Predicting When Condensate Accumulation Nea the Wellbore in a Gas Condensate Reservoir Can Be Identified from a Build Up (SPE-121326)

摘要

A gas condensate reservoir usually displays retrograde behavior when the bottomhole pressure falls below the fluidC 's dew point pressure during production. A two-phase mixture made of gas and liquid condensate forms and creates regions of different mobilities in the reservoir. These yield a fluid induced composite behavior in well test data. Reservoir permeability, relative permeability curves, critical condensate saturation and production rate history are some of the factors which determines when the condensate starts to accumulate during a drawdown and when it becomes mobile. The conditions when the condensate bank becomes visible on the build up derivative response are, however, much less understood. These are addressed in the present study. Compositional simulations of simple drawdown-build up sequences were conducted for various drawdown durations, critical condensate saturations, reservoir absolute permeabilities, oil and gas end point relative permeabilities and gas miscibility. Simulated drawdown and build up well test derivative responses were then compared and analyzed. It was found that, whereas the change in gas mobility is reflected in the drawdown pressure derivative response as soon the pressure falls below the dew point pressure, it is delayed significantly in the build up derivative and only appears when the ratio of the mobility at the start of the build up to that at the end of the drawdown reaches a critical value. This critical value varies little with the parameters investigated in this study. The relationship between the time when the mobility change first appear in the build up derivative and the drawdown rate can be represented in all cases investigated by power equations which can then be used to predict when the condensate bank might be seen on the build up derivative for different rates of production and for different reservoir parameters.