Techniques have been developed to experimentally and numerically evaluate performance of water-alternating-CO2 processes in thin heavy oil reservoirs for the purpose of pressure maintenance and improving oil recovery. Experimentally, a three-dimensional (3D) physical model consisting of three horizontal wells and five vertical wells is used to evaluate the performance of water-alternating-CO2 processes. Two well configurations have been designed to examine their effects on heavy oil recovery. The corresponding initial oil saturation, oil production rate, water cut, oil recovery and residual oil saturation distribution are examined under various operating conditions. Subsequently, numerical simulation is performed to match the experimental measurements and optimize the operating parameters (e.g., slug size and water-CO2 ratio). It has been shown that water-alternating-CO2 processes implemented with horizontal wells can be optimized to significantly improve performance of pressure maintenance and improving oil recovery in thin heavy oil reservoirs. The incremental oil recovery of 12.4% and 8.9% through three water-alternating-CO2 cycles are experimentally achieved for the aforementioned two well configurations, respectively. The excellent agreement between the measured and simulated cumulative oil production indicates that the displacement mechanisms governed water-alternating-CO2 processes have been numerically simulated and matched. The WAG ratios of 0.75 and 1.00 are found to be the optimum values for Scenarios #1 and #2, respectively.