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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 degrees 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. Introduction When a field is put into production, primary depletion is the first mechanism to operate as the field pressure is decreased by fluid removal. For a gas condensate field, when pressure decreases below the dew-point value, a liquid phase condenses on the pore matrix and its concentration increases on a rather large span of decreasing pressure. Knowing the mobility of this condensate phase is extremely important in order to decide on the recovery scheme of the field. When the liquid condensate phase appears, the interfacial tension between the gaseous phase and the oleic phase is very low, in the order of several hundredth of a dyne. The interfacial tension steadily increases as the pressure is reduced. The effect of low interfacial tensions on the flow of fluids has been extensively documented. Many authors pointed out the influence of the capillary number on the trapped saturations for chemical systems; but, when the gas-oil systems are concerned, interfacial tensions rather than the capillary number seem to govern the trapping phenomena. P. 875^

The Industrial Microemulsion Pilot (I.M.P.) test is an application to a significant part of the Chateaurenard field of the enhanced recovery technics already used with success on a small scale during the first Method Pilot. The method involves the injection of a microemulsion slug displaced in the formation by viscous solutions of polymers. The I.M.P. has been initiated by Elf Aquitaine in 1983: the prime objectives were to assess the efficiency of the recovery mechanisms on a large pattern and evaluate the economic constraints of chemical flooding in an industrial environment. The test is implemented on a four adjacent five spot panel, with an enclosed pore volume of 224,000 m3 (1.4 × 10 bbl); the spacing between the production wells is 280 m on average. The chemicals injected consist of a microemulsion of petroleum sulfonates, alcohols, field oil and brine, displaced by solutions of partially hydrolyzed polyacrylamides diluted with field water. The amount of surfactant active matter has been reduced to 0.36 % of the exploitable pore volume, in comparison with 1.02 % on the Method Pilot. The exploitation started in June 1983; the microemulsion injection phase lasted 40 days. The injection of the concentrated polymer buffer amounting to 0.41 of the enclosed pore volume has been completed; it is now being followed by an equal volume of gradually more diluted solutions. A significant increase in oil production has been observed on 7 of the 9 producers of the pilot, with the arrivals of very well defined oil banks on these wells. Between June 16, 1983 and December 31, 1985, the total oil production rose to 35,100 m3 (220,700 bbl) of which 18,800 m3 (118,200 bbl) are attributed to chemical injection; at that time, the overall injection was 142,000 m3. The total cumulated production predicted by numerical simulations is 57,000 m3 (358,500 bbl), i.e. 52 % of the remaining oil in place at the start of the operation. From an economic analysis of the operations, and with the particular cost structure associated with the operation of this field, the cost of tertiary oil is about 40 US $/barrel for micellar recovery. A breakdown of the costs involved shows that the chemical products represent 41 % of the expenses and are a heavy burden to the project. Reducing the amount of chemicals would thus be of great benefit for the chemical process. Recent laboratory studies have shown this could be achieved in Chateaurenard conditions through the injection of an alkaline preflush, ahead of the micellar slug.

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