Mobility of Hydrocarbon Liquids in Gas Condensate Reservoirs: Interpretation of Depletion Laboratory Experiments

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 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^

Critical Condensate Saturation in Porous Media

“…If the decrease in permeability in a set of rocks is due to the decrease in Z, then the critical condensate saturation will increase with the decrease in permeability. This trend is consistent with many experimental observations (4,7,9).…”

“…The critical liquid saturation has been reported to be between 0 to 20% at high permeabilities (ϳ1 Darcy) and up to 70% at low permeabilities (ϳ0.001 Darcy). The critical liquid saturation reported for vertical orientations is often lower than those in horizontal orientations for high permeability cores (4,7,9). The difference is smaller between vertical and horizontal cores at low permeability (e.g., ϳ2% for a 23-md rock, ϳ24% for a 890-md rock).…”