Tunnels In Oil Sand

Abstract

Abstract Deep oil sand deposits may require shafts and tunnels either as access for in-situ oil methods or for direct mining, Only one tunnel, Saline Creek tunnel, has been constructed in oil sand to date (1978). Recent research has made significant contributions to the understanding of oil sand behaviour, but empirical design methods may still have to be used until more experience is gained. Mathematical relationships show some promise in providing a framework for assessing stand-up time and the effects that in-situ recovery processes, such as steam drive, may have. Introduction There are several projects which are being considered for the feasibility or mining oil sand where the depth of overburden exceeds economical mining by open-pit methods (EMR 1977). In these methods shafts, tunnels and underground mining are proposed either to provide access for in-situ processes or for direct mining of oil sand as an ore, However, to date only one Jarge diameter tunnel has been constructed in the oil sands of Alberta and hence the underground behaviour of oil sand poses some uncertainties. The purpose of this paper is to assess tunnel construction in oil sand based on the experience gained from the Saline Creek tunnel (Smith et al. 1978), from observations of the behaviour of oil sand both in natural exposures and man-made excavations and from a review of information available in the literature. The characteristics of oil sand arc discussed, first within the framework of its geological selling and second as a material with unique engineering properties. Consideration is given to the effects of weak clay layers, water-bearing sands and other geological features such as cemented stringers, gas pockets and sink holes as they might affect tunnel construction. Background The construction of shafts and tunnels in Alberta's Athabasca and Cold Lake oil sand deposits has recently assumed practical significance because they may be necessary for access to oil sand under overburden depths too great for economical open-pit mining. A recent study (EMR, 1977) indicates that mining of oil sand as an ore may not be practical until world oil prices are higher than they are at present, but it seems likely that in-situ mining, using shafts and tunnels for access only is feasible. At-least one scheme the Rabbitt Kinetic In-Situ Method (Oilweek, 1978), requires shafts through the oil sand to the underlying Devonian limestone. Conventional design and the construction of shafts and tunnels in soil and in weak or strong rock is a skill largely based on practical experience obtained from previous work. Although oil sand is an oil-impregnated sandstone, observations from present open-pit mine operations have shown that the oil sand does not behave like most sandstones. The oil sand swells and experiences a significant loss of strength as it is exposed by excavation. Recent research at the University of Alberta has provided an understanding of the physical behaviour of oil sand under stress change, and has provided a framework for predicting behaviour.