The use of oil-based fluids in the western Siberian oil industry for hydraulic proppant-fracturing treatments in water-sensitive formations to prevent clay swelling was widespread. The added advantage of using oil-based fluids for cold weather conditions is obvious. However, with the growing number of treatments and increasing treatment sizes, the use of oil or diesel, from an economical point of view, becomes less attractive. In addition, there is a growing environmental concern with respect to using oil for mixing and pumping fracturing fluids. The use of gelled water is cheaper, and from an environmental perspective, more attractive. The addition of 2% KCl to water-based fracturing fluid for temporarily controlling clay swelling is widely accepted as a standard practice. In situations where water-sensitive sandstone formations have been treated and longer protection has been required, very often additional clay stabilizing agents are added to the water-based fracturing fluids. Research on the technology of matrix acidizing treatments has revealed that the use of 2% KCl transforms into 1.5% saltwater as a result of ion exchange. The 1.5% saltwater solution is too weak to prevent clay swelling. Clay swelling can be prevented using a 1 molar (7%) KCl salt solution. Based on acidizing treatment research, it was decided to use 7% KCl as a temporary clay control additive in water-based fracturing fluids for treatments in western Siberia. One-molar salt solutions have been used for all the treatments performed during the last four years. Model 50 viscometer measurements were made to identify the influence of increasing KCl concentration from 2% to 7% on the viscosity development of borate crosslinked fluid. Water retention problems have not been reported since 7% KCl has been used. From a study of the pre- and post-fracturing production data, it was apparent that in general the percentage of water produced with the oil did not reduce. It is postulated that this is an indication of good temporary clay control. This study excluded treatments that clearly contacted a water-bearing zone. Introduction In 1997, it was estimated1that more than 90% of fracturing fluids used during the 1990s were crosslinked water-based fracturing fluids. The assumption is that this percentage has only increased in recent years. Oil-based fracturing fluids are still in use, mostly in water-sensitive, depleted, and undersaturated2 tight sandstones from which water-based fluids are difficult to recover. Factors to take into account when selecting a frac fluid include availability, safety, environmental impact, economics, viscosity, formation compatibility, load recovery, and surface treatment parameters, to name a few. All these factors are discussed in detail in this paper for the two base fluid options, with emphasis on some specific conditions encountered in various oilfields in western Siberia. Availability Water and oil or diesel are considered to be readily available for use as base fluids for fracturing treatments in western Siberia. This is also true in wintertime conditions. Heated water (30° to 40°C) is used during all operations in the area because steam heating equipment is readily available for heating fluids. Water-based fracturing fluids can be used in temperatures as low as -32°C. This is the lowest temperature limit for all operations, including oil-based treatments, because steel becomes brittle at lower temperatures and can result in high-pressure equipment failure. Safety Pumping oil-based fracturing fluids is known to be dangerous. Consequently, pumping hydrocarbon-based treatments in darkness is not permitted. Water-based fluids may be pumped during the nighttime on the condition that adequate and sufficient lighting is provided on location. Environmental Impact Any land spill of fluids in excess of approximately 100 liters (or close to 25 US gallons) is considered a recordable environmental incident. The strategy with respect to environmental issues is to reduce incidents to avoid remediation costs. The risk of spilling for both water- and oil-based fluids is equal; however, from a remediation standpoint, the environmental impact of an oil-based fracturing fluid spill is considered far more costly.