Downhole vibrations is still a problem to solve because models have failed to be predictive and surface measurements do not reproduce downhole phenomena precisely. The paper describes the instrumentation of PDC bits with accelerometers, strain gauges and contact sensors. From these sensors it has been possible to produce direct measurements in lime domain and frequency domain of: instantaneous RPM, accelerations along the three axis, downhole WOB, downhole torque, power dissipated at the bit, eccentricity of hole, bit center trajectory, also indirect measurements of downhole estimated ROP, wear, specific energy. Military type micro electronics of the latest technology have been used to integrate evervthing in the bit shank. With these measurements dynamics can be analyzed : stick slip and whirl are clearly identified. Lab tests have been performed with promising results. Three campaigns have been done in the field to confirm the lab results with different bit profiles in France and UK. Some of the recorded results compare well to the results of existing BHA models. The paper discusses these results and outlines the future developments. P. 457

A corehead was designed, manufactured and tested to reduce fluid invasion of the core. This is obtained by minimizing the exposure time of the core to the drilling fluid in increasing the rate of penetration (ROP). The design incorporates a medium heavyset polycrystalline diamond compact (PDC) cutting structure developed in accordance with cutting models and balancing methods used for drill bits. The highest ROP is achieved by a particular hydraulic design: flow ports shape and positioning to clean the cutting structure enhance the drilled cuttings removal while preventing drilling fluid in the throat of the corehead. Moreover, an internal lip works with a special inner barrel shoe to effectively seal off mud flow from the throat. All the design features have been subjected to laboratory tests, including measurement of pressure drop across the corehead and flow visualization studies. Flow visualization tests include high-speed filming of the flow and paint tracing to indicate the special flow pattern. In conjunction with lab tests, a numerical simulation was performed using fluid dynamics software to optimize hydraulic parameters. The low invasion core bit has been used in numerous applications. The performance achieved was significantly better than the average achieved over a period of years using various PDC coreheads. The rate of penetration was increased by a factor of 4.8 and bit life by 2.3 (often with reusable condition).

This paper was prepared for presentation at the 1998 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, 27-30 September 1998.

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^

No abstract