Benchmark Performance: Zero Barite Sag and Significantly Reduced Downhole Losses with the Industry's First Clay-Free Synthetic-Based Fluid

Abstract

Proposal Since the introduction of invert emulsion fluids in the 1960s, oil-based fluids (OBF) and synthetic-based fluids (SBF) have been formulated with a similar group of components: base oil, organophilic clay and lignite, lime, CaCl2 brine, and emulsifier. The family of invert emulsion fluids has remained closely related in terms of mud properties and performance expectations. In 2001, an SBF formulated entirely without commercial clays or lignites was introduced in the Gulf of Mexico (GOM). Rheological properties are controlled through the emulsion characteristics, a radical departure from accepted solids suspension mechanisms. The behavior of this unique fluid has changed perceptions about what constitutes "good mud." The clay-free, emulsion-based fluid (System) has consistently prevented detectable barite sag on 80+ wells drilled with it to date. Based on observed fluid densities after long static periods (an 8-day logging run in one case) and verified by modular dynamic test (MDT) log data on numerous high-angle wells, the fluid's unique emulsion structure and wetting characteristics prevent settling of barite and other solids. Attempts to control barite sag with conventional clay-based SBFs have produced mixed results. A 14.0-lb/gal SBF treated with sag-prevention organophilic clay showed a 0.3-lb/gal density reduction in the initial return flow and a 0.3-lb/gal density increase in the tail flow after a 52-hr logging run. This 0.6-lb/gal variation in density was deemed "manageable." However, the treatment resulted in a 20% increase in the funnel viscosity at 75°F, demonstrating an adverse effect on the rheology after increasing the clay concentration by only 0.2 lb/bbl. At lower temperatures encountered at the sea floor, this effect would be amplified. In addition to preventing barite sag, the System has provided other important field-documented performance advantages: Whole mud losses reduced by an average of 60% while drilling, running casing, and cementing (with 80% reductions reported on several deepwater wells).Significantly lower ECDs, validated by pressure-while-drilling (PWD) data.High, flat gel strengths that break with minimal initiation pressure, validated by PWD data.Highest standard of compliance with environmental regulations governing GOM operations. The System is currently under consideration for use offshore West Africa, offshore Brazil, and the Asia-Pacific region.