Measurement of Flow Properties and Interfacial Tensions for Gas Condensate Systems
Authors: Zhang H.R. , Bjørkvik B.J.A. , Moffatt B.J. 
Published: December 2007

The Karachaganak gas condensate field in Kazakstan is a one of the largest gas condensate fields in the world. The success of the field development by pressure depletion depends on the mobility of the gas and condensate below dewpoint. An experimental programme is designed to generate phase relative permeabilities and quantify critical condensate saturations at different flow velocities and interfacial tensions. Accurate determination of fluid properties and interfacial tensions at reservoir conditions is essential to the optimisation of the depletion strategy and is strongly associated with the quality of the experimental coreflood data analysis and hence the results of reservoir performance predictions.

This paper presents experimental measurements of pressure-volume-temperature properties of the gas condensate samples from a Karachaganak well. For the first time a relatively detailed comparative discussion is provided on usefulness of the three experimental methods for measurement of low interfacial tensions in gas condensates at reservoir conditions, viz. the drop-shape-based pendant and spinning drop methods, and a laser light scattering method. The laser light scattering technique appears to be the most suitable approach for the determination of low interfacial tension in terms of precision and operation. It has successfully been used to measure interfacial tensions for the Karachaganak gas condensate fluid at elevated reservoir pressures and temperatures.

Introduction

Gas condensate reservoirs are characterised by production of both surface gas and varying additional quantities of stock tank condensate oil. Typical condensate surface yields range from 10 to 300 STB/MMscf in terms of condensate gas ratio (CGR). Isothermal condensation of liquids in the reservoir as pressure drops below the dewpoint pressure, constitutes the process of retrograde condensation. The production of reservoir gas can be handled for the most part with traditional gas engineering tools.1 However, two extra issues arise in a condensate reservoir:how the produced CGR will vary with time; andhow the two-phase gas-oil flow near the wellbore impacts well deliverability.

Both these issues are strongly related to the pressure-volume-temperature (PVT) properties of the fluid system. PVT properties important to the engineering of all gas condensate reservoirs undergoing pressure depletion, include Z-factor, phase viscosities, phase densities, compositional variations with pressure, liquid dropout and dewpoint pressures etc. Accurate experimental determination of these properties is essential to the development or refinement of an appropriate PVT model for use in the reservoir performance simulations.

In principle, however, the well deliverability is more significantly affected by relative permeability effects. Under reservoir conditions the influence of interfacial tension (IFT) on critical/residual liquid saturation and relative permeability is by no means negligible, and much work has been done to investigate its influence on these properties.2–10