No abstract

The 6" PDC bits application of development wells in the Paris Basin (France) has been carried out for a long time. A long directional 6" section of about 1600 meters composed of chalk, sand, shales, hard limestones, with dolomite and anhydrite stringers is currently drilled. In spite of the good results already obtained, several problems were still remaining, the major one was a hard stringer that could not be drilled by PDC bits and was forcing the operator to pull out or to destroy the PDC bit, beside that, nozzle plugging was very frequent. An engineering study was performed and the analysis of the problems encountered led to the choice of new PDC bit body profile with new cutting structure and nozzles design. These choices were improved by means of hydraulic tests run in laboratory and specific methods of cutters positioning. Introduction The typical sedimentary deposits in the Paris Basin can be seen in fig. 1. The 6 inches phase is made up of three main portions. The upper portion, mainly composed of chalk, limestone, shale and sand, is very soft, down to around 1.050 meters. At this depth the medium portion (Purbeckian) of around 40 meters thickness is a very heterogeneous formation of anhydrite, dolomite, interbedded with claystone where drillability with PDC bits can fluctuate from 60 m/hr to 1 m/hr within less than 1 meter. In the past PDC bits were generally destroyed in this formation. The bottom portion below the Purbeckian composed of marl and limestone is a medium to soft formation. About 150 wells have been drilled in that area, very soon it was found that the use of PDC bits were the best option. Over 10 years of development, three phases of bits optimisation were performed. Originally 3 to 4 bits were used to drill the full section. Heavy set PDC bits were run in this first phase of optimization ; balling up or cutter breakage problems were frequently observed. The second phase was focused on improving the hydraulic behaviour of matrix PDC bits (Ref.1). After three years of optimisation, ROP was improved by 25% and footage over 100%. At this point three problems were still remaining : partial bit balling ; nozzle plugging ; PDC breakage in the Purbeckian. Bit performances, dull grading analysis joint with lithology investigation showing that pyrite was only existing as traces, have dictated that PDC bit improvement was still needed. It is why a third phase of optimization starts with these objectives. P. 65^