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This paper presents a simple mechanistic model to describe a gas kick in a drilling riser with water-based mud (WBM) and synthetic-based mud (SBM). This model can estimate key kick parameters such as the change in the wellhead pressure, kick ascent time, and pit gain. In addition, this model also predicts the solubility of the gas kick in SBM at various depths in the annulus. We used the commercial chemical process simulation software, HYSYS, to validate the results of this solubility model. This paper also presents the gas kick experimental results from a 20-ft. tall vertical flow loop at Texas A&M University, Qatar. The base case investigates a gas kick in a vertical 10,000 ft. deep, 12.415 in. drilling riser with WBM. Our analytical model uses the Hasan-Kabir two-phase flow model and develops a set of equations that describe the pressure variation in the annulus. This computed pressure change allows estimates of pit-gain. Our experimental data comes from a 20-ft. tall flow loop with a 2.5 in. steel tube, inside a 4.5 in. Acrylic pipe, that simulates a riser. For these gas kick experiments, we injected specific amounts of gas at the bottom of the setup and recorded the bubble's expansion and migration. The mechanistic model predicted explosive unloading of the riser near the wellhead. A comparison between our model results and HYSYS values for methane liquid-phase mole fraction showed a maximum 8% deviation with complete agreement on bubble point (Pb) pressure and location estimates. Similarly, our model calculated the solution gas-oil ratio (Rs), with a maximum divergence of 3% from HYSYS estimates. From the comparison studies with other empirical Bo & Rs correlations, we note that the estimates of our model agreed best with those of O'Bryan's (Patrick Leon O'Bryan, 1988) correlations. Numerical kick simulators that exist today are notoriously time and power-intensive, limiting their on-field utility. Our mechanistic model minimizes computation time through its simple, analytical form to describe kick migration. Our model offers another layer of novelty through the analytical, thermodynamic solubility modeling as opposed to empirical modeling sused by most of the current gas kick simulators.