1. n. [Drilling Fluids]

A special fluid designed exclusively for drilling through the reservoir section of a wellbore. The reasons for using a specially designed mud are to

• successfully drill the reservoir zone, often a long, horizontal drainhole
• minimize damage and maximize production of exposed zones
• facilitate the well completion needed, which can include complicated procedures.

A drill-in fluid should resemble a completion fluid. It may be a brine containing only selected solids of appropriate particle size ranges (salt crystals or calcium carbonate) and polymers. Only additives essential for filtration control and cuttings carrying are present in a drill-in fluid.

See: bland coring fluid,  brine,  calcium carbonate,  carboxymethyl starch,  carrying capacity,  cesium acetate,  cesium formate,  completion fluid,  conventional mud,  core,  filter press,  filtrate tracer,  filtration,  formate,  formation damage,  hydroxypropyl starch,  particle-plugging test,  sintered,  sized calcium carbonate,  sized salt,  slurry