Predicting and assuring well deliverability is often an important concern during the development of gas-condensate reservoirs, especially offshore fields. In addition, production of gas is usually bound with long-term contracts where it is necessary to assure well deliverability for a long period. Undesirably, well deliverability in gas-condensate reservoirs can be impaired by the formation of a condensate blockage once the bottomhole flowing pressure drops below the dewpoint pressure. In this case, the relative permeability of the gas phase can be significantly reduced due to this condensate accumulation around the wellbore. Therefore, quantifying this negative effect is essentially required to obtain reliable predictions of well deliverability in gas-condensate reservoirs. The evaluation of condensate blockage phenomenon requires an appropriate understanding of flow characteristics of both gas and condensate liquid through reservoir rocks.
In this study, the impact of condensate blockage on gas well deliverability was investigated by examining a number of flow parameters such as absolute permeability (K), critical condensate saturation (Scc), relative permeability shape and end-points (krgmax, kromax). A single well radial and cartesian models were both used in different simulators (modified black-oil and compositional).
The outcomes of this study showed that the condensate blockage can have a severe negative impact at low permeability reservoirs (K≈5md), while the impact can be small at moderate permeability (k≈50md) and diminishable at high permeability reservoirs (k≈200md). This negative impact can double at high critical condensate saturation (Scc>30%). It was found out that the relative permeability curves applied in the model define the magnitude of the blockage, especially in low permeability reservoirs, whereas relative permeability end-points (krgmax and kromax) affect mainly the overall gas recovery. Previous lab measurements showed that gas-condensate relative permeabilities are sensitive to flow velocity only in low permeability reservoirs. This effect was modeled using a compositional model through the capillary number. Velocity-dependent relative permeability model applied in this case showed that the plateau period can be improved by a factor of two in low permeability. On the other hand, no positive effect was observed at moderate and high permeability since the blockage effect was already small or dimensionless.
Finally, the benefit of methanol injection was investigated for improving well deliverability at low permeability reservoirs. Based on this analysis, methanol treatment can improve gas well deliverability and substantially prolong the plateau period by a factor of 2-3.