Evaluation of mass transfer coefficient for gas condensates in porous systems: Experimental and modeling

Abstract

The gas condensate reservoirs are significant resources of energy for various municipal and industrial purposes. An unfavourable phenomenon attributed to this kind of reservoirs is liquid drop-out due to excessive near-wellbore pressure drop during production. Gas injection is normally employed to recover the liquid loss in the underground formation. The amount of condensate recovery is determined by assuming a thermodynamic local equilibrium, which seems irrational due to the limited contact area, high velocity, and low residence time for fluids around the wellbore. In this work, a series of experiments are conducted based on the Taguchi design of experiment (DOE) to study the steady-state mass transfer during gas injection. Three injection gases (CO2, N2, and CH4), three liquids to represent the liquid condensate (C5, C6, and C7), and three mean grain sizes within the range of 150–300 μm are the vital design variables of the experiments where a broad range of gas velocity (0.00472–0.0283 cm/s) is employed. Results of Taguchi DOE approach are then analyzed to find the optimum design levels and influence/contribution of each factor. The experimental data are utilized to develop new models for estimation of the mass transfer coefficient in terms of Schmidt number (Sc), Peclet number (Pe), normalized velocity, and mean grain size. The comparison between the modeling results and real data confirms the reliability and precision of the proposed correlations. Based on the research outputs, the injection rate (60% contribution factor) and grain size (nearly 10% contribution factor) have the highest and lowest effects on the mass transfer coefficient in porous systems, respectively. It is also found that the difference between the equilibrium concentration and the actual concentration of volatile component in the gas phase increases as the gas velocity increases. Based on the DOE, the optimal design levels for the injection rate, gas type, liquid type, and mean grain size are 12 cm3/min, C1, C5, and 300 μm, respectively. The new models provide accurate/reliable mass transfer coefficient (R2 = 0.9906) for non-equilibrium mass transfer cases to simulate re-vaporization of condensate into the gas phase, while implementing gas injection into the gas condensate reservoirs.