This paper was prepared for presentation at the SPE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, USA, 12–16 April 2014. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohi bited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract Millimeter-sized (10 um~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 CO 2 production through high-permeability streaks or conduits (large pore openings), which naturally exist or are aggravated either by mineral solutions 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's opening size 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 impacted injectivity more significantly than did particle opening ratio. Resistance factor increased as the brine concentration and conduit opening size increased. PPGs can significantly reduce the permeability of an open conduit and their plugging efficiency depends highly on the particle strength and the conduit's opening size. The particle size of PPG was reduced during their transport through conduits. Experimental results confirm that the size reduction was caused by both dehydration and breakdown. Based on the lab data, two mathematical models 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. Introduction Excess water production in oil fields is becoming a challenging economical and environmental problem as more reservoirs are maturing. An estimated average of three barrels of water are produced for each barrel of oil produced worldwide (Bailey et al., 2000). It is estimated that the total cost to separate, treat, and dispose of this water is approximately $50 billion per year (Hill et al., 2012). Water can flow into the wellbore as a result of either near-wellbore problems or reservoir-related problems (Seright et al., 2001). The mechanisms that contribute to this undesired water production must be fully understood before the appropriate treatment can be chosen. Water channeling, one of the primary reservoir conformance problems, is caused by reservoir heterogeneities that lead to the development of high-permeability streaks. These streaks include open fractures and fracture like features, such as caves, worm holes, and conduits (Smith et al, 2006). These high-conductivity areas inside the reservoir only occupy a small fraction of the reservoir but will capture a significant portion of injected water. As a result, large amounts of oil remain unswept as a large water flood will bypass oil-rich unswept zones/areas. Gel treatments have been proven to be a cost-effective chemical conformance control technology to reduce the fluid flow in these large opening features. The application of these technologies can not only control water production but also significantly increase the oil production and extend the economic life of a reservoir. Traditionally, in-situ bulk gels have been used for this purpose. However, preformed particle gels recently have attracted much attention because they can solve some of the problems associated with in-situ gel systems, such as the dilution and dispersion of the gelant, chromatographic separation of the gelant solution, and so on. (Chauveteau et al., 2001, 2003; Coste et al., 2000; Bai et al., 2007a, 2007b).