Mechanisms of Capillary Displacement of Residual Oil by Gravity-Assisted Inert Gas Injection

Abstract

Abstract Very high oil recoveries are achieved when an inert gas displaces continuous oil or residual oil downwards in water wet columns of glass beads and in sandstone cores, provided that a semi permeable membrane is used at the production end to prevent gas breakthrough. In this paper, pore level mechanisms that control this displacement process are elucidated, as revealed by experiments in 2-dimensional pore network micromodels and unconsolidated columns of glass beads. Visualizations of the displacement mechanisms are presented and their effect on oil production is discussed. By applying principles of capillarity and by using well defined pore geometries, equations have been developed for predicting both water and oil distributions at the pore level. Introduction Recent laboratory studies have shown that gravity assisted inert gas injection has the potential to become an efficient method of oil recovery. Kantzas et. al demonstrated the potential capabilities of such a method by conducting experiments in unconsolidated cores of various sizes. In their experiments, all core samples were saturated first with brine which was in turn displaced by oil to the residual wetting phase saturation condition. The cores were positioned vertically and air or nitrogen was injected at the top of the column to displace the oil downwards. At the end of the displacement process, 99% of the oil was recovered in all unconsolidated columns while the recovery in consolidated columns was 80% or better. Other experiments were conducted beginning with h cores at waterflood residual oil conditions. At nitrogen were injected at the top and displaced vertically downwards both water and oil. The efficiency of oil recovery was 70% to 94% for the unconsolidated media and 55% to 85% for the consolidated media. The experiments in unconsolidated media provided some initial visualizations of the mechanisms of displacement. A fairly uniform displacement front was developed for the case of displacing continuous oil. In the case of displacing discontinuous oil, two different regimes were identified. One regime was that of controlled drainage conditions where air advanced at very slow flow rates and an oil bank was formed between the water bank and the gas bank zones. Another regime was that of free drainage conditions where gas advanced at high flow rates towards the production end and bypassed the residual oil blobs. Under these conditions lower recovery efficiency was observed. The aforementioned experiments have shown that the isolated blobs can be reconnected and form a continuum which can be efficiently displaced. Leakage mechanisms have been shown to control the displacement of the wetting phase and very low residual wetting phase saturation can be achieved at high capillary pressures. Similar mechanisms can affect the displacement of residual oil in waterwet media when inert gas is the displacing phase. In order to investigate the different types of displacement mechanisms at the pore level, a series of micromodel experiments were conducted. In this paper visualizations of three phase immiscible displacements and theoretical considerations of fluid distributions at the pore level are presented. EXPERIMENTAL a. Experiments in 2-D Micromodels Transparent to flow 2-dimensional micromodels of various pore structure geometries have been fabricated and used by a number of investigators using photofabrication and etching techniques (for details the reader is referred to McKellar and Wardlaw and Chatzis et. al). Using this technique, pore network models were designed on glass plates which were then fused in a muffle furnace. P. 297^