Abstract

Most of the oil sand deposits contain shale barriers of various sizes and with a space distribution difficult to appraise. These shale barriers can lower the reserves and reduce the efficiency of the SAGD process by slowing down steam chamber propagation. The detrimental effect of shale barriers on the SAGD process has been demonstrated by many simulation works and research projects have been launched to develop new technologies favoring breakdown of shale barriers.

The effects of high temperatures on shale are difficult to assess. Field data from SAGD projects show that steam might either bypass shale barriers or breakthrough them, but the conditions favoring these mechanisms are unclear. Lab data show that the constitutive mechanical behavior of clayey materials is strongly sensitive to high temperatures. Of particular importance is the phenomenon of thermal consolidation of clay which was never pointed out to explain the conditions of failure of shale barriers during SAGD. Thermal consolidation denotes an irreversible contraction of clayey materials when heated under drained condition.

This paper presents the results of a project that aimed at evaluating if thermal consolidation could help increase permeability of shale barriers. A comprehensive review of published data shows that the origin of this phenomenon is still poorly understood despite some important factors can be highlighted. It also shows that stress path applied during thermal expansion tests on shale specimens strongly control the thermal consolidation phenomenon, which can lead to erroneous interpretations of these tests. It proposes a very simple model that is used to show 1) under which conditions thermal consolidation of shale barriers might happen; and 2) the consequences of thermal consolidation in terms of permeability changes of shale.

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