Numerical evaluation of phase behavior properties for gas condensate under non-equilibrium conditions

It is generally known that for a gas reservoir if its pressure changes or flow rate is smaller than the phase transition or mass transfer rate, non-equilibrium effect may arise. Commonly, this situation is ignored when the approaches based on the local equilibrium assumption were applied. Previously there are only a few works focused on this subject and now it attracts more and more attentions now. In this work, a mathematical model was formulated to simulate the gas-related properties of Constant Composition Expansion (CCE) tests under non-equilibrium conditions. In this method, the condensed liquid or condensate is assumed to form a continuous film when brought into contact with gas condensate that was depressurized incrementally at a constant temperature. The whole process is controlled by interphase mass transfer and is completed when abandonment pressure or thermodynamic equilibrium is reached. The elapsed time to reach the final state is determined by the pressure dropping rate and the pressure difference between the initial and final states. The results obtained using the current method match satisfactory with the experimental data. The results demonstrate that the larger the pressure drop rate, the lower the retrograde condensate saturation, and the stronger degree of non-equilibrium effect. It proved that a higher pressure dropping rate can not only alleviate the retrograde condensate phenomenon but also improve the recovery of gas condensate.