Cased-Hole Cleaning: Do Annular Velocity, Annular Coverage and Contact Time Equal Mud Removal?

摘要

Mud-to-brine displacement efficiency can be difficult to quantify. Metrics to determine the success or failure of the displacement operation have traditionally been based upon the clarity of the completion brine that follows the spacers, the interface volumes of mud and completion brine, a visual determination of the cleanliness of the drillpipe pulled from the hole, and the time and materials required to filter the fluid. Despite their widespread use, however, these factors may not adequately characterize the quality of displacements in critical wells. Opportunities for improvement are largely affected by how well the displacement process can be understood. This can be difficult with the few direct and indirect parameters used to evaluate mud-to-brine displacement quality. The authors initiated a study of samples captured during displacement circulations to examine the effects of spacer properties, volumes, pump rate, and chemical concentration on mud removal. The laboratory analyses inspected return samples of mud, spacers, and displacing fluids from 27 cased-hole wellbore displacements of oil-based and synthetic-based drilling mud in the Gulf of Mexico. Results from this study suggest that criteria such as annular coverage, contact time, and annular velocity of the wash spacer(s), which have become the basis for most displacement programs, although critical to overall efficiency, are not the primary agents of mud removal during a wellbore cleanup. Data correlations between the volume of mud observed in the samples analyzed and certain spacer characteristics suggest that cased-hole cleaning, when defined as mud removal, is a function of the size of a viscous spacer relative to hole size. This observation should simplify the displacement design process.