In this study, empirical friction factor correlations were developed for two-phase stratified-and intermittent-flow patterns through horizontal fully eccentric annuli. Two-phase flow hydraulics were investigated, and a flow pattern prediction model is proposed. The friction factor correlations were validated using experimental data collected at the multiphase flow loop METU-PETE-CTMFL. Two different geometrical configurations were used during experiments-that is, 0.1143 m inner diameter (ID) casing, 0.0571 m outer diameter (OD) drillpipe; and 0.0932 m ID casing, 0.0488 m OD drillpipe. The eccentric annuli has been represented by representative diameter d(r). A new mixture Reynolds number based on liquid holdup is proposed for friction factor determination.