Four major factors affecting horizontal well gravel pack were studied using a 3-D simulator developed for horizontal well gravel packing. The factors include settling effect, gravel concentration, injection rate, and carrier fluid viscosity. The gravel pack process was simulated and the effectiveness and pack efficiency of the gravel pack job is determined using the simulator. The effect of carrier fluid viscosity on gravel-pack efficiency was studied by varying the viscosity between 1 and 51 cp, injection rate between 1 and 4 barrel per minute, and gravel concentration between 0.50 and 4.0 pound mass per gallon. These three factors affect the settling factor of the gravel as it enters the horizontal section which subsequently affects the pack effciciency of the gravel pack process. Introduction Several authors have investigated the factors affecting gravel transportation and placement towards achieving an effective gravel pack. Gruesbeck et al1 performed experiments to measure pack efficiency as a function of screen parameter, fluid and gravel properties, completion configuration and angle of inclination of the well bore. They also developed a model to determine the height of the equilibrium bank formed during gravel packing of an inclined well bore. They concluded that packing efficiency increases with lower gravel concentration, lower gravel density, higher flow rate and increasing resistance to fluid flow in the tail pipe/screen annulus. They contended that by increasing the ratio of tail pipe diameter to the inside diameter of screen beyond 0.6, better efficient packing could be achieved. Several successful jobs were reportedly performed with this philosophy. Carrier fluid of viscosity less than 10 cp produced an equilibrium bank that allows complete filling of the well bore when the bank height is less than the diameter of the casing. Hodge2 substantiated the Gruesbeck et al. work by determining the accuracy of the predicted equilibrium bank height. Elson, et al3 reported a study conducted to define optimum gravel pack procedures and completion design factors for high angle wells. Result of the study showed that high viscosity carrier fluids with high gravel concentration provides good gravel transport, but are unsuitable in wells with angles of 80o from vertical. Satisfactory transport and improved packing were achieved with lower carrier fluid viscosity and sand concentrations. Tailpipe design requirement recommended by Gruesbeck et al. was used with satisfactory results. Skaggs4 presented the result of a large-scale well bore model he used to study gravel transport through the perforations during a high-density squeeze gravel packing operation. He concluded that the transport efficiency through perforations increases with increased fluid viscosity, gravel concentration, and annular velocity. His work is however based on vertical well bore. Winterfeld and Schroeder5 developed a finite element numerical simulator and used it with a full-scale well bore model to study gravel placement in perforations and annulus. Their model is based on mass and momentum conservation equations. Peden et al6 developed some mathematical design models for predicting the optimum combination of required design parameters such as tailpipe diameter, slurry flow rate, and gravel concentration for an optimum packing efficiency. These models were based on extensive experimental study of factors affecting packing efficiency and dimensional analysis of obtained data. Wahlmeler and Andrews7 in 1988 improved on the earlier works of Gruesbeck et al. and Peden et al. by developing a pseudo-three dimensional mathematical model suitable for designing and evaluating gravel pack treatments. The pseudo-three dimensional model was based on overall aspects of well bore configuration and the mechanism involved in the process of gravel packing. The model was verified both with large-scale laboratory studies and field results Shryock8 worked on a full scale deviated model and had similar conclusions with earlier workers. He discovered that water carrier fluids completely gravel pack well bore deviated at 60o from vertical and that packing efficiency can be improved by increasing the resistance to flow in the liner-tailpipe annulus.