggested that this technology in Obiafu 33 could also improve wellbore conditions and help avoid problems such as stuck pipe and the need to switch prematurely to smaller hole size. The trial with the new drilling system on Obiafu 33 far exceeded expectations. The well established a field record performance. The total time to drill ‘from shoe to shoe’ (the time to drill from 13 3/8-in. casing to cement the 9 5/8-in. casing) was reduced from an average of 31 days for the four previous wells to just under 18 days. BHA efficiency, wellbore stability, and wellbore conditioning for casing runs and logging were all significantly improved. This paper details the successes experienced using the new drilling system.