Effect of Low Permeable Porous Media on Behavior of Gas Condensates
Authors: Shapiro A.A. , Potsch K. , Kristensen J.G. , Stenby E.H. 
Published: December 2007

The aim of the present work is to compare the liquid drop out and the composition of the well stream during the constant volume depletion (CVD) experiments performed in the absence and in the presence of the porous medium. This is important in order to find out, to what extent common PVT experiments may be used for prediction of the behavior of a gas-condensate mixture under development of a petroleum reservoir. For the depletion experiments artificial cores (slim tubes) have been set up. Large discrepancies between the condensate dropout curves in the presence and in the absence of the porous media have been encountered. It has been analyzed whether these discrepancies may be explained by the action of the capillary forces. Algorithms have been created for simulation of the CVD experiments under the effect of the capillary forces. Calculations show that this effect is not sufficient to explain the discrepancies observed.

Introduction

Gas-condensate reservoirs make up nowadays a significant portion of the portfolio of an oil company. All the data relevant to the production scenarios of such reservoirs should be known ahead. Among them, the parameters like dew point, liquid drop out, well stream composition, are determined experimentally in the laboratory.

The study of the thermodynamic behavior of a petroleum mixture is performed in a laboratory PVT-cell. A general assumption is made that the depletion behavior of the mixture in a PVT-cell is the same in a petroleum reservoir. Meanwhile, this behavior may differ due to the effect of the porous medium.

The difference in the mixture behavior in the porous medium and in the free space is usually explained by the action of capillary forces. A non-zero curvature of the gas-liquid interface in a porous medium results in the capillary pressure difference between phases, which, in turn, shifts other thermodynamic properties, like phase compositions and the dew point pressure. The literature data about the actual significance of capillarity are somehow contradictory [1–7].

Another possible explanation of the distinctions between the mixture behavior in a porous medium and in free space is the presence of the nonequilibrium effects. The porous medium causes a chromatographic effect on the mixture, and different components may be produced with different rates. A PVT cell depletion experiment is "static" one. At each pressure step the condensate system is allowed to reach thermodynamic equilibrium. On the other hand, a depletion process from a porous medium - realized in the laboratory by artificial sand packs - is a "dynamic" one, and the condensate system is in the best case in quasi-equilibrium. Under reservoir conditions, both types of behavior may be found, an equilibrium area far away from the well bore and a non-equilibrium zone close to it.

In order to clarify the effect of the porous media on the depletion process, we have performed a comparative experimental study of the CVD processes in a PVT cell and in porous media. Artificial low permeable sand packs with the properties resembling those of Austrian gas-condensate reservoirs have been used in the experiments. The depletion experiments have been carried out with the condensates of different richness, different permeabilities and production rates, with and without connate water saturations. The reproducibility of the depletion experiments was also tested.