In the harsh drilling environments of Siberia where logistics are difficult and expensive, a process of design optimization has led to successful application of PDC bits in the intermediate hole section of development wells, reducing costs and resulting in a record footage performance of 13944 meters - and counting - by a single 11–5/8″ PDC bit. To control costs in the remote Ust-Vakh area of the Samotlor Field of Siberia, bits and equipment must perform at an optimum level. Typically drilled using roller cone bits and turbine motors, the application requires sufficiently rugged equipment to efficiently drill sand/shale formations without costly and time-consuming trips for repair or replacement. This paper describes the practical evolution from use of roller cone bits and turbine motors in the intermediate hole section of these wells, to use of fixed cutter PDC bits and positive displacement motors (PDMs). The resulting performance improvements are documented, including a world footage record of 13944 meters. To facilitate rapid deployment of application-specific bit technology despite the remote location, an in-house bit specialist was located at the field operations base, collaborating with the drilling department in implementing the optimization process. Focusing on the intermediate hole section from approximately 430m to 1100m depth, areas of potential improvement were identified, and solutions proposed. Based on success in similar applications, fixed cutter bits were recommended that incorporated steel body designs with advanced hydraulics, while analysis of various bit/BHA combinations identified the most efficient configurations for this application. Ultimately, large diameter roller cone bits with an expected life of one or two runs were replaced by steel body PDC bits that have been re-run as many as a 20 times or more, with average days per drilling in intermediate section reduced from five to just four days. The paper summarizes savings realized, and describes additional design changes now under investigation. Introduction Western Siberia is the dominant petroleum basin in Russia, producing 75% of all Russian oil and gas. It's the second richest basin in the world, second only to the Middle East, with huge reserves and substantial undiscovered resources. Samotlor Field, subject of this paper, is located at Lake Samotlor in the Urals Federal District. One of seven federal districts, it is the westernmost of the three Asian districts. (Figure 1) Discovered in 1965, the Samotlor Field is one of the largest in the world. Initially more than 16,700 wells were drilled, from which more than 2.3 billion tons of oil was produced, with production peaking in the 1980s. By 1988, intensive development had depleted reserves, a decline exacerbated by extensive water injection resulting in water mixing with the oil. Only 16.74 million tons of oil was produced in 1996. With introduction of new technology at the turn of the century however, the field is being revived. The Samotlor Field extension at Ust-Vakh to the south is a good example of an existing mature field in Russia where the combination of Russian expertise and advanced Western technologies has resulted in improved efficiencies. Production rates there have increased by more than 40 % since 2002. Originally revived through directional drilling, the current phase of development incorporates use of drilling techniques requiring advanced bit technology capable of achieving well objectives. In the on-going effort to improve drilling efficiencies and reduce costs, an optimization process was utilized successfully to introduce and refine application of fixed cutter PDC bit designs in the Ust-Vakh wells.
The traditional drilling fluid of choice in West Siberia is a potassium chloride (KCl) low-solids nondispersed polymer system. The use of KCl has been justified by the need to inhibit smectite-rich clays in the Tertiary and Cretaceous formations. KCl inhibits clay swelling and coagulates dispersive clays and does not impair the rheological and filtration characteristics of a polymer water-based drilling fluid. However, there are a number of concerns about the use of KCl in drilling operations. KCl is used in relatively high concentrations (up to 210 kg/m3), requiring significant logistical resources for delivery and handling of the material, especially on remote ice-road locations. High potassium and chloride ion concentrations can also be considered to be hazardous to the environment. In recent years, regulators and operators have searched for a more environmentally, operationally, and logistically acceptable water-based mud (WBM). This has resulted in the introduction of a new generation of freshwater-based high-performance drilling fluids (Stawaisz et al. 2002). These fluids rely on polymers for clay flocculation instead of KCl. The use of a flocculating high-performance water-based fluid (HPWBF) in the Uvat Field has allowed significant reduction in fluid volumes, fluid cost, and drilling time, as well as helped to reduce logistical costs and control problems such as downhole losses, hole instability, and drilled solids contamination. Introduction In West Siberia, the choice of water-based fluid (WBF) is determined by the requirements of tight environmental and waste disposal controls, as well as by the need for cost-efficient drilling of extended-reach wells. At Uvat, drilling is performed in 311.1-mm (12¼-in.) and 215.9-mm (8½-in.) hole sizes with 127-mm (5-in.) OD drillpipe. Typical wells drilled from each pad have an S-shape profile and three casing string design, with an average depth of ~ 2400 mTVD and a maximum horizontal displacement in excess of 3000 m (Fig. 1). Initial wells in the field were drilled using KCl-polymer WBF. The traditional mud system used during the past decade has been KCl- polymer WBF, which offers several benefits including:Easy conversion of the mud system when drilling different intervalsHole stability in reactive or dispersive claystone sectionsInhibition and control of low-gravity solids (LGS) when drilling long deviated sections of smectite clays in Tertiary and Cretaceous formationsImproved hole cleaning by the use of shear-thinning and thixotropic biopolymersReduced equivalent circulating density (ECD) and mitigation of fracturing in depleted sandstone sectionsUse of the same mud system for most of the wellReduced formation damage by adding specifically sized acid-soluble bridging agents and biodegradable and acid-soluble polymers and controlling drilled solids in the drilling fluid However, the use of KCl is no longer considered to be environmentally acceptable by some operators in terms of waste management and dilution practices. This particular operation is located in the environmentally sensitive and remote Uvat Field, Tyumen Province, West Siberia (Fig. 2). Environmental issues include: water protection zones, protection of aquifers and the permafrost zone, limited waste storage facilities on the drilling pad, and annual re-supply of chemical to the rig site by winter ice road.
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