Summary: | To interpret electrical surveys used to monitor subsurface steam-injection projects,
one needs to know whether steam zones are resistive or conductive relative to initial
conditions. This can be determined through laboratory measurements of the effects of
steam injection on the electrical conductivity of sand. Experiments presented in chapter 2
measured the combined effects of the salinity of the boiler feedwaters and the steam quality
- the fraction of the injected flux that is vapour. The injection of low quality steam, boiled
from a saline solution, into clean sand saturated with the same solution, resulted in a net decrease
in conductivity, and a constant equilibrium conductivity in the steam zone. The injection
of high-quality steam, using the same solutions, resulted in conductivity dropping
first to a minimum, and then increasing to an equilibrium value similar to that seen in the
low quality injection. This localized conductivity minimum became progressively less
conductive with time, and travelled with the steam front. The appearance of the
conductivity minimum at the steam front can be attributed to the formation of a dilution
bank, which temporarily decreases the local salinity. This suggests that many steam
injections will create steam zones with electrically resistive fronts, which can be used to
track the steam.
The effects of clay on the electrical conductivity of steam zones is further
investigated in chapter 3. Experimental and numerical results indicate that clay-bearing
steam zones can be electrically conductive relative to initial conditions, in part due to water
saturations in the steam zones that are higher than those in comparably steam-flooded clean
sands. However, it is still likely that high quality steam injections will result in resistive
leading edges of the steam zones. In low quality steam injections, where dilution banks do
not form around the front, it is more likely that steam zones are entirely conductive relative
to initial conditions, particularly in fresh water environments.
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