CHEMICAL AND ISOTOPIC EXAMINATION OF PRODUCED WATERS FROM THE BP-WOLFLAKE IN-SITU COMBUSTION PILOT

The chemical and isotopic compositions of co-produced waters can be us ed to monitor the processes that take place during in situ combustion. Anticipated processes include mixing of waters, production of CO2, pr oduction of high concentrations of dissolved sulphate and variations i n water chemistry associated with heated zones. Water sources include pore waters in oil-bearing strata, waters in overlying or underlying a quifers, water condensed from previously injected steam, and waters as sociated with combustion. Waters from all sources may mix during produ ction and interpretation of the combustion process can be refined by a n understanding of water sources. Produced fluids from the BP-Wolf Lak e pilot site in Alberta have been examined to evaluate the effectivene ss 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 m ultiple sources of water and other processes, including water-rock rea ctions, act to modify water compositions. At least three sources of pr oduced waters can be recognized and these are interpreted to be format ion water, injected steam and waters that have low Cl and high HCO3 du e to combustion. It is not possible to distinguish waters in the oil-b earing formation from regional waters present in aquifers that underli e the stimulated intervals. Dissolved aqueous species, such as SiO2, N a, K (as Na/K) and Cl can be used to monitor the approach of the combu stion front. Sulphate has been suggested as an indicator of approachin g combustion and, although sulphate concentrations rise as combustion approaches a producing well, this indicator is not reliable in all cas es. The use of all the above chemical parameters is recommended for de tection 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 th at range from -8 to -25 parts per thousand delta(13)C show that oil ox idation is a major contributor of CO2 at high temperatures, but CO2 pr oduced by carbonate mineral dissolution becomes more significant as te mperature decreases. Sulphate concentrations in waters produced during combustion can be an order of magnitude higher than those observed du ring steam stimulation. Both the oil (bitumen) and pyrite (FeS2) are s ignificant sulphur sources. Typically, the sulphur in both phases is i n a reduced state and is available through oxidation associated with c ombustion. The delta(34)S of dissolved sulphate in produced waters doe s not unequivocally identify either of the two major sources of sulphu r. However, the relatively depleted delta(34) values for SO4 suggest t hat the high sulphate concentrations generally associated with the app roach of the combustion front result from the oxidation of pyrite.