Abstract

The concentrations of dissolved silica and Na/K in the produced water from geothermal reservoirs are commonly used as reservoir temperature indicators, and are known as geothermometers. The concentrations of dissolved silica and Na/K in the produced water from cyclic steam-assisted oil recovery in Alberta oil sands reservoirs vary in a regular fashion, and may also reflect temperature changes in the reservoir. Thus the potential exists in thermal oil recovery operations to use the produced water chemistry to monitor process temperatures in the near wellbore region if the water compositions are properly calibrated.

To test this concept, a series of calibration autoclave experiments was performed using extracted oil sands from the Clearwater Formation, Cold Lake area. The dissolved silica concentrations from the oil sands experiments increased with time until a steady state was reached, generating a time-dissolved silica profile similar to that expected for pure quartz. Each of these steady state concentrations is used as a calibration point for an oil sands geothermometer. Such steady state concentrations will be reached in approximately one day when extrapolated to reservoir conditions. Based on these experiments, the silica geothermometer for Clearwater Formation reservoirs can be expressed as: Temperature(°C)=1201/(5.01 - log[Sio₂]) - 273

In addition, historical data collected in one-month autoclave runs on Cold Lake (Clearwater Formation), Athabasca (McMurray Formation), Wabasca (Grand Rapids Formation and Wabiskaw Member) and Peace River (Gething Formation) oil sands at 200°C and 250°C, plus historical data from corefloods between 100°C and 265°C were used to calibrate geothermometers for reservoirs in these areas. All of these oil sands data lie between temperatures defined by quartz and chalcedony geothermometers, regardless of sand mineralogy.

No useful correlation was found for the measured Na/K concentrations as a function of temperature in these experiments. Na/K ratios in aqueous fluids are controlled by complex reactions whereas the silica concentrations seem to behave relatively simply with respect to temperature. Consequently, we recommend use of the silica geothermometer to monitor temperatures in the reservoir near the wellbore in steam-assisted thermal recovery. Downhole thermocouple measurements can be supplemented by simply monitoring produced dissolved silica concentrations.