Well-Test Dynamics in Rich-Gas/Condensate Reservoirs Under Gas Injection

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 121848, ’Well-Test Dynamics in Rich-Gas/Condensate Reservoirs Under Gas Injection,’ by O.A. Aluko, SPE, and A.C. Gringarten, SPE, Imperial College London, prepared for the 2009 SPE Europec/EAGE Annual Conference and Exhibition, Amsterdam, 8-11 June. Well-test behavior of rich-gas/condensate reservoirs with pressure below the dewpoint pressure was studied to analyze revaporization of the condensate bank upon repressurization by gas injection. Compositional-simulation results suggested that the near-wellbore fluid saturation below the dewpoint pressure in a buildup is different from that at the end of the preceding drawdown because of significant differences in fluid properties and saturation distributions. A method was proposed to evaluate the bank storativity, which then is used to calculate the bank radius. Introduction Rich-gas/condensate reservoirs are characterized by a high liquid-hydrocarbon yield and often are developed for their liquid reserves. Compared to lean-gas condensates, rich-gas condensates have a higher percentage of intermediates and C7+ components, higher specific gravity, and lower gas/oil-ratio (GOR). Often, guidelines used to distinguish rich-gas condensate are an initial producing GOR of 3,300 to 5,000 scf/STB, heptanes-plus concentrations of approximately 12.5%, maximum liquid drop-out up to 35%, and an initial liquid yield greater than 100 STB/MMscf. Rich-gas/condensate reservoirs challenge fluid characterization and well-test analysis because of their near-critical nature. A unique problem in reservoir development is the result of condensate deposit at pressures below the dewpoint pressure and the creation of a condensate bank that decreases productivity and may result in a potentially significant loss of liquid reserves. Rich-gas/condensate reservoirs often are developed with secondary- or enhanced-oil-recovery methods to maximize liquid recovery. It is vital to monitor and understand the behavior of the two-phase condensate bank during production when assessing its effect on productivity and ultimate recovery. Gas injection into rich-gas/condensate reservoirs attempt to maintain the average reservoir pressure (and possibly the flowing bottomhole pressure) above the dewpoint pressure. Injecting gas below the dewpoint pressure can improve condensate recovery by displacing condensed liquids toward producing wells and vaporizing the intermediate and heavy components of the condensate. As detailed in the full-length paper, time-lapse well-test-pressure derivatives can be used to monitor changing fluid saturations in these reservoirs. Well-Test Dynamics Below the Dewpoint Pressure Compositional simulation was used to investigate and characterize reservoir-fluid dynamics and well-test behavior in rich-gas/condensate reservoirs. The thermodynamic properties of the fluids were represented by modified Peng-Robinson equation of state (EOS), with EOS parameters based on experimental data from the MTGc reservoir in north Africa. Velocity-dependent parameters matched to multirate-well-test data were included in the numerical modeling. The study focused on the effects of the following.