The Expansion Of Tar Sands During Thermal Recovery

Abstract

Abstract When tar sands are heated by steam injection there is a tendency for the reservoir liquids to expand; this increases the pore pressure. If the bitumen is more confined the effect is larger. Factors which promote the increase of pore pressure include high bitumen viscosity, low reservoir temperature, low permeability, and high steam injection pressure. The pore pressure can rise to the level needed to fracture the reservoir even though the steam injection pressure is below the fracture level. The effect of injecting steam into a permeable layer on the adjacent reservoir has been studied theoretically. It is assumed that heat flows only by thermal conduction and that the expansion of the bitumen is accommodated by flow of the bitumen and by expansion of the pore volume of the reservoir material. The water in the reservoir is assumed to be immobile. It is found that significant disruption and physical expansion of the reservoir matrix may occur. The predicted volumetric expansion is sufficient to lift the surface of the ground above the reservoir several centimeters. The mechanism which is described is useful in explaining the "surface " heave found in steam recovery pilots. It may be important in the recovery process because matrix expansion, particularly of low permeability streaks, improves the effective permeability. It may be possible to choose conditions which optimize the effect. Introduction This paper is concerned with the possibility that, during steam heating, the thermal expansion of bitumen within reservoirs can create pore pressures high enough to produce reservoir expansion, i.e. to move the sand grains apart. The reason that a mechanism such as this can be considered is that the volumetric coefficient of thermal expansion of the liquid pore fluids is far greater than that of the rock; these are compared in Figure 1. The uplifting of the surface above in-situ recovery pilots has been observed. A study reported by Texaco(1) indicated surface heaves of the order of six inches. It is thought that changes of this order are larger than could be accounted for simply by the thermal expansion of the reservoir rock. It is suggested that the mechanism described in this paper is probably responsible for much of the heave. The expansion of the reservoir matrix as a result of thermally induced pore pressures is of more interest than that due to the simple thermal expansion of the rock because it can also modify the recovery process. In particular, it can provide a mechanism for the permeability improvement of low permeability sections of the reservoir. Theoretical Approach As the bitumen becomes heated, its tendency to expand is balanced by three factors;Relief of pressure by flow;Compression of the bitumen; andExpansion of the pore space. The last of these factors can be looked upon as consisting of two parts: The elastic expansion of the pores as a result of an increase in pore pressure and, also the disruption of the pores which arises when the pore pressure exceeds the local geostatic stress.