Special Session 108: New Developments in porous media

Effect of pore size distribution on the state of hydrocarbon phases during pressure depletion

Xiaolong Yin
Colorado School of Mines
In shale gas and shale oil reservoirs, hydrocarbons are generally stored in pores that are nanometers to tens of nanometers in size. In these pores, capillary pressure affects the phase behavior of hydrocarbon mixtures. In this talk, I will present modeling of the effect of pore size distribution on vapor-liquid equilibrium in porous media with narrow pores and strong capillary pressures. Such models are important for predicting the saturations and properties of gas and oil phases during primary production. Take a porous medium that is initially saturated with oil as an example. Gas saturation will appear first in large pores; the initially formed gas changes the composition of the remaining oil, and the compositional change in turn alters the equilibrium condition at which oil is vaporized in smaller pores. To properly predict the state of phases confined in this porous medium, one therefore must trace the entire pressure-saturation history and cannot just rely on a single phase diagram. Our calculations show that the capillary force increases with increasing gas saturation, and it is likely that the smallest pores are always filled with liquid during production, no matter whether the reservoir is initially filled with oil or gas.