Gas Cycling and the Development of Miscibility in Condensate Reservoirs

Abstract

Summary This paper presents a detailed analysis of the development of miscibility during gas cycling in condensate displacements and the formation of condensate banks at the leading edge of the displacement front. Dispersion-free, analytical 1D calculations are presented for enhanced condensate recovery by gas injection. The analytical approach allows investigation of the possible formation of condensate banks (often at saturations that exceed the residual liquid saturation) and allows fast screening of optimal injection-gas compositions. We describe construction of the analytical solutions, a process that differs in some ways from related displacements for oil systems. All analytical solutions are verified by numerical calculations. We use an analysis of key equilibrium tie lines that are part of the displacement composition path to demonstrate that the mechanism controlling the development of miscibility in gas condensates may vary from first-contact miscible drives to pure vaporizing and combined vaporizing/condensing drives. Depending on the compositions of the condensate and the injected gas, multicontact miscibility can develop at or below the dewpoint pressure of the reservoir-fluid mixture. Finally, we discuss the possible impact on performance prediction of the formation of a mobile condensate bank at the displacement front in near-miscible gas-cycling/injection schemes. Introduction A significant portion of current hydrocarbon reserves exists in gas/condensate-carrying formations. In analog to oil reservoirs, production of condensate fields by pressure depletion only may result in significant loss of the heavy ends owing to liquid dropout below the dewpoint pressure. Gas-cycling/injection schemes are often applied to enhanced condensate recovery by vaporization. Successful design and implementation of enhanced condensate recovery schemes require accurate prediction of the compositional effects that control the local displacement efficiency. Many contributions to the development of the analytical theory of gas-injection processes can be found in the literature.1–13 The previously published research in this field has been focused on the understanding and construction of analytical solutions to problems of gas displacing oil as well as prediction of polymer/surfactant flood performance. In this work, we extend the analytical theory to include the important process of enhanced condensate recovery by gas injection. Numerical studies of miscibility variation in compositionally grading reservoirs by Hoier and Whitson14 demonstrated a significant potential for efficient gas cycling in condensate reservoirs below the dewpoint pressure owing to the development of miscibility by the combined condensing and vaporizing mechanism. In their study, rich separator gas was injected to obtain a miscible displacement at pressures far below the dewpoint pressure. With the emerging focus and efforts in the area of greenhouse gas capture and sequestration, CO2 may, in the near future, become widely available for enhanced-oil-recovery and enhanced-condensate-recovery projects. Seto et al.15 demonstrated, based on simulation studies, that CO2 can be used as an effective solvent in enhanced-condensate-recovery processes at pressures well below the dewpoint pressure of the initial condensate. In this paper, we focus on analyzing the development of miscibility during gas cycling in condensate reservoirs that, after primary production, leave significant amounts of retrograde condensate trapped in the formation. We start out by presenting the conservation equations that describe multicomponent two-phase flow in a porous medium, including volume change on mixing, and list the key assumptions made to apply the analytical solution strategy. We then describe, through analytical example calculations, the different mechanisms that control the development of miscibility in retrograde condensate reservoirs. p. 334–341