Four major factors affecting horizontal well gravel pack were studied using a 3D simulator developed for horizontal well gravel packing. The factors included settling effect, gravel concentration, injection rate and carrier fluid viscosity. Three actual Field horizontal well gravel pack jobs obtained from the literature were performed using the simulator to study these factors. The effect of carrier fluid viscosity on gravel pack efficiency was studied by varying the viscosity between 1 and 51 cP, injection rate between 0.159 and 0.636 m(3)/min and gravel concentration between 0.50 and 4.0 Pound mass per gallon. Simulation results demonstrate the validity of the solution routine and the capability of the simulator, because the results were in agreement with the field results. The predicted pack efficiency for cases that considered settling effect are consistently higher than the cases without settling effect. The study also showed that the settling factor decreases with increasing gravel concentration and injection rate.