Some Effects of Overburden Pressure on Oil Shale During Underground Retorting
Authors: Thomas G.W. 
Published: December 2007

Some effects of a simulated overburden pressure on oil shale during retorting are presented. The results are applicable to underground retorting in systems where there is no stress relief.

A core press was fabricated, using pyrex microbeads to transmit pressure to the oil shale samples during retorting. The range of pressures investigated was 25 to 2,500 psig at retorting temperatures up to 1,000 F. Effects of simulated overburden pressure on the degree of thermal fracturing and exfoliation, induced permeability and porosity, bulk volume changes, effective retorting temperature, carbonate decomposition, and thermal conductivity were investigated.

No visible fracturing or exfoliation was found to occur in oil shales retorted under confining stresses of 100 to 2,500 psig. Pore structure, however, is created by removal of oil and water, decomposition of carbonates and the creation of microscopic expansion cracks. Induced porosity is independent of overburden pressure, whereas in beds lying perpendicular to the maximum principal stress, the induced permeability is pressure-dependent. It was also observed that overburden pressure effectively lowers the temperature at which carbonates in oil shale begin to decompose. Thermal conductivities measured on raw and spent shale varied no more than 13 per cent under stress conditions in excess of 1,000 psig.

INTRODUCTION

The possibility of in situ retorting of oil shale is intriguing because mining, crushing and handling large volumes of shale is unnecessary. It is also unlikely that visbreaking of the recovered liquid product is required. Consequently, in situ extraction methods can be more economical than conventional surface retorting, especially where oil shale deposits occur at depth.

Oil shale has no primary permeability or porosity in the usual sense.1 Therefore, prior to underground retorting, inducing a flow path within the shale matrix must be considered. The production mechanism itself would involve heat application to the system to pyrolyze the organic matter to liquid and gaseous products.

A number of ideas have been suggested for in situ retorting of oil shale.2-4 There are two schools of thought for inducing permeability. One suggests the use of nuclear devices to create massive breakage. This would be followed by underground combustion within the broken rock. In support of this idea, Allred et al.5,6 have reported on laboratory combustion studies in crushed oil shale. It is certain that the technical feasibility of the nuclear approach must ultimately be established by a field test.

The other idea is to use hydraulic fracturing to establish interwell communication. A number of thermal processes can presumably be conducted within a single fracture or system of fractures. In 1953, Sinclair Oil & Gas Co. demonstrated the technical feasibility of forward combustion in fractured shale.7

In previous laboratory work, no attempt was made to simulate overburden pressure. This presentation is a report on laboratory studies that are pertinent to underground retorting of shale where there is virtually no stress relief. In such systems, each element of shale undergoing heating is subjected to a confining pressure which limits its ability to spall, crack or exfoliate. We think such a condition exists when retorting is attempted through a system of hydraulically created fractures. For deposits at depth, the principal stress is derived from the overburden. We therefore investigated the effects of a simulated overburden pressure during retorting on the degree of thermal fracturing, induced permeability and porosity, bulk volume changes, the effective retorting temperature, carbonate decomposition and shale thermal conductivity.