Oil Displacement by Water in Vuggy Fractured Porous Media

Abstract

Abstract In this paper experimental results obtained for the oil displacement by water within a two-dimensional vuggy fractured porous cell are presented. The experiments were carried out with an acrylic cell, where fractures were represented by channels of 1.6 mm wide, 1.2 mm deep and 19 cm long, whereas the vugs were represented by cylinders of 1 cm diameter and 1.2 mm deep. The experimental procedure is as follows: the cell, saturated 100% with oil was placed horizontally and at time t=0 water was injected at a constant rate. Photographs were taken to follow the water front in the cell; then, the area corresponding to oil and water was measured. In this way the water saturation in the cell was determined. This water saturation is equal to the oil recovery, considering oil and water as incompressible fluids. Based on the experimental results, a theoretical model was developed. This model considers that the porous space is constituted by two regions: one that can conduct fluids, called flow region and a stagnant region that interchange matter with the flow region through a diffusion-like process. Analytical solutions for water saturation and normalized oil recovery are presented. The model fitted in a reasonable way the experimental data of normalized oil recovery, indicating that the theoretical model can be a useful tool to explain oil recovery processes in vuggy fractured reservoirs. Introduction The natural rocky formations are heterogeneous as a result of the deposits, the tectonic and the environmental effects through millions of years. In terms of the properties of flow, these heterogeneities are manifested in a variety of ways, one of these ways is constituted by fractured formations, with flow paths of high and low permeability. The fractured reservoirs are constituted by two porous systems:intergranular porosity, formed by the empty spaces among the rock grains, andporosity formed by the empty space of the fractures. If the porous system also has small dissolution caverns, there is an additional porosity, the vuggy porosity. The flow of fluids through fractured porous media has had interest in the last years due to the importance that this process has in diverse areas of engineering, in particular oil engineering. The most important reservoirs in the world are naturally fractured. In the case of Mexico, at the present time the main oil areas are in the Gulf of Mexico and in the states of Campeche, Chiapas and Tabasco, from where approximately 90% of the oil production is obtained. There, the reservoirs are located in carbonated rocks with a high amount of fractures. For this reason, it is required a wide knowledge of the mechanisms that are present in the oil displacement by different fluids with the purpose of being able to plan in an adequate way the projects of secondary and enhanced hydrocarbons recovery. The displacement of fluids through fractured porous media is a little studied research area, even thought the naturally fractured reservoir are among the most productive in the world, such as those that are present in Algeria, Iraq, Iran, the North Sea and Mexico. During the productive life of a reservoir, it has three stages. In the stage of primary production, the energy of the fluids and rock in the reservoir expels the fluids to the surface. This stage finishes when the natural mechanisms do not work efficiently. The statistics point out that, on the average, the oil recovery in primary production is about 30% of the original oil in the reservoir. Then the stage of secondary recovery comes. The method of secondary recovery commonly used consists on transforming some producing wells into injectors, or to drill additional wells in appropriate sites. Through these wells, water is injected to the reservoir and water displaces the hydrocarbons towards the producers. By this method, it is obtained about 15% of additional hydrocarbons. As it is observed, after the first two stages it is still 55% of the original oil in the reservoir.