Enhanced Gas-Condensate Recovery in Complex Reservoirs: Pilots and Models
Authors: Bedrikovetsky P. 
Published: December 2007

Abstract
Gas injection in gas-condensate reservoirs requires reservoir studies with cumbersome and complex simulations of flow and of phase behaviour for multiphase multicomponent fluids in heterogeneous formations. A number of fast predictive methods for enhanced/improved gas-condensate recovery are presented in the current paper. The methods are based on analytical models of phase behaviour and of reservoir flow. The model for phase behaviour in low permeable porous media allows to determine liquid saturation from gas samples. 1-D analytical models for 2-phase 3-component displacement allow to compare condensate recovery with injection of different gases and solvents. 2-D stream-line model allows to predict sweep efficiency in a system of wells. Upscaled model allows to compare different recovery technologies in reservoirs with layer cake and stochastic heterogeneity. The methods have been applied for prediction of reservoir and fluid behaviour during the pilot test on the displacement of retrograde condensate by LPG slug and by dry gas in field Vuktyl (Russia, Timan-Pechora region), for reservoir characterisation from this pilot test data, for design and planning of new pilot tests in the same field.

Introduction
During the pressure depletion of gas-condensate fields the condensate liquid precipitates in porous media. The saturation of the dropped down liquid usually does not exceed 10–15%, while liquids start moving in porous media when saturation reaches values of 20–30%, so the precipitated condensate is immobile and almost unrecoverable with the pressure depletion. Therefore, the recovery factor for condensate with blowdown of gas-condensate fields does not exceed 30–35%.
In order to increase condensate recovery, methods of pressure maintenance (injection of different gases and solvents) are applied. If the injection is applied after the pressure depletion and below the dew point pressure, the goals of the project are both pressure maintenance to avoid further precipitation of the condensate liquid, and also recovery of already precipitated condensate. For very low reservoir pressures the second goal of the recovery of so called retrograde condensate dominates.
In 1987-89 and in 1993-95 two pilot tests on the displacement of retrograde condensate by LPG slug and by dry gas were carried out in the Vuktyl field (Russia, Timan-Pechora region). Planning and design of the first pilot (choice of the injected gas composition, slug size, prediction of sweep and displacement efficiency) was performed by using analytical and semi-analytical reservoir and PVT models. After the first pilot the reservoir heterogeneity was determined from the production data using analytical solution for inverse problem. Based on these data, the planning of two new pilot tests was performed by the use of analytical models also.
In the current paper, the practical applications of these models are described. The model for condensate phase behaviour in low permeable porous media allows to determine liquid saturation in rocks from gas samples and to predict the condensate precipitation during the pressure depletion process. 1-D analytical model for displacement of condensate provides with the qualitative hydrodynamic schema of flows and with comparison of injection of different gases. 2-D stream line model allows to estimate sweep in system of wells. Upscaled 2-D model for displacement in heterogeneous reservoirs allows to estimate impact of heterogeneity on the condensate recovery, and also to calculate heterogeneity from the production data.