Accurate relative permeability data are essential for predicting the performance of two-phase flow through porous media. Many factors, such as the rock and fluid properties, may affect the measurement of relative permeability. However, the saturation levels of the fluids flowing through a porous medium have the most impact on the shape of the relative permeability curves. Because relative permeability is a strong function of saturation, an accurate measurement of saturation levels in various types of two-phase flow experiments is required. Weighing and volumetric methods are frequently used to estimate the average saturation during steady-state experiments. However, for unsteady-state flow experiments, material balance methods to determine the saturation levels are relatively difficult to use. This article presents a relatively new non-invasive saturation measurement method and the equipment used to obtain dynamic saturation profiles as a function of time and distance along the core-holder. The new saturation measurement system has been found to be equally good for steady-state and unsteady-state experiments. Typical dynamic saturation profiles, the equipment calibration method, and a set of typical relative permeability curves for a co-current flow experiment are presented. Based on the presented experimental results, it has been found that the new saturation measurement method and the equipment is reliable and can reproduce stable dynamic saturation profiles with a minimum level of uncertainty.
Underlying the extension of the single-phase flow theory to the simultaneous flow of two or more fluids are the concepts of effective and relative permeability. The effective permeability is a relative measure of the conductance of a porous medium for one fluid phase when the medium is saturated with more than one fluid(1). The relative permeability is defined as the ratio of the effective permeability of a phase to a base permeability (e.g., absolute permeability to air or water).
Relative permeability data are essential for almost all two-phase flow studies related to reservoirs. The data are used in making estimations and predictions of the productivity, injectivity and ultimate recovery from reservoirs for evaluation and future development plans. The relative permeability data can also be used to diagnose formation damage expected under various operational conditions. Therefore, unquestionably, these data are one of the most important data sets required in reservoir engineering studies.
Among several methods of obtaining relative permeability curves, laboratory techniques are considered to be the most reliable. These methods of relative permeability measurement are further classified into steady-state unsteady-state methods. Aleman et al.(2) have concluded that the difference in the relative permeabilities obtained by the two approaches is negligible, provided that the magnitude of the local (not macroscopic) capillary number is larger than a limiting value. Bentsen(3), however, disagrees with this conclusion and has demonstrated a considerable difference between the relative permeabilities obtained by the two approaches.
Numerous studies have been conducted to investigate the effect of important parameters during the measurement of relative permeability data. In addition to saturation, the other important parameters affecting relative permeability are wettability, IFT, density, capillarity, viscosity and viscosity ratio, rock properties, flow rates, flow regimes, saturation history, overburden pressure and temperature.