A new process has been developed for converting eastern shales to SNG and/or syncrude. Until now, eastern shales were considered as unsatisfactory feedstocks for synfuels production because of their relatively low yield of oil when analyzed by the conventional Fischer Assay procedure. However, a new hydroretorting process can achieve oil and gas yields nearly as high as those obtained from western shales. Conventional retorting can extract only about 35% of the organic carbon content of these shales, whereas 85% to 90 % can be extracted by the hydroretorting process.
This new hydroretorting process is based on experimental results obtained in laboratory and benchscale tests. The effects of operating variables on the rate and extent of organic matter conversion were determined in a laboratory thermobalance. These tests showed that using hydrogen partial pressures up to 500 psig at low shale-heating rates made possible organic carbon recoveries up to 2.5 times those achievable by conventional retorting,
Based on these results, bench-scale tests were conducted at shale feed rates of about 100 lb/hr. The results agreed well with those obtained in the thermobalance. In addition, the bench-scale tests showed that either syncrude or SNG could be produced by proper selection of operating conditions.
Oil shale research in the United States has been directed almost exclusively to developing the reserves in Colorado, Wyoming, and Utah, even though our eastern Devonian shale reserves exceed those in the West. Eastern shales are considered poor candidates for synfuel production because of their low Fischer Assay oil yields, typically in the 10-gal/ton range. However, the organic carbon contents of many of these shales approach those of western shales. Therefore, their low oil yields by conventional retorting, as indicated by Fischer Assay, are due to differences in the chemical nature of the kerogen. If high-pressure hydro is used to retort these shales, most of the organic carbon can be recovered and the oil yield is increased to a level comparable with that of western shale.
Experimental work has shown that hydroretorting can increase organic carbon recoveries by 250% compared with conventional retorting, from 35% recovery to 85% to 90% recovery. Thus, hydroretorting opens up a new energy source in the eastern United States, where our energy shortage is most critical.
This paper discusses the extent of Devonian shale reserves and the results of our experimental work with these shales. Commercial process designs based on this work are also presented.
During middle and late Devonian and earliest Mississippian time — 330 to 360 million years ago-much of the eastern U. S. was covered by a shallow inland marine sea, the Chattanooga Sea. The extent of the sea inferred from geological studies is shown in Figure 1. Its estimated maximum depth was a few hundred feet and several major rivers flowed into it from the mountainous eastern land mass. The prevailing stagnant conditions and luxuriant algae and prevailing stagnant conditions and luxuriant algae and kelp growth led to deposition of organic-rich black shales throughout the basin. Along the Eastern edge of the sea, the organic-rich material was substantially diluted with river-borne sediments (clastics) leading to a relatively low concentration of hydrocarbon-rich material in very thick rock formations.
Post-depositional erosion has removed the Devonian sediments from much of the area once covered by the Chattanooga Sea. The remaining deposit is shown in Figure 2. The U. S. Geological Survey estimates the total "known resources" of Devonian oil shale in the eastern United States at 400 billion barrels, and the "probable extensions of known resources" at an additional 2600 billion barrels, including both surface and deep deposits.