In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO2 injection into systems containing CH4 hydrate, free CH4 gas, and varying amounts of liquid water. The CH4 hydrate was converted into either pure CO2 or mixed CO2CH4 hydrate to investigate the impact of two primary mechanisms governing the relevant phase transitions: solid-state mass transport through hydrate and heat transfer away from the newly formed CO2 hydrate. Experimentally proven dependence of kinetic conversion rate on the amount of available free pore water was investigated and successfully reproduced in our model systems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO2 or mixed CO2CH4 hydrate, a much slower solid-state mass transport becomes the dominant mechanism. (c) 2015 American Institute of Chemical Engineers.