Numerical Analysis of Production Impairment by Condensate Banking in Gas Condensate Fields

Abstract

Abstract Producing gas-condensate reservoirs below the fluid's dew-point pressure will lead to an increased condensate saturation in the near wellbore region, which impacts the relative permeability to gas. This phenomenon is known as condensate banking and is a cause of productivity impairment. During a field's operational phase, failure to predict condensate banking behaviour accurately will cause problems with a well's ability to attain production targets. This paper explores the effect of both absolute permeability and condensate gas ratio (CGR) in order to quantify production impairment. To achieve this, three sets of PVT data were characterised. The reservoirs Alpha, Bravo and Charlie have an ideal split (C4-/C5+) CGR of 174.1, 44.7 and 13.9 (stb/MMscf) respectively. For each set of PVT data the absolute permeability has been tested within the range 1 to 1000mD. Both porosity and the relative permeability model were adjusted in proportion to absolute permeability. The range of parameters selected provide coverage of most gas-condensate fields. The PVT data from these reservoirs has been tuned and simulated using a Peng-Robinson twenty-two (22) component dynamic compositional model. Primarily, a single well radial model was used, although implementation of a Cartesian model was also explored for full-field modelling. All models discussed represent an ideal reservoir, consisting of homogeneous properties throughout. Under a steady-state dry gas production rate of 50MMscfpd productivity impairment reached a maximum of 15.4% assuming an absolute permeability of 1mD and an initial CGR of 174.1 stb/MMscf. This low value of 15.4% was assisted by the positive effect of Velocity Dependent Relative Permeability (VDRP) and condensate stripping within the near wellbore region. In contrast, for situations where VDRP does not apply such as low production rates or post shut-down production ramp-up operations, the effects of condensate banking were significant. That is, removing VDRP to simulate these conditions, under the same dry gas production rate of 50MMscfpd a productivity impairment of 93.2% was observed. This result suggests that the most detrimental effect of condensate banking is caused by unsteady-state production operations. It is intended that the findings of this study be applied to wells currently on production to screen for potential condensate banking in later well life.