Abstract: A coil tubing string for injecting fluids into a well includes one or more tapered wall tubing sections welded in series with adjacent straight wall tubing sections to form smooth joints therebetween. The tubing string has tubing sections of dissimilar wall thicknesses. Such tapered wall tubing sections are used to connect tubing sections of dissimilar wall thicknesses wherein the transitions from one tubing section to another is smooth and continuous as one progresses from a thin wall tubing section to a heavy wall tubing section in an ascending order.