Retorting of Single Oil-Shale Blocks With Nitrogen and Air
Authors: Huang E.T.S. 
Published: December 2007

Experiments on retorting single oil-shale blocks were carried out to investigate the anomalous, last internal-beating phenomenon reported by the other investigators. Two 0.156-liter/kg (37.5-gal/ton) oil-shale polyhedral blocks (about 18 cm or 7.1 in. in diameter) with almost identical organic content and lithological Properties were heated at atmospheric pressure to 500 degrees C (932 degrees F) with a carrier-gas flow rate of 60 liters/hr (2.1 scf/hr). Carrier gas was air for one test and nitrogen for the other, while all the other experimental parameters were the same. The experimental results showed that the thermal behavior was the same for both runs and no anomalous, fast internal beating was observed. A third run with air at a higher flow rate (173 liters/hr or 6.1 scf/hr) also showed no sign of fast heating.

Production of oil and gas was essentially completed in 5 hours under the beating mode of this study. Oil-recovery efficiency ranged from 80 to 91 percent Fischer assay. About 1 hour after the percent Fischer assay. About 1 hour after the beating started, almost no oxygen was detected in the effluent. The oxygen penetration into the shale was 1.5 cm (0.6 in.) or less.

Introduction

Oil shale is an impervious rock composed of very fine mineral particles intimately combined with solid organic matter. Prerequisites for in-situ retorting are the creation of sufficient surface area and fluid permeability within the rock to allow heat and mass transfer and to provide paths for fluid flow. In in-situ retorting, the oil-shale formation can be fractured by explosives (chemical or nuclear) or other means. The cost of fracturing such a formation varies inversely with the size of fragmented shale. On the other hand, more efficient heating of the fragmented shale and, hence, a higher kerogen conversion efficiency, can be achieved if the size of the fragmented shale is smaller.

During retorting of 150-ton oil-shale experiments, Harak et al. observed that the thermocouple readings inside a 4-ton oil-shale block were higher than those on the block surface. Such an anomalous, fast internal-heating phenomenon (if it really happened) should be beneficial to an in-situ process because the requirement to reduce the process because the requirement to reduce the block size can be relaxed. However, it was found later that the fast internal heating was not real, but was the result of erroneous temperature readings resulting from chemical effects on the thermocouples.

When the work of Harak et al. was first published, several investigations were initiated in published, several investigations were initiated in an attempt to substantiate their findings. Doggett heated single oil-shale blocks (77 to 142 kg) using air or N2 as a carrier gas. The presence of oxygen greatly increased the rate of heating deep in the rock. However, no quantitative comparison between the runs with air and N2 was feasible because of differences in the oil-shale grade, rock size, rock geometry, and thermocouple locations. Tyler cut single oil-shale blocks into hollow, thick-walled cylinders and heated the internal surface with air or N2. The temperatures inside the oil shale for the run with air were reported to be higher than those with N2. However, temperatures at the internal cylindrical surface were not given in the paper to compare with the reported internal temperatures.

Mallon and Miller sealed pressure-sensing devices into oil-shale blocks (6.4 to 130 kg) that were heated. They observed periodic pulsing of the block internal pressure upon heating and concluded that the block internal temperature rose rapidly. Even though the work of the Laramie Energy Research Center did indicate that the phenomenon of fast temperature rise was not real, phenomenon of fast temperature rise was not real, the work of Mallon and Miller implied that the issue was not quite settled. Because of these conflicting reports, three carefully designed experiments were carried out in this report to investigate the so-called anomalous, fast internal-heating phenomenon.

Refs. 3 through 5 report only on thermal aspects of oil-shale retorting. Very little experimental study has been published on the product formation from pyrolysis of single oil-shale blocks and the extent of oxygen penetration into the spent shale. The information is vital to the understanding of the fundamental mechanisms of oil-shale pyrolysis and is useful in verifying mathematical models. Johnson et al., in presenting a mathematical model to describe retorting of an oil-shale sphere, released a portion of their experimental data.

SPEJ

P. 331