With the growing need to explore non-conventional energy sources, the hydrates of natural gases offer a realistic solution in the need for alternative energy sources. Gas hydrates are typically entrapped in the porous media showing sensitive phase stability conditions. Models for phase stability for gas hydrate have not yet been extensively investigated for porous media and thus need attention. In this paper, the phase stability model is developed from the basic Chen and Guo model (Chem Eng J, 1998, 71:145) to accurately predict the phase behavior of the clathrate hydrates of CH4 and CO2 in porous media of varying pore sizes from 6nm to 100nm which mimics the naturally occurring porous environment. We also propose a new equation for calculating the activity of water in porous media as a function of the pore size, the wetting angle, the surface tension, and the shape factor of the pores for varied temperature conditions. The model results are validated against experimental data available in open literature and found satisfactory. The proposed model uses very few input parameters (data intrinsic) and thereby is very beneficial in predicting the stability of the hydrates in virgin gas reservoirs wherein the characteristics of the gas reservoir are largely unknown. The developed model may further be applied to the hydrate systems of other natural gases in porous medium with suitable modifications. © 2014 Elsevier B.V.