Coiled-tubing drilling can be successfully used to drill horizontal wells in overbalanced conditions. Minimizing solids and using “solids-free” rheologically engineered fluids like the F LO -P RO T system are critical to maximizing drilling parameters while minimizing formation damage. A combination of elevated LSRV and back-reaming optimize hole cleaning in through-tubing re-entries. Solids contribute to higher pump pressures, shorter coil life, potential stuck pipe, greater formation damage and higher mechanical friction coefficients.