Predicting In-Situ Physical Properties for Gas Condensates From Fluid Pressure Gradients

Abstract

New hybrid EOS-PVT models have been developed for estimating in-situ fluid properties for retrograde gas condensate fluids. The hybrid models are based on tuned equation of state (EOS) properties for a globally distributed set of quality-controlled (QC) pressure-volume-temperature (PVT) analyses of gas condensates in which the condensate-to-gas ratio (CGR) ranges from ~5 to 350 bbl/MMscf and for which in-situ fluid densities range from ~0.2 to 0.6 g/cm3. The hybrid EOS-PVT models are based on the observation that in-situ fluid densities derived from measured fluid pressure gradients (FPG) are highly correlated with EOS-derived in-situ fluid densities obtained from tuned EOS models and QC PVT data. Using fluid density values calculated using in-situ FPG data, it is now possible to estimate a variety of gas condensate fluid properties such as viscosity, mole fraction of methane (XCH4), density, acoustic velocity, and adiabatic fluid moduli, the latter being a critical input for Gassmann fluid substitution models. The utility of the new hybrid EOS-PVT models lies in their ability to facilitate rapid evaluation of stranded gas resources without having to take a fluid sample for laboratory PVT analysis. Properly executed FPG data also have great utility as a reliable QC for laboratory-measured PVT properties. We present hybrid EOS-PVT models for CGR, viscosity, in-situ fluid density, and acoustic velocity for a broad suite of gas condensates. A significant contribution of this work is the development of a new fluid acoustic properties model that enables estimation of the adiabatic fluid modulus (Kad) for retrograde gas condensates. Using predictive models reported here for fluid density [ρ] and velocity [Vp], the adiabatic fluid modulus is easily calculated. The model reported for fluid velocity was validated with direct measurements of fluid acoustic velocity on a live sample, at in-situ conditions, from the HP/HT Shearwater Field in the Central North Sea (CNS).