An Evaluation of Steady-State-Based Pressure-Saturation Predictions in Gas-Condensate Reservoirs Under Various Reservoir and Well Operating Conditions
Authors: Johnson C.. , Jamiolahmady M.. 
Published: March 2019

The analysis of pressure-transient and rate-transient data in gas-condensate reservoirs (GCRs), operating below dewpoint pressure (Pdew) often requires using two-phase pseudovariables. These are used to handle the nonlinearities that arise in the governing partial-differential equation as a result of pressure-dependent saturation, mobility, and fluid-property changes. The incorporation of relative permeability (kr) data in the computation of such pseudovariables requires a priori knowledge of sufficiently representative pressure-saturation relationships. For this purpose, a number of analytical and semianalytical pressure-saturation prediction methods can be found in the literature.

In this study, a pressure-saturation prediction method, which relates component weight fractions (in the gas phase, the condensate phase, and the reservoir fluid mixture) and gas fractional flow [or gas to total gas-plus-condensate flow rate (GTR)] to pressure, was used to compute pressure-saturation relationships for various reservoir fluids and two rock types. The computed pressure-saturation relationships were compared to the responses observed in the near-wellbore region of corresponding single-well 1D radial homogeneous compositional reservoir-simulation models operated at constant well flowing pressures (Pwf) lower than Pdew. Sensitivities were conducted to highlight the impact of gas-condensate fluid richness, kr characteristics, degree of reservoir undersaturation (Pi – Pdew), and Pwf on observed trends in the representativity of the predicted pressure-saturation relationships.

Like other methods based on two-phase steady-state (SS) assumptions, the condensate saturations predicted by the GTR method generally matched those observed around the wellbore in the simulation models when Pi – Pdew was sufficiently high. For cases in which Pi – Pdew was not sufficiently high, an examination of radial pressure and saturation profiles from the simulation models showed that the trend of increased condensate saturation levels observed around the wellbore with increasing degrees of reservoir undersaturation, as reported in literature, represents one of three possible trends (i.e., increasing, decreasing, and almost-constant condensate-saturation levels). For cases in which the predicted GTR-based pressure-saturation curve showed lower (higher) condensate saturations than the pressure/volume/temperature (PVT) liquid-saturation curves at the given Pwf, increasing Pi – Pdew resulted in decreasing (increasing) condensate-saturation levels around the wellbore. The results indicate that at low Pi – Pdew values, trends in two-phase SS-based analysis techniques are affected by the gas-condensate fluid richness and the nature of the kr characteristics. It is also shown that for cases in which there is not much of a difference between PVT-based and SS-based pressure-saturation curves, variations in the Pi – Pdew do not affect the condensate-saturation levels around the wellbore.