Abstract
This paper investigates the well test behaviour of rich gas condensate reservoirs below the dew point pressure and the impact of re-vaporisation of the condensate bank due to re-pressurisation by gas injection. Results from compositional simulation, verified with field data, suggest that the near-wellbore fluid saturation below the dew point pressure in a build up is different from that at the end of the preceding drawdown, because of significant differences in fluid properties and saturation distributions. A condensate bank is created during drawdowns, which disappears during subsequent build ups. As a result, the corresponding pressure derivatives are different, with the drawdown derivative being the mirror image of the derivative in the subsequent build up. This is different to what happens in lean gas condensate reservoirs and similar to the well test behaviour of volatile oil reservoirs. Drawdowns and build ups in rich gas condensate reservoirs below the dew point pressure exhibit a composite behaviour, which can be analysed to provide the gas mobility in the bank two-phase region. A practical method is proposed to evaluate the bank storativity, which is then used to calculate the bank radius. The study also shows that the loss of productivity due to liquid drop out can be reversed by a properly designed gas injection scheme and that the corresponding changes in saturation distributions can be monitored by well test analysis.
These theoretical investigations are used to explain a series of production tests conducted in a rich gas condensate reservoir in North Africa. A well test analysis methodology is proposed for such systems, which combines conventional well test interpretation methods, deconvolution and verification with both a single phase voranoid grid simulator and a numerical compositional simulator.
