Millimeter-sized (10 μm–mm) preformed particle gels (PPGs) have been used successfully as conformance-control agents in more than 5,000 wells. They help to control both water and CO2 production through high-permeability streaks or conduits (large pore openings), which naturally exist or are aggravated either by mineral solution or by a high injection pressure during the flooding process. This paper explores several factors that can have an important impact on the injectivity and plugging efficiency of PPGs in these conduits. Extensive experiments were conducted to examine the effect of the conduit inner diameter and the PPG strength on the ratio of the particle size to the opening diameter, injectivity index, resistance factor, and plugging efficiency. Five-foot tubes with four internal diameters were designed to emulate the opening conduits. Three pressure taps were mounted along the tubes to monitor PPG transport and plugging performance. The results show that weak gel has less injection pressure at a large particle/opening ratio compared to strong gel. PPG strength affected injectivity more significantly than did particle/opening ratio. Resistance factor increased as the brine concentration and conduit inner diameter increased. PPGs can significantly reduce the permeability of a conduit, and their plugging efficiency depends highly on the particle strength and the conduit inner diameter. The particle size of PPGs was reduced during their transport through conduits. Experimental results confirm that the size reduction was caused by both dehydration and breakdown. On the basis of the laboratory data, two correlations were developed to quantitatively calculate the resistance factor and the stable injection pressure as a function of the particle strength, particle/opening ratio, and shear rate. This research provides significant insight into designing better millimeter-sized particle-gel treatments intended for use in large openings, including open fractures, caves, worm holes, and conduits.