The most widely used drill-in and completion fluids for seepage loss control in payzone are water-based mud containing polymeric additives and sized calcium carbonate or sized salt. These inorganic bridging solids are very effective in sealing the pore throats of the porous matrix and preventing formation damage but owing to their brittleness they will grind down during circulation and handling. This particle degradation makes it difficult to maintain the correct particle size distribution during drilling operations and can cause a deep penetration of the finest solids into the formation.
A novel, solids-free drill-in fluid system containing a biopolymer as viscosifier and a highly crosslinked starch and microfibrous cellulose as fluid loss control agents has been developed for drilling and completing low permeability formations. This fluid composition provides the qualities of both good drilling mud and a non-damaging completion fluid. This paper presents the laboratory results obtained from coreflooding tests carried out on this solids-free drill-in fluid in limestone cores with permeability lower than 100 mD. The synergistic interaction of the biopolymer with a highly crosslinked starch and a microfibrous cellulose material promotes the quick formation of a very thin (ca. 50-60 µm) and impermeable filter cake which prevents polymer and drilled solids invasion into the core. The results show that the filter cake can be completely removed by backflooding and the permeability can be completely recovered. The non-damaging effect of this drill-in fluid was also observed when its density was increased to 1.48 g/cm3 by potassium formate brine. Good filtration and rheological properties were also maintained when simulated drilled solids, e.g. reactive and non-reactive clays were added. Core plug invasion and thickness and the morphological structure of the filter cake, investigated by means of Scanning Electron Microscopy, is also discussed.