ABSTRACT

Triaxial creep behavior of oil shale is being investigated in the laboratory at simulated in-situ conditions. A range of temperature and stress conditions are chosen to represent those within the interchamber pillars of a modified in-situ retorting operation. Oil shale in grades ranging from 10 to 40 gallons per ton is being tested. Tests on low grade samples (<15 GPT) indicate that the nature of triaxial creep, compaction, and dilatation depend upon the initial porosity of the test specimen and the deviatoric stress during creep, and that creep strain rate is very sensitive to temperature.

INTRODUCTION

Creep is a significant aspect of the mechanical behavior of oil shale. This has been confirmed by actual measurements on pillars in experimental underground oil shale mines (Agapito, 1972), and laboratory tests on oil shale samples (Miller et al., 1979; Chu and Chang, 1980). It is, therefore, implied that time-dependent deformational behavior of oil shale will be a governing factor in all oil shale mine designs. In an in-situ, or modified in-situ, retorting operation for recovering oil from oil shale, time dependent deformations are more pronounced due to elevated temperatures. In a modified in-situ retorting operation, part of the oil shale initially is conventionally mined and the retort chambers are prepared by rubblization of shale in place. Creep behavior of oil shale in this case may significantly affect the stability of the interchamber pillars and ground subsidence as well as the functioning of the retorts themselves due to resulting permeability loss. This paper describes the first phase of a complete systematic investigation of the creep behavior of oil shale under temperature and stress conditions expected within an interchamber pillar. The investigation consists of performing a series of triaxial creep tests and constitutive modelling.

Interest in the mechanical properties and behavior of oil shale is quite recent. Sellars, et al. (1972) and Zambas, et al. (1972) seem to be the first to report a limited systematic investigation of time-dependent mechanical behavior in oil shale. They have given the results of long term uniaxial compression tests on oil shale samples from the Anvil Points, Colorado, site. Tests were conducted at a controlled temperature of 50°F with samples held under constant uniaxial load until fa4lure occurred or no deformation was measured during a period of five days. Maximum duration of tests was 1,000 hours. The main findings of these tests were that practically no creep occurred below a critical nominal stress of 2000 psi; very low initial creep strain rates occurred and rapidly reduced to zero (in only a few days) at stress levels between 2000 and 6000 psi (A logarithmic creep strain vs. time behavior was observed during this period.); creep rupture occurred in samples with oil content above 30 gallons per ton (GPT) at stress levels of 8000 psi (corresponding to approximately 80% of compressive strength).

Agapito and Page (1975) have described the long term vertical deformation behavior of actual oil shale pillars in the Colony Oil Shale Mine.

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