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.
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 intervals
Hole stability in reactive or dispersive claystone sections
Inhibition and control of low-gravity solids (LGS) when drilling long deviated sections of smectite clays in Tertiary and Cretaceous formations
Improved hole cleaning by the use of shear-thinning and thixotropic biopolymers
Reduced equivalent circulating density (ECD) and mitigation of fracturing in depleted sandstone sections
Use of the same mud system for most of the well
Reduced 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.