Development of Gas-Saturation During Solution-Gas Drive in an Oil Layer Below a Gas Cap
Authors: Dumore J.M. 
Published: December 2007

Dumore, J.M., Koninklijke/Shell Exploratie En Produktie Laboratorium Rijswijk, The Netherlands

Solution-gas drive is usually described in the literature as a parallel flow of oil and gas, with the gravitational factor being neglected. Under the influence of gravity, however, gas evolving from the oil will migrate upward. The gas then accumulates at the top of the formation where, if not originally present, a gas cap is formed. The gas flows to the producing wells via the gas cap. For an analysis of the drive mechanisms in many oil reservoirs, it is essential to know the magnitude of the gas saturation that develops during solution-gas drive in the oil layer below the gas cap. However, the way in which the gas saturation develops and the parameters on which this development depends are unknown. A study was begun and the results are presented here. Experiments were conducted in packs of glass grains, saturated with a liquid that produced transparence. Gas was injected slowly in the packs via a pore near the bottom. Tests show that packs via a pore near the bottom. Tests show that the upward migration of the gas depends on two conditions related to capillary pressure. At low capillary pressures a conically shaped, gas-saturated region develops, through which the gas is transported upward; whereas at high capillary pressures only one gas channel develops. These pressures only one gas channel develops. These two conditions are called dispersion and nondispersion. Both may occur in oil reservoirs. Under dispersion conditions, the process of solution-gas drive in the oil layer develops in such a way that the entire layer is eventually occupied by disconnected agglomerations of gas bubbles. This results in a-high gas saturation (approximately 20 percent). Under nondispersion conditions, a network of gas channels develops. The lower the pressure-decline rate, the larger the network spacing. pressure-decline rate, the larger the network spacing. Gas saturations of less than 2 percent are often formed.

Introduction

The solution-gas drive process is usually described in the literature on the basis of the following assumptions:during the process a free-gas saturation develops, such that the gas and oil phases remain in equilibrium;the phases to not segregate by gravity; andrelative permeabilities to gas and oil are unique functions permeabilities to gas and oil are unique functions of the saturations.

As a consequence of these assumptions, theoretically, the oil production, hence, the mean-gas saturation developed in the reservoir are related uniquely to the reservoir pressure, and are thus independent of the rate of pressure decline. In reality, gas bubbles are created when a certain critical supersaturation has been reached. As pressure continues to decline, these bubbles grow pressure continues to decline, these bubbles grow and, under the influence of gravity, upward migration of free gas will occur. The gas then accumulates at the top of the formation where, if not originally present, a gas cap is formed. The free gas flows to present, a gas cap is formed. The free gas flows to the producing wells via the gas cap. In order to analyze the drive mechanisms active in many oil reservoirs, we need to know the magnitude of the gas saturation that develops during solution-gas drive in the oil layer below the gas cap. However, the way in which the gas saturation develops and the parameters on which its development depends are unknown. Laboratory experiments have been conducted to investigate this development and to determine whether it is influenced by the rate of pressure decline.

UPWARD MIGRATION OF GAS

The upward migration of gas was observed in experiments carried out in transparent models. Each lucite model contained a pack of crushed pyrex glass of a narrow sieve fraction. The pack was saturated with a kerosene-Novasol* mixture.

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