Experimental Study of Gas Slippage in Two-Phase Flow

Summary

Gas slippage in single-phase gas flow (the Klinkenberg effect) has been investigated extensively. Few papers, however, have been published on the gas slippage in gas/liquid two-phase flow. The gas relative permeabilities at water saturations close to the residual value have been found to be significantly greater than 1 in both nitrogen/water and steam/water flow through rocks. These values became less than 1 after the calculation was calibrated by taking the two-phase gas slip effect into consideration. The gas relative permeabilities have been measured at different mean pore pressures, and the values of two-phase gas slip factors have been computed at different water saturations. The effects of temperature on both nitrogen and steam slip factors also have been studied an.compared. These data have then been used to conduct a calibration to obtain intrinsic gas relative permeabilities that do not vary with the test pressures. It has been found from the present work that neglecting the two-phase gas slip effect may overestimate gas relative perme- abilities. It is the intrinsic gas relative permeabilities instead of those measured at low test pressures that should be used in numerical simulation or other reservoir-engineering calculations.

Introduction

Gas/liquid relative permeabilities are fundamental properties in reservoir engineering. There may be significant effects of gas slippage in gas/liquid two-phase flow. However, few experimental data regarding the gas slip effect in two-phase flow have been published. If the slip effect is not considered correctly, the gas relative permeabilities will vary with test pressures and may be greater than 1 at some water saturations. On the other hand, little attention has been paid to the measurements of steam slip factor, even in single-phase flow. Reliable data for steam flow are essential to the study of steam injection in heavy-oil reservoirs or water injection in geothermal reservoirs where steam is produced.

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The common problems that existed in the previous studies were the small size of the cores and the methods used to establish the liquid saturations. The end effect caused by the capillary discontinuity might have a significant effect on the distribution of the liquid saturation in short cores. Liquid saturations were established often by a slow evaporation process. Apparently, the distributions of the liquid saturations established with such a method would not be uniform.

Satik and Horne⁷ observed an unusual phenomenon in that the values of steam relative permeability (k_rg) at some water saturations appear to be greater than 1. A similar phenomenon was also found in experiments by Mahiya.⁸

Counsil⁹ suggested reducing the slip effect by running experiments at high pressures and temperatures. There are certain difficulties in running experiments at high pressures and temperatures for measuring steam/water properties [for example, when the X-ray computerized tomography (CT) method is used to measure the water saturation in a core (see Ambusso et al.¹⁰)]. On the other hand, increasing experimental temperature will increase the value of b significantly, as reported by Wei et al.¹¹

Herkelrath et al.¹² recognized the importance of gas slippage and incorporated it into a single-phase steam-flow model to model steam pressure-transient experimental data. In their study, the steam slip factor was substituted by the gas slip factor measured using nitrogen. There may be significant differences between steam and nitrogen slip factors, as we found in this study.