Compositional Modeling and PVT Analysis of Pressure Maintenance Effect in Gas Condensate Field: Comparative Study
Authors: Zaitsev I.Y. , Dmitrievsky S.A. , Norvik H. , Yufin P.A. , Bolotnik D.N. , Sarkisov G.G. , Schepkina N.E. 
Published: December 2007

Development of gas-condensate fields is accompanied with condensate dropout. The storage of liquid condensate in the reservoir reduces the condensate recovery factor and well productivity. The highest liquid saturation is observed in the near-well zone. This causes decreasing gas relative permeability and a high pressure drop in the near-well zone. Consequently the productivity of well is reduced. The effect of different injection gases (CH4, CO2, N2, separator gas) to maintain reservoir pressure and decrease the hydrocarbon liquid saturation in the near-well zone was simulated in this study.

This cleaning of the near-well zone was simulated on the one-well radial simulation model. The model has a good conducting layer with low storage of gas. Consequently the gas flow to the well from the main part of reservoir (matrix) is through this layer. Such a model can describe the pressure and fluid properties distribution for a well producing from a fractured reservoir. Before the simulation of gas injection, long term production until the reservoir pressure falls below the dew point was simulated. After that, relatively short gas injections were applied. This is the basis for an evaluation of the effects of different injection gases in increasing the well productivity.

The simulations were performed using compositional models. It allows us to get the components distribution in both gas and liquid hydrocarbon phases which is essential for evaluating increased condensate recovery technologies in gas condensate fields.

The reservoir fluid composition was taken from a real Russian gas-condensate field. PVT modeling was performed using ECLIPSE PVT (Schlumberger Geoquest) and MRS PVT (ONICS, Russia). The EOS systems were corrected to match laboratory data before they were used in compositional simulation.

Introduction

The hydrocarbon condensate dropout in the near-well zone can be caused by several factors, but primarily the high pressure drop and the intense radial gas flow into the well. In highly heterogeneous and fractured reservoirs these factors are accompanied with the significant pressure drop between the good conducting and poor conducting layers (fractures and matrix systems in case of fractured reservoirs). This leads to significant changes of composition and increased liquid saturation as compared with those obtained from the CVD experiment (Constant Volume Depletion) or compositional fluid flow simulation studies.

Dynamic simulations show that the mentioned effects may leads to liquid saturations in the near-well zone above 40% (depending on the initial gas composition and relative permeabilities). Additionally, change of composition may alter an EOS system initially being gas into an oil system. The high liquid saturation in the near-well zone significantly reduces the relative gas permeability and the well productivity. Wells with low initial gas rates may have to be shut down soon after the condensate dropout. So the main objective is to remove the condensate from the near-well zone.

This study is focused on the numerical simulation of a well producing from a fractured reservoir with complex initial fluid composition (high content of non hydrocarbon components) and high initial pressure and temperature.

P. 449