Summary Drilling and completion fluids based on cesium formate brines were selected by Statoil for use in the development of the high-pressure/high-temperature (HP/HT) Kvitebjørn field. CsFo brine was selected primarily to minimize well-control problems and maximize well productivity. These important benefits had been recognized by Statoil in previous HP/HT drilling and completion operations over the past 5 years. The use of the same fluid system for both drilling and completion gives the additional benefits of simplified operations, reduced waste, and elimination of fluid-incompatibility problems. The challenge on the Kvitebjørn field was to drill long, deviated well paths through significant sequences of shales into reservoirs with pressures as great as 81 MPa (11,700 psi) and temperatures up to 155°C (311°F). So far, the CsFo brine has enabled the successful drilling and completion of seven high-angle, HP/HT production wells on Kvitebjørn, two completed with a cemented/perforated liner and five with sand screens. Additionally, an extended-reach exploration well was drilled from the Kvitebjørn platform to the Valemon structure. The 705-m (2,313-ft) -long reservoir section of this 7380-m (24,213-ft) -long well with an inclination of 69° was drilled successfully with the same CsFo fluid system. In all these wells, the CsFo-brine system once again demonstrated clear performance benefits such as very low equivalent circulating densities (ECDs), moderate-to-high rates of penetration (ROPs), good hole cleaning, and excellent wellbore stability while logging. Quick, trouble-free, safe, and robust completion operations were also accomplished, and the wells that have been put on production show high production rates with low skin. Full openhole formation evaluation of the Kvitebjørn reservoir has been carried out successfully with logging-while-drilling (LWD) tools. The evaluation has been aided by the development of a novel logging-interpretation solution for a LWD density tool, in which the extremely high photoelectric effect of cesium-rich filtrate plays a vital role. CsFo brine has helped Statoil to achieve a record of zero well-control incidents in all 15 HP/HT-drilling operations and 20 HP/HT-completion operations in the Kvitebjørn, Kristin, and Huldra fields over a period of 5 years.
The challenging environment in the Kvitebjørn field offshore Norway comprises high-temperature wells, long drilling hours, low rate of penetration (ROP), managed pressure drilling (MPD), and mud additive requirements, all of which are very detrimental for operations and reliability of the positive displacement motor (PDM) power section. In fact, until now, no one has successfully drilled the 5 ¾-in. section in a single run due primarily to motor failures such as elastomer chunking and debonding. This paper presents the steps used for optimizing the selection of a PDM section to achieve a single-run drilling operation with improved ROP. The method includes understanding the drilling environment, type of wells, rig capabilities, formations, temperatures, MPD, and drilling fluid requirements. Furthermore, the usual motor and bottomhole assembly requirements must be evaluated and the mud compatibility with the elastomer must be scrutinized. All of these variables were then input into a modeling engineering workflow to simulate and analyze the power output, the elastomer fatigue life, the hysteresis heating, and the debonding stress to select the best possible PDM candidate for the drilling job. A new long-life elastomer and the drilling parameters recommended by the mud motor modeling resulted in drilling this section in a single run for the first time in the field. Simultaneously, it was possible to drill to the deepest total depth without any need to set the section total depth shallower, as occurred in previous wells due to motor failures. The motor drilled through a very thick cemented sandstone stringer with no stall incidents. This motor set new records for drilling the 5 ¾-in. section with a total run length 60% longer than the previous longest run and a total pumping time 67% greater than the previous record. The combined new technologies of the modeling and the new long-life elastomer were applied for the first time in the anticipated challenging drilling conditions. The successful results demonstrated that with thorough analysis and proper planning, one can achieve a step change in performance and reliability without additional costs. The scope of the operation is even broader than the mud motor application.
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