Experimental study on the effect of confinement on propane phase behavior

Abstract

Condensation of petroleum retrograde gas especially around the wellbore in unconventional reservoirs can decrease well deliverability significantly. Better estimation of the dew point helps to improve production by minimizing the condensate dropout. Over geologic time, organic matter deposited in source rocks turns into oil and gas through a maturation process. These hydrocarbons then migrate away from source rock due to pressure, and find structural or stratigraphic traps and form conventional reservoirs. In unconventional reservoirs such as tight oil and shale gas, hydrocarbons were generated in source rocks with extremely low permeability. As a result, hydrocarbons do not migrate far away from the source rock or even stay within the source rock. Prediction of the productivity of unconventional reservoirs depends on the understanding of the phase behavior of its hydrocarbon content. Unconventional reservoirs have abundant nano-pores, in which the phase behaviors of hydrocarbons are expected to deviate from phase behaviors of bulk fluids; i.e. condensation/ vaporization can happen at conditions different from those measured in conventional PVT cells. Better understanding of phase behaviors in nano-pores should, therefore, yield better analytical and numerical models and better unconventional reservoir engineering practices. Many studies have focused on the theoretical causes of unconventional hydrocarbon phase behavior in nano-pores and modeling. However, direct observations of phase behaviors are scarce and very few experimental data are available. In this study, phase changes of pure propane gas in nano-fluidic devices were directly observed for the first time. Propane gas prefers to condense in nano-pores at pressures lower than the regular vapor pressure and condensation in nano-pores occurs before condensation in larger pores. Calculations show that intermolecular forces in nano-channels lead to lower vapor pressures than those predicted from conventional expectations. These forces do not affect phase behavior in conventional petroleum reservoirs but, their effects can be significant in unconventional reservoirs.