In selecting the most suitable type of drilling fluid, many different factors must be considered. Overall what is required is a mud system that gives the lowest overall cost of drilling each hole section, except for through the reservoir. The direct cost of the fluid itself (the cost per barrel of mud) is but one component of this overall cost. If serious hole problems occur because the mud was not optimized for the formations in an effort to “save money,” obviously much more money will be spent than would have been saved on the mud bill.
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The gypsum-salt formations are complex in drilling fluid. The gypsum-salt formations are characterized by salting-in property, plastic creep and anisotropy. Meantime, the rheological properties of drilling fluids become worse when contaminated by high pressured brine in the gypsum-salt formations. In the process of drilling operation in the long-section gypsum-salt formation, reasonable mud weight and good rheological properties of drilling fluids should be used to ensure the safety drilling in this complex formation. The main matters needing attention is as follows:
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