Abstract
When a rich gas field is put into production, one has to decide on a production mechanism which may involve either depletion, or partial or full pressure maintenance through gas or water injection. In order to reach a decision, it is essential to know not only the thermodynamics of the condensates but also the laws of their migration through the porous medium. This is achieved by a thermodynamic study, measurements of interfacial tensions between separate phases and by running a depletion experiment in bottom hole conditions. The interpretation of the set of measurements provides the necessary tools to run field scale simulations.
We present the study of the fluid behaviour of one of our newly discovered fields, the results of which were integrated in a full field study to define the scenarios of possible field production schemes.
The thermodynamic behaviour of the rich gas was represented by a Peng-Robinson equation of state involving seven pseudo components. The computed interfacial tensions between phases in equilibrium at different pressures ranged between .05 dyne/cm and 3 dyne/cm, they were very close to actual laboratory measurements. A depletion experiment was performed on a reconstituted 2 meters long core in bottom hole conditions (141°C, 400 bars initial pressure), the pressure was decreased very slowly, at less than 1 bar/day, and several stops were imposed to observe pure gravity drainage. Only a small fraction of the condensates was produced but the liquid breakthrough was observed very early during the depletion. The experiment was history matched with a compositional model, relative permeabilities for gravity drainage were deduced. It was necessary to introduce a dependence of the relative permeabilities on interfacial tensions below a threshold value of .15 dyne/cm.
