In this paper, we describe our experience of using the velocity-dependent relative permeability model developed by Heriot-Watt University and the generalized pseudo-pressure model of Fevang and Whitson to simulate condensate banking effects. For cases where the generalized pseudo-pressure model fails, we discuss alternative approaches to simulating condensate banking effects in full field models, together with their limitations. Our methodology is illustrated on a number of different examples, drawn from reservoirs in Indonesia, the Middle East and the North Sea where Total is either the operator or an active partner.
Condensate banking is a well-known phenomenon in gas condensate reservoirs. It occurs when the bottom-hole pressure of a production well falls below the dew-point pressure. Condensate drops out in the reservoir around the well and rapidly accumulates as each cubic metre of fresh gas produced deposits further condensate due to the pressure near the well being lower than the pressure further away from the well. The accumulation of condensate near the well reduces the gas saturation there, in turn reducing the gas relative permeability and hence reducing the the well productivity.
Although the potential for the occurrence of condensate banking is widely recognized, there is no consensus within the industry as to when it occurs and whether or not it will have an effect on well productivity. There are several possible reasons for this:
Detecting the presence of a condensate bank from well test data is not easy because it may be masked by well bore storage effects, because the data may not be of good enough quality, or because velocity-dependent relative permeability effects complicate the analysis.
Decline in productivity with time may be caused by effects other than condensate banking.
Many gas condensate reservoirs are close to their dew point pressure at initial conditions, so there may be no reduction of productivity because the initial productivity is already a "reduced" one due to condensate deposition right from the start of production.
Velocity-dependent relative permeability (VDRP) effects may mean that there are many gas condensate reservoirs where condensate banking either does not occur or has no effect on productivity (i.e. the gas relative permeability near the well is no lower than that far from the well, whether or not the condensate saturation near the well is greater than that in the bulk of the reservoir).