Chemical and isotopic examination of produced waters from the BP-Wolf lake in situ combustion pilot

The chemical and isotopic compositions of co-produced waters can be used to monitor the processes that take place during in situ combustion. Anticipated processes include mixing of waters, production of CO2, production of high concentrations of dissolved sulphate and variations in water chemistry associated with heated zones. Water sources include pore waters in oil-bearing strata, waters in overlying or underlying aquifers, water condensed from previously injected steam, and waters associated with combustion. Waters from all sources may mix during production and interpretation of the combustion process can be refined by an understanding of water sources. Produced fluids from the BP-Wolf Lake pilot site in Alberta have been examined to evaluate the effectiveness of the chemical composition of water and the isotopic compositions of aqueous species for monitoring in situ combustion. Produced waters do not show simple conservative mixing behaviour. This suggests that multiple sources of water and other processes, including water-rock reactions, act to modify water compositions. At least three sources of produced waters can be recognized and these are interpreted to be formation water, injected steam and waters that have low Cl and high HCO3 due to combustion. It is not possible to distinguish waters in the oil-bearing formation from regional waters present in aquifers that underlie the stimulated intervals. Dissolved aqueous species, such as SiO2, Na, K (as Na/K) and Cl can be used to monitor the approach of the combustion front. Sulphate has been suggested as an indicator of approaching combustion and, although sulphate concentrations rise as combustion approaches a producing well, this indicator is not reliable in all cases. The use of all the above chemical parameters is recommended for detection of combustion zones during operation. The isotope composition of produced waters confirms that there has been significant water-rock interaction during combustion. Carbon isotope compositions of HCO3 that range from −8 to −25% δ13C show that oil oxidation is a major contributor of CO2 at high temperatures, but CO2 produced by carbonate mineral dissolution becomes more significant as temperature decreases. Sulphate concentrations in waters produced during combustion can be an order of magnitude higher than those observed during steam stimulation. Both the oil (bitumen) and pyrite (FeS2) are significant sulphur sources. Typically, the sulphur in both phases is in a reduced state and is available through oxidation associated with combustion. The δ34S of dissolved sulphate in produced waters does not unequivocally identify either of the two major sources of sulphur. However, the relatively depleted δ34 values for SO4 suggest that the high sulphate concentrations generally associated with the approach of the combustion front result from the oxidation of pyrite.