Abstract:

Gas-condensate wells experience a significant decrease in gas productivity once the flowing bottom-hole pressure drops below the dew-point pressure. However, there is still a lack of understanding how the condensate bank affects the deliverability because of the complex phase and flow behaviors. The difficulty of understanding the phase and flow behaviors lies in the variation of the composition due to the existence of two-phase flow and the relative permeability effect (each phase has different mobility). The change of composition will also brings about a large change in saturation and phase properties such as surface tension, viscosity, etc. of the fluids. These effects will impact mobilities and hence productivity.

The composition variation has been observed in the field but its effects have been studied only rarely in the literature. This work studied the impact of compositional variation on the flow behavior of the gas-condensate system through numerical simulations and a series of laboratory experiments. The study verified claims made about effect of flow through porous media on the apparent phase behavior of a gas-condensate mixture, namely compositional variation during depletion, saturation profile around the well, experience on shutting in the wells in an attempt to achieve condensate revaporization, and the effect of bottom-hole pressures on condensate banking. Finally, the work was extended to the case that we normally see in the field: gas-condensate reservoirs where immobile water is present.

Results from this study show that composition varies significantly during depletion. Due to the difference in mobilities caused by relative permeability, the composition of the mixture will change locally. The overall composition near the wellbore becomes richer in heavy components. As a result, the phase envelope will shift to the right. Near-well fluids can undergo a transition from retrograde gas to a volatile oil, passing through a critical composition in the process. The condensate bank can be reduced with proper producing sequence, hence the productivity of the well can be improved. The study also showed that the presence of immobile water did not have any significant effect on the compositional variation of the gas-condensate mixture, at least in the cases investigated.

The ultimate objective of the research was to gain a better understanding of how the condensate blocking affects the well productivity, with the focus on the effect of compositional variation on the flow behavior. This is important for optimizing the producing strategy for gas-condensate reservoirs, reducing the impact of condensate banking, and improving the ultimate gas and condensate recovery.

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