ABSTRACT

This paper is an introduction to the ongoing study of the elastic and mechanical properties of oil shales as a function of temperature and pressure. Many of these properties were previously studied and published mostly at room temperature. However, only few studies are available at higher temperatures. The exploitation of oil shales has been carried out in different ways; usually surface mining. However, the economical and environmental impacts of these old methods have hindered oil shales from competing with the conventional oils. The outcome of this work is intended to help characterize oil shale formations during in-situ retorting (heating), and provide relevant acoustic results for seismic monitoring processes.

INTRODUCTION

Oil shales consist of the host rock and organic material (kerogen). Oil Shales are shallow formations near the surface down to about 1 Km. The kerogen found in oil shales is not mature enough to be converted into oil. Under elevated temperature and pressure, oil shales can generate high quality oil through the conversion of kerogen. According to the Oil Shale Exploration Company (OSEC) [1], it is estimated globally that there are about 3 trillion barrels of oil in place. This number is comparable to the original world assets of conventional oil. Moreover, nearly two thirds of the oil shale reserves are found in the USA near the common borders of Wyominh, Utah, and Colorado. Oil Shales are shallow formations near the surface down to about 1 Km.

GEOLOGICAL BACKGROUND

According the USGS report [2] on the geology of the world oil shale resources, lacustrine sediments of the Green River Formation were deposited in two large lakes that occupied 65,000 km2 in several sedimentary basins in Colorado, Wyoming, and Utah during early through middle Eocene time. The Uinta Mountain uplift and the Axial Basin anticline separated these basins. The Green River lake system was in existence for more than 10 million years during a time of a warm temperate to subtropical climate. Fig. 2, from the same report, also shows a stratigraphic column from the Uinta and Green River formations in northwestern Colorado. Fig. 2 predicts the potential oil resources from the different oil shale strata according to their burial depths and stratigraphy.

EXTRACTION METHODOLOGIES

In the past, the common production technology of oil shale has been surface mining followed by processing in above-ground retorts. High temperatures, up to about 500o C are required for this technique in order to convert the kerogen to oil. Despite its simplicity, this approach requires costly surface facilities in addition to some of the environmental problems resulting from waste disposal.

An alternative process still in the development phase is the in situ conversion. This technique involves inserting heaters in wells to heat the oil shale reservoir slowly to about 350°C. At this temperature, kerogen starts to be converted to oil and gas. At pilot scale, Shell has successfully managed to extract good quality oils requiring no further upgrading (Kleinberg [3]). Nevertheless, the efficient use of heat with the ability to seismically monitor the performance of the reservoir, and map the areas of high temperature is a key to any oil shale in-situ process.

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