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Fassihi, Mohammad Reza*, ARCO Oil and Gas Co.; Abu-Khamsin, Sidqi*, and Brigham, William E.*, Stanford University Pet. Res. Inst., Pet. Res. Inst., Williams, Loretta A., and Graham, Steve A.*, Applied Earth Science, Stanford University *Members SPE-AIME
Abstract Diatomaceous sediments of California host large reserves of oil and gas tut are incompletely exploited. The matrix of these sediments is comprised largely of the frustules of diatoms (microscopic marine plants). Punctae in the frustules are responsible for the high porosities Punctae in the frustules are responsible for the high porosities characteristic of diatomites (up to 70 vol.%), but because of the very small size of the pores, permeabilities are low (commonly around 1 md). Furthermore, the oil contained in the reservoirs is often either immature or heavily biodegraded, hence viscous.
In order to test the feasibility of applying in-situ combustion techniques to diatomaceous reservoirs, a laboratory test was conducted for a section fo core taken in the south plunge of the anticline in the Lest Hills field of the San Joaquin Valley in California.
During the experiment, a fast stream plateau, good oxygen utilization, and an unusual front were observed. Fingering caused formation of a second front downstream. This finger stabilized and later became very hot (1600F). Velocity of front movement through the core almost doubled after the two fronts joined. API gravity of the oil extracted from the core ranged between 28 and 45, compared to the original value of 28 API.
Tests were conducted to compare the cores before and after combustion, using-scanning microscopy, powder x-ray diffraction, and extraction techniques. After burning, the sediments changed from dark brown to red in color as a result of oxidation of organic and iron phases. Small numbers of diatom frustules were transformed from amorphuos opol to quartz, with accompanying occlusion of Tore spaces. With the exception of the color change, however, the sediments remained largely unaltered.
It is especially interesting to note that the kerogen within the sediments appears to supply a portion of the fuel required for combustion. Kerogen is detrital organic matter than can be burned but cannot be extracted easily from a core in the laboratory. Kerogen's presence could make in-situ combustion more economical and efficient than other EOR methods in diatomites.
Introduction Diatomaceous sediments are composed mainly of diatom frustules of microscopic algae which comprise a large part of the marine planktonic flora. Diatom accumulations in a reducing environment are potential oil source rocks. Because diatomites have high porosities, they may also be reservoir rocks. Indeed, large diatomite oil reservoirs occur in California in the oil producing San Joaquin Valley and the coastal borderland.
Diatoms are very fine-grained sediments ranging in diameter from tens to hundreds of microns, with a high porosity largely formed by microscopic punctae of individual frustules. The geometry of this microporosity punctae of individual frustules. The geometry of this microporosity results in excellent bulk porosities (as high as 70%) and poor permeabilities (as low as 1 md or less). permeabilities (as low as 1 md or less).In the Cenozoic, diatoms were an important component of earth's biota. During the Tertiary, active wrench tectonism altered the major organic-siliceous deposits in California. In the San Joaquin Valley, diatomite ultimately is preserved on the crests of structural features. Oil found in these shallow sediments is either immature or highly biodegraded and, hence, very viscous.
It was not until the recent oil price increase that producing oil from diatomites became profitable. As a result, increased recovery efforts were undertake in areas of California where oil-saturated diatomites and cherts (the diagenetic end product of these microfloral accumulations) occur.
Diatomite reservoirs are often highly fractured, significantly increasing permeabilities and improving recovery rates. Still, primary production from these reservoirs amounts to only about 5% of original oil production from these reservoirs amounts to only about 5% of original oil in place. Consequently, enhanced recovery methods must be used to make the venture economical.
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