Determination of Relative Permeability and Recovery for North Sea Gas Condensate Reservoirs

Abstract

SPE Members Abstract Laboratory experiments on gas condensate flow behavior were conducted under reservoir conditions. Two North Sea gas condensate reservoirs that have distinct rock and fluid properties were studied. The objectives of the corefloods were to investigate the effects of rock and fluid characteristics on critical condensate saturation (CCS), gas and condensate relative permeabilities during in-situ condensation, hydrocarbon recovery and trapping by water injection, and incremental hydrocarbon recovery by subsequent blowdown. It was found that both CCS and relative permeability were sensitive to flow rate and interfacial tension. The results on gas relative permeability rate sensitivity suggest that gas productivity curtailed by condensate drop out can be somewhat restored by increasing production rate. Phase behavior and interfacial tension influence the extents of gas relative permeability reduction and condensate mobility. High interfacial tension ultimately caused condensate relative permeability to decrease with increasing condensate saturation. Condensate immobile under gas injection could be recovered by water injection, but more immediate and efficient condensate recovery was observed when the condensate saturation prior to water injection exceeded the CCS. Approximately 27 %PV gas was trapped by water injection. Subsequent blowdown recovered additional gas, but incremental condensate recovery was insignificant. Introduction Reservoirs bearing gas condensates are becoming more commonplace as developments are encountering greater depths, higher pressures, and higher temperatures. In the North Sea, gas condensate reservoirs comprise a significant portion of the total hydrocarbon reserves. Accuracy in engineering computations for gas condensate systems (e.g., estimating reserves, sizing surface facilities, and predicting productivity trends) depends upon a basic understanding of phase and flow behavior interrelationships. For example, gas productivity may be curtailed as condensate accumulates by pressure depletion below the dew point pressure (Pd). Conceptual modeling on gas condensate systems suggests that relative permeability (kr) curves govern the magnitude of gas productivity loss. Unfortunately, available gas and condensate relative permeability (krg and krc) results for gas condensates are primarily limited to synthetic systems. Such results show that higher CCS and less krg reduction were observed for a conventional gas/oil system compared to a gas condensate system. If condensate accumulates as a continuous film due to low interfacial tension (IFT), then high IFT gas/oil and water/oil kr data may not be applicable to gas condensates. Water invasion of gas condensate reservoirs may enhance hydrocarbon recovery or trap potential reserves. Laboratory results suggest water invasion of low IFT gas condensates may not be represented using high IFT water/oil and water/gas displacements. Subsequent blowdown may remobilize hydrocarbons trapped by water invasion. The presence of condensate may hinder gas remobilization, thus conventional gas/water blowdown experiments may not be appropriate in evaluating the feasibility of depressurization for gas condensates. Other laboratory evaluations of gas condensate flow behavior indicate measured results depend upon experimental procedures, fluid properties, and rock properties. Factors to consider include the history of condensate formation (i.e., imbibition or drainage), how condensate was introduced (i.e., in-situ drop out versus external injection or in-flowing gas), flow rate, differential pressure, system pressure, IFT, connate water saturation, core permeability, and core orientation. Experiments performed to evaluate the consequences of water invasion suggest optimum conditions depend upon IFT, initial gas saturation, and core permeability. P. 699

Relative Permeability Estimation for Rich Gas-Condensate Reservoirs

“…This approach to modeling multiphase flow through porous media has been validated by numerous investigators through coreflood experiments (Chen et al, 1995;Henderson et al, 1995). For gas condensate fluids it has experimentally been shown that the mobility of both the gas and condensate phases increases with increasing capillary numbers.…”

“…Investigators have experimentally demonstrated an improvement in both gas and condensate relative permeability at high velocities (Chen et al, 1995;Henderson et al, 1995). A greater enhancement was observed in the gas relative permeability as compared to the condensate relative permeability.…”

Gas Condensate Reservoir Performance

“…It shows a critical condensate saturation of about 19% at which point the gas permeability had decreased to 17% of its original value. This Scc was considered high, but some porous media has been reported with Scc values well in excess of this (1) . Once the Scc had been attained, the stack was then flooded with equilibrium condensate at 3,000 psi and the displacement history-matched with a two-phase compressible relative permeability model.…”

“…Capillary pressure causes phase interference which usually reduces gas productivity. A cross-section of interesting topics that show the complexities of gas-condensate reservoir production have been reported in the literature (1)(2)(3)(4)(5)(6)(7) . All of the relevant parameters, if well understood, will lead to more accurate evaluation of the amount of hydrocarbon in place, the rate at which the resource can be produced and the optimization strategies as the reservoir matures.…”

Measurement and Modelling of Gas Condensate Flow Around Rock Perforation

“…The improvement of relative permeability of condensing systems at low interfacial tension (IFT) due to an increase in velocity, referred to as the "positive coupling effect", was first reported by Danesh et al (1994). Since then many investigators have worked on this flow behaviour both experimentally (e.g., Henderson et al, 1996;Chen et al, 1997) and theoretically (Jamiolahmady et al, 2000(Jamiolahmady et al, and 2005. The coupling effect has also been shown to be in strong competition with that of inertia, which tends to reduce relative permeability at very high velocities (Henderson et al, 2001).…”